Recent advances in Co-based electrocatalysts for the oxygen reduction reaction

In recent years, global environmental problems caused by burning fossil fuels have become increasingly serious. Proton exchange membrane fuel cells (PEMFCs) that use highly efficient clean renewable fuels (hydrogen, H-2) to generate electricity have received a lot of attention. The main drawback of current PEMFC technology is the high cost caused by the high content of precious metal platinum (Pt) in catalysts for the electrocatalytic oxygen reduction reaction (ORR). Therefore, the development of stable and efficient low Pt content or Pt-free catalysts for the ORR is one of the current research hotspots. Currently, cobalt (Co)-based electrocatalysts for the ORR have attracted extensive attention. This research provides potential to reduce the Pt content or prevent the use of Pt in electrocatalysts, while keeping high activity and stability. This review summarizes recent advances in Co-based electrocatalysts including Co-Pt-based materials, Co,N-doped carbon (Co-N-C) materials, heterometal-doped Co-N-C materials and nonmetal-doped Co-N-C materials. In addition, this review also discusses the structure-activity relationship and the way in which the structural characteristics of electrocatalysts, such as the surface area and accessible active sites, and heteroatom doping affect their ORR activities. We hope that this review could shed some light on the design and development of electrocatalysts that can be industrialized in PEMFCs.