Activity versus stability of atomically dispersed transition-metal electrocatalysts

Polymer electrolyte fuel cells operating on clean and sustainable hydrogen are an attractive solution for clean transportation. However, polymer electrolyte fuel cells are costly owing to the use of considerable amounts of platinum group metal (PGM) catalysts, which are needed to catalyse the very slow oxygen reduction reaction at the cathode. The most attractive path in that regard is a complete replacement of precious metal catalysts by PGM-free materials with similar or better performance. Since 2010, numerous promising catalysts have been proposed for PGM-free electrocatalysis. However, the best-performing catalysts do not yet meet the requirements of practical systems. One important hurdle in catalyst discovery is relying heavily on empirical rather than rational design-based approaches. This Perspective article focuses on the most promising PGM-free oxygen reduction reaction catalysts based on atomically dispersed, nitrogen-coordinated single-atom metal sites (M-N-C catalysts). We specifically concentrate on the active-site structure and critical factors governing catalytic activity and performance durability. We propose potentially effective strategies for improving performance by controlling the catalyst structure at the atomic scale, mesoscale and nanoscale. We highlight the importance of overcoming often-observed activity-stability trade-offs and the importance of advanced modelling for the rational design of catalysts. Platinum group metal-free electrocatalysts that utilize atomically dispersed, nitrogen-coordinated transition-metal sites in carbon are a promising replacement for platinum-based oxygen reduction reaction catalysts in fuel cells. This Perspective article offers a concise discussion on addressing remaining challenges related to activity-stability trade-offs by precisely controlling catalyst structures at multiple scales.