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.

The rational design of metal single-atom catalysts (SACs) for electrochemical nitrogen reduction reaction (NRR) is challenging. Two-dimensional metal–organic frameworks (2DMOFs) is a unique class of promising SACs. Up to now, the roles of individual metals, coordination atoms, and their synergy effect on the electroanalytic performance remain unclear. Therefore, in this work, a series of 2DMOFs with different metals and coordinating atoms are systematically investigated as electrocatalysts for ammonia synthesis using density functional theory calculations. For a specific metal, a proper metal-intermediate atoms p-d orbital hybridization interaction strength is found to be a key indicator for their NRR catalytic activities. The hybridization interaction strength can be quantitatively described with the p−/d- band center energy difference (Δd-p), which is found to be a sufficient descriptor for both the p-d hybridization strength and the NRR performance. The maximum free energy change (ΔG_max) and Δd-p have a volcanic relationship with OsC₄(Se)₄ located at the apex of the volcanic curve, showing the best NRR performance. The asymmetrical coordination environment could regulate the band structure subtly in terms of band overlap and positions. This work may shed new light on the application of orbital engineering in electrocatalytic NRR activity and especially promotes the rational design for SACs.

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.

Carbon dots (CDs) have attracted much attention due to their excellent photoelectric properties and potential applications. Although previous studies have shown that almost all organic molecules can be converted into CDs via chemical carbonization, the mechanism of the conversion process remains unclear. The hydrothermal/solvothermal method commonly used to prepare CDs is complicated and leads to the generation of many by-product CDs with similar structures. Considering that the purification of the synthesized by-products is difficult, the process of CDs formation cannot be readily analyzed and understood. Herein, we use ethanol as a carbon source to synthesize white-emitting CDs (W-CDs). Column chromatography separation shows that the synthesized W-CDs are composed of blue-, cyan-, and yellow-emitting CDs that fluoresce at wavelengths corresponding to the three emission centers of W-CDs. Although the samples have similar graphitic structure, they exhibit different surface states due to variations in the degree of oxidation and carbonization. Therefore, the red-shift in their emission peaks is attributed to an increased degree of carbonization in their polymer structure. Theoretical calculations verify the experimental results, and the prepared CDs are successfully used to develop multi-color and white light-emitting diodes (LEDs).

The most widely used method of identification of microbial morphology and structure is microscopy, but it can be difficult to distinguish between pathogens with a similar appearance. Existing fluorescent staining methods require a combination of a variety of fluorescent materials to meet this demand. In this study, unique concentration-dependent fluorescent carbon dots (CDs) were synthesized for the identification and quantification of pathogens. The emission wavelength of the CDs could be tuned spanning the full visible region by virtue of aggregation-induced narrowing of bandgaps. This tunable emission wavelength of the specific concentration response to diverse microbes can be used to distinguish microorganisms with a similar appearance, even in a same genus. A hyperspectral microscopy system was demonstrated to distinguish Aspergillus flavus and A. fumigatus based on the results above. The identification accuracy of the two similar-looking pathogens can be close to 100%, and the relative proportions and spatial distributions can also be profiled from the mixture of the pathogens. This technique can provide a solution to the fast detection of microorganisms and is potentially applicable to a wide range of problems in areas such as healthcare, food preparation, biotechnology, and health emergency.