Kinetics of oxidation of H2 and reduction of H2O in Ni-YSZ based solid oxide cells

Reduction of H2O and oxidation of H-2 was studied in a Ni-YSZ electrode supported Solid Oxide Cells produced at DTU Energy conversion (former Riso DTU). Polarisation (i-V) and electrochemical impedance spectroscopic characterisation show that the kinetics for reduction of H2O is slower compared to oxidation of H-2. The kinetic differences cannot be explained by the reaction mechanisms which are similar in the two cases but are rather an effect of the thermodynamics. The preliminary analysis performed in this study show that the slow kinetic for reduction is partly related to the endothermic nature of the reaction, cooling the active electrode, thereby leading to slower kinetics at low current densities. Likewise, the increased kinetic for oxidation was found to be related to the exothermic nature of the reaction, heating the active electrode, and thereby leading to faster kinetics. At higher current densities an increase caused by diffusion becomes more pronounced.