The electrochemical oxygen reduction reaction (ORR) is pivotal in energy conversion via a 4e⁻ ORR pathway and green hydrogen peroxide production via 2e⁻ ORR pathway. Transition metal single atom catalysts (TM SACs) have attracted intense attention in recent years for ORR due to their high activity and near maximum metal atom utilization. The future development of TM SACs for ORR requires improved understanding of reaction pathways, since currently the true origin of activity remains contentious owing to the lack of qualitative/quantitative information about active sites. Knowledge-guided design is imperative for the optimization of TM SACs performance in terms of activity and selectivity. This review focuses on the latest progress in the design of TM SACs for ORR, placing particular attention on efforts to elucidate reaction mechanisms. Experimental evidence based on in-situ/operando characterization measurements, along with theoretical predictions, are summarized to deepen understanding of the structure-performance relationships at both atomic and molecular level. Finally, some perspectives are offered relating to the fundamental science needed for TM SACs to find practical application in energy storage and conversion devices. We hope this review will inspire the development of new synthetic routes towards high-performance ORR electrocatalysts for the energy sector.

Hydrogen energy, a new type of clean and efficient energy, has assumed precedence in decarbonizing and building a sustainable carbon-neutral economy. Recently, hydrogen production from water splitting has seen considerable advancements owing to its advantages such as zero carbon emissions, safety, and high product purity. To overcome the large energy barrier and high cost of water splitting, numerous efficient electrocatalysts have been designed and reported. However, various difficulties in promoting the industrialization of electrocatalytic water splitting remain. Further, as high-performance electrocatalysts that satisfy industrial requirements are urgently needed, a better understanding of water-splitting systems is required. In this paper, the latest progress in water electrolysis is reviewed, and experimental evidence from in situ/operando spectroscopic surveys and computational analyses is summarized to present a mechanistic understanding of hydrogen and oxygen evolution reactions. Furthermore, some promising strategies, including alloying, morphological engineering, interface construction, defect engineering, and strain engineering for designing and synthesizing electrocatalysts are highlighted. We believe that this review will provide a knowledge-guided design in fundamental science and further inspire technical engineering developments for constructing efficient electrocatalysts for water splitting.

Electrolysis of seawater offers a highly promising and sustainable route to attain carbon-neutral hydrogen energy without demanding on high-purity water resource. However, it is severely limited by the undesirable chlorine oxidation reaction (ClOR) on the anode and the releasing toxic chlorine species, inducing anode corrosion and multiple pollutions to reduce the efficiency and sustainability of this technology. The effective way is to limit the overpotential of oxygen evolution reaction (OER) below 480 mV and thus suppress the ClOR. Herein, we demonstrate that nitrogen-doped carbon dots strongly coupled NiFe layered double hydroxide nanosheet arrays on Ni foam (N-CDs/NiFe-LDH/NF) can efficiently facilitate OER with an ultralow overpotential of 260 mV to deliver the geometric current density of 100 mA·cm⁻² and a Tafel slope of as low as 43.4 mV·dec⁻¹ in 1.0 M KOH. More importantly, the N-CDs/NiFe-LDH/NF electrode at 100 mA·cm⁻² shows overpotentials of 285 and 273 mV, respectively, by utilizing 1.0 M KOH with 0.5 M NaCl and 1.0 M KOH with 1.0 M NaCl as the simulated seawater, well avoid triggering ClOR. Notably, despite the complex environment of real seawater, N-CDs/NiFe-LDH/NF still effectively promotes alkaline seawater (1.0 M KOH + seawater) electrolysis with a lifetime longer than 50 and 20 h, respectively, in 1.0 M KOH and alkaline seawater electrolytes. The investigation result reveals that M–N–C bonding generated between N-CDs and NiFe-LDH intrinsically optimizes the charge transfer efficiency, further promoting the OER kinetics.