Metalloporphyrin- and metallocorrole-based catalysts for the oxygen reduction reaction: from molecules to materials

Efficient oxygen reduction reaction (ORR) catalysts are pivotal for advancing clean energy technologies, such as fuel cells and metal-air batteries. Metalloporphyrins and metallocorroles, inspired by biological systems, represent promising molecular catalysts for the ORR due to their tunable structures and redox properties. This review systematically explores recent progress made in developing metalloporphyrin- and metallocorrole-based catalysts for the ORR, spanning from fundamental molecular design to advanced material engineering. We first introduce the fundamentals of the ORR and its significance. The discussion then delves into molecular catalysis, covering both homogeneous and heterogeneous catalytic systems. For heterogeneous systems, in addition to directly loading molecular catalysts on electrode materials through physical adsorption, we discuss covalent grafting of molecular catalysts on carbon supports (e.g., carbon nanotubes, graphene, and carbon black) and other support materials (e.g., metal oxides and gold electrodes). Moreover, the other focus of this review is placed on elucidating structure-property relationships, particularly on analyzing the effect of substituents, trans axial ligands, proton relay groups, electrostatic effects, and binuclear structures on the ORR mechanism and performance. Furthermore, the integration of these molecular catalysts into structured porous materials, including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), is discussed, highlighting how material design enhances catalytic activity, stability, and electron/proton transport. Finally, this review summarizes key achievements, identifies current challenges, and offers perspectives on future research directions for developing next-generation, high-performance ORR catalysts based on metalloporphyrins and metallocorroles. This work aims to provide valuable insights for the rational design of efficient and durable metalloporphyrin- and metallocorrole-based ORR catalysts and for the development of molecule-based functional materials for the future application of molecular electrocatalysis.