Controllable dispersion of nickel phthalocyanine molecules on graphene oxide for efficient electrocatalytic CO2 reduction†

Abstract

Single-atom electrocatalysts with Ni–N_x–C sites usually possess excellent activity for the CO₂ reduction reaction (CO₂RR). However, it still remains a challenge to synthesize them using unmodified nickel phthalocyanine (NiPc) with an intrinsic Ni–N₄–C moiety at room temperature. Here, NiPc molecules are controllably dispersed on graphene oxide (GO) in the form of single molecules, dimers, or aggregates through a simple hydrolysis of protonated NiPc in a GO-containing aqueous phase. Systematic characterization shows the existence of π–π interaction, hydrogen bond and axial coordination between NiPc and GO in NiPc–GO composites. Electrochemical tests demonstrate that these NiPc–GO composites have high activity for electrocatalytic CO₂RR to CO. After optimizing the GO content in NiPc–GO, a CO Faraday efficiency of >90% is achieved over a work potential range of −0.8 to −1.1 V_RHE, reaching up to 98.6% at −0.9 V_RHE. Further experiments confirm that GO in NiPc–GO benefits CO₂ adsorption and formation of the *COOH intermediate. The change in the Ni²⁺/Ni³⁺ ratio with the GO amount in NiPc–GO composites reveals that the Ni(II)/Ni(III)/GO heterojunction structure is the most conductive to the CO₂RR process. This work provides an insight into the design and synthesis of single-atom Ni–N₄–C electrocatalysts for the CO₂RR.