Detailed H2 and CO Electrochemistry for a MEA Model Fueled by Syngas

SOFCs are capable of directly oxidizing CO in addition to H₂, which allows them to be coupled to a gasifier that converts coal, biomass or other carbonaceous fuels to synthesis gas.   Most membrane-electrode-assembly (MEA) models neglect CO electrochemistry because H₂ oxidizes more readily and most CO presumably shifts to H₂ by the water-gas-shift reaction in the presence of H₂O.  In this paper we examine this assumption, especially at conditions of high current density and high CO-content syngas.  The comprehensive 1D-MEA model presented here incorporates detailed mechanisms for both H₂ and CO oxidation.  The rate parameters and rate-limiting steps for the H₂ mechanism are selected by fitting the model to experimental porous anode data for H₂/H₂O mixtures.  The kinetic parameters of the CO mechanism were determined by matching the data to EIS measurements. The anode’s physical parameters, triple-phase-boundary length and nickel pattern width, are then determined by fitting the CO mechanism to porous anode data for CO/CO₂ mixtures.  The resulting H₂ and CO mechanisms are then combined, along with surface reforming kinetics, into a single model that iterates through anode activation overpotential to resolve the individual current contributions of each fuel.  We find that the model under-predicts experimental data for H₂/CO mixtures when CO electrochemistry is neglected, but fits the data well at high current densities when CO oxidation is included.  Furthermore, the combined model fits H₂/CO data best when a single charge-transfer step in the H₂ mechanism is assumed to be rate-limiting over the full range of current densities.  This single rate-limiting step assumption for H₂/CO mixtures differs from a previous finding that the H₂ adsorption step becomes rate-limiting at high current densities for H₂/H₂O mixtures.  This implies that adding CO to the fuel stream can fundamentally alter the H₂ oxidation process.  The individual H₂ and CO current contributions for CO-rich syngas confirm that the addition of CO oxidation can delay H₂ current saturation at high anode activation overpotentials, hence improving cell performance.  These results indicate that CO oxidation cannot be neglected in MEA models running on CO-rich synthesis gas, and further studies are needed on the combined CO and H₂ mechanism on Ni-YSZ.