PEM fuel cell operation at -20°C. I. Electrode and membrane water (charge) storage

An experimental procedure using isothermal galvanostatic operation was developed to quantify the charge (water) accumulation in proton exchange membrane (PEM) fuel cells at subfreezing conditions prior to voltage failure (i.e., zero cell voltage). The charge passed until voltage failure was compared to charge (water) storage estimates in the membrane phase and the cathode electrode void volume. Cryo-scanning electron microscope images of electrodes following voltage failure were used to assess ice filling of the cathode electrode void volume. At very low current densities, the membrane absorbs a maximum of ≈14 to ≈15 water molecules/per sulfonate group (λmax ≈14-15) and cathode electrode voids are completely ice filled. It is shown that the maximum charge storage of a membrane electrode assembly increases with electrode void volume and the difference between λmax and λinitial. With increasing current densities, decreasing fractions of the maximum charge storage can be utilized, which is shown to be related both to water transport resistances in the membrane phase and to reduced ice filling of the electrode void volume. Experimental results show that the charge storage utilization is mainly controlled by the current density and is less dependant on initial water content or electrode thickness.