||Y. Yamazaki, A. Kuwabara, J. Hyodo, Y. Okuyama, C.A.J. Fisher, S.M. Haile, Oxygen affinity: the missing link enabling prediction of proton conductivities in doped barium zirconates, Chemistry of Materials, https://doi.org/10.1021/acs.chemmater.0c01869, 32, 17, 7292-7300, 2020.09, Proton-conducting oxides, specifically doped barium zirconates, have garnered much attention as electrolytes for solid-state electrochemical devices operable at intermediate temperatures (400−
600 °C). In chemical terms, hydration energy, Ehyd, and proton− dopant association energy, Eas, are two key parameters that determine whether an oxide exhibits fast proton conduction, but to date ab initio studies have for the most part studied each parameter separately, with no clear correlation with proton conductivity identified in either case. Here, we demonstrate that the oxygen affinity, EO.dopant, defined as the energy released when an oxide ion enters an oxygen vacancy close to a dopant atom, is the missing link between these two parameters and correlates well with experimental proton conductivities in doped
barium zirconates. Ab initio calculations of point defects and their complexes in Sc-, In-, Lu-, Er-, Y-, Gd-, and Eu-doped barium
zirconates are used to determine Ehyd, Eas, EO.dopant, and the hydrogen affinity, EH.host, of each system. These four energy terms are
related by Ehyd = EO.dopant + 2EH.host + 2Eas. Complementary impedance spectroscopy measurements reveal that the stronger the
calculated oxygen affinity of a system, the higher the proton conductivity at 350 °C. Although the proton trapping site is also an
important factor, the results show that oxygen affinity is an excellent predictor of proton conductivity in these materials..
||Junji Hyodo, Koki Kitabayashi, Kenta Hoshino, Yuji Okuyama, Yoshihiro Yamazaki, Fast and Stable Proton Conduction in Heavily Scandium-Doped Polycrystalline Barium Zirconate at Intermediate Temperatures, Advanced Energy Materials, 10.1002/aenm.202000213, 10, 25, 2000213, 2020.07, The environmental benefits of fuel cells and electrolyzers have become increasingly recognized in recent years. Fuel cells and electrolyzers that can operate at intermediate temperatures (300–450 °C) require, in principle, neither the precious metal catalysts that are typically used in polymer-electrolyte-membrane systems nor the costly heat-resistant alloys used in balance-of-plant components of high-temperature solid oxide electrochemical cells. These devices require an electrolyte with high ionic conductivity, typically more than 0.01 S cm<sup>−1</sup>, and high chemical stability. To date, however, high ionic conductivities have been found in chemically unstable materials such as CsH<sub>2</sub>PO<sub>4</sub>, In-doped SnP<sub>2</sub>O<sub>7</sub>, BaH<sub>2</sub>, and LaH<sub>3−2</sub><sub>x</sub>O<sub>x</sub>. Here, fast and stable proton conduction in 60-at% Sc-doped barium zirconate polycrystal, with a total conductivity of 0.01 S cm<sup>−1</sup> at 396 °C for 200 h is demonstrated. Heavy doping of Sc in barium zirconate simultaneously enhances the proton concentration, bulk proton diffusivity, specific grain boundary conductivity, and grain growth. An accelerated stability test under a highly concentrated and humidified CO<sub>2</sub> stream using in situ X-ray diffraction shows that the perovskite phase is stable over 240 h at 400 °C under 0.98 atm of CO<sub>2</sub>. These results show great promises as an electrolyte in solid-state electrochemical devices operated at intermediate temperatures..
