Recent progress in active sites for non-noble metal carbon-based oxygen reduction catalysts

Abstract

Platinum-based materials are widely applied to catalyze cathodic oxygen reduction reaction in H2/O2 fuel cell. Due the high price and scarcity of Pt, developing novel non-noble metal catalysts is of great importance for wide application of H2/O2 fuel cell due to their low price and abundance compared to platinum-based materials. Non-noble metal carbon-based oxygen reduction reaction catalysts, including transition-metals-nitrogen co-doped carbon composites and heteroatom-doped carbon composites, are intensively studied. Recognizing the active sites for such catalysts is of great importance for further improving the catalytic performance and scaled up synthesis, thus it has attracted tremendous research efforts. For the M–N–C type catalysts, M-N x /C ( x =1,2,3,4), N x -C, as well as active graphene layers in iron encapsulated within carbon nanotubes or graphene layers have been regarded as active sites in varied research, respectively. For the heteroatoms-doped carbon catalysts, carbon atoms adjacent to pyridinic nitrogen are accepted as active sites. However, it is difficult to eliminate trace amount of metal that introduced from precursors and during the synthesis course, thus synthetizing absolute non-metal heteroatoms-doped carbon and identifying the active sites are almost impossible. Therefore, distinguishing active sites from several components and improving catalysts active sites density are issues that we must resolve. This short review focuses on our recent results acquired regarding the activity and active sites for ORR, and we expect that this effort will promote further research on this challenging and important topic.