Catalysis, electrocatalysis and electrochemical promotion of the steam reforming of methane over Ni film and Ni-YSZ cermet anodes

Abstract

The kinetics of the steam reforming reaction of CH₄ were investigated at temperatures 750 to 950°C under both open-circuit and closed-circuit conditions on Ni-YSZ (Yttria Stabilized Zirconia) solid oxide fuel cell (SOFC) anodes and polycrystalline Ni film SOFC anodes of measured Ni surface area. It was found that the rate of methane reforming on the Ni surface exhibits a Langmuir-Hinshelwood type dependence on$P_{CH_4 }$ and$P_{H_2 O}$ which results from competitive adsorption of carbonaceous species and oxygen or OH. Consequently the rate is maximized for intermediate$P_{CH_4 }$ to$P_{H_2 O}$ ratios. The reaction kinetics are affected significantly by cell current and potential under closed-circuit conditions. Over a rather wide range of operating conditions the observed rate changes are Faradaic, which implies negligible variation in the catalytic properties of the Ni surface with potential. At lower temperatures, however, and particularly under conditions of carbon deposition, the rates of CO, H₂, CO₂ and, more importantly, carbon formation exhibit pronounced non-Faradaic (NEMCA), or electrochemical promotion, behaviour. Some non-Faradaic behaviour is also observed for higher H₂O to CH₄ ratios but in this case the effect of applied potential is reproducible but not readily reversible.