New insights into efficient oxygen evolution were obtained by developing robust evaluation protocols and understanding interfacial behaviors.

Electrolytes and electrodes in protonic ceramic electrolysis/fuel cells (PCECs/PCFCs) can exhibit significant chemical strains upon incorporating H2O into the lattice. To increase PCEC/PCFC durability, oxides with lower hydration coefficients of chemical expansion (CCEs) are desired. We hypothesized that lowering symmetry in perovskite-structured proton conductors would lower their CCEs and thus systematically varied the tolerance factor through B-site substitution in the prototypical BaCe0.9–x Zr x Y0.1O3−δ (0 ≤ x ≤ 0.9) solid solution. X-ray diffraction (XRD) confirmed that symmetry decreased with decreasing Zr content. CCEs were measured by isothermal XRD, dilatometry, and thermogravimetric analysis (TGA) in varied pH2O over 430–630 °C. With decreasing Zr content, the isothermal H2O uptake was greater, but the corresponding chemical strains were smaller; therefore, CCEs monotonically decreased. Density functional theory simulations on end-member BaCe1–y Y y O3−δ and BaZr1–y Y y O3−δ compositions showed the same trend. Lower CCEs in this solid solution correlate to decreasing symmetry, increasing unit cell volume, increasing oxygen vacancy radius, decreasing bulk modulus, and inter- vs intraoctahedral hydrogen bonding. Microstructural constraints may also contribute to lower macroscopic CCEs in lower-symmetry bulk ceramics based on the observed anisotropic chemical expansion and enhanced strains in powder vs bulk BaCe0.9Y0.1O3−δ. The results inform design principles for the rational tailoring of CCEs and materials choice for chemomechanically durable devices.

Zero-strain materials are desired for high chemo-mechanical stability in energy conversion/storage devices, where operational stoichiometry changes can cause large chemical stresses. Here, we demonstrate near-zero redox coefficients of chemical expansion (CCEs) for mixed-and triple-conducting Pr-oxide perovskites. PrGa 0.9 Mg 0.1 O 3 − δ (PGM) and BaPr 0.9 Y 0.1 O 3 − δ (BPY), having Pr on the A-and B-site, respectively, were synthesized and characterized with in situ high temperature, variable atmosphere X-ray diffraction, dilatometry, and thermogravimetric analysis to obtain isothermal stoichiometry changes, chemical strains, and CCEs. Despite empirical model predictions of smaller CCEs for Pr on the A-site, both compositions yielded unprecedented low average CCEs (0.004−0.011), 2−5× lower than the lowest reported perovskite redox CCEs. Simple empirical models assume pseudo-cubic structures and full charge localization on multivalent cations like Pr. To evaluate actual charge distribution, in situ impedance spectroscopy and density functional theory calculations were performed. Results indicate that the anomalously low CCEs in these compositions likely derive from a combination of decreased crystal symmetry (vs cubic), partial charge delocalization through hybridization of Pr-4f and O-2p orbitals, and redox/multivalence on O rather than just Pr (with or without hybridization). On this basis, we suggest band structure design principles for near-zero redox-strain perovskites, highlighting the benefit of locating holes partially or fully on oxygen.

Chemical expansion is a strain induced by a change in stoichiometry, such as oxygen loss. It can enable a material to actuate when exposed to different gas environments, and it can significantly impact device longevity when there are large concentration gradients across small material dimensions in multilayer devices. The desired magnitude of chemical expansion depends on the application; typically, the goal is a large expansion in the former and a small expansion in the latter, where compatibility with neighboring materials is of concern. Therefore, design rules for tailoring expansion from oxygen exchange are needed for the multitude of applications in which it is a contributing factor. As oxygen leaves an oxide lattice during reduction, electrons are generated to preserve charge neutrality, which may move to multivalent cations; this process results in significant expansion of the multivalent cation, and the surrounding lattice, as the oxidation state of the multivalent cation decreases. Recently, an empirical formula describing the pseudo-cubic lattice constant of perovskite materials was developed [1], which relates the lattice parameter to the ionic radii of each cationic and anionic component. This equation predicts that changes in the B-site cation size will have a larger effect on the lattice parameter than an equal change at the A-site. If the multivalent cation is the only one changing size during redox processes, this equation suggests that its placement on the A or B site will have a significant effect on the magnitude of the overall lattice strain during oxygen loss or gain. To test this theory, model perovskite bulk ceramic compositions with multivalent Pr(3+,4+) on the A site and on the B site were fabricated: PrGa0.9Mg0.1O2.95+ δ and BaPr0.9Y0.1O2.95- δ, respectively. Strain values from dilatometry and defect concentrations from thermogravimetric analysis were used to calculate the coefficients of chemical expansion (CCE) in both materials, to determine the effect of multivalent cation placement. These measurements were performed under isothermal conditions while varying the pO2 to avoid effects from nonuniform thermal expansion at different stoichiometries. Multiple temperatures were analyzed to observe any temperature-related trends in CCEs and to determine any effects from crystal structure changes. The resulting CCEs will be interpreted not only in terms of multivalent A vs. B site placement but also considering other factors that may impact CCEs: crystallographic distortions, temperature, and charge localization. [1] Marrocchelli, D., Perry, N. H., & Bishop, S. R. (2015). Understanding chemical expansion in perovskite-structured oxides. Physical Chemistry Chemical Physics, 17(15), 10028-10039.

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