The high cost and limited supply of platinum has driven intensive research into the use of non-platinum group metals (PGM-free) as cathode oxygen reduction reaction (ORR) catalysts for proton exchange membrane fuel cells (PEMFCs), which is crucial in promoting the large-scale applications of PEMFCs and the most important step to achieve the DOE ultimate stack cost target. As the most promising group of PGM-free ORR catalysts, the metal/N/C (M–N–C) catalysts have been extensively explored. However, several problems still exist in the development of high-performance M/N/C catalysts, including limited intrinsic activity and durability, severe concentration polarization and ohmic polarization. Therefore, our aim is to conduct a comprehensive understanding and recognize the strengths and weaknesses of the state-of-the-art PGM-free ORR catalysts for PEMFCs. Herein, we summarize three classes of the most active M/N/C ORR catalysts, namely, conductive PANI-based catalysts and their analogues, ZIFs-based catalysts and their derivatives, and other high-performance ORR catalysts, thoroughly expound their deactivation mechanisms, and deeply discuss the oxygen reduction mechanisms and approaches for identifying active sites. The challenges and perspectives on PGM-free cathode catalysts of PEMFCs are also discussed.

中文翻译：

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酸性介质和 PEMFCs 中氧还原非贵金属电催化剂的最新进展：活性、稳定性和机理研究

铂的高成本和有限的供应推动了对使用非铂族金属（不含 PGM）作为质子交换膜燃料电池（PEMFC）的阴极氧还原反应（ORR）催化剂的深入研究，这对于促进PEMFC 的大规模应用以及实现 DOE 最终堆栈成本目标的最重要步骤。作为最有前途的一组不含 PGM 的 ORR 催化剂，金属/N/C（M-N-C）催化剂已被广泛探索。然而，高性能M/N/C催化剂的开发仍存在一些问题，包括固有活性和耐久性有限，浓差极化和欧姆极化严重。所以，我们的目标是全面了解并认识到最先进的 PEMFC 无 PGM ORR 催化剂的优缺点。在此，我们总结了三类活性最高的 M/N/C ORR 催化剂，即导电 PANI 基催化剂及其类似物、ZIFs 基催化剂及其衍生物以及其他高性能 ORR 催化剂，并详细阐述了它们的失活机制，并深入讨论氧还原机制和识别活性位点的方法。还讨论了 PEMFC 的无 PGM 阴极催化剂的挑战和前景。深入阐述了它们的失活机制，并深入探讨了氧还原机制和活性位点识别方法。还讨论了 PEMFC 的无 PGM 阴极催化剂的挑战和前景。深入阐述了它们的失活机制，并深入探讨了氧还原机制和活性位点识别方法。还讨论了 PEMFC 的无 PGM 阴极催化剂的挑战和前景。