Isolated asymmetric CoN₄ sites on nitrogen-doped hollow carbons as electrocatalysts for hydrogen evolution reactions...

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-N₄ 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 Co²⁺-containing polypyrrole (Co-PPY) precursor, and the subsequent removal of embedded Co particles. Thanks to the high density of atomically distributed asymmetric Co-N₄ moieties, which facilitate efficient water activation andoptimal 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 H₂SO₄ 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.