Ultralow platinum loading proton exchange membrane fuel cells: Performance losses and solutions

In an effort to reduce the platinum group metal (PGM) content in proton exchange membrane fuel cells (PEMFCs) to a value comparable with conventional vehicles, a concerted global effort over the past decade has been made by the fuel cell community to develop high activity oxygen reduction reaction (ORR) catalysts. However, as PGM loadings in membrane electrode assemblies (MEAs) decrease below 0.125 mg/cm2, it has become clear that ORR mass activity is just one of the challenges which must be overcome. At the anode, it appears that a kinetic limitation may have been reached and new research may now be required to develop hydrogen oxidation reaction (HOR) catalysts with higher turnover frequencies vs. conventional Pt. At the cathode, transport limitations have been uncovered which are related to ionomer/catalyst interactions. Recently, the critical role of the carbon support (used to disperse the PGM catalyst) on these ionomer/catalyst interactions has been revealed. Thus, rational design of carbon structures either through modification of commercial carbons or design of completely new structures, is now of utmost importance. The challenges and discoveries related to these concepts are critically analyzed in this review, along with suggestions for future research directions to help overcome the remaining hurdles.