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.