||Junie Jhon M. Vequizo, Shunta Nishioka, Junji Hyodo, Yoshihiro Yamazaki, Kazuhiko Maeda, Akira Yamakata, Crucial impact of reduction on the photocarrier dynamics of SrTiO3 powders studied by transient absorption spectroscopy, Journal of Materials Chemistry A, 10.1039/c9ta08216f, 7, 45, 26139-26146, 2019.01, Inducing oxygen vacancy defects in SrTiO3 powders via high temperature treatment in the presence of a mixture of Ar, O2, dry air, and H2 ambient gases is a promising strategy to produce homogeneously defective SrTiO3 photocatalysts with a remarkable 40-fold enhancement of H2 evolution activity. Electron doping of SrTiO3 due to oxygen vacancies triggers the development of a highly active SrTiO3 photocatalyst; however, the photodynamic processes involved in these modifications of SrTiO3 have not been fully elucidated yet. In this work, we investigated the impact of high temperature treatment based on the dynamics of photocarriers by transient absorption spectroscopy (TAS). TAS results revealed that upon band gap excitation of SrTiO3, most of the photoexcited electrons in non-reduced SrTiO3 are deeply trapped in the intrinsic defects as evident from the strong broad absorption signals peaking at 11 000 cm-1 (909 nm, 1.36 eV) and 20 000 cm-1 (500 nm, 2.48 eV), whereas the absorption intensities in this wavenumber region largely decreased in highly reduced SrTiO3, suggesting a possible electron filling of deeply trapped states via reduction treatment (or electron doping). Interestingly, the photoexcited electrons in oxygen-deficient SrTiO3 preferably occupy the shallower electron traps. The lowest energy limit of the electron trap filled by photoexcited electrons is estimated to be at the absorption edge located at 1000 cm-1 (∼0.12 eV below the conduction band), which is much shallower than that of non-reduced SrTiO3 (>0.7 eV). Furthermore, it was found that the electron population in the shallow traps in highly reduced SrTiO3 is nearly 2 orders of magnitude higher compared to that in non-reduced SrTiO3, indicating a large improvement in the electron lifetime. The findings herein offer significant insights into the crucial impact of the reduction of SrTiO3via induced oxygen vacancy defects to provide available photoexcited electrons that can be readily utilized for the H2 generation reaction..
||Shunta Nishioka, Junji Hyodo, Junie Jhon M. Vequizo, Shunsuke Yamashita, Hiromu Kumagai, Koji Kimoto, Akira Yamakata, Yoshihiro Yamazaki, Kazuhiko Maeda, Homogeneous Electron Doping into Nonstoichiometric Strontium Titanate Improves Its Photocatalytic Activity for Hydrogen and Oxygen Evolution, ACS Catalysis, 10.1021/acscatal.8b01379, 8, 8, 7190-7200, 2018.08, Water splitting using a semiconductor photocatalyst has been extensively studied as a means of solar-to-hydrogen energy conversion. Powder-based semiconductor photocatalysts, in particular, have tremendous potential in cost mitigation due to system simplicity and scalability. The control and implementation of powder-based photocatalysts are, in reality, quite complex. The identification of the semiconductor-photocatalytic activity relationship and its limiting factor has not been fully solved in any powder-based semiconductor photocatalyst. In this work, we present systematic and quantitative evaluation of photocatalytic hydrogen and oxygen evolution using a model strontium titanate powder/aqueous solution interface in a half reaction. The electron density was controlled from 1016 to 1020 cm-3 throughout the strontium titanate powder by charge compensation with oxygen nonstoichiometry (the amount of oxygen vacancy) while maintaining its crystallinity, chemical composition, powder morphology, and the crystal and electronic structure of the surface. The photocatalytic activity of hydrogen evolution from aqueous methanol solution was stable and enhanced by 40-fold by the electron doping. The enhancement was correlated well with increased Δabsorbance, an indication of prolonged lifetime of photoexcited electrons, observed by transient absorption spectroscopy. Photocatalytic activity of oxygen evolution from aqueous silver nitrate solution was also enhanced by 3-fold by the electron doping. Linear correlation was found between the photocatalytic activity and the degree of surface band bending, ΔΦ, above 1.38 V. The band bending, potential downhill for electronic holes, enlarges the total flux of photoexcited holes toward the surface, which drives the oxygen evolution reaction..
||Chih Kai Yang, Yoshihiro Yamazaki, Aykut Aydin, Sossina M. Haile, Thermodynamic and kinetic assessments of strontium-doped lanthanum manganite perovskites for two-step thermochemical water splitting, Journal of Materials Chemistry A, 10.1039/c4ta02694b, 2, 33, 13612-13623, 2014.09, Solar-driven thermochemical water splitting using non-stoichiometric oxides has emerged as an attractive technology for solar fuel production. The most widely considered oxide for this purpose is ceria, but the extreme temperatures required to achieve suitable levels of reduction introduce challenges in reactor design and operation, leading to efficiency penalties. Here, we provide a quantitative assessment of the thermodynamic and kinetic properties of La 1-xSrxMnO3-δ perovskites, targeted for a reduced temperature operation of thermochemical water splitting. Sr-doping into lanthanum manganite increases the thermodynamic fuel production capacity, which reaches 9 ml g-1 for 0.4 Sr for a thermochemical cycle operated between 1400 and 800 °C. The hydrogen yields are moreover in good agreement with expected values based on analysis and extrapolation of thermogravimetric data available in the literature. High levels of Sr doping, however, result in low steam-to-hydrogen conversion rates, implying high energy penalties in an operational reactor. Furthermore, the rate of fuel production decreases with increasing Sr content, suggesting that intermediate compositions may yield the most suitable combination of properties. This journal is.
