Kinetics in an internal reforming fuel cell, high temperature PEM fuel cell performance, and a fundamentals-based impedance model.

摘要

A study of the kinetics of the methanol steam reforming reaction within an idealized tube reactor and with a non-ideal Internally Reforming Fuel Cell (IRFC) was performed. Kinetic expressions were calculated from the reaction rate data obtained from the tube reactor by least squares fitting to general power law model as well as a mechanism-based model put forth by Peppley et. al. Reaction rate data obtained from an IRFC with and without an H₃PO ₄ containing membrane electrode assembly (MEA) was compared to the reaction rates predicted by the kinetic model. It was found that methanol conversion rates in the IRFC were significantly less than would be for an ideal PFR with an equal amount of catalyst, due to the non-ideal flow through the reactor bed. However, despite the non-ideal flow caused by the design compromises inherent in an IRFC and the resulting drop in effective catalyst activity, it was projected that for fuel cell systems with a current density greater than 400 mA cm−2, the IRFC would require less catalyst mass than a traditional system with external reformer.; A parallel study evaluating the performance of fuel cell membrane electrode assemblies (MEA's) made from commercially available ELAT electrodes and PBI/H ₃PO₄ electrolyte operating at ambient pressure was conducted. These fuel cells showed an increase in performance with increasing temperature, however this effect diminished at ∼200°C, above which, performance was temperature insensitive. Operation of these cells with anodic feeds containing 1% CO showed a small performance loss as compared with pure H₂. A much more significant performance loss occurred when the cathode feed was switched from pure O₂ to air. Transport related losses were confirmed by impedance spectroscopy. Impedance studies of these fuel cells suggested that at low current densities, significant resistance, apart from kinetic resistance and membrane resistance, was observed and attributed to ionic resistance within the electrodes.; To verify this hypothesis, a phenomenological based impedance model was derived. Results from this model demonstrated the effectiveness of the model for the simulation of impedance data, and for the estimation of physical and electrochemical parameters of the fuel cell. Impedance simulations of the fuel cell model demonstrated that the Nernst terms were the source of a low frequency inductive loop commonly observed in experimental data from high temperature PEM fuel cells. Estimated parameters obtained from the fitting of the model to fuel cell impedance data showed that the ionic resistance within the membrane and cathode decreases as the current of the cell increases, suggesting that the ionic conductivity within the electrodes is a crucial factor for the performance of these types of cells.