Atomically Dispersed MnN4 Catalysts via Environmentally Benign Aqueous Synthesis for Oxygen Reduction: Mechanistic Understanding of Activity and Stability Improvements

Development of platinum group metal (PGM)-free and iron-free catalysts for the kinetically sluggish oxygen reduction reaction (ORR) is crucial for proton-exchange membrane fuel cells. A major challenge is their insufficient performance and durability in the membrane electrode assembly (MEA) under practical hydrogen-air conditions. Herein, we report an effective strategy to synthesize atomically dispersed Mn-N-C catalysts from an environmentally benign aqueous solution, instead of traditional organic solvents. This innovative synthesis method yields an extremely high surface area for accommodating an increased density of MnN4 active sites, which was verified by using advanced electron microscopy and X-ray absorption spectroscopy. The Mn-N-C catalyst exhibits promising ORR activity along with significantly enhanced stability, achieving a peak power density of 0.39 W cm(-2) under 1.0 bar H-2-air condition in a MEA, outperforming most PGM-free ORR catalysts. The improved performance is likely due to the unique catalyst features, including the curved surface morphology and dominant graphitic carbon structure, thus benefiting mass transport and improving stability. The first-principles calculations further elucidate the enhanced stability, suggesting that MnN4 sites have a higher resistance to demetallation than the traditional FeN4 sites during the ORR.