1. Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China;2. School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026, China
Abstract:
Proton-exchange membrane fuel cells (PEMFCs) are an efficient and clean energy conversion technology with the advantage of zero pollution for transportation applications. The oxygen reduction reaction (ORR) is the key step in the energy conversion at the cathode, but the slow kinetics requires a high content of expensive platinum-group-metal (PGM) catalysts. Therefore, research on high-performance and inexpensive catalysts to replace PGM-based catalysts are essential to promote the commercialization of fuel cells. Single-atom catalysts (SACs) with highly active sites that are atomically dispersed on substrates exhibit unique advantages, such as maximum atomic utilization, abundant chemical structures, and extraordinary catalytic performances for multiple important reactions. Inspired by macrocyclic compounds with MN4 active centers, the application of pyrolyzed M-Nx/C type SACs (M = Fe, Co, Mn, Ru, Cr, Zn, etc.) in the ORR has significantly progressed within the last ten years. Particularly, single-atom Fe-N-C catalysts have been extensively investigated, demonstrating high ORR activity, which indicates that the initial electrochemistry and fuel cell performance are similar to that of conventional Pt/C catalysts. However, in the oxidizing and acidic PEMFC cathode, Fe-N-C catalysts are degraded rapidly, which hinders the application of these nonprecious metal M-Nx/C-type catalysts. Several degradation mechanisms have been proposed over the past few years, such as carbon oxidation, demetallation, and waterflooding. However, the degradation mechanisms remain unknown and require further investigation of the underlying causes of the mechanism, degradation process, and coping strategies. To achieve the future commercialization of high-performance M-Nx/C catalysts, several key challenges are summarized with potential research guidelines proposed to overcome bottlenecks. This review summarizes the development history and state-of-the-art research progress on nonprecious metal M-Nx/C-type catalysts in PEMFCs. First, we introduce the basic theory of the ORR and the methods of advanced characterization techniques for active site identification and reaction mechanism analysis to gain a comprehensive understanding of the structure–performance relationship. Subsequently, the representative studies and recent advancements in M-Nx/C-type catalysts by experimental and theoretical calculations are presented. Additionally, we analyze the root cause of the stability problems and propose the corresponding solution strategies to promote the intrinsic electrocatalytic ORR activity and durability, including regulating the electronic structure and coordination environment, as well as altering the central metal atoms and guest groups. Finally, we propose that the future direction of M-Nx/C-type catalysts is the rational design of catalysts with a high site density and high stability. Moreover, improving the lifetime of nonprecious metal catalysts remains essential for feasible applications in the future.
