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Atomically dispersed iron–nitrogen–carbon (Fe–N–C) catalysts have emerged as the most promising alternative to the expensive Pt-based catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs), however suffer from low site density of active Fe-N4 moiety and limited mass transport during the catalytic reaction. To address these challenges, we report a three-dimensional (3D) metal–organic frameworks (MOF)-derived Fe–N–C single-atom catalyst. In this well-designed Fe–N–C catalyst, the micro-scale interconnected skeleton, the nano-scale ordered pores and the atomic-scale abundant carbon edge defects inside the skeleton significantly enhance the site density of active Fe-N4 moiety, thus improving the Fe utilization in the final catalyst. Moreover, the combination of the above mentioned micro- and nano-scale structures greatly facilitates the mass transport in the 3D Fe–N–C catalyst. Therefore, the multiscale engineered Fe–N–C single-atom catalyst achieves excellent ORR performance under acidic condition and affords a significantly enhanced current density and power density in PEMFC. Our findings may open new opportunities for the rational design of Fe–N–C catalysts through multiscale structural engineering.

The electrocatalytic N2 reduction reaction (eNRR) – which can occur under ambient conditions with renewable energy input – became a promising synthetic pathway for ammonia (NH3) and has attracted growing attention in the past few years. Some achievements have been made in the eNRR; however, there remain significant challenges to realize satisfactory NH3 production. Therefore, the rational design of highly efficient and durable eNRR catalysts with NN bond activating and breaking ability is highly desirable. Two-dimensional (2D) materials have shown great potential in electrocatalysis for energy conversion and storage. Although most 2D materials are inactive toward the eNRR, they can be activated by various modification methods. Heteroatom doping engineering can impact the charge distribution and spin states on catalytic sites, therefore accelerating the dinitrogen adsorption and protonation process. This review summarises the recent research progress of heteroatom-doped 2D materials, including carbon, molybdenum disulfide (MoS2) and metal carbides (MXenes), for the eNRR. In addition, some existing opportunities and future research directions in electrocatalytic nitrogen fixation for ammonia production are discussed.

Cost-effective carbon-based catalysts are promising for catalyzing the electrochemical N2 reduction reaction (NRR). However, the activity origin of carbon-based catalysts towards NRR remains unclear, and regularities and rules for the rational design of carbon-based NRR electrocatalysts are still lacking. Based on a combination of theoretical calculations and experimental observations, chalcogen/oxygen group element (O, S, Se, Te) doped carbon materials were systematically evaluated as potential NRR catalysts. Heteroatom-doping-induced charge accumulation facilitates N2 adsorption on carbon atoms and spin polarization boosts the potential-determining step of the first protonation to form *NNH. Te-doped and Se-doped C catalysts exhibited high intrinsic NRR activity that is superior to most metal-based catalysts. Establishing the correlation between the electronic structure and NRR performance for carbon-based materials paves the pathway for their NRR application.

Zinc–air battery has drawn increasing attention, but however, suffers from low-power density. Here, we show an S-doped Fe–N–C catalyst with hierarchical porous architecture as an oxygen reduction reaction (ORR) catalyst for a high-power-density zinc–air battery. We demonstrate that the macropores and mesopores in the interconnected hierarchical morphology of Fe–N–C catalyst significantly increase the accessible active Fe–N4 sites during ORR; the combination of microstructure and nanostructure in the Fe–N–C catalyst provides an ideal aerophilic/hydrophobic surface, promoting the O2 transport during ORR; the heteroatom S-doping in the carbon skeleton boosts the intrinsic activity of the single Fe–N4 moiety. Therefore, the zinc–air battery assembled with such Fe–N–C catalyst exhibits superior performance with a super high-power-density of 453 mW cm−2.