Asymmetric electrode ionomer for low relative humidity operation of anion exchange membrane fuel cells
The operation of fuel cells under low relative humidity (RH) conditions gives substantial cost and performance benefits. Nonetheless, it is not currently feasible to operate anion exchange membrane fuel cells (AEMFCs) at low RH conditions because current materials for membrane electrode assembly cannot provide sufficient water for the oxygen reduction reaction. Here we synthesized polyfluorene ionomers with different ammonium concentrations for anode and cathode to control water management. We designed several asymmetric electrodes that enable high performance under low RH conditions via not only conventional backward water diffusion (anode to cathode) but also forward diffusion (cathode to anode). The AEMFCs using optimized asymmetric electrodes exhibited high H2/CO2-free air performance (rated power density of circa 540 mW cm−2 at 90 °C under 75% (anode) and 50% RH (cathode) conditions), which is comparable to those of state-of-the-art AEMFCs under nearly water-saturated conditions. The durability of the AEMFCs is excellent, generating 0.6 A cm−2 for >900 h at 80 °C under 50% RH (cathode) conditions. This study demonstrates that high-performance and durable AEMFCs under low RH and high current generating conditions are possible.