Molecularly dispersed nickel complexes on N-doped graphene for electrochemical CO2 reduction

Abstract

In this work, new hybrid catalysts based on molecularly dispersed nickel complexes on N-doped graphene were developed for electrochemical CO₂ reduction (ECR). Nickel(II) complexes (1-Ni, 2-Ni), and a new crystal structure ([2-Ni]^Me), featuring N₄-Schiff base macrocycles, were synthesized and investigated for their potential in ECR. Cyclic voltammetry (CV) in NBu₄PF₆/CH₃CN solution demonstrated that the nickel complexes bearing N–H groups (1-Ni and 2-Ni) showed a substantial current enhancement in the presence of CO₂, while the absence of N–H groups ([2-Ni]^Me) resulted in an almost unchanged voltammogram. This indicated the necessity of the N–H functionality towards ECR in aprotic media. All three nickel complexes were successfully immobilized on nitrogen-doped graphene (NG) via non-covalent interactions. All three Ni@NG catalysts exhibited satisfactory CO₂-to-CO reduction in aqueous NaHCO₃ solution with the faradaic efficiency (FE) of 60–80% at the overpotential of 0.56 V vs. RHE. The ECR activity of [2-Ni]^Me@NG also suggested that the N–H moiety from the ligand is less important in the heterogeneous aqueous system owing to viable hydrogen-bond formation and proton donors from water and bicarbonate ions. This finding could pave the way for understanding the effects of modifying the ligand framework at the N–H position toward fine tuning the reactivity of hybrid catalysts through molecular-level modulation.