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.

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In oxide materials, an increase in oxygen vacancy concentration often results in lattice expansion, a phenomenon known as chemical expansion that can introduce detrimental stresses and lead to potential device...

The exchange of ions between a lattice and the gaseous phase makes mixed conducting oxides ideal for a range of electrochemical applications. Altering oxygen ion concentration is accompanied by a change to electronic species concentrations, and this influences electrical, chemical, kinetic, and mechanical properties. The stability of electrochemical devices like fuel cells and batteries can heavily rely on the mechanical response to changes in chemical defect concentrations. Under both dynamic and steady-state operation of these devices, large volume strains and strain mismatch at interfaces can result in fracture, warping, and delamination that can cause performance degradation and/or failure. Strains between different materials are compared using the coefficient of chemical expansion (CCE), which normalizes the isothermal chemical strain by the change in defect concentration. Here, we advance the understanding of chemo-mechanical coupling through the study of PrGa0.9Mg0.1O3-δ and BaPr0.9Y0.1O3-δ by demonstrating CCEs 2-5x lower than any previously reported perovskite oxide1. Isothermal CCEs were evaluated with in situ, high temperature, and variable atmosphere x-ray diffraction and dilatometry for chemical strains, and with thermogravimetric analysis for stoichiometry changes. The experimental results show chemical strains to be significantly lower than predictions from simple empirical models that 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. The collaboration of experimental and computational work combines accurate and reliable material characterization with insights into atomic and electronic structures that are difficult to probe experimentally. Our results for the studied compositions indicate 2 primary factors that can be used to modify CCEs: 1) Altering the crystal structure away from the isotropic, cubic phase encourages anistropic expansion and lower CCEs in polycrystalline materials, and 2) Varying the distribution of charge along B-O bonds is shown to dramatically alter the CCE. While the first factor provides rather clear guidance to tailor expansion, we elaborate on the second by suggesting band structure design principles for near-zero redox-strain perovskites, and the benefit of locating holes partially or fully on oxygen is highlighted. These new findings add to the growing collection of crystal-chemical design rules for the rational tailoring of chemo-mechanical coupling in perovskite oxides. (1) Anderson, L. O.; Yong, A. X. Bin; Ertekin, E.; Perry, N. H. Toward Zero-Strain Mixed Conductors: Anomalously Low Redox Coefficients of Chemical Expansion in Praseodymium-Oxide Perovskites. Chem. Mater. 2021, 33 (21), 8378–8393. https://doi.org/10.1021/ACS.CHEMMATER.1C02739.

Chemical expansion is a strain induced by a change in stoichiometry, such as oxygen loss, and it can have a significant impact on device performance and lifetime. While large coefficients of chemical expansion (CCE) are needed for high displacements in an actuator, the same, large CCE can be deleterious to device longevity in a fuel cell where large chemical potential gradients exist across very small thicknesses. The breadth of CCE values needed in various devices calls for the development of design rules to tailor CCE for optimal material response, so our work targets the establishment of such structure-property insights. Oxygen-loss-induced, stoichiometric chemical expansion in oxides involves the formation of an oxygen vacancy; when oxygen leaves the lattice, charge compensating electrons are left behind and can localize on nearby multivalent cations. As cations are reduced, their atomic radii and the surrounding lattice expand. An empirical formula describing the pseudo-cubic lattice constant of perovskite materials has been developed [1] which relates the lattice parameter to the ionic radii of cation and anion components. 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. In an effort to develop and understand design rules to tailor CCE, two compositions, PrGa0.9Mg0.1O3 (PGM) and BaPr0.9Y0.1O3 (BPY), have been synthesized. These compositions allowed for a comparison between A and B-site multivalent Pr (nominally 3+/4+); however, we found that the empirical model did not adequately predict the differences in CCEs on this basis. Other factors including crystal symmetry, charge localization, and location of charge (anion or cation) were instead found to be significantly impactful for both compositions [2]. Values of CCE have been determined by characterizing isothermal changes in stoichiometry with thermogravimetric analysis (TGA) and corresponding changes in strain with dilatometry and in situ, high temperature XRD (HTXRD) as a function of oxygen partial pressure (pO2). The degree of charge localization has been interpreted from impedance measurements of the temperature dependence of conductivity, and the experimental results have been compared to density functional theory (DFT+U) calculations. Over the pO2 and temperature range studied, PGM and BPY have low CCEs, therefore making them of potential interest for fuel/electrolysis cell electrodes. The effects of the abovementioned design rules are discussed to provide insights into rational material design for tailored CCE. [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. [2] Ricote, S., Hudish, G., O’Brien, J. R. & Perry, N. H. Non stoichiometry and lattice expansion of BaZr0.9Dy0.1O3-δ in oxidizing atmospheres. Solid State Ionics (2019) doi:10.1016/j.ssi.2018.12.006.

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.