UNDERSTANDING CO OXIDATION IN SOFC'S USING NICKEL PATTERNED ANODE

Abstract

The ability of solid oxide fuel cells (SOFC's) to operate with carbonaceous fuels depends on their ability to oxidize CO through direct electrochemical oxidation and/or water-gas-shift reactions with subsequent H2 electrochemical oxidation. Many recent SOFC studies assume that all gaseous CO undergo water-gas-shift and all charge transfer involves H2. This assumption seems questionable when comparing rates of charge transfer for CO and H2 in Ni/YSZ anodes. However, electrochemical reaction rates are clouded by transport-controlled, gas-phase concentration gradients and non-electrochemical surface reactions in porous anodes. To eliminate those complicating factors, this study has implemented micro-fabricated, patterned Ni anodes on single crystal YSZ electrolytes to evaluate CO electrochemical oxidation under dry and wet conditions. Electrochemical impedance spectroscopy (EIS) and voltammetry were used to quantify the patterned anode performance. Adding ~ 3% H2O to CO/CO2 anode flows increased electrochemical oxidation rates by almost 2X for temperatures between 700 and 800 °C. This can suggest a role in water gas shift for creating H2 to increased charge transfer reaction rates. Fitting experimental results from EIS studies with equivalent circuit models elucidates the role of charge transfer activation in high frequency impedance and the impact of surface diffusion and adsorption on low-frequency impedance. A surface mechanism was developed to predict electrochemical oxidation of CO and H2 on Ni surface. This mechanism was implemented into a numerical model which integrated the transient governing equations and provided a linearization for rapid impedance spectra calculations. Model results, utilizing surface chemistry adapted and modified from the literature, tracked the dependency of current per unit length of three-phase boundary for dry CO and also predicted qualitatively the influence of H2O on electrochemical oxidation rates with CO feeds. Predicted characteristic frequencies for the impedance spectra were too high by an order of magnitude or more. Further assessment with this and other patterned anode studies will be critical for refining the mechanism for CO oxidation on Ni/YSZ anodes and such a refined mechanisms will provide a basis for improved design and operation of porous Ni/YSZ anodes with CO rich streams. This study has provided a basis for continuing development of that mechanism.