Recent progress on transition metal macrocycles for electrochemical CO2 reduction

With increasing global energy demand and excessive utilization of conventional fossil fuels, implementing carbon emission reduction to curb climate change has been recognized internationally as a way forward. Electrochemical carbon dioxide reduction (CO2RR) is a promising approach to close the anthropogenic carbon cycle and address the intermittent issue of renewable electricity by converting CO2 into value-added chemicals, which are beneficial to establishing a carbon-neutral economy. It remains a significant challenge in practical applications due to the chemical inertia of CO2 molecules and the involved multiple-electron transfer steps. Although considerable efforts have been made to improve the catalytic performance of CO2RR, high reaction overpotential, low selectivity and relatively poor stability are the key bottlenecks that hinder the commercial application of the technology. Hence, it is urgent to develop cost-effective, efficient, high-performance catalysts for electrocatalytic CO2 reduction. Among various candidates, transition metal macrocycles (such as phthalocyanines and porphyrins) have attracted extensive attention and flourished in CO2RR. Compared with metal-based materials, transition metal macrocycles have the advantages of well-defined structures and functional diversities in tailoring molecular structures and regulating electronic structures, making it possible to realize the desired performances of CO2RR and provide an ideal model for the understanding of structure-activity correlations. Importantly, the central metal-nitrogen (M-N-4) unit is formed by coordinating the central metal with the surrounding macrocycle ligand, which provides an open site for interaction with CO2 molecules and promotes their activation and conversion. Although significant progress has been made in the exploration of CO2RR on molecular catalysts, further improvements are essential to achieve high activity, large current density, and long-term stability. In this perspective, we summarize the recent progress of transition metal macrocyclic for electrochemical CO2 reduction to C-1 products. Transition metal macrocycles can reduce CO2 to CO as the primary product via a two-electron pathway. Among various transition metal centers, iron, cobalt, and nickel-based macrocycles have been extensively explored and demonstrated to be highly efficient catalysts for CO2-to-CO conversion. However, the inevitable aggregation via intermolecular pi-pi stacking limits the achievement of remarkable chemical stability and accessible active site exposure. To this end, we focus on the key issues in electrocatalytic CO2-to-CO conversion and discuss effective strategies, including covalent or non-covalent attachment to carbon support, ligand modification and conjugated network construction. Based on these strategies above for optimizing catalytic performance, we emphasize the effect of coordination environment, central metal, and support-metal interaction in improving catalytic performance and stability. To facilitate the commercialization of CO2RR technology, we describe the advanced applications of transition metal macrocycles in zero-gap electrolyzers, demonstrating their availability for industrialized production at commercially relevant current densities. In addition to CO, transition metal macrocyclic can occasionally reduce CO2 to produce multi-electron (>2e(-)) reduction products at more negative potentials. Taking the most intensively studied cobalt-based macrocycles for example, due to their favorable binding energies for *CO, we summarize relevant theoretical and experimental results and discuss their potential in the production of CH4 or CH3OH. As an effective strategy for boosting the generation of multi-carbon products, constructing tandem catalysis by combining molecular catalysts with a second site capable of producing multi-carbon products is summarized. Finally, we aim at the current crucial challenges and propose future research directions in performance optimization and technological breakthrough for a commercial application.