Porphyrin-based frameworks and derivatives for the oxygen reduction reaction

The global energy crisis and environmental pollution issues caused by the continuous consumption of fossil fuels are increasingly becoming severe. Developing clean and sustainable hydrogen (H2) energy and corresponding storage and conversion devices have attracted increasing attention. The utilization of H2 energy highly depends on the fuel cell (FC). However, the slow kinetics of oxygen reduction reaction (ORR) on the cathode of FC cannot be improved without efficient catalysts. Currently, precious metal platinum (Pt) and its alloys are commonly used catalysts for ORR process. However, the scarcity and high cost of Pt hinder their large-scale practical application. To replace typical Pt-based catalysts, non-precious metal electrocatalysts have been investigated for ORR. Inspired by the catalytic ORR active center Fe porphyrin of cytochrome c oxidase in nature, metalloporphyrin-based frameworks and their derivatives have become promising electrocatalysts due to their abundant conjugated electronic structure, tunable functional groups, and large specific surface area. First, this review briefly introduces porphyrins and catalytic ORR mechanisms. Second, recent progress on porphyrin-based ORR catalysts including porphyrin-based frameworks, framework@substrate composites, and framework-based derivatives was summarized. Porphyrin-based frameworks mainly include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs), and polymers constructed with porphyrin building blocks. Porphyrin-based framework@substrate composites were usually assembled and constructed to improve the electron transfer rate of frameworks. To further improve the conductivity, framework-based derivatives, usually named metal, nitrogen-doped carbon (M-N-C) materials, were synthesized through pyrolyzing frameworks at high temperatures. Finally, research challenges and directions of porphyrin-based electrocatalysts for ORR were discussed. This review is meaningful and enlightening for designing and developing other porphyrin-based electrocatalysts for ORR.