Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1149/2.0581709jes

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DOI： 10.1016/j.ijhydene.2016.08.024

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DOI： 10.1021/acs.chemmater.6b01990

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DOI： 10.1038/srep13126

Language：English   Publishing type：Research paper (scientific journal)  

本研究は、燃料電池の高効率化に向けたシステム解析で、市販の燃料電池と比べ、約3割の発電効率向上が望めることを示した。高温型燃料電池システムにおいて、システム安定作動のために、発電に使用しなかった燃料を構造改良で活用しつつ、材料変更による燃料の希釈化を防ぐことで、相乗的な性能向上効果が得られ、供給した燃料のエネルギーの約80%を電気に変換可能である事を示すことができた。

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/srep12640

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DOI： 10.1038/srep11807

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Language：English   Publishing type：Research paper (scientific journal)  

To simulate realistic operating conditions in SOFC systems, we investigate the influence of thermal cycling on the performance of electrolyte-supported planar SOFCs. Thermal cycling is often associated with interruption of fuel supply, with three main modes; hot standby, cold standby, and shutdown. Cell performance degradation is most significant during shutdown cycles. Nickel oxidation and agglomeration are more pronounced when SOFCs are subjected to lower temperatures for longer periods of time, leading to significant performance degradation. Ostwald ripening at the anode leads to degradation as Ni grains increase in size with cycling. Ni particle precipitation on the anode zirconia grains and along electrolyte grain boundaries is found for the first time in shutdown cycling tests. When H₂S is mixed with the fuel, the internal reforming reactions and electrode reactions are inhibited by sulfur poisoning of the Ni anodes, accelerating degradation. The SOFC cycling degradation mechanisms are discussed in detail.

DOI： 10.1149/2.0421409jes

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijhydene.2012.08.062

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Language：English   Publishing type：Research paper (scientific journal)  

Carbon-free Pt-based electrocatalysts supported on semiconducting SnO ₂ (Pt/ SnO₂) have been developed for polymer electrolyte fuel cells (PEFCs) by various electrochemical and microstructural characterizations. Pt/ SnO₂ exhibited comparable current-voltage characteristics to conventional Pt electrocatalysts and, in particular, a considerable tolerance against 10,000 cycles of voltages up to 0.9 and 1.3 VRHE (RHE denotes reversible hydrogen electrode) vs the RHE. These results indicate that the carbon-free oxide-supported Pt/ SnO₂ can be a promising alternative electrocatalyst with long-term durability against voltage cycling up to higher potentials, as a possible fundamental solution to the PEFC degradation caused by carbon support corrosion.

DOI： 10.1149/1.3152325

Language：English   Publishing type：Research paper (scientific journal)  

Poisoning effects by various fuel impurities, including H₂S, CH₃SH, COS, Cl₂, and siloxane, to Ni-ScSZ cermet anodes have been analyzed and compared. Degradation of cell performance caused by these impurities was characterized by measuring cell voltage and anode polarization at a constant current density of 0.2 Acm^{- 2} for humidified H₂ and CH₄ fuels. Poisoning for hydrogen-based fuels containing 5 ppm sulfur compounds, H₂S, CH₃SH, and COS, caused an initial cell voltage drop of about 15 mV at 1000 °C. The initial voltage drop was independent of the kind of sulfur compounds, whereas in the case of poisoning by CH₃SH, an additional gradual decrease in cell voltage was clearly detected after the initial voltage drop. Thermochemical calculation and FESEM-EDX analysis also indicated that the poisoning by Cl₂ caused the formation of nickel nano-particles on zirconia grains via NiCl₂ (g), while the poisoning by siloxane formed segregated silica (SiO₂) in porous cermet anodes.

DOI： 10.1016/j.ssi.2008.02.062

Language：English   Publishing type：Research paper (scientific journal)  

The influence of H2 S fuel impurity on power generation characteristics of solid oxide fuel cells (SOFCs) has been analyzed by measuring cell voltage at a constant current density, as a function of H2 S concentration, operational temperature, and fuel gas composition. Reversible cell voltage change was observed around 1000°C, while fatal irreversible degradation occurred at a lower operational temperature, at a higher H2 S concentration, and at a lower fuel H2 CO ratio. Sulfur tolerance of SOFCs was improved by using Sc2 O3 -doped Zr O2 instead of Y2 O3 -doped Zr O2 as electrolyte and/or as electrolyte component in the anode cermets. It has been found that H2 S poisoning consists of at least two stages, i.e., an initial cell voltage drop within a short time period to a metastable cell voltage, followed by a gradual larger cell voltage drop. Possible H2 S poisoning processes are discussed.

DOI： 10.1149/1.2336075

Language：English   Publishing type：Research paper (scientific journal)  

One of the most attractive features of solid oxide fuel cells is their flexibility for fuels so that internal reforming and/or simple external reforming may be possible. In this study, equilibria in various possible fuel gases are considered, and C-H-O diagrams are constructed. Power generation characteristics for these fuels are measured and compared with those for simulated reformed gas in equilibrium compositions. We have succeeded to demonstrate direct-alcohol SOFCs for e.g. methanol, ethanol, and propanol. Mixtures of CH4 and CO2 are used as simulated biogas, and iso-octane (C8H18) and n-dodecane (C12H26) are highlighted as simulated gasoline and kerosene, respectively. Influence of fuel impurities on power generation characteristics is also discussed.

DOI： 10.1007/s10832-004-5174-z

Language：English   Publishing type：Research paper (international conference proceedings)  

Nanostructured Platinum-based electrode catalysts were prepared via in-situ colloidal impregnation for polymer electrolyte fuel cells. Crystallite size, grain size, and distribution of Pt nanoparticles on carbon support materials were characterized by XRD, TEM, high-resolution FESEM, and STEM. Effective surface area and kinetically-controlled current density of Pt electrode catalysts were analyzed by cyclic and hydrodynamic voltammetry using rotating disk electrodes. PEFCs with these electrode catalysts were also prepared and their I-V characteristics were examined at 80oC.

We have succeeded to develop Pt electrode catalysts with a diameter of a few nm, supported on carbon nanofibers with different structures (including herringbone-type fibers, platelet-type fibers, and highly-conductive vapor-grown fibers), carbon nanotubes, as well as carbon black. The dependencies of nanostructure and electrochemical properties on crystallographic structure of carbon support materials and preparation conditions of electrode catalysts are analyzed and discussed. Nanostructural design of PEFC electrode catalyst layers using carbon nanofibers as catalyst supports and electrode fillers is also discussed.

Language：English   Publishing type：Research paper (scientific journal)  

Power generation characteristics of solid oxide fuel cells (SOFCs) for alcohol-based fuels are measured and compared with those for simulated reformed gas in equilibrium compositions. We demonstrate direct-alcohol SOFCs for methanol, ethanol, propanol, and butanol. Current-voltage characteristics for the methanol-based fuel are comparable to those for the simulated reformed gas at 1000degreesC because of fast kinetics for thermal decomposition of methanol with H2O. With increasing carbon number of alcohols and/or with decreasing operational temperature of SOFCs, a decrease in cell voltage is observed associated with slower decomposition and/or reforming kinetics of alcohols.

DOI： 10.1149/1.1756884

Language：English   Publishing type：Research paper (scientific journal)  

Because chemical compositions in thermodynamic equilibrium for various fuel gases are specified by the ratios among carbon, hydrogen, and oxygen, carbon-hydrogen-oxygen (C-H-O) ternary diagrams are useful to present relevant operational conditions of fuel cells. Such C-H-O ternary diagrams are constructed, describing the carbon deposition region, the fuel gas region, partial pressures of gaseous species including O-2, H-2, and CO, and electromotive force corresponding to the theoretical open circuit voltage of fuel cells. Such ternary diagrams present these operational conditions for any given C-H-O ratios without any additional calculations.

DOI： 10.1149/1.1577338

Language：English   Publishing type：Research paper (scientific journal)  

Using thermochemical data of ca. 300 compounds consisting of carbon, hydrogen, and oxygen, the amounts of equilibrium products have been calculated for various fuel cell fuels including alkanes (CH4, C3H8, C8H18, C12H26), alcohols (CH3OH, C2H5OH, C3H7OH), alkenes, alicyclic hydrocarbons, and dimethyl ether, as well as for other hydrocarbon-containing fuels such as biogas and coke oven gas, in the temperature range between 100 and 1000degreesC. It has been revealed that the major constituents in typical fuel cell gases in thermodynamic equilibrium are H-2(g), H2O(g), CO(g), CO2(g), CH4(g), and C(s). Possible minor constituents are specified while their concentrations were negligibly low. Derived are the minimum amounts of H2O and CO2 for reforming and O-2 for partial oxidation, thermodynamically essential to prevent carbon deposition.

DOI： 10.1149/1.1577337

Language：English   Publishing type：Research paper (scientific journal)  

The low-temperature defect chemistry of oxides is considered, characterized by frozen-in interaction with the ambient oxygen and reversibility of internal interactions, in particular the redistribution of electronic carriers. Analytical relations describing ionic and electronic defect concentrations are derived for various conditions. The presence of redox-active, i.e., deep-level, dopants proves to be of special interest in this context. The analytical relations permit the detailed discussion of the dependencies of the charge carrier concentrations on the control parameters. Such analytical relations are useful for understanding and tailoring defect concentrations and thus related properties of electroceramics.

DOI： 10.1063/1.371542

Language：English   Publishing type：Research paper (scientific journal)  

Using oxides as examples, the defect chemistry is systematically analyzed for a low-temperature regime, at which the oxygen exchange equilibrium reaction is no longer reversible, while the internal defect equilibrium reactions (in particular, the electronic transfer processes) may still be reversible. For the partially frozen-in states as well as for the complete equilibrium cases, defect concentrations are numerically calculated for idealized model oxides including pure, acceptor-doped, and donor-doped oxides. Foreign ions (major/minor, shallow/deep, acceptor/donor), oxygen vacancies, and oxygen interstitials are taken into account as redox-active defects. The deep-level (redox-active) defects often dominate defect concentrations in the partially frozen-in states, while the major dopants fix the concentrations in complete equilibrium. The temperature and oxygen partial pressure dependencies of defect concentrations in the partially frozen-in states are discussed. The description does not only allow one to extend the defect chemistry to lower temperatures, such as room temperature, but also offers a quantitative basis for manipulation and prediction of defect concentrations in ionic crystals. Thereby, the physical and chemical performance of such materials may be controlled at temperatures lower than those at which the oxygen nonstoichiometry is established. The results are equally relevant for applications in solid state physics (e.g., compound semiconductors) and in solid state chemistry (e.g., solid electrolytes, mixed conductors).

DOI： 10.1063/1.371541

Language：English   Publishing type：Research paper (scientific journal)  

Microstructure, cathodic polarization, and ohmic resistance on the cathode side of ZrO₂-based solid oxide fuel cells have been studied for the intermediate temperature operation range between 700 and 900°C. Starting powder characteristics, powder calcination temperature, and sintering temperature strongly influence the final microstructure of cathodes. Electrochemical performance depends on these processing parameters as well as on the cathode thickness and the contact spacing of current collectors. A decrease in effective electrode area occurs both on the microscopic level with coarse and inhomogeneous cathode microstructure and on the macroscopic level with a wide contact spacing of the current collectors. The smaller effective electrode area causes inhomogeneous current density distribution and results consequently in higher ohmic losses originating from the electrolyte and higher cathodic polarization. These losses are evaluated using La_0.35Sr_0.15MnO₃ cathodes with different microstructures and on the ZrO₂-8 mole percent Y₂O₃ electrolyte. The influence of current path constrictions on the ohmic and nonohmic losses is demonstrated using Pt current collectors of different geometric spacings.

DOI： 10.1149/1.1836476

Language：English   Publishing type：Research paper (scientific journal)  

Phase equilibria in the system ZrO₂─InO_1.5 have been investigated in the temperature range from 800° to 1700°C Up to 4 mol%, InO_1.5 is soluble in t‐ZrO₂ at 1500°C. The martensitic transformation temperature m→t of ZrO₂ containing InO_1.5 is compared with that of ZrO₂ solid solutions with various other trivalent ions with different ionic radii. The diffusionless c→t′ A phase transformation is discussed. Extended solid solubility from 12.4 ± 0.8 to 56.5 ± 3 mol% InO_1.5 is found at 1700°C in the cubic ZrO₂ phase. The eutectoid composition and temperature for the decomposition of c‐ZrO₂ solid solution into t‐ZrO₂+InO_1.5 solid solutions were determined. A maximum of about 1 mol% ZrO₂ is soluble in bcc InO_1.5 phase. Metastable supersaturation of ZrO₂ in bcc InO _1.5 and conditions for phase separation are discussed.

