Tunable Synthesis of Hollow Metal–Nitrogen–Carbon Capsules for Efficient Oxygen Reduction Catalysis in Proton Exchange Membrane Fuel Cells

Atomically dispersed metal catalysts anchored on nitrogen-doped (N-doped) carbons demand attention due to their superior catalytic activity relative to that of metal nanoparticle catalysts in energy storage and conversion processes. Herein, we introduce a simple and versatile strategy for the synthesis of hollow N-doped carbon capsules that contain one or more atomically dispersed metals (denoted as H–M–Nx–C and H–Mmix–Nx–C, respectively, where M = Fe, Co, or Ni). This method utilizes the pyrolysis of nanostructured core–shell precursors produced by coating a zeolitic imidazolate framework core with a metal–tannic acid (M–TA) coordination polymer shell (containing up to three different metal cations). Pyrolysis of these core–shell precursors affords hollow N-doped carbon capsules containing monometal sites (e.g., Fe–Nx, CoNx, or Ni–Nx) or multimetal sites (Fe/Co–Nx, Fe/Ni–Nx, Co/Ni–Nx, or Fe/Co/Ni–Nx). This inventory allowed exploration of the relationship between catalyst composition and electrochemical activity for the oxygen reduction reaction (ORR) in acidic solution. H–Fe–Nx–C, H–Co–Nx–C, H–FeCo–Nx–C, H–FeNi–Nx–C, and H–FeCoNi–Nx–C were particularly efficient ORR catalysts in acidic solution. Furthermore, the H–Fe–Nx–C catalyst exhibited outstanding initial performance when applied as a cathode material in a proton exchange membrane fuel cell. The synthetic methodology introduced here thus provides a convenient route for developing next-generation catalysts based on earth-abundant components.