Impact of electrode kinetics on the dynamic response of a DMFC to change of methanol feed concentration

A dynamic one-dimensional rigorous process model of a single-cell direct methanol fuel cell (DMFC) is presented. Multi-component mass transport in the diffusion layers and the polymer electrolyte membrane (PEM) is described using the generalised Maxwell–Stefan equation for porous structures. In the PEM, local swelling behaviour and non-idealities are accounted for by a Flory–Huggins activity model. This model is used as basis of a model family with different anode and cathode reaction mechanisms (single-step and multi-step with and without adsorption to catalyst surface sites).The model variants were used to simulate the dynamic (transient) response of the DMFC to stepwise changes in the methanol feed concentration from typical operating levels down to zero, while maintaining the cell current. For validation, similar experiments were carried out. In the experiments, the cell voltage broke down only after an unexpectedly long period of time, and for a variety of operating conditions even a cell voltage overshoot could be observed. Such overshoot behaviour is also predicted by those model variants, which feature anode reaction mechanisms with reaction intermediates (e.g. CO) adsorbed to the anode catalyst, while models without such detailed anode reaction mechanisms fail in this respect. The model-based analysis reveals that the observed overshoots result from the different time constants of the responses of the anode and cathode overpotentials to the feed change.