DOI： 10.1111/j.1151-2916.1993.tb03661.x

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DOI： 10.1016/0022-3115(89)90604-1

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DOI： https://doi.org/10.1149/MA2023-02391923mtgabs

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DOI： https://doi.org/10.1149/11205.0121ecst

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DOI： https://doi.org/10.1149/11205.0141ecst

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DOI： https://doi.org/10.1149/11204.0215ecst

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DOI： https://doi.org/10.1149/11204.0315ecst

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DOI： https://doi.org/10.1149/11204.0353ecst

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DOI： https://doi.org/10.1149/11204.0369ecst

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DOI： https://doi.org/10.1149/11204.0083ecst

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DOI： https://doi.org/10.1149/11205.0129ecst

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DOI： https://doi.org/10.1149/MA2023-02401938mtgabs

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DOI： https://doi.org/10.1149/1945-7111/ad1165

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DOI： https://doi.org/10.1016/j.apsusc.2023.157136

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DOI： https://doi.org/10.1016/j.ijhydene.2023.03.195

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DOI： https://doi.org/10.1149/11106.1901ecst

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DOI： https://doi.org/10.1149/11106.0313ecst

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DOI： https://doi.org/10.1149/11106.0223ecst

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DOI： https://doi.org/10.1149/11106.1623ecst

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DOI： https://doi.org/10.1149/11106.0111ecst

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DOI： 10.1016/j.jpowsour.2023.232826

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DOI： 10.1016/j.jpowsour.2022.232500

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DOI： 10.1016/j.ijhydene.2022.09.172

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DOI： 10.1016/j.jpowsour.2022.232098

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DOI： 10.1016/j.jpowsour.2022.232098

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DOI： 10.1149/10911.0071ecst

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DOI： 10.1149/10911.0003ecst

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DOI： 10.1016/j.micromeso.2022.112141

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DOI： 10.1016/j.micromeso.2022.112141

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DOI： 10.1016/j.memsci.2022.120734

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DOI： 10.1149/10909.0241ecst

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DOI： 10.1149/10909.0349ecst

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DOI： 10.1149/10909.0369ecst

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DOI： 10.1149/10911.0025ecst

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DOI： 10.1016/j.ijhydene

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DOI： 10.3390/membranes12070658

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DOI： 10.1016/j.ijhydene.2022.03.164

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DOI： 10.1149/1945-7111/ac662d

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DOI： 10.1149/10408.0359ecst

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DOI： 10.1016/j.ijhydene.2021.09.025

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DOI： 10.1149/10301.1981ecst

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DOI： 10.1149/10301.2033ecst

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DOI： 10.1149/10301.2007ecst

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DOI： 10.1039/D0TA12150A

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DOI： 10.1016/j.jpowsour.2021.229600

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DOI： 10.1021/acsanm.1c00006

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DOI： 10.3390/molecules26030724

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DOI： 10.1007/s10570-020-03593-w

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DOI： 10.1002/app.50328

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DOI： 10.1149/09809.0687ecst

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DOI： 10.1149/09809.0517ecst

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DOI： 10.1149/09809.0573ecst

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DOI： 10.1149/1945-7111/abb37d

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DOI： 10.1149/1945-7111/ababd8

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Repeated reduction and oxidation of metallic nickel in the anodes of solid oxide fuel cell (SOFC) causes volume changes and agglomeration. This disrupts the electron conducting network, resulting in deterioration of the electrochemical performance. It is therefore desirable to develop more robust anodes with high redox stability. Here, new cermet anodes are developed, based on nickel alloyed with Co, Fe, and/or Cr. The stable phases of these different alloys are calculated for oxidizing and reducing conditions, and their electrochemical characteristics are evaluated. Whilst alloying causes a slight decrease in power generation efficiency, the Ni-alloy based anodes have significantly improved redox cycle durability. Microstructural observation reveals that alloying results in the formation of a dense oxide film on the surface of the catalyst particle (e.g. Co-oxide or a complex Fe-Ni-Cr oxide). These oxide layers help suppress oxidation of the underlying nickel catalyst particles, preventing oxidation-induced volume changes/agglomeration, and thereby preserving the electron conducting pathways. As such, the use of these alternative Ni-alloy based cermets significantly improves the redox stability of SOFC anodes.

DOI： 10.1149/1945-7111/abac87

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Membrane electrode assemblies (MEAs) with various catalyst-loading were made using a standard 46.5%Pt/KB (TEC10E50E) with the aim of developing low Pt-loading PEFCs. Since increase in concentration overvoltage for low catalyst-loading MEAs was observed, the mechanism of such increase in concentration overvoltage was considered, and then reducing concentration overvoltage was tried through two methods, by adding extra carbon and using a more hydrophobic carbon support, graphitized Ketjen black, within the cathode layer (CL). As a result, we have experimentally proven that increase in hydrophobicity of CL can promote water drain and reduce the concentration overvoltage for low catalyst-loading MEAs, and hydrophobicity is an important factor to be controlled within CL in addition to porosity.

DOI： 10.1149/1945-7111/abac28

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Carbon-based electrodes in polymer electrolyte fuel cells (PEFCs) are prone to corrosion. Therefore, alternative "carbon-free"materials are required. Here, the use of a catalyst-coated porous metal support is proposed as a gas diffusion electrode. As a proof-of-concept, commercially available porous titanium sheets comprising sintered titanium fibers are chemically etched with NaOH, followed by heat treatment. This results in the formation of oxidized titanium nanostructures (such as nanosheets and nanotubes) at the surface. Subsequently, platinum decoration is performed via arc plasma deposition (APD). This porous composite structure is then attached to the membrane, and used as the gas diffusion electrode for PEFC membrane electrode assemblies (MEAs). This concept integrates the catalyst, catalyst support, gas diffusion layer, and current collector in a single structure, cutting down on the number of cell components and reducing total device thickness. The carbon-free nature of this integrated gas diffusion electrode is demonstrated to successfully avoid carbon corrosion during start-stop potential cycling over 60,000 potential cycles. However, further improvements in initial electrochemical activity are still required.

DOI： 10.1149/1945-7111/ab9cd4

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Solid oxide fuel cells (SOFCs) with proton-conducting solid electrolyte, instead of the oxide-ion conducting solid electrolyte have attracted attentions because of their high potential to reduce operating temperatures and to enhance the electrical efficiencies of SOFCs. In addition, the proton-conducting SOFCs with multistage electrochemical oxidation configuration will be promising technology for critically-high electric efficiencies. However, it is known that there are non-negligible charge -carriers other than protons in typical proton-conducting solid oxide electrolytes at relatively high temperatures. The existence of the partial conductivities of holes and/or electrons will cause the internal leakage current that consumes fuel but never generates any electrical power output. The higher ratio of the leakage current to external current will more deteriorate the electrical efficiency. In this study, the effects of blocking -layers formed on the air side surface of base electrolyte layer consisting of BaZr_0.8Y_0.2O_3-δ (BZY82) for suppressing the leakage current have been investigated by using electrochemical parameters of the partial conduction of the materials. The chemical potential profile and leakage current showed large dependence on the material of the blocking-layer. Lanthanum tungstate was found to play a role as unique and strong blocking-layer against the leakage current.

DOI： 10.1149/1945-7111/ab904f

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Porous Pt electrocatalysts have been developed as an example of carbon-free porous metal catalysts in anticipation of polymer electrolyte membrane (PEM) fuel cells and PEM water electrolyzers through the assembly of the metal precursor and surfactant. In this study, porous Pt was structurally evaluated and found to have a porous structure composed of connected Pt particles. The resulting specific electrochemical surface area (ECSA) of porous Pt was 12.4 m² g⁻¹, which was higher than that of commercially available Pt black. Accordingly, porous Pt showed higher oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity than Pt black. When the activity was compared to that of a common carbon-supported electrocatalyst, Pt/ketjen black (KB), porous Pt showed a comparable ORR current density (2.5 mA cm⁻² at 0.9 V for Pt/KB and 2.1 mA cm⁻² at 0.9 V for porous Pt), and OER current density (6.8 mA cm⁻² at 1.8 V for Pt/KB and 7.0 mA cm⁻¹ at 1.8 V), even though the ECSA of porous Pt was only one-sixth that of Pt/KB. Moreover, it exhibited a higher durability against 1.8 V. In addition, when catalyst layers were spray-printed on the Nafion^® membrane, porous Pt displayed more uniform layers in comparison to Pt black, showing an advantage in its usage as a thin layer.

DOI： 10.3390/molecules25102398

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DOI： 10.1088/1361-6528/ab76ed

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DOI： 10.1039/c9nj06156h

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DOI： 10.1002/er.5265

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Numerical simulations can be used to visualize and better understand various distributions such as gas concentration and temperature in solid oxide fuel cells (SOFCs) under realistic operating conditions. However, pre-existing models generally utilize an anode exchange current density equation which is valid for humidified hydrogen fuels – an unrealistic case for SOFCs, which are generally fueled by hydrocarbons. Here, we focus on developing a new, modified exchange current density equation, leading to an improved numerical analysis model for SOFC anode kinetics. As such, we experimentally determine the exchange current density of SOFC anodes fueled by fully pre-reformed methane. The results are used to derive a new phenomenological anode exchange current density equation. This modified equation is then combined with computational fluid dynamics (CFD) to simulate the performance parameters of a three-dimensional electrolyte-supported SOFC. The new modified exchange current density equation for methane-based fuels reproduces the I–V characteristics and temperature distribution significantly better than the previous models using humidified hydrogen fuel. Better simulations of SOFC performance under realistic operating conditions are crucial for the prediction and prevention of e.g. fuel starvation and thermal stresses.

DOI： 10.1016/j.ijhydene.2019.12.089

Language：English   Publishing type：Research paper (scientific journal)  

We developed polymer electrolyte membranes (PEMs) utilizing charge-transfer (CT) interactions for polymer electrolyte fuel cells (PEFCs). CT complex formation was applied to control the position of proton conductive groups in the membranes. To understand the effect of CT complex formation on PEM performance, heat treatment was performed to enhance the extent of CT complex formation in the membrane. In this work, sulfonated polyimide (SPI) was used as the electron-accepting polymer, while polyether-containing electron-rich dialkoxynaphthalene (Poly-DAN) was used as the electron-donating polymer. After heat treatment at 150 °C for 50 h, the concentration of CT complex in the membrane was significantly enhanced by about 13 times. Heat-treated SPI/Poly-DAN membranes showed higher mechanical strength (50.8 MPa) than Nafion 212 (15.5 MPa) and highly chemical durability compared to the untreated membrane by the synergetic effect of enhanced CT complex formation and chemical cross-linking. Heat-treated SPI/Poly-DAN membranes also showed reasonable proton conductivity (32.3 mS cm^-1, 80 °C, and 90% RH), although some cross-linking occurred between sulfonic acid units due to the heat treatment process. In single cell tests, heat-treated SPI/Poly-DAN membranes had maximum power densities of 255 mW cm^-2 at 80 °C and 95% RH and 59.0 mW cm^-2 at 110 °C and 31% RH, indicating that these heat-treated CT complex membranes could be used for fuel cell applications.

DOI： 10.1021/acsaem.9b01697

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DOI： 10.1149/09208.0833ecst

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DOI： 10.1149/09208.0493ecst

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DOI： 10.1149/09208.0507ecst

Language：English   Publishing type：Research paper (scientific journal)  

The oxygen deficiency or excess, as reflected in the nonstoichiometry of oxide films, plays a crucial role in their functional properties for applications such as micro solid oxide fuel cells, catalysis, sensors, ferroelectrics, and memristors. High concentrations of oxygen vacancies may be beneficial or detrimental according to the application, and hence there is interest in controlling the oxygen content of films without resorting to compositional changes. Here, we demonstrate that substantial changes in the nonstoichiometry of Pr_0.1Ce_0.9O_2-δ (PCO), a model mixed ionic electronic conductor, can be achieved by fabricating multilayers with an inert material, SrTiO₃ (STO). We fabricated heterostructures using pulsed laser deposition, keeping the total thickness of PCO and STO constant while varying the number of layers and thickness of each individual layer, to probe the effects of the PCO/STO interfaces. Conductivity measurements as a function of oxygen partial pressure (P_O2 ) and temperature showed a significant weakening of the P_O2 dependence compared to bulk PCO, which scaled with the density of interfaces. We confirmed that this change was due to variations in nonstoichiometry, by optical transmission measurements, and show that the lower oxygen content is consistent with a decrease in the effective oxygen reduction enthalpy of PCO. These results exemplify the dramatic differences in properties between films and their bulk counterparts, achievable by interface engineering, and provide generalized insight into tailoring the properties of mixed ionic electronic conductors at the nanoscale.

DOI： 10.1021/acsami.9b08864

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DOI： 10.1149/09101.0847ecst

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DOI： 10.1149/09101.1009ecs

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DOI： 10.1149/09101.1889ecst

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DOI： 10.1149/09101.1905ecst

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DOI： 10.1149/09101.2299ecst

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/fuce.201800187

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/fuce.201800181

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.actamat.2018.12.058

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DOI： 10.1016/j.ijhydene.2019.02.134

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijhydene.2019.02.136

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1021/acsami.8b21285

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.electacta.2019.01.108

Language：English   Publishing type：Research paper (other academic)  

Surface oxide layer of Fe-Cr-Al alloy was investigated to apply for metal support material of SOFCs. We already found that electrical resistance of the surface oxide layer can be decreased by La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) coating and heat-treatment. The morphology of the surface oxide layer changed to a columnar structure consisting of γ-Al₂O₃ polycrystal and Sr₃Al₂O₆ growing outward in the same direction. In this study, we investigated heat-treatment condition to increase durability of the oxide layer. The Fe-Cr-Al alloy was firstly coated with LSCF and pre-heat treated in a vacuum at 1000^oC for 1 h. Stability of mass gain and electrical resistance in air at 700^oC was significantly improved. The morphology of the surface oxide layer was a complex structure consisting of α-Al₂O₃/Sr₃Al₂O₆.

DOI： 10.1149/09101.2299ecst

Language：English   Publishing type：Research paper (other academic)  

In order to improve the stability under high fuel utilization, alternative anodes are fabricated with ionic (mixed) conducting GDC (Ce_0.9Gd_0.1O₂) and electronic conducting LST (Sr_0.9La_0.1TiO₃), both of which act as stable ion- and electron-conducting frameworks against reduction-oxidation (redox) cycles, respectively. Noble metal catalyst nanoparticles (Rh, Pt, or Pd) are incorporated via impregnation with GDC on the LST-GDC backbones. The electrochemical characteristics, such as the stability against redox cycling and under high fuel utilization, of SOFC single cells using these anodes are characterized in humidified H₂ at 800°C. Moreover, the changes of the noble metal catalyst nanoparticles before/after the high fuel utilization durability tests are analyzed and discussed.

