A macroscopic study of the electrochemical modeling parameters of a 6 kW PEM electrolyzer.

摘要

This study examined the thermodynamic electrochemistry of the process of proton exchange membrane electrolysis of a 6 kW electrolyzer manufactured by Proton Energy Systems. Each modeling parameter was examined based on the definition and the theories which characterize their behavior using experimental data generated at the University of North Dakota hydrogen laboratory. The exchange current densities, membrane resistance and charge transfer coefficients were examined at both electrodes for their temperature dependence. While most of the temperature studies revealed an Arrhenius behavior, the exchange current density at the anode showed a deviation. This leads me to believe that stacking cells together in a commercial electrolyzer may have altered the temperature dependence of the anode exchange current density. I also suspected electo-osmotic drag as a contribution factor.;The stack parameters were compared where possible to cell parameters determined under similar experimental conditions to verify the effect of stacking. Although it was difficult to obtain cell parameters determined under similar conditions as the stack experiment, there is no reason to believe that stacking has had a significant effect on the parameters beyond the non-Arrhenius behavior at the anode.;A theoretical procedure was developed for the estimation of the symmetry factor based on the cathode reaction mechanism. It was determined that the charge transfer at the cathode is between charges that are not chemically bonded. As a result Marcus equation of charge transfer can be applied. Hence the symmetry factor is the first derivative of the Marcus equation or the Bronsted coefficient. The exact value of the symmetry factor could not be determined because the Gibbs free energy of the charge transfer could not be determined. However, by apply the boundary conditions for which the Marcus equation is valid, a range of values for the symmetry factor was determined for the cathode. This range happens to includes 0.5 which has become acceptable for electrolyzer modeling. It was not possible to prove that the anode reaction could also be modeled using Marcus equation because of the complexity of the reaction mechanism. However, deductions were made assuming the validity of Marcus equation for the anode reaction.