Dependence of hydrogen oxidation reaction on water vapor in anode-supported solid oxide fuel cells

In order to understand the hydrogen oxidation reaction (HOR) mechanism in real Ni/YSZ porous anodes, the electrochemical properties as a function of hydrogen partial pressure p(H-2) and water vapor pressure p(H2O) (dry, 1% - 60% humidity) at 973 K of a commercial anode-supported solid oxide fuel cell (SOFC) were measured. The impedance spectra from electrochemical impedance spectroscopy (EIS) measurement were analyzed by 1-D approximation transmission line model (TLM) to observe changes in the reaction resistance (r(P)) between the Ni/YSZ and reaction capacitance (C-p) from the anode. From the analysis, r(p) is essentially independent over p(H-2) in dry atmosphere but decreased with increasing p(H2O) at the anode side. In contrast, the r(p) shows a large dependency on p(H-2) in wet atmosphere indicating that the rate-limiting step of HOR is probably different between dry and wet atmospheres. On the other hand, the C-p is independent of p(H-2) in dry and wet condition. However, similar to r(p), it has strong dependency on p(H2O). From these results, the difference in HOR between dry and wet atmospheres is assumed to be an additional reaction pathway formed in the latter where the formation of OHad- on the Ni metal surface possibly enhanced the HOR rate.