DOI： 10.1149/09101.1905ecst

Language：English   Publishing type：Research paper (scientific journal)  

Perovskite La_0.9Sr_0.1Ga_0.9Mg_0.1O_3-δ (LSGM) is one of the fastest known oxide ion conductors, with reported enhanced p-type electronic transference numbers at grain boundaries, attributed to space charge effects. As this material is applied as a solid oxide fuel/electrolysis cell electrolyte, it is of interest to learn how its mixed conductivity may be tailored. Field assisted sintering technique/spark plasma sintering (FAST/SPS) and conventional sintering without field or pressure were employed to prepare pellets with various grain sizes, in order to systematically assess the influence of processing route on the mixed conductivity. AC-impedance spectroscopy and the brick layer model were applied to determine local conductivities as a function of temperature, oxygen partial pressure, and dc bias. With increasing sintering temperature and grain size, the following trends were observed: larger electrical grain boundary (GB) widths, higher GB potentials, lower specific GB conductivity, greater dc-bias dependence of GB conductivity, higher pO2-dependence of GB conductivity indicating higher electronic transference numbers, and lower pre-exponential factor for specific GB conductivity. These results suggest an increasing GB space charge effect with increasing sintering temperature/grain size, which coincided with increasing compositional uniformity by TEM and EDS. The results confirm that sintering route is an important variable for tailoring mixed conduction.

DOI： 10.1149/2.0151910jes

Language：English   Publishing type：Research paper (scientific journal)  

An analytical method to determine the electrochemical energy efficiencies of electrolytes with partial electronic conduction has been developed previously and reported in the literature. However, this analytical method does not address the effects of differing ionic species in electrolytes, i.e., the oxide-ions or protons. Therefore, we aimed to modify this analytical method to account for the effects of differing ionic species, and applied it to compare the energy efficiencies of oxide-ion conducting solid electrolytes such as yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) to proton-conducting solid electrolytes, such as yttria-doped barium zirconate (BZY). With the modification, difference in the influence of the fuel consumption between the oxide-ion conducting electrolyte and the proton-conducting electrolyte has been successfully taken into account. The energy efficiency of the BZY electrolyte relatively increased against those of YSZ or GDC electrolytes by the modification. Additionally, partial oxide-ion conduction in the proton-conducting electrolyte was successfully estimated using the modified analytical method.

DOI： 10.1002/fuce.201800181

Language：English   Publishing type：Research paper (other academic)  

Solid oxide fuel cells (SOFCs) with proton-conducting solid electrolyte, instead of the oxide-ion conducting solid electrolyte have attracted attentions because of their high potential to reduce operating temperatures and to enhance the electrical efficiencies of SOFCs. In addition, the proton-conducting SOFCs with multistage electrochemical oxidation configuration will be promising technology for critically-high electric efficiencies. However, it is known that there are non-negligible charge -carriers other than protons in typical proton-conducting solid oxide electrolytes at relatively high temperatures. The existence of the partial conductivities of holes and/or electrons will cause the internal leakage current that consumes fuel but never generates any electrical power output. The higher ratio of the leakage current to external current will more deteriorate the electrical efficiency. In this study, the effects of blocking -layers formed on the air side surface of base electrolyte layer consisting of BaZr_0.8Y_0.2O_3-δ (BZY82) for suppressing the leakage current have been investigated by using electrochemical parameters of the partial conduction of the materials. The chemical potential profile and leakage current showed large dependence on the material of the blocking -layer. Lanthanum tungstate was found to play a role as unique and strong blocking -layer against the leakage current.

DOI： 10.1149/09101.1009ecst

Language：English   Publishing type：Research paper (other academic)  

The influence of current load on the growth of SrZrO₃ formed at the interface of a gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte was analyzed using high-resolution electron microscopy. A cell with LNF (LaNi_0.6Fe_0.4O₃) cathode was prepared after removing the LSCF cathode using HCl. The LNF cathode was used to eliminate the effect of Sr diffusion during cell operation. These cells were operated under 0.2 A cm⁻² at 800°C. For the cell with the LNF cathode, no significant change was observed in the amount of SrZrO₃. At the SrZrO₃/GDC interface, crystal orientation was the same from the GDC side to the SrZrO₃ side. Before cell operation, the GDC grain had some defects and no clear boundary was distinguished between GDC and SrZrO₃. After cell operation, the SrZrO₃/GDC interface was clearer and crystallization of SrZrO₃ proceeded.

DOI： 10.1149/09101.0847ecst

Language：English   Publishing type：Research paper (other academic)  

Solid oxide fuel cell (SOFC) is a promising electrochemical energy conversion device that can directly produce electricity from chemical fuels. On the other hand, in the Ni-zirconia cermet currently used for the SOFC anode, the electron conducting pathways through the Ni metal phase can be easily destroyed by redox processes, where Ni oxidation/reduction (redox) results in significant volume changes, leading to deterioration of the electrochemical performance. In this study, various anodes using Ni-based alloys as alternative materials for Ni are prepared. Their electrochemical performance and redox stability are evaluated. In particular, Ni-Co alloy cermet exhibits better durability against redox cycling.

DOI： 10.1149/09101.1889ecst

Language：English   Publishing type：Research paper (scientific journal)  

Achievements of NEDO durability projects on SOFC mode are summarized with a focus on the physicochemical mechanisms characterized by diffusion properties of cell components and chemical reactions of cell components with gaseous impurities. Ni sintering and depletion including impurity (P, B, S) effects have been examined in terms of the surface/interface energies of Ni/oxide cermet anodes. The conductivity degradation due to the transformation of the cubic YSZ electrolyte was found to be characterized in terms of two time constants for the reductive and the oxidative regions to be determined by the Y-diffusivity and its enhancement on NiO internal reduction in YSZ, while observed gaps in conductivity degradation behavior between stacks and button cells were ascribed to differences in those physicochemical properties involved, namely cation diffusion and kinetics associated with NiO internal reduction. The cathode performance degradation due to sulfur poisoning exhibits a variety of dependences on the microstructure (dense or porous) of doped-ceria interlayers, the thickness of YSZ electrolyte and the humidity in the anode atmosphere, suggesting effects of protons in the cathode vicinity and the SrO activity changes during fabrication the LSCF/GDC/YSZ multilayers. Some defect chemical considerations were made on how such defects are affected by fabrication processes.

DOI： 10.1002/fuce.201800187

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DOI： 10.1149/08613.0541ecst

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DOI： 10.1149/08613.0719ecst

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DOI： 10.1149/08613.0497ecst

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DOI： 10.1149/08613.0673ecst

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DOI： 10.1149/08613.0461ecst

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DOI： 10.1149/08613.0531ecst

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DOI： 10.1149/08613.0069ecst

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1149/2.1081809jes

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DOI： 10.1016/j.dib.2018.02.031

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DOI： 10.1016/j.ijhydene.2018.01.118

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DOI： 10.1016/j.jpowsour.2017.07.025

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DOI： 10.1149/2.0791803jes

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DOI： 10.1016/j.memsci.2017.11.025

Language：English   Publishing type：Research paper (scientific journal)  

SrZrO₃ formation at the interface of gadolinia-doped ceria (GDC) interlayer and yttria-stabilized zirconia (YSZ) electrolyte is analyzed using high-resolution electron microscopy. SrZrO₃ is dispersed in the inter-diffusion layer on the GDC side from the Ce/Zr border. Zr, which diffuses into the GDC grain, contributes to the formation of SrZrO₃. The crystallographic relationship among the SrZrO₃ grains and its neighboring GDC grains reveals that SrZrO₃ is formed at the surface, at the grain boundary, and inside the grain, while maintaining a highly matched boundary with the adjacent GDC grain. The matching of the interface boundary is confirmed by the O-lattice theory, according to which the threshold Zr/Ce ratio is 13/34. If Zr/Ce ratio in the GDC grain is higher than the threshold, SrZrO₃ may significantly grow into the grain. The conduction path for the oxygen ion is retained because the GDC grain containing Zr is split into the SrZrO₃ grain and the less-Zr-containing GDC grain. If Zr/Ce ratio is lower than the threshold, SrZrO₃ may be formed but will be limited by the amount of Zr diffusing from the adjacent region. Thus, the morphology of SrZrO₃ is strongly affected by the state of GDC grains in the inter-diffusion layer.

DOI： 10.1149/2.0551811jes

Language：English   Publishing type：Research paper (other academic)  

In order to improve the mass productivity of polymer electrolyte fuel cells (PEFCs), it is useful to have a fully-integrated structure from electrocatalysts to gas diffusion layer. Although carbon support is widely used for PEFCs, carbon corrosion may occur on the cathode side. To solve these technological issues, in this study, non-carbon support is directly prepared on the microporous layer (MPL) coated on the gas diffusion layer (GDL) by arc plasma deposition (APD). Pt catalyst nanoparticles are then deposited by the APD on the non-carbon support to prepare a "Pt/non-carbon support/MPL/GDL" integrated structure. Microstructure of the electrocatalyst layers and electrochemical properties of such MEAs are analyzed.

DOI： 10.1149/08613.046ecst

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijhydene.2017.10.122

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DOI： 10.1002/app.46087

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DOI： 10.1016/j.memsci.2017.07.012

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DOI： 10.1016/j.jpowsour.2017.10.009

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DOI： 10.1149/08008.0907ecst

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DOI： 10.1149/08008.0897ecst

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DOI： 10.1149/08008.0789ecst

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DOI： 10.1149/08008.0781ecst

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DOI： 10.1149/08008.0773ecst

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DOI： 10.1016/j.jpowsour.2017.05.077

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DOI： 10.1002/fuce.201600186

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DOI： 10.1149/07801.0441ecst

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DOI： 10.1149/07801.2467ecst

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DOI： 10.1149/07801.2563ecst

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DOI： 10.1149/07801.1253ecst

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DOI： 10.1149/07801.1179ecst

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DOI： 10.1149/07801.2431ecst

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DOI： 10.1149/07801.0993ecst

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DOI： 10.1149/07801.0197ecst

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DOI： 10.1149/07801.2497ecst

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DOI： 10.1149/07801.2069ecst

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DOI： 10.1149/07801.1523ecst

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DOI： 10.1149/2.0071710jes

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DOI： 10.1016/j.ijhydene.2017.04.267

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DOI： 10.1149/2.0711707jes

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DOI： 10.1016/j.ijhydene.2016.10.123

Language：English   Publishing type：Research paper (scientific journal)  

Ru-based solid oxide fuel cell (SOFC) cermet anodes are presented. The preparation conditions of the Ru-based anodes are adjusted by preventing the sublimation of Ru-oxides at elevated temperatures by using an oxidizing atmosphere. SOFC single cells with zirconia-electrolyte and cathode are prepared, and the electrochemical performance is examined using realistic fuels containing low concentrations of higher hydrocarbons and trace sulfur impurities. The degradation rate is relatively high under simulated high fuel utilization operating conditions. However, under the operational condition near the fuel inlet of SOFC systems, the Ru-based anode satisfies 5000-h durability by using hydrocarbon-containing fuels. While a much higher durability is needed for stationary applications, the cells with the Ru-based anode may be applicable to e.g. automobile applications with hydrocarbon-containing fuels as high internal reforming activity, carbon deposition tolerance, and sulfur impurity tolerance have been verified.

DOI： 10.1016/j.ijhydene.2017.01.028

Language：English   Publishing type：Research paper (scientific journal)  

The stable conductive oxide Sr_0.9La_0.1TiO₃ (SLT), instead of the conventional Ni-zirconia, is applied as supporting porous substrate for an SOFC single cell. Four different composite anode materials: Ni-SSZ (SSZ: (ZrO₂)_0.89(Sc₂O₃)_0.1(CeO₂)_0.01), Ni-GDC (GDC: Ce_0.9Gd_0.1O₂), Ni-GDC-SLT, and GDC-SLT were prepared. The porous thick Sr_0.9La_0.1TiO₃ substrate was stable enough during 50 redox cycles so that OCV showed no decrease during the cycling test. Moreover, the electrochemical performance, redox stability, and performance degradation during the cycling tests were mainly determined by the different electrical and catalytic properties of the composite anode materials.

DOI： 10.1016/j.ijhydene.2016.11.160

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DOI： 10.1016/j.ijhydene.2016.12.066

Language：English   Publishing type：Research paper (scientific journal)  

We investigated the treatment of Fe-Cr-Al alloy for application in solid oxide fuel cells (SOFCs). The electrical resistance of the Al₂O₃-based surface oxide layer on the alloy decreased and was stable when La_0.6Sr_0.4Co_0.2Fe_0.8O₃(LSCF), La_0.8Sr_0.2MnO₃(LSM), LaNi_0.6Fe_0.4O₃(LNF), or Pr_0.8Sr_0.2MnO₃(PrSM) were first coated on the alloy and heat treated at 700 °C in air. The activation energy, calculated from the resistance, also suggested that the surface oxide became more conductive with treatment. The surface oxide layer after treatment had a microstructure of columns growing outward in the same direction, containing small amounts of elements such as Sr, Ni, Fe, La, Mn, and Pr. The microstructure consists of polycrystalline γ-Al₂O₃and small amounts of Al compounds with these elements. In the case of the LNF coating, the formation of NiAl₂O₄was observed_.The enhanced electrical conductivity may have resulted from the arrangement of the columnar structure, along with the electronic conduction path generated by the reaction of γ-Al₂O₃with these elements.

DOI： 10.1002/fuce.201600038

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1007/s11581-016-1816-9

Language：English  

Solid oxide fuel cells (SOFCs) are the most flexible fuel cells, and internal reforming and/or simple external reforming may be possible. In this chapter, the multifuel capability of SOFCs is considered from both thermochemical and kinetic viewpoints. Equilibria in various possible SOFC fuel gases are examined. Power generation characteristics for various types of fuels are described. Extrinsic degradation affected by fuel impurities and intrinsic degradation during long-term and cycle operation are also described and discussed.