||Yoshihiro Yamazaki, Frédéric Blanc, Yuji Okuyama, Lucienne Buannic, Juan C. Lucio-Vega, Clare P. Grey, Sossina M. Haile, Proton trapping in yttrium-doped barium zirconate, Nature Materials, 10.1038/nmat3638, 12, 7, 647-651, 2013.07, The environmental benefits of fuel cells have been increasingly appreciated in recent years. Among candidate electrolytes for solid-oxide fuel cells, yttrium-doped barium zirconate has garnered attention because of its high proton conductivity, particularly in the intermediate-temperature region targeted for cost-effective solid-oxide fuel cell operation, and its excellent chemical stability. However, fundamental questions surrounding the defect chemistry and macroscopic proton transport mechanism of this material remain, especially in regard to the possible role of proton trapping. Here we show, through a combined thermogravimetric and a.c. impedance study, that macroscopic proton transport in yttrium-doped barium zirconate is limited by proton-dopant association (proton trapping). Protons must overcome the association energy, 29 kJ mol -1, as well as the general activation energy, 16 kJ mol -1, to achieve long-range transport. Proton nuclear magnetic resonance studies show the presence of two types of proton environment above room temperature, reflecting differences in proton-dopant configurations. This insight motivates efforts to identify suitable alternative dopants with reduced association energies as a route to higher conductivities..
||Yoshihiro Yamazaki, Raul Hernandez-Sanchez, Sossina M. Haile, Cation non-stoichiometry in yttrium-doped barium zirconate: phase behavior, microstructure and proton conductivity, Journal of Materials Chemistry, 20, 8158-8166, 2010.08.
||Yoshihiro Yamazaki, Raul Hernandez-Sanchez, Sossina M. Haile, High total proton conductivity in large-grained yttrium-doped barium zirconate, Chemistry of Materials, 10.1021/cm900208w, 21, 13, 2755-2762, 2009.07, Barium zirconate has attracted particular attention among candidate proton conducting electrolyte materials for fuel cells and other electrochemical applications because of its chemical stability, mechanical robustness, and high bulk proton conductivity. Development of electrochemical devices based on this material, however, has been hampered by the high resistance of grain boundaries, and, due to limited grain growth during sintering, the high number density of such boundaries. Here, we demonstrate a fabrication protocol based on the sol - gel synthesis of nanocrystalline precursor materials and reactive sintering that results in large-grained, polycrystalline BaZr 0.8Y 0.2C 3-δ of total high conductivity, ∼ 1 × 10 -2 Scm -1 at 450 °C. The detrimental role of grain boundaries in these materials is confirmed via a comparison of the conductivities of polycrystalline samples with different grain sizes. Specifically, two samples with grain sizes differing by a factor of 2.3 display essentially identical grain interior conductivities, whereas the total grain boundary conductivities differ by a factor of 2.5-3.2, depending on the temperature (with the larger-grained material displaying higher conductivity)..
||Yoshihiro Yamazaki, Peter Babilo, Sossina M. Haile, Defect chemistry of yttrium-doped barium zirconate
A thermodynamic analysis of water uptake, Chemistry of Materials, 10.1021/cm800843s, 20, 20, 6352-6357, 2008.10, Thermogravimetry has been used to evaluate the equilibrium constants of the water incorporation reaction in yttrium-doped BaZrO3 with 20-40% yttrium in the temperature range 50-1000°C under a water partial pressure of 0.023 atm. The constants, calculated under the assumption of a negligible hole concentration, were found to be linear in the Arrhenius representation only at low temperatures (≤500 °C). Nonlinearity at high temperatures is attributed to the occurrence of electronic defects. The hydration enthalpies determined here range from -22 to -26 kJ mol-1 and are substantially smaller in magnitude than those reported previously. The difference is a direct result of the different temperature ranges employed, where previous studies have utilized higher temperature thermogravimetric measurements, despite the inapplicability of the assumption of a negligible hole concentration. The hydration entropies measured in this work, around -40 J K-1 mol -1, are similarly smaller in magnitude than those previously reported and are considerably smaller than what would be expected from the complete loss of entropy of vapor-phase H2O upon dissolution. This result suggests that substantial entropy is introduced into the oxide as a consequence of the hydration. The hydration reaction constants are largely independent of yttrium concentration, in agreement with earlier reports..