Mesoporous electrocatalysts for oxygen reduction reaction: From mechanisms, characterizations to designs

Oxygen reduction reaction (ORR) as the core process of sustainable energy technologies such as metal–air batteries, plays a decisive role in energy conversion efficiency but usually suffers from sluggish intrinsic kinetics. Thereby highly active and stable ORR electrocatalysts are desired for accelerating the reaction process, while mesoporous design is one promising solution benefitted from high specific surface area, abundant active sites, and rapid mass transfer. In this review, the proposed mechanisms of ORR process are firstly theoretically discussed from both thermodynamic and kinetic perspectives, accompanied by the advantage analysis of mesoporous electrocatalyst design. Subsequently, recent research progresses of a series of mesoporous ORR electrocatalysts with distinct structural and kinetic characteristics are demonstrated and compared to clarify their correlation with activity, including noble metal-, transition metal-, carbon-based, and composite material classes, etc., together with energy-related applications. Meanwhile, spectroscopic insights contribute to more understanding on intermediates and reaction routes. Finally, design rules and prospects are systematically provided in terms of structural design, characterization techniques, theoretical simulations, and data standardization perspectives, for inspiring the development of mesoporous ORR electrocatalysts and meeting future challenges.