Achievements, challenges and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation

Proton exchange membrane fuel cells can use hydrogen and air to power clean electric vehicles. However, technical barriers including high cost, limited lifetime and insufficient power density limit their broad applications. Advanced cathode catalysts for the kinetically-sluggish oxygen reduction reaction (ORR) in acidic media are essential for overcoming these barriers. Here, we highlight breakthroughs, challenges and future directions for both platinum group metal (PGM) and PGM-free ORR cathode catalysts. Among PGM catalysts, highly-ordered PtM intermetallic nanostructures exhibit enhanced activity and stability relative to PtM random alloys. Carbon supports, with optimal balance between graphitization degree and porosity, play an important role in enhancing overall performance. Among PGM-free catalysts, transition metal and nitrogen co-doped carbons (M-N-C) perform best. However, degradation at practical voltages (>0.6 V) still prevents their practical application. For all catalysts, translating intrinsic activity and stability into device performance requires electrodes with robust three-phase interfaces for effective charge and mass transfer. Proton exchange membrane fuel cells can efficiently provide clean power for electric vehicles, although more efficient and economic cathode catalysts are still required. This Review highlights recent breakthroughs, challenges and future research directions for Pt group metal (PGM) and PGM-free oxygen reduction catalysts.