via the overall reaction of H 2 with O 2 , only giving rise to water without pollution. Beyond that, hydrogen fuel cells have also high safety, quick-refueling, fast-charging, power grid compatibility, and high energy conversion efficiency (40-70%) even restricted by polarization in practical conditions, which make them potential for long-range and high-utilization transportation as electric vehicles. [2] The energy conversion efficiency mainly depends on the cathodic oxygen reduction reaction (ORR) due to its naturally complex and sluggish kinetics as compared to the anodic hydrogen oxidation reduction during electron transfer. [3] It is, as such, highly desirable to employ efficient electrocatalysts, capable in alleviating overpotentials of electrocatalytic ORR while also, promoting the slow transformation process. Currently, the expensive platinum (Pt) electrocatalysts still exhibit unparalleled catalytic performance for ORR; [4] however, the scarcity of resources, sluggish ORR kinetics, inferior operational stability, and susceptibility to poisoning of the Pt catalysts result in high cost, which remains to be one of key factors that hamper commercial applications of fuel cells. [1a,5] At this juncture, the replacement of costly Pt-based catalysts with other cost-effective, high-performing and durable nonprecious metal catalysts (NPMCs) for ORR is urgently needed. [6] In pursuit of a diverse class NPMCs, M/N codoped M-N-C (M = Fe, Co, Ni, etc.) especially Fe-N-C catalysts, widely prepared by annealing a compound of carbon carriers, nitrogen-containing precursors and transition metal salts or without carbon supports under inert or reactive gas, attract substantial interest due to comparable activity with that of benchmark Pt/C for ORR in alkaline media. [7] However, their volumetric activity is still far from that of Pt/C. Furthermore, their stability remains poor in acid because of the dissolution of Fe-based species associated with active sites. [4b,8] As such, Fe-N-C catalysts demonstrate low competency for proton exchange membrane fuel cell (PEMFC) application at present. It is greatly challenging but yet, extremely desirable to exploit novel NPMCs having outstanding activity and durability in ORR. Elucidation of the active site nature and pursuit of unprecedented high ORR activity and stability motivates design of unique NPMCs. Recently, transition-metal carbides, such as Fe 3 C, Fuel cells represent the most suitable energy conversion, capable of addressing energy crises and environmental pollution. Recently, as one of nonprecious metal catalysts (NPMCs), the MC@N-C (M = Fe, Co, Ni, Mo, W) catalysts, especially for Fe 3 C encased in carbon layer (denoted as Fe 3 C@N-C) have emerged as promising replacements for costly Pt-based catalysts for oxygen reduction reaction (ORR). This review highlights the synthetic strategies undertaken such as hard template, soft template, and template-free methods for deriving enhancements in electrocatalytic activity and durability. It also provides a comparison on the synth...
