Dynamic characteristics of water transport in the membrane of PEM fuel cell

The dynamic characteristics of the water transport across the membrane of the PEM fuel cell is an important phenomenon to be understood before the design of fuel cells can be soundly implemented, especially when the application of the fuel cell in mobile systems is concerned. To attack this issue, we develop an unsteady mathematical model and investigate the transient behavior of water transport across the membrane as well as the influences due to the presence of foreign impurity ions, which turn out to be a crucial factor affecting the unsteady state features of water transport across the membrane. The results suggest that a higher initial current density to start the operation, a higher water flux from the anode side, a smaller operational current density, and a lower level of contamination in the membrane (especially at the cathode side) can all result in a shorter time for the water transport to reach the steady state, and thus a better dynamic performance of the fuel cell can be obtained. More precisely to say, both a larger starting operational current density i₀ and a smaller operational current density i lead to a smaller dynamic response time t_ss, the time for the water distribution across the membrane to reach the steady state. The most powerful as well as the most feasible control scheme is to control the humidification parameter k, i.e. to adjust the water content of the feeding fuel, so that the t_ss would remain steadily in a reasonably low value in a wide range of water flux fraction β, another control-parameter of the membrane.