A Unitized Regenerative PEM Fuel Cell Energy Storage System

Unmanned High Altitude Long Endurance (HALE-UAV) aircraft, offer a unique combination of capabilities for use as observation platforms and relay links. Staying aloft indefinitely requires that the aircraft be energetically self-sufficient, and the most promising way of achieving this is the use of solar electric power. Keeping a solar powered aircraft aloft through the night, requires storing surplus electric power during the day for use at night. A lightweight proton exchange membrane (PEM) high pressure unitized regenerative fuel cell (URFC) operating as an electrolyzer during the day to convert hydrogen into electricity and as a fuel cell to generate electricity at night offers the means for achieving this. Current configurations for URFCs are either complex or inadequate to achieve the desired energy density. The URFC proposed here uses a unique oxygen electrode design. This electrode uses a mixed (Pt and IrO2-RuO2) electrocatalyst with components optimized for both oxygen evolution and oxygen reduction reactions. These electrodes are used in conjunction with a novel gas diffusion structure based on a conductive oxide matrix with both hydrophilic and hydrophobic regions arranged to optimize both gas and water transport in both electrolyzer and fuel cell modes to produce superior performance.