The correlation between surface kinetic and bulk diffusion properties of proton conducting oxides

The correlation between surface kinetic and diffusion properties of proton conducting oxides is addressed in this work. Thermogravimetric relaxation measurements have been undertaken for three different oxides, and the same correlation between surface exchange coefficients (k) and bulk diffusion coefficients (D) is found to hold for both hydrogen tracer transport and chemical transport of water across several orders of magnitude. On account of the prevailing kinetic regimes we argue that the observed k-D correlations must be ascribed the materials transport properties rather than an inherent parameter interrelation. It is conceivable that the same elementary step in the reaction scheme for dissolution of water is rate limiting for all the data sets approximating the same linear power law. The only data sets significantly departing from this trend are recorded under conditions where the surface exchange appears to be dominated by a reaction with molecular hydrogen. When evaluating surface exchange coefficients it may also be necessary to take the resistivity stemming from separation of charge carriers in subsurface space charge regions into account.