Improved charge delivery within a covalently ligated cobalt phthalocyanine electrocatalyst for CO2 reduction

Abstract

Cobalt(II) phthalocyanine (CoPc) complexes are some of the most active catalysts for the CO₂ electroreduction reaction (CO₂ERR). However, these organic complexes are non-conductive, thus the CO₂ERR rate is hindered by the slow electron delivery to the active centers. Herein, our recently developed variable frequency square wave voltammetry (VF-SWV) was employed to directly image the charge transfer between the covalently ligated polymeric CoPc and the carbon fiber paper electrode. The VF-SWV shows that the conjugated structure of the covalently ligated CoPc provides a direct path for the charge migration between the active centers and the electrode. Combined with the density-functional theory (DFT) calculations, our mechanistic studies show that Co^I is the catalyst resting state and the doubly reduced Co⁰ is the key catalytically active species driving the CO₂ERR. The conductive nature of the macromolecular framework evidenced by VF-SWV allows for faster replenishing of dianionic Co⁰ species and, subsequently, boosts the CoPc-cov catalyst performance with a high selectivity to carbon monoxide (CO) evolution with the Faradaic efficiency up to 85% and turnover frequency TOF(CO) of up to 38.1 s⁻¹, which is nearly double that of the noncovalently ligated counterpart. The immobilization strategy developed in this work provides an opportunity for the development of cheap, efficient, and stable heterogeneous molecular catalysts for the CO₂ERR.