Pyrolyzed M–N x catalysts for oxygen reduction reaction: progress and prospects

Fuel cells, a sustainable technology that assures a cleaner earth, once experienced disillusion due to the issue of economic viability associated with the massive usage of Pt-based catalysts. To address this issue, research on cost-effective catalysts, especially on the cathode side, has been intensively carried out where the latest achievements have made the M–N x /C material on its verge of application. Here, we present a comprehensive overview of the fundamental chemistry of oxygen reduction reaction (ORR) and the progress of the M–N x /C catalysts. Efforts aimed at clarifying the nature of catalytic sites are summarized, highlighting the indispensable role of advanced characterization tools. A variety of successful attempts are exemplified to describe the synthetic methodologies, followed by close tracking of newly emerging single-atom or dual-atom sites. Future directions for this research area are believed to be the rational and/or controllable synthesis of catalysts with adequate active sites and the use of strategies to substantially alleviate catalyst degradation.