Water Management Issues for Direct Borohydride/Peroxide Fuel Cells

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 $10Wh$ to $57Wh$⁠, 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.