A review of the development of porphyrin-based catalysts for electrochemical CO2 reduction

The reduction of greenhouse gas carbon dioxide (CO2) to carbon-based fuels provides a potential solution to the sustainable and scalable energy storage challenges. To solve the issues with activity, selectivity and stability, much attention has been put on porphyrin catalysts, and more recently, the integration of porphyrins on conductive supports that operate in heterogeneous conditions. This review summarizes the main principles and strategies explored for the application of porphyrin catalysts to electrochemical CO2 reduction, ranging from homogeneous catalysis and heterogeneous catalysis to structure modification and immobilization techniques. The first section discusses mechanistic study of porphyrin catalysts including the catalytic steps involved in CO2 reduction, the effects of porphyrin ligands on catalytic activities. The second section provides insight into the kinetic study of porphyrin catalysts. The third section presents examples of porphyrins in homogeneous catalysis with particular focus on iron porphyrins and the boosting effects brought by Lewis acid and Brønsted acid. The next section summarizes the main techniques for the heterogenization of porphyrin catalysts on conductive supports, including non-covalent, covalent and periodic immobilization, whereas periodic immobilization comprises porphyrin scaffolds or frameworks in the structure. The last section gives an update of porphyrins employed in a flow cell. This review surveys the recent advances and basic principles of porphyrins in CO2 reduction, and the findings can be instrumental for designing efficient and selective catalysts for CO2 reduction.