Recent Progress in Proton-Exchange Membrane Fuel Cells Based on Metal-Nitrogen-Carbon Catalysts

Proton-exchange membrane fuel cells (PEMFCs) directly transform chemical energy into electrical energy with high energy density and zero carbon emissions, thereby offering a clean energy alternative for fossil fuels and vehicle electrification. However, the existing PEMFCs rely on Pt-based catalysts, especially at the cathode side wherein the sluggish oxygen reduction reaction (ORR) takes place, resulting in high cost and limiting their commercial applications. Therefore, there is a strong interest in developing platinum group metal-free (PGM-free) PEMFCs. Although impressive advancements have been made since metal-nitrogen-carbon (M-N-C) catalysts have been developed as promising candidates for low-cost cathode catalysts, PGM-free PEMFCs still suffer from insufficient activity and durability. Owing to the intricate structure of the tri-phase interface and mass transport limitation, the M-N-C catalysts with high ORR activity in rotating disk electrode (RDE) tests still suffer from unexpected problems such as showing low activity and undesired rapid degradation process in real fuel cell conditions. Therefore, a comprehensive understanding of the active sites and influences of the M-N-C catalyst structure and cathode structure on the PEMFC performance will promote the development of PGM-free PEMFCs. Herein, with an aim to increase the activity and durability of PEMFCs based on M-N-C catalysts, we summarize the recent progress in understanding the active sites of M-N-C catalysts and the relationships between the structures of catalysts/catalyst layers and device performances. At the catalyst level, multiple delicately designed synthetic strategies suggest that attractive device performances can be obtained by tailoring the intrinsic activity and density of the catalyst active sites while engineering the porosity of catalysts to improve the utilization of active sites. Additionally, integrating the catalyst ink into the cathode catalyst layers in PGM-free PEMFC is pivotal for transforming the impressive ORR performance of catalysts in the RDE test to fuel cell performance. Accordingly, the recent advances in the enhancement of mass transfer and charge transport to achieve remarkable fuel cell performance were also included by rationally designing ionomer contents, catalyst morphology, and fabrication process of cathodic catalyst layers. Moreover, durability is the Achilles heel of PEMFCs with M-N-C catalysts, which is currently far behind the commercial requirements. The possible degradation mechanisms and the recent progress in seeking the corresponding solutions are also discussed in this review, including the decomposition of metal species, protonation of nitrogen sites, corrosion of carbon support, and micropore flooding. Based on these insights, the perspective is proposed by articulating open challenges and opportunities in materials innovations and device engineering with an aim to achieve practical M-N-C based PEMFCs.