The proton conductivity of mixed hole-, oxygen ion- and proton-conducting Ba0.5Sr0.5Fe0.8Zn0.2O3-δ (BSFZ), a potential cathode material for fuel cells based on oxidic proton-conducting electrolytes, was determined from the weight changes of dense pellets upon changing pH2O (and pD2O). The obtained proton concentrations at 20 mbar pH2O range from 1.3 to 0.32 mol% (350-600 °C). The effective diffusion coefficients extracted from the transients and ranging from 1.4 to 29 × 10(-7) cm(2) s(-1) (350 to 600 °C) represent a lower bound for the proton diffusivity and the directly related proton mobility. The calculated proton conductivities reach values in the range of 0.9 to 3 × 10(-4) S cm(-1). Since the real proton conductivity might be underestimated, these values are sufficiently high to render the bulk path in the oxygen reduction mechanism dominant in dense, thin-film electrodes.

Thermogravimetric investigations on the perovskite Ba(0.5)Sr(0.5)Fe(0.8)Zn(0.2)O(3-δ) (BSFZ, with mixed hole, oxygen vacancy and proton conductivity) from water vapor can occur by acid-base reaction (hydration) or redox reaction (hydrogen uptake), depending on the oxygen partial pressure, i.e. on the material's defect concentrations. In parallel, the effective diffusion coefficient of the stoichiometry relaxation kinetics also changes. These striking observations can be rationalized in terms of a defect chemical model and transport equations for materials with three mobile carriers. Implications for the search of cathode materials with mixed electronic and protonic conductivity for application on proton conducting oxide electrolytes are discussed.

Materials with three independent mobile charge carriers, in the sense of not being in local defect‐chemical equilibrium though naturally coupled through electroneutrality, are encountered in various cases of scientific and technological relevance. Examples are proton conducting perovskites under conditions at which hole and also oxygen vacancy conductivity may become significant, and mixed conducting cathode materials suited for fuel cells using proton conducting oxide electrolytes. Already the thermodynamics of the equilibrium situation is complex as a increase can lead to proton incorporation by water uptake (pure acid–base reaction) or by hydrogenation (redox reaction). As far as the even more complex transport kinetics are concerned, diffusion equations are derived which are exact for the interaction‐free (ideally dilute) situation. Kinetic implications are discussed and checked by exemplary numerical simulations. The treatment includes simple sub‐cases such as onefold relaxation on change, as well as complex patterns characterized by the appearance of more than one characteristic time scales (“twofold relaxation”) or apparent “moving boundary” kinetics. Implications for stability and functionality of ceramic materials are discussed.

The presence of protons in mixed conducting cathode materials contacted with proton conducting electrolytes introduces additional possible mechanistic pathways for the surface oxygen reduction reaction compared to cathodes on oxide ion conducting electrolytes. A reaction network that includes these additional paths with protonated intermediate species is developed. The corresponding rate equations are derived, reflecting the point defects' contributions to the overall oxygen and water partial pressure dependencies. An equivalent circuit is derived for the general case of electrodes that exhibit proton, oxygen ion and electronic conductivity on proton conducting electrolytes. A characteristic circuit element is a chemical capacitance comprising proton and oxygen ion concentration changes. Complications due to nonnegligible electronic transference numbers arising in proton conducting electrolytes at high pO 2 are discussed.