Defect Chemistry of Mixed Conducting Pcfc Cathode Materials with Protons, Oxygen Vacancies and Holes

Abstract

In recent years significant progress was achieved for protonic ceramic fuel cells (PCFC) concerning fundamental understanding, materials processing, and device performance. PCFC cathode materials require sufficient proton conductivity to extend the reaction zone beyond the three phase boundary. The hydration thermodynamics of BaFeO3- d-related perovskites was studied using thermogravimetry.[1] Despite a high oxygen vacancy concentration, the degree of hydration is lower for cathode materials compared to Ba(Zr,Y)O3-x electrolytes. A partial substitution of iron by redox-inactive, oversized Zn2+ or Y3+ drastically increases the proton uptake. Measurements of oxygen nonstoichiometry and proton uptake indicate pronounced deviations from ideally dilute defect chemistry (assigned to hole-hole and hole-proton defect interactions [1]). Based on DFT calculations [2] and EXAFS/XRS measurements [3], these interactions are related to the partial transfer of electron holes from iron to adjacent oxygen ions, which in turn disfavors protonation. The obtained detailed defect-chemical understanding serves as the basis for further PCFC cathode optimization, in particular since proton uptake, catalytic activity, electronic conductivity, and stability show conflicting dependencies on cation composition.[1] R. Zohourian, R. Merkle, G. Raimondi, J. Maier, Adv. Funct. Mater. 28 (2018) 1801241.[2] M.F. Hoedl, D. Gryaznov, R. Merkle, E. A. Kotomin, J. Maier, J. Phys. Chem. C 124 (2020) 11780.[3] G. Raimondi, F. Giannici, A. Longo, R. Merkle, A. Chiara, M. F. Hoedl, A. Martorana, J. Maier, Chem. Mater. 32 (2020) 8502.We thank GIF (I-1342-302.5/2016) for financial support.Figure 1: (a) Increase of proton uptake by Y or Zn doping [1]. (b) Dependence of hydration enthalpy on formal Fe oxidation state [2]. (c) Increased local deformations by Y doping [3]. (d) Conflicting trends; e.g. Ba is beneficial for proton uptake and catalytic activity but detrimental for electronic conductivity and chemical stability. Figure 1