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

Thursday, 30 July 2015: 08:40
Alsh (Scottish Exhibition and Conference Centre)
W. Y. Lee, K. M. Ong, and A. F. Ghoniem (Massachusetts Institute of Technology)
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.