Water Management Issues for Direct Borohydride/Peroxide Fuel Cells

Abstract

This study evaluated water management strategies to lengthen the run time of a batch fueled direct sodium borohydride/peroxide (NaBH4/H2O2) proton exchange membrane fuel cell. The term “batch fueled” refers specifically to a fuel tank containing a fixed volume of fuels for use in the run. The length of a run using a fixed fuel tank is strongly influenced by water dynamics. The water that reacts at the anode is produced at the cathode, and is transported through the membrane via drag and diffusion. Resulting concentration changes in the fuel of the NaBH4/H2O2 fuel cell were modeled to evaluate the run lifetime. The run time is defined as the amount of time required for NaBH4 or for NaBO2 (the byproduct compound) to reach either solubility limit or until the fuel is depleted, whichever occurs first. As part of the evaluation, an “effective” H2O drag coefficient (net drag minus back diffusion) with Nafion® 112 was experimentally determined to be 1.14 and 4.36 at 25°C and 60°C, respectively. The concentrations of the NaBH4 and NaBO2 solutions were calculated as a function of initial concentration, and for the case where H2O was supplied to the anode compartment during operation. Several strategies to increase the run time by both passive and active water management were considered. It is found that the run time is increased from 10 W h to 57 W h, with a decrease in the initial NaBH4 concentration from 30 wt % (typically employed in these cells) to 10 wt %. Adding 0.125 ml/min H2O to the bulk anode solution increases the run time of a 10 wt % NaBH4 solution by a factor of 1.6. Adding 0.225 ml/min H2O to 30 wt % NaBH4 bulk solution increases the run time by a factor of 4.4. While attractive for increasing run time, the practicality of water addition depends on its availability or requires incorporation of an added unit, designed to separate and recirculate water from the cathode solution.