Cathode materials for protonic ceramic fuel cells: Bulk defect chemistry and surface reaction kinetics
September 4-8, 2016
Fuel cells based on ceramic proton conductors receive growing interest because these electrolytes offer a higher ionic conductivity compared to oxide ion conductors, in particular at 300-600 °C. To extend the oxygen reduction reaction to water beyond the three-phase boundary, cathode materials for such cells should have a certain proton conductivity. Since the perovskites considered as cathode material exhibit perceptible oxygen vacancy, proton and hole concentrations, proton uptake can occur by water incorporation (acid-base reaction) and by hydrogen incorporation (redox reaction) . The presence of three mobile carriers can lead to a complex two-fold stoichiometry relaxation kinetics, requiring four diffusion coefficients for complete description [1,2].
The different regimes of proton uptake are explored by thermogravimetry (pH2O changes in different pO2) for perovskites such as La0.5Sr0.5FeO3- and Ba0.5Sr0.2Fe0.8Zn0.2O3- (BSFZ). The obtained maximum proton concentrations are significantly lower than in BaZrO3 electrolyte materials at same T and pH2O . The proton mobility in BSFZ extracted from the transient behavior is comparable to that in BaZrO3 electrolytes. Correlations between cation composition and amount of incorporated protons are discussed.
The kinetics of oxygen reduction to water is measured by impedance spectroscopy at dense thin-film BSFZ microelectrodes on proton-conducting Ba(Zr,Y)O3 as substrate. The dependence of surface reaction resistance on electrode area demonstrates that the proton conductivity of BSFZ in the range of » 10-3 S/cm at 400 °C  indeed suffices to transport protons from the Ba(Zr,Y)O3- electrolyte through the dense BSFZ film to the gas interface. The reduction of O2 to water can in principle proceed without oxygen incorporation into the cathode material. The values of the pO2 and pH2O dependence of the effective rate constant indicate that molecular oxygen species participate in the rate determining step, and that protonated oxygen species appear only after this step .
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Rotraut Merkle, "Cathode materials for protonic ceramic fuel cells: Bulk defect chemistry and surface reaction kinetics" in "Nonstoichiometric Compounds VI", ECI Symposium Series, (2016). https://dc.engconfintl.org/nonstoichiometric_vi/18