DOI： 10.1016/B978-0-08-101102-7.00003-9

Language：English   Publishing type：Research paper (other academic)  

Electrocatalyst layers in polymer electrolyte fuel cells (PEFCs) have complicated 3-dimensional microstructure, so that control and optimization of the structure are essential for higher cell performance. Carbon black supported cathode electrocatalyst is widely used, whilst carbon black support can be degraded through electrochemical oxidation on the cathode side. In this study, stable and electronic conductive Nb-doped SnO₂ support deposited on conductive carbon nanotube (CNT) filler is considered as an alternative support material for increasing cell performance. Consequently, high current-voltage (I-V) performance and voltage cycle durability of membrane electrode assemblies (MEAs) using Nb-doped SnO₂ supports deposited on CNTs are successfully demonstrated under the PEFC operational condition.

DOI： 10.1149/08008.0897ecst

Language：English   Publishing type：Research paper (scientific journal)  

The rate of oxygen exchange at the surface of mixed conductors is a critical property impacting the performance of elevated temperature energy conversion/storage devices. Microstructural features, such as grain boundary density and crystalline quality, are expected to impact the surface exchange kinetics, but their effect has not yet been widely studied. In this work, mixed conducting perovskite SrTi_0.65Fe_0.35O_3-δ (STF35) thin films grown by pulsed laser deposition were applied as a model system to systematically study the effect of microstructure on oxygen surface exchange kinetics. The impact of growth temperature on crystalline quality, orientation, grain size, surface roughness, and surface chemistry was evaluated by X-ray diffraction, scanning probe microscopy, transmission electron microscopy, and angle-resolved X-ray photoelectron spectroscopy (AR-XPS). A contact-free, continuous, in situ optical transmission relaxation approach was then applied to quantify the films' native surface oxygen exchange coefficients (k_chem). Amorphous films, grown at low temperatures (25 °C), did not exhibit measurable oxygen exchange ability. Highly crystalline films, grown at high temperatures (800 °C), exhibited reasonable, but not optimal k_chem. The most rapid k_chem was found for intermediate growth conditions, i.e., for amorphous-grown thin films just after crystallization at higher temperatures (550 °C) or for films grown near (580 °C) the crystallization temperature. Combined with the AR-XPS results showing greater surface Sr concentrations in films grown at higher temperatures, results suggest rapid k_chem is obtained as a trade-off between good crystalline quality and low Sr surface concentration. Degradation of k_chem over time was correlated to increased Sr surface concentration. Additionally, (100)-oriented epitaxial vs. nano-columnar grained (110)-oriented thin films with excellent crystalline quality exhibited very similar k_chem and aging behavior, suggesting that neither grain boundaries nor film orientation cause observable changes in surface exchange kinetics in this composition.

DOI： 10.1039/c7ta04940d

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1002/ente.201600576

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DOI： DOI: 10.1002/fuce.201600038

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DOI： 10.1016/j.electacta.2016.10.090

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DOI： 10.1002/macp.201600397

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DOI： 10.1016/j.ijhydene.2016.07.265

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DOI： 10.1016/j.jpowsour.2016.07.059

Language：English   Publishing type：Research paper (scientific journal)  

Solid oxide fuel cells (SOFCs) have the potential to efficiently convert chemical energy into electricity and heat and are expected to be implemented in stationary combined heat and power (CHP) systems. This paper presents the heat balance analysis for a 5-kW medium-sized integrated SOFC system and the evaluation of the effect of anode gas recirculation on the system performance. The risk of carbon deposition on an SOFC anode due to anode gas recirculation is also assessed using the C-H-O diagram obtained from thermodynamic equilibrium calculations. These results suggest that a higher recirculation ratio increases net fuel utilization and improves the electrical efficiency of the SOFC system. Furthermore, cost simulation of the SOFC system and comparison with the cost of electricity supply by a power grid indicates that the capital cost is sufficiently low to popularize the SOFC system in terms of the total cost over one decade.

DOI： 10.1016/j.jpowsour.2016.06.045

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1016/j.ijhydene.2016.05.160

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DOI： DOI: 10.1149/2.0141607jes

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (international conference proceedings)  

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DOI： 10.1149/2.0631609jes

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DOI： 10.1016/j.ijhydene.2016.05.216

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DOI： 10.1016/j.jpowsour.2016.04.111

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DOI： 10.1016/j.jpowsour.2016.03.108

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DOI： 10.1016/j.memsci.2016.02.017

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DOI： 10.1021/acsami.6b01670

Language：English  

SOFCs have the solid-state ceramic construction and operate at high-temperatures, with flexibility in fuel choice, high efficiency, stability, and reliability. The most attractive characteristics of SOFCs should be high fuel-to-electricity conversion efficiencies of as high as 50 to 60 percent LHV. For further improving the electrical efficiencies, preceding studies on multi-stage electrochemical oxidation with SOFCs have been reported. However, there are many parameters for the multi-stage oxidation, and effects of the parameters on the efficiency remains to be identified. We have investigated the multi-stage oxidation by using a symbolic analysis method. In the case of n-stage electrochemical oxidation, the fuel utilization ratio in the individual stage was found to decrease with increasing the n value at a fixed fuel utilization ratio of an entire system, resulting in the enhancement of robustness against the operation at a high fuel utilization ratio of the entire system as well as against a gas-leakage.

DOI： 10.1002/9781119234531.ch4

Language：English   Publishing type：Research paper (scientific journal)  

Novel charge-transfer (CT) complex hybrid films for high temperature polymer electrolyte fuel cell (PEFC) application based on sulfonated polyimide (SPI) with hydrophobic unit (4,4 (sulfonylbis(4,1-phenylene)bis(oxy)dianyline (BAPPS)) are synthesized and characterized. In this study, property changes of the polymer electrolytes prepared by a unique CT complex postmodification process are evaluated. The effect of the CT complex formation in the obtained films on the properties of the polymer films is evaluated, because the CT complex can work as a binder between SPIs. Water uptake of the CT complex hybrid films shows a lower value than that of the original SPI film. The CT complex hybrid films also exhibit comparable proton conductivity to Nafion 115 under high-temperature operational conditions (100-120 °C). These results suggest that the CT complex hybrid films developed are promising alternatives for high temperature PEFC application.

DOI： 10.1002/macp.201500320

Language：English   Publishing type：Research paper (other academic)  

The electrocatalyst layer of the polymer electrolyte fuel cells (PEFCs) has a complicated porous microstructure. Since the porous nanostructure allows the transport of reactant gas, water vapor, protons, and electrons, cell performance depends strongly on their 3D microstructure. Understanding the correlations between the electrocatalyst microstructure and the cell performance is therefore essential for further improving the PEFC electrochemical performance. In this study, these correlations are quantitatively considered by using the focused-ion-beam coupled scanning electron microscopy (FIB-SEM) technique. In particular, while it is difficult to distinguish between the solid and the pore of the electrocatalyst layer due to the complicated porous nanostructure, this problem has been solved by the auto-image thresholding technique.

DOI： 10.1149/07514.0347ecst

Language：English   Publishing type：Research paper (other academic)  

Electrocatalyst layer in polymer electrolyte fuel cells (PEFCs) has complicated 3D nanostructure, and control and optimization of the structure are essential for higher cell performance. Carbon black supported cathode catalyst is widely used, whilst carbon support can be degraded through electrochemical oxidation on the cathode side. The use of Nb-doped SnO₂ supports for Pt electrocatalysts prevents support corrosion, while the use of carbon nanotube (CNT) based conductive fillers assists electronic transport in the electrocatalyst layers. High I-V performance of membrane electrode assemblies (MEAs) using stable SnO₂ supports deposited on conductive CNT fillers is successfully demonstrated under the strongly acidic PEFC operational condition.

DOI： 10.1149/07514.0841ecst

Language：English   Publishing type：Research paper (other academic)  

Overpotentials included in electrochemical hydrogen pump are separated with using a reference electrode. Separation result shows that non-ohmic overpotential in cathode is larger than that in anode. This result is also confirmed by electrochemical impedance spectra measurement, which shows impedance spectra of cathode is larger than that of anode. Volmer-Heyrovsky-Tafel mechanism is used to explain the separation result. Simulation result with using this mechanism suggests that reactions in anode and cathode are dominated by different mechanisms, and reaction rate of cathode is 2 orders of magnitude slower than that of anode.

DOI： 10.1149/07514.1155ecst

Language：English   Publishing type：Research paper (other academic)  

SrTi_o65Fe_o35C_3.δ (STF35) thin films grown by pulsed laser deposition were applied as a model system to explore the impact of microstructure on oxygen surface exchange kinetics, of importance for solid oxide cell electrodes. The impact of growth temperature on crystalline quality, orientation, grain size, and surface roughness was evaluated. A contact-free, continuous, in situ Optical Transmission Relaxation (OTR) approach was applied to quantify the films' native surface oxygen exchange coefficients (k_chem). In situ crystallization was used to obtain fast surface exchange kinetics via a dynamic nanostructuring process.

DOI： 10.1149/07543.0023ecst

Language：English   Publishing type：Research paper (other academic)  

Novel iridium-basedelectrocatalysts supported on SnO₂/Vapor-Grown Carbon Fiber (VGCF)havebeendevelopedand characterized by FESEM, STEM, and XPS.The membraneelectrode assembly (MEA)made withtheIrO₂/Sn_0.98Nb_0.02O₂/VGCFelectrocatalystexhibited a decrease inovervoltageespeciallyunderthelow current densitycompared to the MEA made withconventionalIrO₂/Ti electrocatalyst.Iridium-based SnO₂/VGCF can bea good candidate for anode catalystsof water electrolysis cells.

DOI： 10.1149/07514.1129ecst

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Pt/C is widely used as a PEFC electrocatalyst. However, Pt detachment and aggregation due to carbon corrosion can occur. Therefore, the conventional Pt/Celectrocatalysthas a difficulty in durability.Our research group have developed Pt/SnO₂/VGCF-H electrocatalystsand achieved both high catalyticactivity and high voltage-cycle durability. In this study,we have focusedon Pt_xCo_yalloying anddevelopedPt_xCo_y/Nb-SnO₂/VGCF-H electrocatalystsin order to achieve higher oxygen reduction reaction (ORR)activity. XRD measurementsexhibited alloying of Pt_xCo_ybut not reduced SnO₂. In addition, half-cell measurements revealedthat a high ORRactivity due to possible Pt_xCo_yalloyinghas been achievedafterperforming the proper heat treatment.

DOI： 10.1149/07514.0851ecst

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Toward the development of electrocatalysts for future higher temperature PEFCs, extraction of critical issues for higher temperature PEFCs was aimed. Degradation phenomena and mechanisms at 105°C were evaluated for both Aquivion® MEA and Nafion® MEA. The correlation between proton conductivity and changes in cathode structures on the acceleration degradation test was studied in detail. As a result, even though the macroporous structure of the cathode layer was homogeneous in Aquivion® MEA, larger pores almost intensely formed near the Nafion® membrane side. We have found that the carbon oxidation reaction is deeply related to H2O produced by a fuel cell reaction if it is under the low humidification condition. Therefore, when proton conductivity is low like Nafion® MEA, H₂O is mainly generated at the interface between the cathode layer and Nafion® membrane and carbon oxidation intensely occurs at the interface.

DOI： 10.1149/07514.0329ecst

Language：English   Publishing type：Research paper (other academic)  

Pt nanoparticles have been encapsulated into nano-channels of mesoporous carbon (MC), in order to suppress the mobility of Pt particles and then increase the durability. For further enhancing IV performance, especially in the high current density region, addition of carbon nanofiber (CNF) to the cathode catalyst layers was explored, and an optimal condition was evaluated. As a result, MEA with 10 wt.% CNF addition showed the highest IV performance even though porosity itself was better for 20 wt.% CNF addition. Therefore, thickness is another important factor in addition to porosity, especially in terms of Pt active sites per unit thickness.

DOI： 10.1149/07514.0827ecst

Language：English   Publishing type：Research paper (scientific journal)  

The mechanism of Cr deposition was investigated using NiO/YSZ or NiO/GDC composite cathodes, controlling cathode polarization, and observing the change in Cr distribution. By applying cathode polarization at 200 mV, Cr deposited on the surface of the electrolyte (YSZ or GDC) near the electrode reaction site similarly to the case of typical LSM cathodes. In these cathodes consisting of NiO, the Cr deposition occurred only on the electrolyte surface. After removing the polarization, the deposited Cr partially detached from the electrolyte surface and agglomerated as crystalline Cr₂O₃ at the interface between NiO and the electrolyte. The deposited Cr may decrease over time by continuous vaporization, and may be agglomerated to form crystalline Cr₂O₃ transiently. The disappearance of Cr was faster for the NiO/GDC cathode than for the NiO/YSZ cathode. By applying reverse polarization at -200 mV for 1 h, the deposited Cr disappeared from the electrolyte surface for both cathodes. Therefore, the deposition of Cr on the electrolyte seems to be a reversible reaction.

DOI： 10.1149/2.0141607jes

Language：Japanese   Publishing type：Research paper (scientific journal)  

Language：Japanese   Publishing type：Research paper (scientific journal)  

Potential for critically-high electrical efficiency by using multi-stage electrochemical oxidation with solid oxide fuel cells (SOFCs)

Language：English   Publishing type：Research paper (scientific journal)  

To develop more highly-efficient SOFCs, we have investigated the thickness of highly resistive layer which consist of solid solutions of CeO₂-ZrO₂ system generally observed between YSZ electrolyte and cathode-interlayer made of doped CeO₂. In terms of the effect of the dopant in the CeO₂-based interlayer materials on the thickness of the solid solution, the use of YDC or LDC for the interlayer results in a thinner solid solution compared to that obtained when a GDC interlayer was used. When adapted into SOFCs, I-V tests at 800 °C indicated that the cell with a YDC interlayer exhibited substantially better performance than the cell with a GDC interlayer.

