© 2020, The Author(s), under exclusive licence to Springer Nature Limited. The use of nitrogen fertilizers has been estimated to have supported 27% of the world's population over the past century. Urea (CO(NH2)2) is conventionally synthesized through two consecutive industrial processes, N2 + H2 → NH3 followed by NH3 + CO2 → urea. Both reactions operate under harsh conditions and consume more than 2% of the world's energy. Urea synthesis consumes approximately 80% of the NH3 produced globally. Here we directly coupled N2 and CO2 in H2O to produce urea under ambient conditions. The process was carried out using an electrocatalyst consisting of PdCu alloy nanoparticles on TiO2 nanosheets. This coupling reaction occurs through the formation of C-N bonds via the thermodynamically spontaneous reaction between *N=N* and CO. Products were identified and quantified using isotope labelling and the mechanism investigated using isotope-labelled operando synchrotron-radiation Fourier transform infrared spectroscopy. A high rate of urea formation of 3.36 mmol g-1 h-1 and corresponding Faradic efficiency of 8.92% were measured at-0.4 V versus reversible hydrogen electrode.

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure–activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO–Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO–Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co–OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in‐depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal‐ion batteries, lithium–sulfur batteries, and metal–air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.

An earth-abundant and highly efficient electrocatalyst is essential for oxygen evolution reaction (OER) due to its poor kinetics. Layered double hydroxide (LDH)-based nanomaterials are considered as promising electrocatalysts for OER. However, the stacking structure of LDHs limits the exposure of the active sites. Therefore, the exfoliation is necessary to expose more active sites. In addition, the defect engineering is proved to be an efficient strategy to enhance the performance of OER electrocatalysts. For the first time, this study prepares ultrathin CoFe LDHs nanosheets with multivacancies as OER electrocatalysts by water-plasma-enabled exfoliation. The water plasma can destroy the electrostatic interactions between the host metal layers and the interlayer cations, resulting in the fast exfoliation. On the other hand, the etching effect of plasma can simultaneously and effectively produce multivacancies in the as-exfoliated ultrathin LDHs nanosheets. The increased active sites and the multivacancies significantly contribute to the enhanced electrocatalytic activity for OER. Compared to pristine CoFe LDHs, the as-exfoliated ultrathin CoFe LDHs nanosheets exhibit excellent catalytic activity for OER with a ultralow overpotential of only 232 mV at 10 mA cm and possesses outstanding kinetics (the Tafel slope of 36 mV dec ). This work provides a novel strategy to exfoliate LDHs and to produce multivacancies simultaneously as highly efficient electrocatalysts for OER.

Layered double hydroxide (LDH)‐based materials have attracted widespread attention in various applications due to their unique layered structure with high specific surface area and unique electron distribution, resulting in a good electrocatalytic performance. Moreover, the existence of multiple metal cations invests a flexible tunability in the host layers; the unique intercalation characteristics lead to flexible ion exchange and exfoliation. Thus, their electrocatalytic performance can be tuned by regulating the morphology, composition, intercalation ion, and exfoliation. However, the poor conductivity limits their electrocatalytic performance, which therefore has motivated researchers to combine them with conductive materials to improve their electrocatalytic performance. Another factor hampering their electrocatalytic activity is their large lateral size and the bulk thickness of LDHs. Introducing defects and tuning electronic structure in LDH‐based materials are considered to be effective strategies to increase the number of active sites and enhance their intrinsic activity. Given the unique advantages of LDH‐based materials, their derivatives have been also used as advanced electrocatalysts for water splitting. Here, recent progress on LDHs and their derivatives as advanced electrocatalysts for water splitting is summarized, current strategies for their designing are proposed, and significant challenges and perspectives of LDHs are discussed.

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Carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) in alkaline medium have been extensively investigated with the aim of replacing the commercially available, but precious platinum-based catalysts. For the proper design of carbon-based metal-free electrocatalysts for the ORR, it would be interesting to identify the active sites of the electrocatalyst. The ORR was now studied with an air-saturated electrolyte solution droplet (diameter ca. 15 μm), which was deposited at a specified position either on the edge or on the basal plane of highly oriented pyrolytic graphite. Electrochemical measurements suggest that the edge carbon atoms are more active than the basal-plane ones for the ORR. This provides a direct way to identify the active sites of carbon materials for the ORR. Ball-milled graphite and carbon nanotubes with more exposed edges were also prepared and showed significantly enhanced ORR activity. DFT calculations elucidated the mechanism by which the charged edge carbon atoms result in the higher ORR activity.

This paper reports a hydrothermal preparation of NiO-graphene sheet-on-sheet and nanoparticle-on-sheet nanostructures. The sheet-on-sheet nanocomposite showed highly reversible large capacities at a common current of 0.1 C and good rate capabilities. A large initial charge capacity of 1056 mAh/g was observed for the sheet-on-sheet composite at 0.1 C, which decreased by only 2.4% to 1031 mAh/g after 40 cycles of discharge and charge. This cycling performance is better than that of NiO nanosheets, graphene nanosheets, NiO-graphene nanoparticle-on-sheet, and previous carbon/carbon nanotube supported NiO composites. It is believed that the mechanical stability and electrical conductivity of NiO nanosheets are increased by graphene nanosheets (GNS), the aggregation or restacking of which to graphite platelets are, on the other hand, effectively prevented by NiO nanosheets.