Breaking the Activity and Stability Trade-Off of Platinum-Free Catalysts for the Oxygen Reduction Reaction in Hydrogen Fuel Cells.

Hydrogen fuel cells, which use hydrogen as fuel to generate electricity, hold great promises as future energy conversion devices for heavy-duty transport, due to their zero CO₂ emissions, high energy conversion efficiency, and high power density. However, the adoption of hydrogen fuel cells has been slow due to their reliance on large amounts of costly and scarce platinum (Pt) for the oxygen reduction reaction. The replacement of Pt with Earth-abundant transition metals such as Fe, Co, Mn, and Sn with oxygen reduction reaction affinity has thus been a holy grail of electrocatalysis research. Pt-free catalysts must combine both high power density and high stability in hydrogen fuel cells to be considered viable alternatives to Pt. Despite promising progress on both fronts, a trade-off has emerged: Pt-free catalysts either achieve high power densities (≥1.5 W cm^-2) but suffer from low stabilities (≥70% loss after 25 h) or more recently demonstrate improved stability (≤25% loss after 150 h), while delivering considerably lower power densities (<1 W cm^-2) in hydrogen fuel cells. Herein, we summarize the recent progress in the synthesis of high power density M-N-C catalysts for hydrogen fuel cells and highlight the critical importance of uncovering the underlying mechanisms using operando methods. We then discuss the primary causes of catalyst degradation in hydrogen fuel cells and the most promising strategies to enhance the stability of the M-N-C catalysts. Finally, a roadmap is proposed to overcome the activity stability trade-off for Pt-free catalysts in hydrogen fuel cells.