DOI： 10.1016/j.ssi.2015.09.005

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DOI： 10.1016/j.jpowsour.2015.07.096

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Language：English   Publishing type：Research paper (scientific journal)  

The aims of this study are immobilization of formaldehyde dehydrogenase (FDH) into nano-space of nanostructured carbon and evaluation as a possible sensor detecting low concentration formaldehyde. In order to understand the effect of carbon pore size on activity and stability of FDH, mesoporous carbon (MC), originally made in our lab, and commercially available ketjen black (KB) were used in this study. Enzyme activity and electrochemical sensing ability of FDH encapsulated into such two carbon materials were compared. Our original MC resulted in favourable in some evaluated conditions but not in other conditions. MC adsorbed FDH less than KB, but enzyme activity was higher on MC per FDH. Stability against methanol increased on MC, but stability against water was rather lower on MC. Electrochemical sensing ability toward formaldehyde resulted in much better on KB, which was able to detect the sub-ppb level of formaldehyde. Consequently, such dependence is resulted by available nano-space of carbon, and so tuning the pore size of carbon is an important factor in order to develop enzyme based electrochemical sensors with high sensitivity and stability.

DOI： 10.1166/jnn.2015.10618

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DOI： 10.1002/pola.27419

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DOI： 10.1016/j.ijhydene.2015.07.016

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DOI： 10.1039/c5nr03484a

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Language：English   Publishing type：Research paper (scientific journal)  

Mg/Al-hydrotalcite (HDT)-dispersed paper-structured catalyst (PSC) was prepared by a simple paper-making process. The PSC exhibited excellent catalytic activity for the steam reforming of model biodiesel fuel (BDF), pure oleic acid methyl ester (oleic-FAME, C₁₉H₃₆O₂) which is a mono-unsaturated component of practical BDFs. The PSC exhibited fuel conversion comparable to a pelletized catalyst material, here, conventional Ni-zirconia cermet anode for solid oxide fuel cell (SOFC) with less than one-hundredth Ni weight. Performance of electrolyte-supported cell connected with the PSC was evaluated in the feed of oleic-FAME, and stable operation was achieved. After 60 h test, coking was not observed in both SOFC anode and PSC.

DOI： 10.1016/j.jpowsour.2015.02.116

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DOI： 10.1021/acs.chemmater.5b00501

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PEFC electrocatalyst microstructure strongly influences cell performance. In this study, Nafion ionomer content in the cathode electrocatalyst layer is systematically varied to modify the electrocatalyst microstructure. Current-voltage characteristics of the membrane-electrode-assemblies are measured and then the electrocatalyst microstructure is observed by the focused-ion-beam coupled scanning electron microscopy (FIB-SEM) to examine the correlations between the 3D microstructure and the cell performance. Although technical limitation was found for FIBSEM analysis, certain correlations were also established.

DOI： 10.1149/06917.0709ecst

Language：English   Publishing type：Research paper (other academic)  

Rapid oxygen transport across the gas/solid interface (oxygen exchange) is critical for solid oxide fuel cell electrode performance. In this work, La and Sm are shown to dramatically improve the oxygen exchange rate on Pr_0.1Ce_0.9O_2-δ thin films (>50 x at 450 °C) as compared to a freshly deposited film without La or Sm oxide. A surface cleaning effect from the La and Sm is suspected as the source of the enhancement, though catalytic activity from La and Sm may also play a role.

DOI： 10.1149/06602.0157ecst

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We have investigated the property of Fe-Cr-Al-type stainless steel as a porous alloy substrate for metal-supported SOFCs especially on the cathode side. We confirmed not only good heat resistance but also low electrical resistance at the interface between the porous substrate and a La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) coating at 700°C in air. Long-term durability of the oxidation resistance of the LSCF-coated Fe-Cr-Al alloy at 700°C was investigated by measuring the mass gain, surface oxide thickness, and electrical resistance at different temperatures from 700 to 900°C.

DOI： 10.1149/06801.1715ecst

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We have been developing a rapid evaluation method for durability of SOFC stacks for thermal cycles during their lifetimes based on the assumption of residential use. The durability for thermal cycles is expected to be affected by the degradation during their long-term operation. In order to accelerate the evaluation, treatments to intentionally cause degradation were investigated. A degradation factor was determined depending on the SOFC stacks with different structures respectively because each degradation mechanism during their long-term operation also depends on them. The SOFC stacks were supplied by four SOFC stack manufactures in Japan. In this work, we investigated the Cr poisoning treatment to tubular SOFC (TOTO) and the S poisoning treatment to singlestep co-fired planar SOFC (Murata Manufacturing). As results of both cases, 10 years' worth of degradation was successful to be intentionally caused in short period.

DOI： 10.1149/06801.2209ecst

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We utilized stable Sr_0.9La_0.1TiO₃ (SLT) with Ce_0.9Gd_0.1O₂ (GDC) and (Sc₂O₃)_0.1(CeO₂)_0.01(ZrO₂)_0.89 (SSZ) to fabricate robust anode for SSZ electrolyte-supported SOFC single cell. The anode consisted of three layers: a layer of 60wt% GDC-SLT or 40wt% SSZ-SLT as a functional layer and two layers of SLT as current conducting layers. Cathode was composed of a layer of 50wt% SSZ-LSM as a functional layer and a layer of LSM as a current conducting layer. Impregnation method has been employed to decorate nano-sized Ni and CeO₂ into the ceramic anode. We measured electrochemical properties of single cells in 3%-humidified H₂ at 800°C, and investigated long-term redox stability of the nano-sized catalyst impregnated ceramic anodes by fuel interruption method.

DOI： 10.1149/06801.1447ecst

Language：English   Publishing type：Research paper (other academic)  

The solid-state ceramic construction of SOFCs enables high fuel to electricity conversion efficiencies of as high as 50 to 60 percent LHV in high temperature operation, and allows more flexibility in fuel choice. In this study, we have developed a symbolic analysis method to investigate the availability of variable parameters appearing in multi-stage electrochemical oxidation mechanism that is expected to further improve the electric efficiencies of SOFCs. In the flow system of the multi-stage oxidation, the fuel utilization, Uf, at the most downstream stack, Uf_M, is expressed as a function of number of stacks, n, and total fuel utilization, Uf_T. When n = 10 and Uf_T = 85%, Uf_M is calculated to be 36%, which is much smaller than Uf_T. Therefore, if the most downstream stack has high robustness against lean fuel gas, Uf_T could be set to higher values without serious degradation by using this flow system.

DOI： 10.1149/06801.1961ecst

Language：English   Publishing type：Research paper (scientific journal)  

As a theoretical consideration on electrode defect chemistry, general relations of exchange current density quantitatively representing Solid Oxide Fuel Cell (SOFC) electrode performance are systematically derived as a function of gas partial pressures, equilibrium constants of adsorption and dissociation reactions on electrode surfaces, and electrochemical reaction rate constants for possible elemental reactions at the cathode and the anode, in the case that an electrochemical reaction is the rate-determining electrode reaction. Simplified expressions are also derived, under the condition that one kind of neutral or charged adsorbed species is predominant at the electrode, to derive gas partial pressure dependence of exchange current density for given rate-determining electrochemical reactions. Importance of considering elementary steps is highlighted to derive rate equations and to clarify various dependencies. Partial pressure dependencies of the exchange current density are compiled and discussed by simulating normalized exchange current density values for given partial pressures. The applicability and limitation of the Butler-Volmer type expressions of exchange current density for SOFC electrodes are carefully discussed.

DOI： 10.1149/2.0561501jes

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Thermography imaging can be applied for the surface of an SOFC electrode to evaluate the spatial distribution of reforming and electrode reactions. For getting a correct temperature distribution by thermography, emissivity change on the object surface caused by chemical reactions or material coverage must be precisely evaluated. In-situ observation using both infrared and visible light dual cameras enables us to obtain precise emissivity change and thus temperature distribution, because the influence of emissivity change can be corrected by the visible light imaging technique. In this study, this imaging method is applied for an anode material which was exposed to a fuel flow causing coke formation, and the information on the carbon distribution was successfully separated on the anode surface to obtain the true temperature distribution.

DOI： 10.1149/06801.1115ecst

Language：English   Publishing type：Research paper (scientific journal)  

The hetero-interfaces between the perovskite (La_1-xSr_x)CoO₃ (LSC₁₁₃) and the Ruddlesden-Popper (La_1-xSr_x)₂CoO₄ (LSC₂₁₄) phases have recently been reported to exhibit fast oxygen exchange kinetics. Vertically aligned nanocomposite (VAN) structures offer the potential for embedding a high density of such special interfaces in the cathode of a solid oxide fuel cell in a controllable and optimized manner. In this work, VAN thin films with hetero-epitaxial interfaces between LSC₁₁₃ and LSC₂₁₄ were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC₂₁₄ domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC₁₁₃ domains above 250 °C in 10^-3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC₂₁₄ with LSC₁₁₃ also provides for a more stable cation chemistry at the surface of LSC₂₁₄ within the VAN structure, as compared to single phase LSC₂₁₄ films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC₁₁₃ and LSC₂₁₄ in the temperature range of 320-400°C. The higher reactivity of the VAN surface to the oxygen reduction reaction is attributed to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation. The instability of the LSC_113/214 hetero-structure surface chemistry at temperatures above 400°C, however, was found to lead to degraded ORR kinetics. Thus, while VAN structures hold great promise for offering highly ORR reactive electrodes, efforts towards the identification of more stable hetero-structure compositions for high temperature functionality are warranted. This journal is

DOI： 10.1039/c4ta04993d

Language：English   Publishing type：Research paper (other academic)  

This paper introduces a challenge to realize a fuel-cell-powered campus at Kyushu University where SOFC technology plays a major role. The Smart Fuel Cell Demonstration Project, supported by Cabinet Secretariat/Office of Japan, enables us to install one 250 kW SOFC power generation system, other SOFC units, and the world-first university-owned fuel cell vehicle to which renewable hydrogen gas is supplied from the hydrogen refueling station on the campus using electrolyzers. The experience in this demonstrative project is described along with related efforts to accelerate industry-academia collaborations and fundamental scientific studies using advanced analytical facilities.

DOI： 10.1149/06801.0171ecst

Language：English   Publishing type：Research paper (other academic)  

For polymer electrolyte fuel cell (PEFC) electrocatalysts, carbon corrosion is one of the important technological issues to be solved. TiO₂ is a possible alternative support material. In this study, vaporgrown carbon fiber (VGCF-H) conductive fillers are applied as the built-in electron-conductive backbone, on which Nb-doped TiO₂ is coated and Pt nanoparticles are then impregnated. FESEM images of Pt/Ti_0.95Nb_0.05O₂/VGCF-H reveal selective impregnation of Pt nanoparticles on the TiO₂ grains rather than the VGCF-H conductive fillers. This conductive-filler-integrated electrocatalyst exhibits relatively higher electrochemical surface area (ECSA) and oxygen reduction reaction (ORR) activity, compared to the electrocatalysts without conductive fillers.

DOI： 10.1149/06917.0603ecst

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A multi-stage fuel supply SOFC system is studied, which has additional fuel supply inlets between each SOFC stack. The anode offgas from the first stack is supplied to the next stack as reformed fuel gas being mixed with additional fresh fuel. In this paper, the effect of the additional fuel flow ratio is evaluated. The electric efficiency and the fuel utilization of the system can be improved in applying multi-stage fuel supply design.

DOI： 10.1149/06801.3107ecst

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In the downstream of SOFC systems, higher oxygen partial pressure can cause oxidation-induced Ni anode degradation. In this study, we have investigated cell performance at high fuel utilizations for simulating situations around the system downstream. When the anode voltage was higher than a voltage threshold, the cell performance was stable. On the other hand, it became unstable associated with cell voltage oscillation when anode voltage was around or less than the threshold value. The threshold value was consistent with the anode potential derived from the oxygen partial pressure at the phase boundary at which both Ni and NiO coexist.

DOI： 10.1149/06801.1345ecst

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One of the major phenomena to shorten SOFC durability is the formation of insulating phases, such as SrZrO₃, between the cathode and the electrolyte. It is known that SrZrO₃ is formed and grown during sintering processes as well as during long-term operation. A systematic study is made to examine the SrZrO₃ formation mechanisms and their influence on electrochemical properties.

DOI： 10.1149/06801.2463ecst

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For the next generation polymer electrolyte fuel cell (PEFC), increase in operating temperature above 100 °C is desired in order to improve the cathode overvoltage. The objective of this study is to understand degradation phenomena of cathode catalysts by simulating fuel cell vehicle-s "load cycles", where Pt dissolution/reprecipitation reaction occurs. Standard membrane-electrode-assemblies (MEAs) made with Nafion^® membrane were evaluated under increased temperature like 105 °C and also under reduced humidity condition like RH57%. As a result, comparing to the standard operating condition (80 °C and RH100%), Pt enlargement was suppressed at RH57% owing to the presence of smaller amount of water. However, degradation of Nafion^® ionomer under low humidity condition led to lowering MEA performance. Furthermore, at higher temperature, in addition to Nafion^® degradation, enhanced Pt enlargement was recognized and became a serious problem on MEA performance.

DOI： 10.1149/06917.0701ecst

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It has been clarified that Cr deposition occurs significantly on the electrolyte surface near cathode reaction sites as a consequence of cathode polarization. In this study, we investigated the influence of a change in the cathode polarization on the Cr deposited on the electrolyte surface by using NiO/YSZ or NiO/GDC as the cathode material. The deposited Cr segregated at the interface of NiO and YSZ in the case of the NiO/YSZ cathode after a decrease in the cathode polarization, which suggests nucleation under a cathode polarization of 200 mV and growth of Cr compounds after decreasing the cathode polarization. In contrast, the amount of deposited Cr decreased in the case of the NiO/GDC cathode after decreasing the cathode polarization.

DOI： 10.1149/06801.1031ecst

Language：English   Publishing type：Research paper (scientific journal)  

The temperature distribution in internal reforming SOFC anodes can be visualized by thermography. However, thermal imaging techniques are limited since they cannot take into account the influence of emissivity changes caused by carbon deposition on the anode. This problem can be solved by simultaneous visualization using thermography and visible light imaging. By combining these techniques, carbon deposition can be monitored via visible light imaging, whilst temperature changes due to the reforming reaction can be monitored by thermal imaging. This in operando visualization technique enables the evaluation and prediction of the distribution of multiple chemical reactions on the SOFC electrodes.

