| Abstract | The high cost and limited supply of platinum for Pt-based catalysts in proton exchange membrane fuel cells (PEMFCs) have driven intensive research into the use of non-noble metal catalysts in recent years. As the most promising non-noble metal catalysts for PEMFC oxygen reduction reactions (ORR), metal/N/C class of catalysts has been extensively explored. Earlier efforts (1964–2004) were mainly focused on the exploration of various synthesis routes and the investigation of active site mechanisms. During recent years (2005–2010), great progress in the development of these types of non-noble metal catalysts in real PEMFC environments has been achieved both in terms of catalytic activity and stability. From 2011 to present, several new synthetic approaches have been explored to produce highly dense catalytically active sites decorated within micropores using rationally designed zeolite imidazolate frameworks (ZIFs) and porous organic polymers (POPs). Currently, the most active non-noble metal catalysts are derived using this method and are able to deliver a kinetic volumetric current density of 450 A/cm3 at 0.8 V under fuel cell operating conditions. These results are superior to the US DOE 2020 target of 300 A/cm3. In terms of fuel cell maximum power density, the best non-noble metal catalysts for cathodes can achieve results as high as 0.98 W/cm2 and 0.41 W/cm2 with feeds of pure O2 and air respectively. In terms of stability, some non-noble metal catalysts have remained stable for over 1000 h with only minor degradation under PEMFC conditions. Nonetheless, activity and stability still remain major challenges for non-noble metal catalysts when compared to Pt-based ones in PEMFCs. Improvements in the structure of both catalysts and catalyst layers are urgently needed to realize the activity targets established for automobile fuel cell applications as well as the US DOE Hydrogen and Fuel Cell (H&FC) program. In the long term and the sustainable commercialization of fuel cells, replacing Pt-based catalysts with non-noble metal catalysts is, in the present authors' opinion, the most sustainable solution. Therefore, further intensive research into fundamental studies is critical to uncovering the workings of active site mechanisms. Once controllable design and synthesize of non-noble metal catalysts with high active site densities and utilization can be achieved, the goal of cost-effective, non-noble metal catalysts in automobile fuel cells can become reality. |
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