Modeling and simulation of an alkaline fuel cell

Abstract

The alkaline fuel cell (AFC) is a promising energy system in the sense of the energy transition, as it only emits water and has the potential to be very efficient and cost-effective. Its best-known application is probably the use of an AFC to power the spacecraft in NASA's Apollo mission. However, for more conventional applications the AFC needs to be improved in terms of power density, lifetime and manufacturing costs. In this dissertation a two-dimensional (2-D) mathematical model of an AFC has been developed with the software COMSOL Multiphysics® for stationary operation. The model considers the transport of species, mass, momentum, charge and energy. The kinetics of the electrode reactions is modelled on the basis of the Butler-Volmer expression and taking into account the temperature dependences of the exchange current densities and of the transfer coefficients. The transport of ions in the electrolyte is described based on the concentrated electrolyte theory, by adapting the transport properties for use in the Nernst-Planck equation. The model is validated by comparing the predicted polarization curve with measurements and verified by examining the gradients and discussing the physical processes in the cell. The significance of the cathode performance for the behavior of the entire cell is worked out. In order to find optimization potentials and to investigate the sensitivity of the model, specific parameter studies are carried out. It turns out that, under the selected base case conditions (60 °C, 1 atm, H2 on the anode side, and air on the cathode side), the optimal electrolyte concentration with respect to cell performance is in the range of 3 M KOH. In addition, the increases in pressure and temperature are verified as effective strategies to improve the cell performance. The influence of the temperature dependences of the kinetic parameters are investigated individually. It turns out that particularly the temperature dependence of the cathode exchange current density should not be neglected in mathematical models. The investigation of the sensitivity of the model with respect to parameters specific to the catalysts shows that research in the field of catalysts for the oxygen reduction reaction (ORR) in alkaline media is very promising and could strongly improve cell performance. Consequently, the AFC model developed in this dissertation allows the investigation of the physical processes in the cell and the identification of optimization potentials. This enables the development of AFCs for different applications, thus ensuring the economic success of the AFC and contributing to the success of the energy transition.