DOI： 10.1149/2.0031511eel

Language：Japanese   Publishing type：Research paper (scientific journal)  

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For large-scale SOFC power generation systems, a shorter start-up time of SOFC cell stacks with relatively large heat capacity is one of the most important technological issues to determine the flexibility in SOFC system operation. In this study, start-up procedures have been examined to shorten the start-up time period. The conventional heating procedure using pre-heated hot air and self-heating by SOFC operation at low temperatures had a difficulty to shorten the start-up time period because of the limitation in power generation at lower temperatures. In this study, as an alternative start-up procedure, catalytic combustion at the SOFC cathodes is, for the first time, demonstrated to be useful on the system level. The applicability of the catalytic combustion to shorten the start-up time period has been verified numerically as well as experimentally by using a large-scale cell stack cartridge. This unique start-up procedure enables to operate SOFC-based large-scale power generation systems.

DOI： 10.1002/fuce.201400014

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The electrical, mechanical, and compositional characterization of a graphene oxide membrane is presented, and its application as an electrolyte material in a polymer electrolyte membrane fuel cell is explored. Self-supporting graphene oxide membranes were prepared by a simple vacuum filtration process and, for the first time, characterized as the electrolyte in a fuel cell operating in an elevated temperature range (30-80 °C), with a maximum power density of ∼34 mW cm^-2, approaching that of a Nafion electrolyte based cell prepared and tested under similar conditions. Evidence for partial membrane reduction was found at higher temperatures and is believed to originate from more easily released, higher energy oxide groups, such as epoxides. We also discuss the morphology, the mechanical properties, chemical composition, and electrical conductivity of the graphene oxide membranes, with comparisons made to conventional Nafion membranes.

DOI： 10.1016/j.jpowsour.2014.08.071

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Large-scale, facile synthesis of nitrogen-doped graphene foam (GF) from low cost precursors is reported as a support material for platinum in fuel cell cathodes. The nitrogen-doped GF was produced by combustion of nitrogen-containing sodium alkoxide, followed by washing and heat treatment in various gases. A nitrogen-free reference sample was also synthesized. The BET surface area is higher than 700 m² g^-1, and the material is highly defective, partly due to the inclusion of nitrogen, and partly due to the low temperature, catalyst-free synthesis method. These defects result in an excellent distribution of platinum nanoparticles on the surface. The electrochemical performance of the resulting electrocatalysts was characterized using cyclic voltammetry and linear sweep voltammetry, revealing that nitrogen doping has potential to increase the durability of graphene-based catalysts. Therefore, this material is a potentially useful catalyst support for use in polymer electrolyte membrane fuel cells.

DOI： 10.1002/fuce.201300258

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Sulfonated polyimide (SPI)/dihydroxynaphthalene (DHN) charge-transfer (CT) complex hybrid films were investigated as possible alternative for polymer electrolyte membranes in polymer electrolyte fuel cells. SPI/DHN CT complex hybrid films include CT complexes, which might work as electronic conductors, and sulfonic acid units, which could work as proton conductors. Therefore, the origin of the conductivity of SPI/DHN complex hybrid films was evaluated by four-probe impedance measurements in the through-plane direction of the films. The obtained conductivity of the CT complex hybrid films increased with the increase of ion exchange capacity of the CT films and the decrease of CT complex concentration in the films. These results indicated that proton transfer dominantly occurred in the CT complex hybrid films. Proton conductivity of the CT complex hybrid films consisting of 2,6- or 1,5-DHN showed the similar values, although the molecular geometries of the CT complex were different. The activation energy values for proton conductivity in the CT films were approximately the same as that of Nafion 212. Water uptake (WU) results were also conducted and suggest that CT complex formation could control the degree of WU of the films and prevent dissolution of SPI.

DOI： 10.1002/pola.27343

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Eu-doped cerium oxide was synthesized and its crystal structure was analyzed by X-ray diffraction. The electrical conductivity was measured by impedance spectroscopy at 350-900 °C over a wide range of controlled oxygen partial pressures, from H₂ to O₂, using gas mixtures. In the high pO₂ region, ionic conductivity is predominant, and the activation energy for total conductivity is 0.8 ± 0.01 eV. In low pO ₂, the conductivity is found to obey a - 1/4 power law dependence on pO₂, with an enthalpy of reduction of 4.2 ± 0.16 eV. These results demonstrate that Eu has a similar effect on electrical properties as Gd or Sm acceptor dopants in ceria, namely, that Eu shows no evidence for multivalent behavior; rather, it appears to be a fixed valent dopant in the fluorite structured ceria, in contrast to prior studies finding multivalency of Eu in perovskite oxides under similar conditions.

DOI： 10.1016/j.ssi.2014.05.010

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Solid Polymer Water Electrolysis (SPWE) is a method to efficiently produce high-purity hydrogen gas using a polymer electrolyte membrane-based system. SPWE systems that utilize natural water circulation (resulting from a difference in buoyancy) are a promising technology, which need no additional circulation pump for water supply to the electrolysis cells, and generate no pressure difference between the hydrogen generation and oxygen generation chambers. However, despite not needing an accurate pressure control, gas bubbles formed and trapped within the cell stacks can inhibit heat convection, leading to hot-spot formation and consequent destructive degradation. Improving the reliability is therefore one of the most important technological issues in natural circulation SPWEs. In this study, hot-spot formation is studied both by numerical heat and flow analysis, and by experimental in-situ visualization. This leads to insights into the degradation mechanisms of SPWE stacks, and their possible solutions. An improved design for an SPWE cell stack is successfully fabricated, and reliable operation up to 5000 h is demonstrated.

DOI： 10.1016/j.ijhydene.2014.07.153

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Chromium poisoning phenomena of solid oxide fuel cells (SOFCs) were investigated using (La_0.8Sr_0.2)_0.98MnO ₃ (LSM), Pr_0.8Sr_0.2MnO₃ (PrSM), Nd_0.8Sr_0.2MnO₃ (NdSM), and Br _0.5Sr_0.5Co_0.8Fe_0.2O₃ (BSCF) for the cathode materials and yttria-stabilized zirconia (YSZ) as the electrolyte material at 700 °C under constant cathode polarization conditions. Deposition of chromium increased with increasing cathode polarization similarly for the four cathodes, although position of the deposition was different for the BSCF cathode. Chromium deposited near the cathode/electrolyte interface for the LSM cathode, the PrSM cathode and the NdSM cathode. Chromium deposition on the surface of the zirconia electrolyte was observed for the PrSM cathode and the NdSM cathode as previously observed in the LSM cathode. Oxygen deficiency in the deposited chromium on the surface of the zirconia electrolyte was also observed, thus the reaction mechanism of chromium vapor with the oxygen vacancy induced by cathode polarization was supported. The oxygen vacancy on the surface of the zirconia electrolyte seemed to be generated via metal oxides such as manganese oxide or neodymium oxide segregated from the cathode materials. Chromium deposited on the surface of the BSCF cathode. Cathode polarization seems to increase reactivity of BSCF and enhance trapping of chromium vapor near the cathode surface.

DOI： 10.1016/j.ssi.2014.01.047

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DOI： 10.1017/S143192761401126X

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In order to develop durable electrocatalysts for polymer electrolyte fuel cells, six different metal oxides, namely MoO₃, SnO₂, Nb₂O₅, Ta₂O₅, TiO₂, and WO₃, are selected as thermochemically stable, carbon-free platinum electrocatalyst support materials. The stability of Pt on these alternative oxide support materials is systematically analyzed, for the first time, using common experimental protocols simulating realistic fuel cell vehicle operation. Pt/SnO₂ shows the best performance in terms of both electrochemical activity, and stability against dissolution. Pt dissolution rates in Pt/SnO ₂ are comparable to those of conventional Pt/C electrocatalysts. These results suggest that SnO₂ is a promising candidate as an alternative electrocatalyst support.

DOI： 10.1016/j.ijhydene.2014.01.094

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The nanostructure of sulfonated polyimide (SPI)/dihydroxynaphthalene (DHN) derivative charge-transfer (CT) complex hybrid films, which are noble alternative polymer electrolyte membranes (PEMs), is determined by a combined visible spectroscopy/quantum mechanical approach. From the visible spectra of SPI/2,6-dihydroxynaphthalene (2,6-DHN) and 1,5-dihydroxynaphthalene (1,5-DHN) CT complex hybrid films, it is confirmed that these films have different maximum wavelength, although difference of the molecular structure is small. From the calculation based on the experimental result, SPI and DHNs form multiple interactions consisting of not only CT interaction, but also hydrogen bonding in multilayered structures. CT interaction between SPI and DHN defines the DHN position in the SPI matrix, with DHN sitting in the cavity formed between SPIs. The molecular structure of the CT films derived from the multiple and complex interactions can recognize small differences in the structural isomers and bring changes of the optical property. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 293-298 Many sulfonated polyimides (SPIs) have been actively investigated as polymer electrolyte membranes (PEMs) for polymer electrolyte fuel cells (PEFCs) as alternatives to Nafion. In this work, the molecular geometry of (SPI)/dihydroxynaphthalene (DHN) derivative charge-transfer complex hybrid films is determined by a combined visible spectroscopy/quantum mechanical approach. From the experimental and calculated results for SPI/2,6-DHN and 1,5-DHN, while the structural difference is small, different molecular geometries and optical properties can be seen.

DOI： 10.1002/polb.23411

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For polymer electrolyte fuel cells (PEFCs), corrosion of the carbon support of electrocatalysts under cathode conditions is one of the important technical issues to solve, for which SnO₂ is a promising alternative support material. In this study, Pt/SnO₂ electrocatalysts with integrated conductive fillers have been developed, where the fillers act as the backbone and the built-in electron-conductive pathway. Electrochemical activity and durability of such electrocatalysts against voltage cycling between 1.0 and 1.5 VRHE are investigated. This durable electrocatalyst exhibits higher the oxygen reduction reaction (ORR) activity than Pt/SnO₂, and comparable ORR activity to that of the standard Pt/C electrocatalysts.

DOI： 10.1149/06403.0215ecst

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For the application of polymer electrolyte fuel cell (PEFC) as a next generation fuel cell vehicle, an increase in the operation temperature up to 120 °C is desired in order to improve the power generation efficiency. The objective of our research is to extract problems on catalytic degradation above 100 °C with the aim of developing durable electrocatalysts for future high temperature PEFC. In this study, we have prepared standard membrane-electrode-assemblies made with Nafion® membrane and evaluated their durability against carbon oxidation in the cathode layer under increased temperature like 105 °C. As a result, comparing to the standard operation condition (80 °C and RH100%), the degradation rate of their electrochemical performance was much more increased at higher temperature. Furthermore, a collapsed carbon structure in the cathode layer was clearly observed under the high temperature operation.

DOI： 10.1149/06403.0755ecst

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Chromium poisoning phenomena were compared among three SOFC cathodes using (La_0.8Sr_0.2)_0.98MnO₃ (LSM), La _0.6Sr_0.4Fe_0.8Co_0.2O₃ (LSCF) and LaNi_0.6Fe_0.4O₃ (LNF) at 700 C by changing cathode polarization (0-400 mV). Chromium vapor deposited near the electrolyte for LSM and LNF, and the amount of the deposition increased with increasing cathode polarization. In the case of LSCF, chromium deposited near the cathode surface under smaller cathode polarization (≤200 mV). Under larger cathode polarization (≥300 mV), however, chromium deposition near the cathode/electrolyte interface similarly increased for the three cathodes. Cathode polarization facilitated the chromium deposition and there seemed to be no correlation with the current density. Microscopic distribution of the deposited chromium, which was located on the surface of LSM, LSCF, LNF grains, and also on the surface of zirconia and ceria, seemed to correspond to the distribution of oxygen vacancy by cathode polarization at the electrode reaction sites. Chromium deposition on the zirconia surface seemed to be assisted by metal oxides segregated from the cathode material, which can conduct electron required for generating oxygen vacancy continuously. Oxygen deficiency on the surface of the deposited chromium was confirmed and interdiffusion of chromium and zirconium caused by cathode polarization was also suggested.

DOI： 10.1016/j.ijhydene.2013.11.030

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High temperature solid oxide fuel cells (SOFCs) can accept the direct feed of hydrocarbon fuels which are reformed in the porous anode materials producing H₂-rich syngas, subsequently used to generate electricity and heat through electrochemical oxidation. This type of SOFC is called direct internal reforming SOFC (DIR-SOFC). Now we try to overcome problems associated with the DIR-SOFC, such as carbon deposition caused by internal reforming, by applying a flexible structured-catalyst called paper-structured catalyst (PSC). Here, recent progresses are summarized focusing on the catalytic activity of hydrotalcite-containing PSCs for the dry reforming of methane.

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Hydrogen storage is a crucial technology for the realization of a carbon-neutral society. However, few materials have been able to approach useful hydrogen storage capacity at reasonable temperatures and pressures. Graphene has an extremely high surface-area-to-weight ratio, is strong, cheap, chemically inert, and environmentally benign. As such it may be an ideal substrate for hydrogen storage. Here we present synthesis of graphene foam by combustion of sodium ethoxide. This technique is low-cost, scalable, and results in a three-dimensional graphene network with a surface area of more than 1200 m ²/g. It is applied as a hydrogen storage material at liquid nitrogen temperature, with a capacity of 2.1 wt%.

DOI： 10.1016/j.ijhydene.2013.10.044

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The characterization and application of graphene oxide membranes as fuel cell electrolytes is explored and presented. Morphology, chemical composition, mechanical and electrochemical properties of vacuum-filtration prepared graphene oxide membranes and their performance as fuel cell electrolytes are discussed. Our graphene oxide membrane fuel cell (GOMFC) showed a maximum power density of ∼ 6 mW/cm² at 30 °C. Power density exhibited a decrease with increasing temperature and operation time, possibly due to partial loss of oxygen through reduction, and a resulting decrease in water-mediated proton transport.

