Mixed protonic−electronic conducting oxides are key functional materials for protonic ceramic fuel cells. Here, (Ba,Sr,La)(Fe,Zn,Y)O 3−δ perovskites are comprehensively investigated by X-ray spectroscopy (in oxidized and reduced states). Extended X-ray absorption fine structure shows that Zn,Y doping strongly increases the tendency for Fe−O−Fe buckling. X-ray absorption near-edge spectroscopy at the Fe K-edge and X-ray Raman scattering at the O K edge demonstrate that both iron and oxygen states are involved when the samples are oxidized, and for the Zn,Y doped materials, the hole transfer from iron to oxygen is less pronounced. This can be correlated with the observation that these materials show the highest proton uptake.

Oxides with mixed protonic and p-type electronic conductivity (and typically containing also mobile oxygen vacancies) are important functional materials, e.g., for oxygen electrodes in protonic ceramic electrochemical cells or for permeation membranes. Owing to the presence of three carriers, their defect chemical behavior is complex. Deviations from ideal behavior (defect interactions) have to be taken into account, which are related to the partially covalent character of the transition metal–oxygen bonds. Compared to acceptor-doped Ba(Zr,Ce)O3− z electrolytes, perovskites with redox-active transition-metal cations typically show smaller degrees of hydration. Trends in the proton uptake of (Ba,Sr,La)(Fe,Co,Y,Zn)O3−δ perovskites are analyzed and correlated to structural features (local lattice distortions) and electronic properties (the position of oxygen states on an absolute energy scale). The proton mobility in such mixed-conducting perovskites is estimated. Specific aspects of the application of protonic and electronic mixed-conducting oxides in protonic ceramic electrochemical cells are discussed, and an overview of recent materials and device developments is given.

Protonic–electronic mixed-conducting perovskites are relevant as cathode materials for protonic ceramic fuel cells (PCFCs). In the present study, the relation between the electronic structure and the thermodynamics of oxygen nonstoichiometry and hydration is investigated for BaFeO3−δ and Ba0.5Sr0.5FeO3−δ by means of density functional theory. The calculations are performed at the PBE + U level and yield ground-state electronic structures dominated by an oxygen-to-metal charge transfer with electron holes in the O 2p valence bands. Oxygen nonstoichiometry is modeled for 0 ≤ δ ≤ 0.5 with oxygen vacancies in doubly positive charge states. The energy to form an oxygen vacancy is found to increase upon reduction, i.e., decreasing concentration of ligand holes. The higher vacancy formation energy in reduced (Ba,Sr)­FeO3−δ is attributed to a higher Fermi level at which electrons remaining in the lattice from the removed oxide ions have to be accommodated. The energy for dissociative H2O absorption into oxygen vacancies is found to vary considerably with δ, ranging from ≈−0.2 to ≈−1.0 eV in BaFeO3−δ and from ≈0.2 to ≈−0.6 eV in Ba0.5Sr0.5FeO3−δ. This dependence is assigned to the annihilation of ligand holes during oxygen release, which leads to an increase in the ionic charge of the remaining lattice oxide ions. The present study provides sound evidence that p-type electronic conductivity and the susceptibility for H2O absorption are antagonistic properties since both depend in opposite directions on the concentration of ligand holes. The reported trends regarding oxygenation and hydration energies are in line with the experimental observations.

Phenomenologically, the enthalpy of the dissociative water incorporation (hydration) of oxides is often found to be more favorable for more basic oxides. In the present work, we investigate proton, hydroxide ion, and oxide ion affinities (PA, HA, and OA) for 19 closed-shell oxides ranging from Li 2 O and Cs 2 O to TiO 2 , SnO 2 , and SiO 2 , including also perovskites such as SrTiO 3 and BaZrO 3 using first-principles defect calculations and thermochemical cycles. The proton affinity is found to play a predominant role in the hydration thermodynamics. The ion affinities are strongly correlated with the oxides' electronic structure (specifically, the ionization potential (IP)). This intriguing correlation between PA and IP holds also for gaseous O species, suggesting a very general origin. Understanding the major factors controlling a metal oxide's susceptibility for dissociative hydration of oxygen vacancies is not only of fundamental interest but also key to the successful development of novel mixed proton−electron conducting oxides for protonic ceramic fuel and electrolyzer cells. In addition to elucidating the hydration reaction, these ion affinities also serve a more general purpose, as they can be used to predict the oxides' tendency to in-/ excorporate a specific ion.

Oxides with mixed protonic-electronic conductivity are relevant as cathode materials for protonic ceramic fuel cells and electrolyzers. We investigate the modification of the electronic structure of (Ba,La)(Fe,Zn,Y)O3- when iron is...