Electrocatalytic Reduction of Carbon Dioxide in Acidic Electrolyte with Superior Performance of a Metal–Covalent Organic Framework over Metal–Organic Framework

Abstract

CO₂ electroreduction (CO₂RR) to generate valuable chemicals in acidic electrolytes can improve the carbon utilization rate in comparison to that under alkaline conditions. However, the thermodynamically more favorable hydrogen evolution reaction under an acidic electrolyte makes the CO₂RR a big challenge. Herein, robust metal phthalocyanine(Pc)-based (M = Ni, Co) conductive metal-covalent organic frameworks (MCOFs) connected by strong metal tetraaza[14]annulene (TAA) linkage, named NiPc–NiTAA and NiPc–CoTAA, are designed and synthesized to apply in the CO₂RR in acidic electrolytes for the first time. The optimal NiPc–NiTAA exhibited an excellent Faradaic efficiency (FE_CO) of 95.1% and a CO partial current density of 143.0 mA cm^–2 at −1.5 V versus the reversible hydrogen electrode in an acidic electrolyte, which is 3.1 times that of the corresponding metal–organic framework NiPc–NiN₄. The comparison tests and theoretical calculations reveal that in-plane full π–d conjugation MCOF with a good conductivity of 3.01 × 10^–4 S m^–1 accelerates migration of the electrons. The NiTAA linkage can tune the electron distribution in the d orbit of metal centers, making the d-band center close to the Fermi level and then activating CO₂. Thus, the active sites of NiPc and NiTAA collaborate to reduce the *COOH formation energy barrier, favoring CO production in an acid electrolyte. It is a helpful route for designing outstanding conductive MCOF materials to enhance CO₂ electrocatalysis under an acidic electrolyte.