DOI： 10.1149/06403.0441ecst

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SnO₂ can act an alternative PEFC electrocatalyst support to the conventional carbon black, for preventing catalyst support corrosion. In this study, current-voltage characteristics of MEAs with SnO₂ support are improved by controlling the content of electron-conductive fillers and proton-conductive Nafion ionomer.

DOI： 10.1149/06403.0207ecst

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With the aim of developing durable electrocatalysts for polymer electrolyte fuel cells, we have been working on Mesoporous Carbon (MC), composed by a nano-channel structure with the diameter of ca.10 nm. Based on the increased stability of metal particles on the curved surface comparing to the flat surface, Pt particles are encapsulated into the pores of MC and used as an electrocatalyst (Pt/MC). In this study, membrane-electrodeassemblies were developed with Pt/MC as a cathode catalyst. Their durability on the current-voltage response was evaluated using the potential cycle test at the potential range where the dissolution and re-precipitation of Pt particles occurs. As a result, increased durability has been demonstrated most likely because of reducing Pt mobility on the carbon support. Successful Nafion® introduction into nano-cahnnels of MC comes to a key factor to further increase both performance and durability.

DOI： 10.1149/06403.0137ecst

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Platinum-free oxygen reduction reaction (ORR) catalysts could help reduce the cost of future generations of polymer electrolyte membrane fuel cells (PEFCs). One class of non-precious catalyst for PEFCs are nanostructured Fe/C/N-based materials. In these, the nature of the active site is still hotly contested. Resolving this issue could lead to the development of better catalysts. One approach to achieve this is to study nitrogen-doped carbons, without any Fe content. Such materials have been studied, but largely in alkaline media where high activity is routinely obtained. Studies of metal-free catalysts in acid are rare, and Fe-contamination is often an issue. To truly shed light on the ORR mechanism of Fe/C/N-based catalysts, measurements on metal-free catalysts in acid media are required to simulate proton-based PEFC systems. Here we present synthesis of a metal-free defective nitrogen-doped graphene powder with remarkable surface area. We apply this as an ORR catalyst in acid medium and comment on the reaction mechanism.

DOI： 10.1149/2.095404jes

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Iron-free, nitrogen-doped graphene foam is presented as a model electrocatalyst system for studying the role of nitrogen in the oxygen reduction reaction in non-precious Fe/N/C-based electrocatalysts. Due to the large surface area and high porosity, these electrocatalysts display high activity in rotating ring-disk electrode voltammetry measurements. The electron transfer number is as high as 3.6, despite the metal-free nature of this electrocatalyst. The sample with the highest activity has a significantly larger proportion of tertiary/graphite-like nitrogen, and therefore this is proposed as a 4-electron oxygen reduction active site in acid environment.

DOI： 10.1149/06403.0271ecst

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The oxygen reduction reaction has faster kinetics in anion exchange membrane fuel cell, opening the possibility of using nonprecious materials as electrocatalysts. Therefore, defective nitrogen-doped graphene foam is applied as an electrocatalyst in alkaline medium. The large surface area and highly porous structure results in high oxygen reduction reaction activity in rotating ring-disk electrode measurements. During potential cycling tests, the nitrogen-doped graphene foam displayed comparable durability to a Pt/CB catalyst.

DOI： 10.1149/06403.1161ecst

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Possible alternative electrocatalyst support materials to the conventional carbon black have been examined. Among others, doped-SnO₂ can be a promising support for Pt nanoparticles well connected to the oxide with a certain crystallographic orientation. Pt/doped SnO₂ exhibits suitable voltage cycle durability, while further improvement in oxygen reduction reaction (ORR) activity is still desired.

DOI： 10.1149/06403.0221ecst

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Gram-scale synthesis of defective graphene foam from low-cost precursors is reported as a catalyst support material for platinum in fuel cell cathodes. The material was produced by combustion of sodium ethoxide, followed by washing and heat-treatment in various gases. The BET surface area is higher than 1500 m²/g. The defects in the material result in excellent distribution of platinum nanoparticles on the surface. The electrochemical performance is compared with platinum-decorated carbon black and commercially obtainable graphene using cyclic voltammetry, linear sweep voltammetry, andmembrane electrode assemblies. Pt-decorated grapheme foam has larger electrochemical surface area (101 m²/g) and higher mass activity (176 A/gPt). However, durability and fuel cell power density still require improvements. This graphene foam is a potentially useful catalyst support, especially for use in polymer electrolyte membrane fuel cells.

DOI： 10.1149/2.0231409jes

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Using tin oxide (SnO₂) and niobium-doped tin oxide (Nb-SnO₂) as alternative electrocatalyst support materials can effectively solve the issue of carbon corrosion in polymer electrolyte fuel cell (PEFC) cathodes. Here, we systematically explore the effect of support surface area, Pt loading, and Pt nanoparticle size on the electrochemistry of these carbon-free electrocatalysts. Reducing the Pt loading leads to an increase in electrochemical surface area, but the specific activity decreases as previously observed in conventional carbon based electrocatalysts. Removing residual chlorine impurities by replacing the H₂PtCl₆ nanoparticle precursor with Pt(acac)₂ increases the specific activity. Niobium-doping of the SnO₂ support also results in an increase in specific activity, due to the increased electronic conductivity. Consequently, the oxygen reduction reaction activity of optimized Pt-decorated Nb-SnO₂ is approaching to that of Pt-decorated carbon black, the current state-of-the-art PEFC electrocatalyst.

DOI： 10.1149/2.0431412jes

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Niobium-doped SnO₂ is selected as an alternative carbon-free support material to negate carbon corrosion in polymer electrolyte fuel cells (PEFCs) electrocatalysts. The durability is measured using a membrane electrode assembly (MEA) over 60,000 start-stop cycles at high potential, equating to the lifetime of fuel cell vehicles. Using the Nb-doped SnO₂ support results in retention of 99% of the initial cell voltage. The current-voltage characteristics are improved by adding carbon nanofibers as fillers in the Nb-doped SnO₂, indicating that electronic conduction in the electrocatalyst layer is critical in the application ofmetal oxide-supported electrocatalysts.

DOI： 10.1149/2.005404eel

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Architectonics of the paper-structured catalyst for the application to the biofuel reformer or direct internal reforming SOFC (DIRSOFC) was studied. Inorganic fiber network, "paper", composed of yttria-stabilized zirconia (YSZ) fiber (Zf), alumina fiber (Af) and inorganic binder (Al ₂O₃ sol (As) or ZrO₂ sol (Zs) or CeO ₂ sol (Cs)) was prepared by a simple paper-making process. Then, the catalytic activities of the Ni and Ni-MgO loaded papers called "paper-structured catalysts (PSCs)" for the steam reforming of biodiesel fuels (BDFs) were evaluated. Ni-MgO loaded PSC using Cs as an inorganic binder, Ni-MgO/ZfAfCs, exhibited excellent performance over Ru/γAl₂O₃ catalyst beads. Formation of light hydrocarbons, especially C₂H₄, was eliminated and water-gas shift reaction was more promoted compared to the catalyst beads.

DOI： 10.1016/j.ijhydene.2013.06.080

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Inorganic fiber network including YSZ fiber which acts as catalyst support was created by the simple paper-making process, and novel Ni-loaded paper-structured catalysts(PSCs) with excellent catalytic activity for the dry reforming of methane were designed and developed. The PSCs exhibited high fuel conversion comparable to the conventional powdered catalysts with less than one-tenth catalyst weights. The significant advantages of the PSCs are their high mechanical flexibility and material workability. So far, a functionally-graded catalytic reaction field which leads to uniform temperature distribution during biogas reforming resulting in stable operation of planar SOFC was successfully developed by the PSC array based on the kinetic simulation model built in this research.

DOI： 10.1016/j.ijhydene.2013.06.046

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In this study, performance of solid oxide fuel cell (SOFC) connected with paper-structured catalyst (PSC) was evaluated in the direct feed of wet oleic fatty acid methyl ester (oleic-FAME, C₁₉H₃₆O₂), which is a mono-unsaturated component of practical biodiesel fuels (BDFs), in the steam to carbon ratio (S/C) range between 2.0 and 3.5, and high current density of 1 A cm^-2 (at 0.7 V) was recorded at 800 C. Long term stability of oleic-FAME fueled SOFC was achieved by the incorporation of PSC into SOFC even under severe operating condition prone to coking (direct feed of unsaturated hydrocarbon with carbon number 19 and low S/C ratio of 2.0). After 100 h test, coking was not observed in both SOFC anode and PSC.

DOI： 10.1016/j.ijhydene.2013.05.128

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Low durability of polymer electrolyte fuel cell (PEFC) is a major drawback that should be solved. Recent studies have revealed that leaching of liquid phosphoric acid (PA) from both polymer electrolyte membrane and catalyst layers causes inhomogeneous PA distribution that results in deterioration of PEFC performance during long-term operation. Here we describe the finding that a novel PEFC free from acid leaching shows remarkable high durability (single cell test: >400,000 cycling) together with a high power density at 120 C under a non-humidified condition. This is achieved by using a membrane electrode assembly (MEA) with Pt on poly(vinylphosphonic acid)-doped polybenzimidazole wrapped on carbon nanotube and poly(vinylphosphonic acid)-doped polybenzimidazole for the electrocatalst and electrolyte membrane, respectively. Such a high performance PEFC opens the door for the next-generation PEFC for "real world" use.

DOI： 10.1038/srep01764

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Much attempt was made to improve PEFC perfonnance to optimize its operating condition and its structure by experimental and numerical trials. One of the most critical issues about PEFC improvement is the durability enhancement at its membrane electrode assembly (MEA) to keep its robust control. The objective of this study is to keep the optimal water distribution at MEA with applying the hydrophilic anode support. In this study, the influence of modification to use two hydrophilic support materials is calculated by the case of weak hydrophilized carbon support and strong hydrophilic SnC>2 support. The distribution of liquid water shows that hydrophilic anode support has the effect to hold the liquid water in anode catalyst layer under the low humidification, and water saturation ratio in cathode catalyst layer is also controlled by the use of hydrophilic materials. Hydrophilic anode support works as a water transport pathway to anode across polymer electrolyte membrane. Its effect also prevents from drying-up of the anode and flooding of the cathode.

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The feasibility of a direct internal reforming (DIR) solid oxide fuel cell (SOFC) running on wet palm-biodiesel fuel (BDF) was demonstrated. Simultaneous production of H₂-rich syngas and electricity from BDF could be achieved. A power density of 0.32Wcm^-2 was obtained at 0.4 A cm^-2 and 800°C under steam to carbon ratio of 3.5. Subsequent durability testing revealed that a DIR-SOFC running on wet palm-BDF exhibited a stable voltage of around 0.8V at 0.2 A cm^-2 for more than 1month with a degradation rate of approximately 15 % / 1000h. The main cause of the degradation was an increase in the ohmic resistance.

DOI： 10.1002/er.2883

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DOI： 10.1149/05030.0021ecst

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DOI： 10.1149/05040.0053ecst

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DOI： 10.1016/j.electacta.2013.02.062

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With the aim of developing durable electrocatalysts for PEFC, their durability with different carbon supports was evaluated electrochemically. During our study, Pt deposited Ketjen Black carbon was found to lose most of electrochemical active surface area after the potential cycle tests, and then a possible mechanism for that was investigated. We believe two different mechanisms probably occur at the same time. Many of Pt particles are dissolved into the electrolyte solution either as Pt ion or Pt nanoparticles. Some of Pt particles are also embedded into the hole formed on the carbon surface owing to carbon corrosion. However, the effect of these two factors was rather small for Vulcan carbon. Moreover, on the graphitized surface, such problems were mostly overcome. Consequently, graphitization of the carbon surface is a simple but useful technique to improve the durability of electrocatalysts.

DOI： 10.1149/05312.0023ecst

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With the aim of developing durable electrocatalysts for polymer electrolyte fuel cells, their durability with different carbon supports have been evaluated electrochemically in our study. During our previous investigation, we have found that graphitization of carbon surface through the heat treatment at appropriate temperature is a simple but useful technique to improve the durability of electrocatalysts. Especially, a electrocatalyst made by heat treatment of Vulcan carbon at 1600 °C, Pt/GVC1600, has shown much more improved durability than one made without heat treatment, Pt/VC, in the half-cell evaluation. In this study, MEA with Pt/GVC1600 for the cathode was developed and evaluated. Initial performance of MEA was found to be almost the same between these two catalysts, but regarding to durability, Pt/GVC1600 showed much more improved durability than Pt/VC. Heat treatment of carbon supports at optimum temperature lead to increasing in durability not only in the half-cell but also in the full-cell.

DOI： 10.1149/05824.0007ecst

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With the aim of developing durable electrocatalysts for polymer electrolyte fuel cells, we have been working on Mesoporous Carbon (MC) with the nano-channel structure, whose diameter is ca. 10 nm. By encapsulating Pt catalysts into the pores of MC, improvement of oxygen reduction reaction activity and durability, comparing to Pt deposited on conventional carbon, has been observed in the half-cell evaluation. Improvement on its durability is mainly due to the reduced mobility of Pt particles within pores of MC. In this study, MEAs were newly developed using Pt deposited MC as a cathode catalyst. By controlling the amount of Nafion^® and adding a heat treatment step after ball-milling of MC, MEA with IV performance, comparable to Pt deposited on conventional carbon, was successfully developed.

DOI： 10.1149/05801.1105ecst

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The solubility of Mn in CeO2 was investigated for 3-50 mol% Mn. Powders were synthesized by the Pechini method in air. Raman spectroscopy and lattice parameter analysis revealed that the solubility limit of Mn in CeO2 is less than 3 mol% for the present preparation conditions. The low solubility limit was rationalized in terms of the much smaller ionic radius of Mn versus Ce. As a consequence, an approach to increase the Mn radii size through annealing in reducing conditions (humidified H2 at 1100 °C) was attempted. In this case, the MnO phase was observed by XRD, thereby indicating difficulty in obtaining Mn doped CeO2 under reducing conditions, as well.

