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Water Management in a 25cm2 Pem Fuel Cell with Electroosmotic Pumping

Shawn E. Litster, Cullen R. Buie, Tibor Fabian, Jonathon D. Posner, and Juan G. Santiago. Stanford University, Stanford, CA 94305

Efficient water management is commonly a compromise of proper membrane humidification to avoid flooding which induces oxidant starvation. A number of water management studies have identified that significant drying and flooding occur simultaneously in a single cell as a result of these inherent design compromises. Dry-out commonly occurs near inlets for under-saturated gases and flooding is most severe at the oxidant outlet. We recently presented the integration of electroosmotic (EO) pumps into a 1.2 cm2 PEM fuel cell. Our experiments demonstrated that this pumping can prevent flooding, increase power density, and ensure stable performance by removing water from flooded regions and redistributing it to under-saturated regions.

EO pumps have no moving parts and scale proportionally with fuel cell size. We are currently investigating the process of scaling up EO pumps and integrating them with PEM fuel cells of a size closer to those of commercial interest. We will present the integration of an EO pump structure into a fuel cell with a 25cm2 active area. This cell has multiple (parallel) serpentine channels and thin gas separators for maximum volumetric power density. Importantly, the EO pump is designed to add minimal volume to the fuel cell and represents only 8% of the fuel cell volume in our preliminary work. Our pumping structure evacuates liquid water from the gas channels and pumps it out of the fuel cell. The pumping structure also redistributes the water to regions of the fuel that would otherwise be under-saturated. The design results in higher current density and should also aid in membrane hydration uniformity. The system has favorable energetics as the power consumed by the EO pump is negligible compared to that generated by the fuel cell. Preliminary experiments suggest the EO pump's power consumption is less than 1% of the total fuel cell power. Moreover, the increase in fuel cell power is significantly greater than the power consumed by the pump, and so a considerable net power increase is achieved.