| Summary: | The proton exchange membrane fuel cells (PEMFCs) have been considered as promising future energy conversion devices, and have attracted immense scientific attention due to their high efficiency and environmental friendliness. Nevertheless, the practical application of PEMFCs has been seriously restricted by high cost, low earth abundance and the poor poisoning tolerance of the precious Pt-based oxygen reduction reaction (ORR) catalysts. Noble-metal-free transition metal/nitrogen-doped carbon (M−N<sub>x</sub>C) catalysts have been proven as one of the most promising substitutes for precious metal catalysts, due to their low costs and high catalytic performance. In this review, we summarize the development of M−N<sub>x</sub>C catalysts, including the previous non-pyrolyzed and pyrolyzed transition metal macrocyclic compounds, and recent developed M−N<sub>x</sub>C catalysts, among which the Fe−N<sub>x</sub>C and Co−N<sub>x</sub>C catalysts have gained our special attention. The possible catalytic active sites of M−N<sub>x</sub>C catalysts towards the ORR are also analyzed here. This review aims to provide some guidelines towards the design and structural regulation of non-precious M−N<sub>x</sub>C catalysts via identifying real active sites, and thus, enhancing their ORR electrocatalytic performance.
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