基于过渡金属大环分子的电化学二氧化碳还原研究进展

Recent progress on transition metal macrocycles for electrochemical CO_(2) reduction

利用电化学方法还原二氧化碳(CO_(2)RR)制备高附加值化学品是实现碳中和的重要途径.开发具有低成本、高性能的电催化剂是该技术发展的核心关键.在众多CO_(2)还原候选材料中,过渡金属卟啉、酞菁等大环分子化合物因具有结构明确和功能可调等特点,在实现高效CO_(2)RR催化性能和探究结构-性能内在关系等方面表现出良好的发展潜力.基于此,本文总结了过渡金属大环分子催化剂电化学CO_(2)还原制备碳一(C_(1))产物的最新研究进展.首先,重点讨论了不同改性策略及电解池设计对于生成一氧化碳的选择性、稳定性、单位催化活性以及电流密度等性能的影响.随后,探讨了分子催化剂在生成甲醇和甲烷等多电子还原产物的催化潜力.最后,聚焦该材料体系在实际应用中面临的关键挑战,对该领域未来的研究发展方向进行了讨论与展望.

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(CO_(2)RR)is a promising approach to close the anthropogenic carbon cycle and address the intermittent issue of renewable electricity by converting CO_(2)into value-added chemicals,which are beneficial to establishing a carbonneutral economy.It remains a significant challenge in practical applications due to the chemical inertia of CO_(2)molecules and the involved multiple-electron transfer steps.Although considerable efforts have been made to improve the catalytic performance of CO_(2)RR,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 CO_(2)reduction.Among various candidates,transition metal macrocycles(such as phthalocyanines and porphyrins)have attracted extensive attention and flourished in CO_(2)RR.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 CO_(2)RR and provide an ideal model for the understanding of structure-activity correlations.Importantly,the central metal-nitrogen(M-N4)unit is formed by coordinating the central metal with the surrounding macrocycle ligand,which provides an open site for interaction with CO_(2)molecules and promotes their activation and conversion.Although significant progress has been made in the exploration of CO_(2)RR 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 CO_(2)reduction to C_(1)products.Transition metal macrocycles can reduce CO_(2)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 CO_(2)-to-CO conversion.However,the inevitable aggregation via intermolecularπ-πstacking limits the achievement of remarkable chemical stability and accessible active site exposure.To this end,we focus on the key issues in electrocatalytic CO_(2)-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 CO_(2)RR 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 CO_(2)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 CH_(4)or CH_(3)OH.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.