Pore engineering to improve oxygen transport in cathode oxygen reduction catalysts of proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) is a device that converts chemical energy into electrical energy, and the design of catalytic layer of its air electrode should not only contain abundant and easily accessible reactive sites, but also have highly connected electrons, protons, and reactants mass transfer channels. Therefore, the electrode must possess a specific three-dimensional (3D) geometric structure and orderly distributed functionalized channels to ensure full utilization of catalytic active sites and enable continuous reaction processes. Herein, we review the recent research progress of porous carbon-based oxygen reduction reaction (ORR) catalysts for cathodic catalytic layer in PEMFC. Firstly, the reaction mechanism of PEMFC as well as the optimization principles were briefly introduced, followed by a detailed discussion on the design and preparation of PEMFC cathode ORR catalysts with hierarchical porous structures from the main methods, such as hard template, soft template, combination of hard and soft templates, self-assembled template, electrostatic spinning, and 3D printing. Additionally, the performance characterization of PEMFC cathode catalysts with porous structure was elaborated from three aspects: electrochemical performance testing, numerical investigations of oxygen transport, and in-situ characterization and operation techniques. Finally, the future challenges and opportunities for the PEMFC cathodic catalytic layer were envisioned.