The emerging coupled low-PGM and PGM-free catalysts for oxygen reduction reaction

Proton exchange membrane (PEM) fuel cells are high-efficiency, clean energy devices that require highly active and stable catalysts for, particularly, the oxygen reduction reaction (ORR) at the cathode. Platinum-group metals (PGMs) are currently the most used ORR catalysts; the high cost and scarce resources of PGMs, however, preclude their practical application. Significant efforts have thus been made in developing both low-PGM (LP) and PGM-free (PF) ORR catalysts, yet the coupling of LP and PF to form composite LP/PF ORR catalysts has been rarely explored. When LP and PF are coupled, beneficial structural and electronic modifications in catalysts could actually be induced from some complementary aspects, allowing a simultaneous enhancement in both the activity and stability of LP/PF ORR catalysts. Based on the milestones and challenges of LP and PF catalysts and the recent research progress on LP/PF catalysts, the coupling (synergistic) effects of LP/PF catalysts are summarized herein, in terms of enhanced intrinsic activity and stability. The perspectives for the future R&D of ORR catalysts through LP and PF coupling are also provided.