Nanocarbon electrocatalysts for oxygen reduction in alkaline media for advanced energy conversion and storage

Alkaline oxygen electrocatalysis, targeting anion exchange membrane fuel cells, Zn-air batteries, and alkaline-based Li-air batteries, has become a subject of intensive investigation because of its advantages compared to its acidic counterparts in reaction kinetics and materials stability. However, significant breakthroughs in the design and synthesis of efficient oxygen reduction catalysts from earth-abundant elements instead of precious metals in alkaline media remain in high demand. Carbon composite materials have been recognized as the most promising because of their reasonable balance between catalytic activity, durability, and cost. In particular, heteroatom (e.g., N, S, B, or P) doping can tune the electronic and geometric properties of carbon, providing more active sites and enhancing the interaction between carbon structure and active sites. Importantly, involvement of transition metals appears to be necessary for achieving high catalytic activity and improved durability by catalyzing carbonization of nitrogen/carbon precursors to form highly graphitized carbon nanostructures with more favorable nitrogen doping. Recently, a synergetic effect was found between the active species in nanocarbon and the loaded oxides/sulfides, resulting in much improved activity. This report focuses on these carbon composite catalysts. Guidance for rational design and synthesis of advanced alkaline ORR catalysts with improved activity and performance durability is also presented. The development of highly efficient catalysts from earth-abundant elements rather than precious metals is crucial for advanced electrochemical energy conversion and storage technologies. Recent breakthroughs in the nanocarbon composite cathode catalysts in alkaline-based fuel cells and metal-air batteries are summarized and guidance for the rational design and synthesis of advanced catalysts with improved activity and performance durability is presented.