DOI： 10.1149/05701.1607ecst

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In principle, direct internal reforming of practical biodiesel fuels (BDFs) in a solid oxide fuel cell (SOFC) is viable. However, direct feed of practical BDFs to a SOFC caused severe carbon deposition accompanied by C ₂H₄ formation, which were more significant for higher degree of unsaturation of BDFs. Performance of SOFC running on BDF can be enhanced by increasing the content of a fatty acid methyl ester (FAME) with lower degree of unsaturation. To suppress formation of carbon and C ₂H₄, a flexible catalyst composed of inorganic fiber network was applied for the operation of SOFC running on FAMEs. In this study, rather high power density of 0.84 W cm^-2 at 0.7 V was achieved in the feed of a mixture of oleic- and palmitic-FAMEs.

DOI： 10.1149/05701.3005ecst

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Polymer electrolyte fuel cells (PEFCs) are one of the promising future power sources. Even though technologies for PEFC are mostly developed, durability of their electrocatalysts is one of the most important issues for further development. In order to improve the durability against carbon corrosion problem, we have been working on carbon-free Pt electrocatalysts using SnO ₂ and demonstrating improved durability. However, SnO ₂-supported electrocatalysts often result in lower performance mainly due to relatively high contact resistance between SnO₂ particles. In this study, an attempt has been made to develop an alternative way to fabricate the SnO₂-based cathode using pulse laser deposition (PLD) and ion plasma sputtering in order to reduce the contact resistance. Consequently, the use of the doped-SnO₂ thin layer has lowered the overpotential of cathode reactions, resulting in successful development of PEFC consisting of SnO₂-supported cathode with both high power performance and durability.

DOI： 10.1149/05801.1293ecst

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Gram-scale synthesis of graphene-like foam from low cost precursors is reported as a support material for platinum in fuel cell cathodes. The graphene-like foam was produced by combustion of sodium ethoxide, followed by washing and heat treatment in various gases. The BET surface area is higher than 1500 m²/g, although the material is highly defective. These defects result in an excellent distribution of platinum nanoparticles on the surface. The electrochemical performance of the resulting electrocatalysts are characterized using cyclic voltammetry and linear sweep voltammetry techniques, which reveal that this Pt-decorated graphene has large electrochemical surface area (101 m²/g) and high mass activity for the oxygen reduction reaction (176 A/gPt), making this graphene-like foam a potentially useful catalyst support for use in polymer electrolyte membrane fuel cells.

DOI： 10.1149/05801.1751ecst

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The durability and reliability of segmented-in-series (SIS) type cellsstack was investigated by multimodal assessment in which Tokyo Gas collaborated with research institutes under NEDO project, "Development of system and elemental technology on SOFCs". The SIS cells-stack, developed by Tokyo Gas in cooperation with Kyocera, has many advantages such as reduced temperature operation, highvoltage / low-current power generation, and lower in material cost of electrical insulating substrate compared to Ni based substrates for anode-supported cells. Another key advantage is that there is no need for alloy interconnects. This would make the cell-stack more durable than other types of cell-stacks having metallic interconnects. Durability of the stacks was investigated by the operations in electric furnaces at Tokyo Gas, Central Research Institute of Electric Power Industry (CRIEPI), and Kyushu University. After the operations post analyses were conducted by the research institutes, such as National Institute of Advanced Industrial Science and Technology (AIST), Tohoku University, Kyoto University, The University Tokyo, and Kyushu University. Through the multimodal assessment durability and reliability of the SIS stacks for long-term operation and thermal cycles have been shown to be high enough for 40,000 h life and more.

DOI： 10.1149/05701.0325ecst

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We already clarified that chromium deposition near the cathode/electrolyte interface was predominantly affected by the cathode polarization and the chromium deposition occurred not only on the cathode materials (LSM, LSCF etc.) but also on the electrolyte material, which seems to be involved in the electrode reaction sites. In this study, influence of cathode polarization change on the distribution of the deposited chromium was investigated by using LSM cathode. Microstructure at the cathode/electrolyte interface seems to change by cathode polarization and the microscopic distribution of the deposited chromium can be affected by the change in the microstructure.

DOI： 10.1149/05701.1859ecst

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Donor doping of the p-type solid oxide fuel cell cathode, Sr(Ti,Fe)O _3-δ, with La was pursued to examine the role of minority charge carriers (electrons) on the oxygen exchange rate. Model dense thin film cathodes of (La,Sr)(Ti,Fe)O_3-δ were fabricated by pulsed laser deposition. Optical transmission spectroscopy measurements demonstrated that the addition of the donor La resulted in a change in valence state of Fe from 4+ to 3+, suggesting a rise in Fermi level. The activation energy of the cathode's area specific resistance, studied by impedance spectroscopy, was found to be lower with La addition; however, the magnitude of the area specific resistance was not lowered.

DOI： 10.1149/05701.1719ecst

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SOFC can directly utilize not only hydrogen but also various fuels such as city gas, kerosene, and others by internal reforming reaction on anodes. However, impurities and un-reformed hydrocarbon in fuels can cause degradation associated with carbon deposition. In this study, in-plane distribution of carbon deposition was evaluated using planar cells with an electrode area of 4 cm by 4 cm. The 2-dimensional distribution of deposited carbon has been visualized, where the carbon deposition was accelerated in the coexistence of C3H8 and H2S as minor constituents in simulated prereformed CH4-based fuels.

DOI： 10.1149/05701.1593ecst

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Decreasing the operating temperature in solid oxide fuel cells (SOFCs) will aid in increasing their portability and durability. However, the electrolyte and electrode resistances increase significantly at lower temperatures. Pulsed Laser deposition (PLD) is a promising method to prepare thin films, thus reducing the ohmic resistance of the electrolyte and electrode, with microstructural and morphological control depending on deposition conditions, keys to maximizing electrode electrochemical performance. In this research, SOFC cells consisting of anode/electrolyte/cathode thin film layers grown by PLD on electrically conductive porous supports were fabricated. The microstructure control of cathode and anode functional layers in different deposition conditions are discussed.

DOI： 10.1149/05701.0843ecst

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While Solid Oxide Fuel Cells (SOFCs) have attracted increasing attention, there are only a limited number of papers on exchange current density, representing electrode activity to describe and simulate SOFC performance. In this study, theoretical relationships on anode exchange current density have been derived as a function of various operational parameters by taking into account elementary reaction processes. Analytical relations under various boundary conditions are also derived with respect to gas partial pressures.

DOI： 10.1149/05701.1585ecst

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For polymer electrolyte fuel cells (PEFCs), carbon corrosion, leading to lower activities of catalysts under cathode conditions, is one of the important technological issues to be solved. We have been developing SnO₂ and Nb-SnO₂ as alternative support materials. In this study, the oxygen reduction reaction (ORR) activity and the durability of Pt/SnO₂ and Pt/Sn_0.98Nb_0.02O₂ against voltage cycling between 1.0 and 1.5 V_RHE are quantitatively investigated. Longer ORR durability was verified, compared to the standard catalyst, Pt/Vulcan. Furthermore, we have found that the degradation mechanism of ORR is probably different between carbon black and SnO₂-based supports. In the case of SnO₂-based catalysts, degradation arose mainly from the loss of electrochemical surface area (ECSA), while other factors were also involved for Pt/Vulcan. Consequently, SnO₂ is a promising candidate electrocatalyst support material to solve carbon corrosion problems of PEFCs.

DOI： 10.1149/05801.1251ecst

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Non-precious oxygen reduction reaction (ORR) catalysts could help reduce the cost of future generations of polymer electrolyte membrane fuel cells (PEFCs). One class of Pt-free catalysts for PEFCs are nanostructured Fe/C/N-based materials. In these, the nature of the active site is still hotly contested. Resolving this issue could lead to the development of better catalysts. One approach to achieve this is to study nitrogen-doped carbons which do not contain any Fe. Such materials have been studied, but largely in alkaline media where high activity is obtained. To truly understand the ORR mechanism of Fe/C/N-based catalysts, measurements in acid media are required to simulate the activity in proton-based PEFC systems. Here we present bulk synthesis of a metal-free nitrogen-doped graphene powder with remarkable surface area. We apply this as an ORR catalyst in acid media and comment on the reaction mechanism.

DOI： 10.1149/05801.1529ecst

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Hydrogen gas is compressed with a new electrochemical pump, where internal humidifier is built in, and dead-end at anode is considered for a practical situation. Both the voltage and current efficiency of the hydrogen pump is evaluated experimentally. The pump is operated up to 2 MPaG under the temperature condition of 293 and 333 K. The voltage efficiency indicates nearly 50% at most and the maximum current efficiency is 96%. Overpotentials inherently involved in the pump are discussed with electrochemical impedance spectroscopy.

DOI： 10.1149/05801.0681ecst

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In SOFCs, a wide range of fuel gases can be applied, but fuel impurity tolerance may be desired. By applying gadolinia-doped ceria (GDC) anode material, we analyzed power generation characteristics and internal reforming followed by high-resolution electron microscopy in order to moderate sulfur poisoning. Effect of ceria addition into the anodes layers on sulfur poisoning is systematically analyzed and discussed.

DOI： 10.1149/05701.1395ecst

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For the further commercialization of SOFCs, the cost reduction for materials is one of the most important technological issues. Currently, Y or Sc is widely used as a dopant of zirconia-based electrolytes, but these rare-earth elements are relatively expensive. In this study, we have examined the possibility to use Ca as an alternative dopant since it is abundant in the earth and inexpensive. We have experimentally found that Ca diffusion is relatively fast, causing cell performance degradation. Even so, such fast diffusion could be depressed by decreasing the concentration gradient of Ca dopant, leading to an improvement in the durability.

DOI： 10.1149/05701.0807ecst

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Durability of cells and stacks against thermal cycling and redox cycling is essential for practical SOFCs, in which the system experiences various kinds of cycling conditions, including the shutoff of fuel supply. In this study, we have investigated the influence of thermal cycling conditions, such as hot-standby, cold-standby, and shut-down, on the cell performance degradation.

DOI： 10.1149/05701.0691ecst

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Flexible Ni-loaded paper-structured catalysts (PSCs) were prepared by the simple paper-making process. Stable performance of DIRSOFC fuelled by simulated biogas mixture (CH₄/CO₂ = 1) was demonstrated by the integration of electrolyte-supported cell with PSC array which leads to uniform in-plane reaction distribution. Anode-supported cell stacked with PSC exhibited maximum power density of 820 mW cm^-2 and degradation rate of 1.4 %/1000 h at 750 °C in the direct feed of biogas mixture without humidification.

DOI： 10.1149/05701.2997ecst

Language：English   Publishing type：Research paper (scientific journal)  

Metal supported cells with porous metallic substrates made of stainless steels are expected to drastically improve remaining issues of durability and cost for solid oxide fuel cells (SOFCs). In this study, we succeeded in fabricating a porous alloy substrate for SOFC using a Fe-Cr-Al type stainless steel, which showed excellent heat resistance. We investigated the application of the alloy substrate for the cathode side and good heat resistance and relatively low contact resistance between the alloy substrate and La _0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) was obtained at 700°C in air.

DOI： 10.1149/05701.2289ecst

Language：English   Publishing type：Research paper (scientific journal)  

For solid oxide fuel cell (SOFC) systems operating with practical fuels, it is important to simulate the distribution of sulfur poisoning and its dependence on the operating conditions in planar-type SOFCs. In this paper, by taking into account the sulfur poisoning effect and its distribution in planar-type SOFC simulation, numerical analysis is performed using anode exchange current density data and Temkin-like sulfur adsorption isotherm on Ni surface. From this numerical analysis, it is found that the distribution of current density after sulfur poisoning is changed, depending on operating temperature and fuel utilization.

DOI： 10.1149/05701.2841ecst

Language：English   Publishing type：Research paper (scientific journal)  

Impurities on the surface of SOFC electrodes can dramatically reduce their interfacial transport. In this work, we use a novel in situ optical absorption relaxation technique to derive the oxygen exchange rate in the absence of electrodes, thereby studying the "innate" material. Si is found to be an impurity in high concentration, with its presence associated with an orders of magnitude decrease in oxygen exchange rate. Addition of La to the electrode surface is found to substantially improve oxygen exchange rate.

DOI： 10.1149/05701.2003ecst

Language：English   Publishing type：Research paper (scientific journal)  

Durability of SOFCs is one of the most important requirements for commercialization. In this paper, we analyze chemical degradation phenomena caused by both extrinsic and intrinsic origins besides by thermal/redox cycling. As external degradation, impurity (sulfur, phosphorus etc.) poisoning has been systematically analyzed and classified based on both thermochemical and kinetic consideration. We present typical intrinsic chemical degradation phenomena observed, mainly diffusion-related processes (interdiffusion, grain boundary diffusion, dopant dissolution, phase transformation etc.) around interfaces between the electrolyte and the electrode, which has been revealed through high-resolution STEM-EDX analysis of cells after long-term operation. Degradation mechanisms have been classified.

DOI： 10.1149/05701.0315ecst

Language：English   Publishing type：Research paper (other academic)  

For the purpose of studying electrochemical reactions in nanospace of structurally controlled carbon, electrochemistry inside the hydrophobic 8 nm pores with platinum deposited mesoporous carbon (Pt/MC) was investigated, and reaction selectivity within carbon nano-space was demonstrated. Distinctive activities toward hydrogen evolution/oxidation and oxygen evolution reactions were observed, comparing to reactions in larger pores (>30 nm). Only small amount of water is expected to exist within such small nano-space even though a proton pass is effectively maintained. Consequently, selectivity in reactions, such as activation of proton-related reactions and suppression of water-related reactions, is achieved by Pt/MC electrocalatysts.

DOI： 10.1149/1.3684482