Protons in Triple-Conducting Perovskites—Concentration, Mobility and Surface Reaction Aspects

Triple-conducting oxides with mobile oxygen vacancies, protons and electronic defects are of fundamental interest, and find application as electrode materials of protonic ceramic electrochemical cells (PCFC, PCEC) operating in an intermediate temperature range 400-600°C. In electrolysis mode, PCECs enable the direct generation of dry, compressed H₂ with high efficiency.[1]. Compared to Ba(Zr,Ce,Acc)O_3-z electrolytes, the degree of hydration in triple-conducting Ba(Fe,Co,Zn,Y)O_3-_d perovskites is significantly lower. This is attributed to the high covalency of (Fe,Co)-O bonds, which decreases the basicity of the oxide ions.[2-4]<br/>The experimental determination of proton conductivity or mobility in triple-conducting perovskites is challenging. Proton diffusivities from chemical diffusion experiments and SIMS measurements will be presented.[5] They are complemented by extended DFT and ab-initio MD calculations for BaFeO_3-_d, which analyze also trapping effects.[6] While the overall proton transfer process is similar, the high covalency and redox activity in BaFeO_3-_d lead also to specific differences compared to electrolyte materials.<br/>The role of protons for the surface oxygen reduction reaction is ambivalent. A certain bulk proton concentration is beneficial for proton transfer from the electrolyte to the extended surface of porous thick-film electrodes. However, triple-conducting perovskites show a high coverage by (dissociatively) adsorbed water even at elevated temperature, which may block catalytically active centers such as oxygen vacancies. Microelectrode measurements of Ba(Fe,Co,Zn,Y)O_3-_d in different pO₂ and pH₂O did not indicate a contribution of protons before or in the rate determining step of the oxygen reduction reaction so far. Overall, a comprehensive picture of proton stability, mobility and reactivity is derived, which can be employed for further materials optimization.<br/><br/>A.C. and D.G. thank the Latvian Council of Science (project no. lzp-2021/1-0203) for financial support, E.A.K. acknowledges the M-Era.Net HetCat project for funding.<br/><br/>[1] E. Vollestad et al., <i>Nat. Mater</i>. 18 (2019) 752-759<br/>[2] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, <i>Adv. Funct. Mater.</i> 28 (2018) 1801241<br/>[3] R. Merkle, M. F. Hoedl, G. Raimondi, R. Zohourian, J. Maier, <i>Ann. Rev. Mater. Res</i>. 51 (2021) 461-493<br/>[4] G. Raimondi, F. Giannici, A. Longo, R. Merkle, A. Chiara, M. F. Hoedl, A. Martorana, J. Maier, <i>Chem. </i><i>Mater</i>. 32 (2020) 8502-8511<br/>[5] C. Berger et al., to be submitted (2024)<br/>[6] M. F. Hoedl, A. Chesnokov, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, <i>J. Mater. </i><i>Chem. A</i> 11 (2023) 6336-6348