Gas Diffusion Media for Open-Cathode Fuel Cells in Atmospheric Flight

Abstract

Operating a proton exchange membrane fuel cell (PEMFC) in a planar, open cathode configuration can enable significant reductions in overall device weight and system complexity compared to a conventional fuel cell stack. This decrease in device weight makes an array of open-cathode, planar fuel cells a compelling candidate power source for unmanned aerial vehicles (UAVs) that can utilize the flow of ambient air over the vehicle-mounted cells to improve oxygen transport, waste heat rejection, and ultimately, water transport in the open-cathode. The effectiveness of each of these processes is largely influenced by the thickness and porosity of the cathode gas diffusion layer (GDL). We relate these cathode GDL properties to open-cathode fuel cell polarization behavior in a broad range of atmospheric conditions in active air flow and in passive, air-breathing mode to study the effect of gas diffusion layer thickness and porosity on cell performance through polarization and AC impedance measurements. Reducing cathode GDL porosity leads to a decrease in cell temperature and ohmic resistance, which improves water retention. Reducing cathode GDL thickness facilitates O2 and H2O transport, increasing oxygen availability to the electrocatalyst at the expense of improved water retention. After observing open-cathode fuel cell performance in a broad range of environmental conditions including ambient temperature, relative humidity, altitude, and in active and stagnant air flows, we find that application of a thicker cathode GDL (315 μm) with lower porosity (75%) yields the highest open-cathode fuel cell voltages and power generation; the open-cathode fuel cell achieves a maximum power-to-weight ratio of 1.44 kW kg-1.