Nitrogen-coordinated metal sites (MNx) in metal- and nitrogen-codoped carbon (M-N-C) catalysts offer promising electrocatalytic activity, but selective synthetic design of MNx sites with specific coordination environments remains challenging. Here, we manipulate the formation statistics of MNx sites by using sacrifice alkali metals (AM = Li, Na, and K) to form metal vacancy-Nx carbon (AM-MVNx-C) templates, which are used to direct the solution-phase formation of CoN4 sites in Co-N-C catalysts. We build a probability weight function based on the embedding energy of M in MNx sites as the descriptor for MNx formation statistics, and we predict that the alkali metals are prone to induce the formation of MVN4 sites. By coordinating Co2+ ions with AM-MVNx-C templates, we synthesize Co-N-C with CoN4 sites, demonstrating remarkable oxygen reduction activity in anion exchange membrane fuel cells. These results highlight the statistical thermodynamics of MNx formation and open up the possibility for the rational design of complex M-N-C electrocatalysts with well-defined MNx sites.
Atomic transition metal-nitrogen (M-N x ) active sites dispersed on carbon matrix have emerged as highly promising electrocatalysts for energy conversion systems, owing to their maximized metal center utilization and excellent conductivity of the carbon substrate. However, conventional symmetric M-N 4 configurations often exhibit limited hydrogen evolution reaction (HER) activity in alkaline due to the inferior water dissociation capacity. In this work, isolated Co single atoms anchored on N -doped hollow carbon substrates (Co SA /NHCs) are successfully fabricated by the pyrolysis of Co 2+ -containing polypyrrole (Co-PPY) precursor, and the subsequent removal of embedded Co particles. Thanks to the high density of atomically distributed asymmetric Co-N 4 moieties, which facilitate efficient water activation and optimal adsorption of the active hydrogen (H*), the synthesized Co SA /NHCs electrocatalysts show remarkable HER activity and stability in alkaline electrolyte. Besides, they can effectively electrolyze HER in acidic medium, undergoing the Volmer-Heyrovsky pathway. Remarkably, Co SA /NHC-800, pyrolyzed at 800 °C, requires small overpotentials of 230 and 228 mV to achieve the current density of −10 mA cm −2 in 1.0 M KOH and 0.5 M H 2 SO 4 solutions, respectively. Moreover, it displays small Tafel slopes, high Faradic efficiencies (∼100 %) and superior long-term stability. This study paves a strategic approach for designing high-performance single-atom electrocatalysts through asymmetric structural engineering.
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