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N x 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N x moieties and Pt NPs to the catalytic activity toward the hydrogen evolution reaction. Combined electrochemical measurements and theoretical calculations identify that the atomic PtN x moieties by themselves possess negligible activity, but they can significantly boost the activity of the Pt NPs via the synergistic effect.