A General Strategy to Remove Metal Aggregates toward Metal–Nitrogen–Carbon Catalysts with Exclusive Atomic Dispersion

Metal- and nitrogen-doped nanocarbons (M–N–Cs) are promising alternatives to precious metals for catalyzing electrochemical energy conversion processes. However, M–N–Cs synthesized by high-temperature pyrolysis frequently suffer from compositional heterogeneity with the simultaneous presence of atomically dispersed M–Nx sites and crystalline metal nanoparticles (NPs), which hinders the identification of active sites and rational optimization in performance. Herein, a universal and efficient strategy is reported to obtain both precious- and nonprecious-metal-based M–N–Cs (M = Pt, Fe, Co, Ni, Mn, Cu, Zn) with exclusive atomic dispersion by making use of ammonium iodide as the etchant to remove excessive metal aggregates at high temperature. Taking Pt–N–C as a proof-of-concept demonstration, the complete removal of Pt NPs in Pt–N–C enables clarification on the contributions of the atomic Pt–Nx moieties and Pt NPs to the catalytic activity toward the hydrogen evolution reaction. Combined electrochemical measurements and theoretical calculations identify that the atomic Pt–Nx moieties by themselves possess negligible activity, but they can significantly boost the activity of the Pt NPs via the synergistic effect.