In order to extend the reactive zone for oxygen reduction to water beyond the gas/electrode/electrolyte three phase boundary, cathode materials for proton conducting ceramic fuel cells should exhibit a certain proton conductivity, exceeding 10^-5 S/cm [1]. Proton uptake into (Ba,Sr,La)(Mn,Fe,Co)O_3-d perovskites considered as cathode material can occur by incorporation of H₂O (for ) or by incorporation of only H (for ) assuming ideally dilute defect concentrations [2,3]. These two regimes can be identified by thermogravimetric investigations over an extended range of pO₂, pH₂O and temperature.

Independent of the actual proton incorporation regime the proton concentrations in (Ba,Sr,La)(Fe,Co)O_3-d cathode materials amount to less than 3% [3]. They are much lower than for acceptor-doped Ba(Zr,Ce)O_3-xelectrolytes, which remain completely hydrated (10-20% protons) up to about 400-500 °C. Correlations between cation composition and proton uptake will be explored [6]. The basicity of the oxide ions is known to favour proton uptake in electrolyte materials. In p-type conducting and redox-active cathode materials, additional defect interactions between protons and electron holes lead to a perceptible decrease of proton uptake already for low hole concentrations. This is probably related to the partially covalent bonding between the transition metal and oxygen, by which one localized hole affects the basicity of six adjacent oxide ions.

Proton mobility in cathode materials has so far only been measured for Ba_0.5Sr_0.5Fe_0.8Zn_0.2O_3-d [4], where it is comparable to that in Y-doped BaZrO_3-x electrolytes. The estimated proton conductivity of » 10^-3 S/cm at 350-450 °C [1] suffices to transport protons from the electrolyte through the dense cathode film to the gas interface, as evidenced from the diameter dependence of microelectrode resistances. The reduction of O₂ to water can in principle proceed without oxygen incorporation into the cathode material. The measured pO₂ dependence of the effective rate constant indicates that molecular oxygen species participate in the rate determining step. From the pH₂O dependence for Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-d microelectrodes we conclude that protonated oxygen species appear only after this step [5]. Together, these findings suggest that for Ba_0.5Sr_0.5Fe_0.8Zn_0.2O_3-d and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-dthe oxygen reduction to water proceeds via intermediate uptake of O into surface oxygen vacancies.

[1] R. Merkle, D. Poetzsch, J. Maier, ECS Transact. 66(2) (2015) 95

[2] D. Poetzsch, R. Merkle, J. Maier, Adv. Funct. Mater. 25 (2015) 1542

[3] D. Poetzsch, R. Merkle, J. Maier, Faraday Discussions 182 (2015) 129

[4] D. Poetzsch, R. Merkle, J. Maier, Phys. Chem. Chem. Phys. 16 (2014) 16446

[5] D. Poetzsch, R. Merkle, J. Maier, J. Electrochem. Soc. 162 (2015) F939

[6] R. Zohourian, R. Merkle, J. Maier, Solid State Ionics doi:10.1016/j.ssi.2016.09.012