Design of a new Ni@NCNT/graphene hybrid structured catalyst for high-performance electrochemical CO₂ reduction: unravelling the roles of N-doping

Doping strategies have been recognized as effective approaches for developing cost-effective and durable catalysts with enhanced reactivity and selectivity in the electrochemical synthesis of value-added compounds directly from CO₂. However, the reaction mechanism and the specific roles of heteroatom doping, such as N doping, in advancing the CO₂ reduction reaction are still controversial due to the lack of precise control of catalyst surface microenvironments. In this study, we investigated the effects of N doping on the performances for electrochemically converting CO₂ to CO over Ni@NCNT/graphene hybrid structured catalysts (Ni@NCNT/Gr). Ni nanoparticles (Ni NPs) were encapsulated in N-doped carbon nanotubes (NCNTs) which were <i>in situ</i> generated from g-C₃N₄ during the annealing process due to the thermal catalysis of the existing Ni NPs. Our results show that the optimized pyrrolic N doping level, coupled with stable NCNT/Gr hybrid structures, high electrochemically active surface area, rich active sites, and reduced Ni NP size, synergistically contribute to the distinguished electrocatalytic performances. The as-prepared Ni@NCNT/Gr-R catalyst demonstrated a high CO faradaic efficiency (>90%) with negligible differences in CO FE across a wide potential range (-0.71--0.91 V <i>vs.</i> RHE) in an H-cell while maintaining magnificent stability with negligible current density loss for 24 hours at -0.71 V (<i>vs.</i> RHE). Our findings provide evidence and insight into the optimization of pyrrolic N doping levels together with reducing NP size within the stable NCNT/Gr hybrid substrate for designing efficient CO₂ reduction catalysts.