Advances and Perspective of Noble-Metal-Free Nitrogen-Doped Carbon for pH-Universal Oxygen Reduction Reaction Catalysts

With the increasing demand for diversification of renewable energy sources, the high activity and stability of electrocatalysts in all pH ranges have been highlighted as one of the future directions in electrochemical energy generation and storage systems. Noble-metal-free nitrogen-doped carbon (M-N/C) has been explored as a substitute for the expensive platinum-based oxygen reduction reaction (ORR) electrocatalysts. However, its ORR activity remains limited to alkaline electrolytes. In acidic medium, its low activity and stability correspond to metallic site dissolution and protonation of N-functional groups, which are even worse in neutral electrolytes because of low ionic conductivity and low H+ concentration. This review summarizes strategies to improve the stability and activity of M-N/C as pH-universal ORR electrocatalysts. First, the ORR mechanism focusing on active site identification for each pH condition is discussed. Four strategies, including engineering pore structure, adding carbon shell wrapping, and introducing multiple nonmetal dopants and dual metallic active sites on the carbon substrate, are then evaluated to design pH-universal ORR electrocatalysts with distinguished activity and stability. Lastly, future perspectives are given to show the viewpoint of further development and potential applications of M-N/C electrocatalysts