Heat-Treated Non-precious Metal Catalysts for Oxygen Reduction

Polymer electrolyte fuel cells (PEFCs) are a green alternative to the internal combustion engine, particularly in the transportation sector. System costs unfortunately are still too high from a techno-economic standpoint, with approximately half of the total stack cost arising from the platinum catalysts. Therefore, the development of entirely non-platinum group metal (non-PGM) catalysts for the inherently slow oxygen reduction reaction (ORR) is highly desirable. Among several non-PGM catalysts, the most promising are heat-treated metal, nitrogen, carbon (M–N–C) catalysts. In this work, we have reviewed critical factors affecting the performance and durability of these catalysts, such as precursors and heat treatment temperature/time. We have also provided background information on the electrochemical and non-electrochemical evaluation techniques applicable to these catalysts, along with active site structure discussions. In terms of catalyst performance, while fuel cell activity for heat-treated non-PGM catalysts has been dramatically improved over the past two decades, further improvements are still required to compete with state-of-the-art Pt catalysts. Though indispensable, the heat treatment destroys the initial structure of precursors, producing a highly heterogeneous, complicated structure that makes it very difficult to elucidate and understand the active site(s) of the resulting non-PGM catalysts. At this stage, these authors think that fundamental knowledge surrounding the active site structure(s) and their formation is required in order to propel non-PGM catalyst development to new levels of achievement through rationally guided research efforts.