Protonic ceramic fuel and electrolysis cells (PCFC, PCEC) require fast kinetics of the oxygen reduction/water oxidation reaction at the air electrode (positrode). To extend the reactive zone beyond the three-phase boundary, positrode materials are desirable which combine mobile electronic defects, oxygen vacancies and protons ("triple conductors"). Because of the more covalent TM-O bonds, triple conducting (Ba,Sr)(Fe,Co,Zn,Y)O_3-d perovskites typically show a lower degree of hydration than Ba(Zr,Ce)O₃ electrolyte materials (see e.g. [1,2,3]).

Owing to the dominating electronic and competing oxygen vacancy conductivity, the experimental determination of the proton conductivity and mobility of (Ba,Sr)(Fe,Co,Zn,Y)O_3-d positrode materials is challenging. The experimental approaches comprise water chemical diffusion measurements, and measurements of deuterium concentration profiles by secondary ion mass spectroscopy (SIMS). They show that proton mobilities in Ba(Fe,Y)O_3-d are of comparable magnitude as in Ba(Zr,Ce)O₃ electrolytes. However, the results also indicate that dopants such as Y³⁺ may lead to proton trapping effects in barium ferrates.

To obtain a detailed picture at atomistic level, proton migration in BaFeO_3-d is also investigated by DFT calculations.[4] The proton migration barriers are found to depend both on the initial O^...O and O-H distances. Interestingly, oxygen deficiency and correspondingly decreased formal Fe oxidation state lower the barriers. Proton barriers calculated with Sc³⁺ and Y³⁺ dopants further elucidate electrostatic and lattice distortion contributions to proton trapping. The characteristics of the proton transfer - being assisted by phonons providing a suitable O^...O distance - is demonstrated by ab initio molecular dynamics.

Depending on the positrode morphology (dense thin film or porous thick film), the transport of oxygen vacancies may also become relevant for the overall surface reaction rate. Measurements in moderately reducing conditions (pO₂ = 10^-10 to 10^-20 bar in an oxygen pumping cell) indicate that oxygen vacancies may also be affected by trapping at dopants in BaFeO_3-d. Furthermore, the effect of dopant concentration on vacancy trapping may differ from the respective variation for protons (cf. related investigation for Ba(Zr,Y)O_3-d [5]).

Overall, the detailed understanding of proton concentration and mobility as function of the specific materials composition is a key element for the rational optimization of PCFC/PCEC positrode materials, which have to fulfil mutually conflicting requirements (protonic and electronic conductivity, catalytic activity, long-term stability).