Single-Iron Site Catalysts with Self-Assembled Dual-size Architecture and Hierarchical Porosity for Proton-Exchange Membrane Fuel Cells

[Display omitted] •We report a self-assembled strategy to synthesize 3D superstructure single Fe site electrocatalysts.•Unlike homogeneous particle morphologies, we developed a catalyst with small particles uniformly dispersed onto large particles.•The unique dual sized particle morphology can mitigate the particle aggregates during the synthesis and reaction.•The catalyst with optimal size combination represents one of the best ORR activity in both acidic electrolyte and MEAs. Atomically dispersed and nitrogen coordinated single iron site (i.e., FeN4) catalysts (Fe-N-C) are the most promising platinum group metal (PGM)-free cathode for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, current Fe-N-C catalysts are limited by the inferior exposure of active FeN4 sites due to the inevitable agglomeration of particles in cathodes. Herein, we report a self-assembled strategy to synthesize the atomically dispersed FeN4 site catalysts with a hierarchically porous matrix derived from dual-size Fe-doped ZIF-8 crystal precursors by using large particles to support small particles. The tailored structure is effective in mitigating the particle migration, agglomeration, and spatial overlap, thereby exposing increased accessible active sites and facilitating mass transport. The best performing catalyst composed of 100 nm “nucleated seed” assembled by 30 nm “satellite” demonstrates exceptional ORR activity in acidic electrolyte and membrane electrode assembly. This work provides new concepts for designing hierarchically porous catalysts with single metal atom dispersion via self-assembly of ZIF-8 crystal precursors with tunable particle sizes and nanostructures.