Hydrogen surface exchange on proton conducting oxides studied by gas phase analysis with mass spectrometry

The surface exchange rates of H2 (g) on unsubstituted, Nd-substituted and Mo-substituted lanthanum tungstates, Y-doped barium cerate, and Yb-doped strontium cerate have been determined by monitoring the distribution of H2/D2/HD isotope couples over the oxide surfaces as a function of time by mass spectrometry. On bases of the differences between the exchange rates of the materials and with different pretreatments we propose that point defects play a more important role in the exchange than surface defects such as grain boundaries. The exchange rates of unsubstituted and Nd-substituted lanthanum tungstate have activation energies of 130 and 90 kJ mol−1, respectively, whereas the exchange rate on the Mo-substituted specimen has virtually no temperature dependency. It is speculated whether this difference between the tungstates is related to the differences in the concentration of electronic charge carriers. Based on analyses of the pH2-dependecies, rate-determining steps of the surface exchange of H2 (g) are proposed for the different compositions of tungstates. The exchange rates of barium cerate and strontium cerate have activation energies of 50 and 130 kJ mol−1, respectively. The critical thicknesses of the different materials are estimated based on surface exchange rates and ambipolar proton–electron conductivities, and are by far greatest for barium cerate. For unsubstituted lanthanum tungstate, the estimated critical thickness suggests that surface kinetic limitations must be overcome to reach a target flux of 1 mL min−1 cm−2 at 800 °C.