Toward Durable Protonic Ceramic Cells: Hydration-Induced Chemical Expansion Correlates with Symmetry in the Y-Doped BaZrO₃–BaCeO₃ Solid Solution

Abstract

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.

Tailored and Improved Protonic Conductivity through Ba(ZxCe10−x)0.08Y0.2O3−δ Ceramics Perovskites Type Oxides for Electrochemical Devices

“…[42,46,47] These properties ultimately relate to the bonding strengths of oxide ions and protons, as previously hypothesized by Bjørheim et al, [48] and are consistent with the present observation. Even though no clear-cut dependence on Zr content or the B-site ionic radius is apparent, the trend for both values is somewhat similar to previously reported [48,49] trends observed for the proton concentration discussed above. A summary of the extracted thermodynamic parameters is presented in Table 2.…”

Tuning the Co/Fe ratio in BaCoxFe0.8−xZr0.1Y0.1O3−δ, a promising triple ionic and electronic conducting oxide, to boost electrolysis and fuel cell performance

“…Dehydration causes a chemical contraction in protonconducting oxides, which can offset thermal expansion and lead to plateau-like expansion behavior. [36][37][38][39] BaFe 0.6 Co 0.3 Nb 0.1 -O 3−d and BaCo 0.7 Yb 0.2 Bi 0.1 O 3−d , which are very similar Bacontaining Co and/or Fe-doped perovskite TCO materials, have been reported to show negative thermal expansion below 300 °C aer intentional exposure to wet atmosphere and also upon exposure to ambient air at room temperature overnight due to water adsorption/desorption. 40 This nding demonstrates that water incorporation in Ba-containing perovskites can occur even at low temperatures and ambient humidity, which is in accordance with our hypothesis that the BCFZY samples were already hydrated under ambient air at room temperature before HT-XRD measurement, and therefore underwent dehydration during heating, leading to the TEC plateau behaviour.…”

High proton conduction by full hydration in highly oxygen deficient perovskite

“…Since the activation energy for proton (H + ) conductivity is generally lower than that for oxide ion (O 2− ) conductivity, it is known that H + conductors exhibit higher conductivity than O 2− conductors at low and intermediate temperatures (50-500 °C). [10][11][12][13][14][15][16][17][18] Therefore, as a technology alternative to the SOFCs, protonic ceramic fuel cells (PCFCs) have recently been attracting much attention. [10][11][12]19 Furthermore, there has been great interest in proton-conducting electrolysis cells (PCECs), because they can effectively convert electrical energy into chemical energy.…”

“…Perovskite-type oxides such as BaCeO 3 -and BaZrO 3 -based materials are representative proton conductors. 17,[26][27][28][29][30][31][32][33][34][35] A major problem with these conventional proton conductors is proton trapping, which results in low proton conductivity at low and intermediate temperatures. 18 The conventional method to improve the proton conductivity is acceptor M 3+ doping into Ba 2+ B 4+ O 3 perovskite where M 3+ is an acceptor with lower valence 3+ than 4+ of the host B 4+ cation.…”