Proton uptake in the H(+)-SOFC cathode material Ba(0.5)Sr(0.5)Fe(0.8)Zn(0.2)O(3-δ): transition from hydration to hydrogenation with increasing oxygen partial pressure.

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.

Proton conductivity in mixed-conducting BSFZ perovskite from thermogravimetric relaxation.

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.