Nanopore-Confined Sub-Nanometer Co and Ni Species on Porous Carbon Nitride for Efficient Water Electrolysis

Abstract

Electrochemical water splitting is a promising route in clean energy production. Therefore, the development of efficient, stable, and cost-effective electrocatalysts remains highly desirable. Here, we report a facile nanopore-confinement strategy for growing well-dispersed Ni and Co subnanometer metal species within a three-dimensional porous carbon nitride (PCN) network using an SBA-15 template. The porous template facilitates the confinement and uniform dispersion of subnanometer metal species within the PCN framework. Comparative studies using nontemplated control samples suggest that, in the absence of nanopore confinement, cobalt readily forms crystalline oxide phases, whereas templated synthesis maintains highly dispersed metal species. HRTEM and HAADF-STEM reveal uniformly distributed subnanometer Ni and Co species, while XPS and Raman analyses indicate strong metal–nitrogen coordination that stabilizes the confined active sites. Among the tested catalysts, Co-f-PCN exhibited the best electrochemical performance with significantly reduced overpotential for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) to 210 and 70 mV at 10 mA cm–2, respectively. Moreover, the synthesized Co-f-PCN demonstrates a low Tafel slope (51 mV dec–1 for OER and 60 mV dec–1 for HER), a large electrochemically active surface area (ECSA) of 675 cm–2, and reduced charge transfer resistance as evidenced from Nyquist plots, suggesting rapid reaction kinetics and superior electrocatalytic activity. More significantly, Co-f-PCN achieves overall water splitting with a potential of 1.5 V at 10 mA cm–2, reaching a maximum current density of 600 mA cm–2 and an H2 evolution rate of 0.08 cm3 s–1, with excellent stability. Overall, this work highlights nanopore confinement within porous carbon nitride as an effective strategy for stabilizing highly dispersed subnanometer transition-metal species for alkaline water electrolysis.