Design, synthesis, and electronic structure modulation of ORR electrocatalysts

Due to the environmental deterioration caused by modern industrialization, the key focus of researchers worldwide is diverted toward clean and sustainable energy sources, such as fuel cells and metal-air batteries, where the oxygen reduction reaction (ORR) at the cathode plays a central role. Unfortunately, the sluggish kinetics at the cathode of the ORR, in which the platinum-based (Pt-based) catalyst plays a critical role, is the principal impediment to commercialization. Pt and Pt-based, cutting-edge ORR electrocatalysts suffer from high cost, low methanol tolerance, and poor stability. Researchers have made tremendous efforts towards substitution of Pt-based catalysts, and significant achievements were accomplished in the form of alternatives such as metal oxides, nitrides, chalcogenides, carbides, metal-free nanocarbon, and metal-nitrogen/carbon (M-N/C) nanocomposites. Among these innumerable substitutes of Pt-based catalysts, M-N/C shows great potential because of their high stability, high methanol tolerance, and admirable ORR activities. In this review, we have summarized the principles and factors determining the rate of ORR, which are related to different surface planes of the crystal, surface area, and electronic structure tunability, where the morphology, multiple heteroatom-doping, and controlled particle size play a vital role. Furthermore, we have also discussed the synthetic protocol of different ORR materials such as Pt-based, metal chalcogenides, metal oxides, and M-N/C nanocomposites and the challenges related to these materials. Hopefully, this review will provide a status update as well as new directions to the scientific community engaged in designing novel catalytic materials for energy and environmental applications.