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

Abstract

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 MnN 4 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 MnN 4 sites have a higher resistance to demetallation than the traditional FeN 4 sites during the ORR.

Dynamically Unveiling Metal–Nitrogen Coordination during Thermal Activation to Design High‐Efficient Atomically Dispersed CoN4 Active Sites

“…Despite the most encouraging ORR activity achieved with Fe‐N‐C catalysts, [11–17] their unsatisfied stability is the primary barrier for vital applications [18] . Even worse, the Fenton reactions between Fe 2+ and H 2 O 2 are significant concerns due to the generation of radical oxygen species (ROS) [19–21] . Both ROS and H 2 O 2 could attack the FeN 4 active sites, carbon support, organic ionomers, and polymer membranes, thereby accelerating performance degradation [22–24] .…”

Engineering Atomically Dispersed FeN4 Active Sites for CO2 Electroreduction

“…Along with the dominant nitrogen ligands and porosities, the Zn evaporation from ZIF‐8s during the thermal activation (e.g., >700 °C) leaves behind a defect and nitrogen‐rich hierarchical carbon structure suitable for hosting highly unstable isolated single metal sites [21, 22] . ZIF‐8 precursors have been used either directly as a template for doping active M [7, 16, 17, 23] or carbonized to produce an N‐doped C host for subsequent active metal adsorption, [18] or a combination of both [24–26] …”

“…Like traditional precious metal particle catalysts, [27] we developed various synthetic chemistry to tune the particle sizes of M‐N‐C catalysts in a wide range (e.g., 20 to 1000 nm) due to the feasibility of controlling the size of ZIF‐8 nanocrystals [16, 25, 28] . In principle, smaller sizes often lead to an increase in low‐coordination atoms situated on edges, corners, and vertices, whereas for larger particles, the planar face dominates [29] .…”

“…[21,22] ZIF-8 precursors have been used either directly as atemplate for doping active M [7,16,17,23] or carbonized to produce an N-doped Ch ost for subsequent active metal adsorption, [18] or ac ombination of both. [24][25][26] Like traditional precious metal particle catalysts, [27] we developed various synthetic chemistry to tune the particle sizes of M-N-C catalysts in awide range (e.g., 20 to 1000 nm) due to the feasibility of controlling the size of ZIF-8 nanocrystals. [16,25,28] In principle,s maller sizes often lead to an increase in low-coordination atoms situated on edges,corners, and vertices,w hereas for larger particles,t he planar face dominates.…”

“…[24][25][26] Like traditional precious metal particle catalysts, [27] we developed various synthetic chemistry to tune the particle sizes of M-N-C catalysts in awide range (e.g., 20 to 1000 nm) due to the feasibility of controlling the size of ZIF-8 nanocrystals. [16,25,28] In principle,s maller sizes often lead to an increase in low-coordination atoms situated on edges,corners, and vertices,w hereas for larger particles,t he planar face dominates. [29] Thea ctive MN 4 sites can be located either on the basal plane or the low-coordination sites on edge,bridging two graphene planes together or on the vertices between these graphene planes.Edge-hosted MN 4 moieties have been postulated to be more active due to the synergistic role of the dangling Ca nd the out-of-plane Ms ites.…”