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Single-atom catalysts (SACs) have been rapidly rising as emerging materials in the field of energy conversion, especially for CO2 reduction reaction. However, the selectivity and running current are still beyond practical applications. Herein, we report new unsaturated SACs with CO2 to CO selectivity of nearly 100% at 51 mA cm−2, and 91% at 100 mA cm−2. Such unsaturated SACs (M-Nx, M = Ni, Fe and Co, x < 4) are rationally prepared through a novel CO2-to-carbon process in large quantity and confirmed by X-ray absorption spectroscopy. As electrocatalysts for CO2 reduction, unsaturated Ni–N2 centered SACs exhibit outstanding activity for CO2 reduction, outperforming state-of-the-art unsaturated SACs. Operando X-ray absorption spectroscopy and theoretical calculation reveal that such unsaturated Ni sites with rich vacancies are favorable for production of more unpaired 3d electrons, and consequently reduce the free energy for COOH* formation, therefore boosting CO2 reduction performance. Not only does this work provide a new method towards unsaturated SACs in large quantity, but also contributes fundamental understanding of the unsaturated single-atom sites in electrochemical catalysis.

Heterostructures exhibit considerable potential in the field of energy conversion due to their excellent interfacial charge states in tuning the electronic properties of different components to promote catalytic activity. However, the rational preparation of heterostructures with highly active heterosurfaces remains a challenge because of the difficulty in component tuning, morphology control, and active site determination. Herein, a novel heterostructure based on a combination of RuMo nanoalloys and hexagonal N-doped carbon nanosheets is designed and synthesized. In this protocol, metal-containing anions and layered double hydroxides are employed to control the components and morphology of heterostructures, respectively. Accordingly, the as-made RuMo-nanoalloys-embedded hexagonal porous carbon nanosheets are promising for the hydrogen evolution reaction (HER), resulting in an extremely small overpotential (18 mV), an ultralow Tafel slope (25 mV dec-1 ), and a high turnover frequency (3.57 H2 s-1 ) in alkaline media, outperforming current Ru-based electrocatalysts. First-principle calculations based on typical 2D N-doped carbon/RuMo nanoalloys heterostructures demonstrate that introducing N and Mo atoms into C and Ru lattices, respectively, triggers electron accumulation/depletion regions at the heterosurface and consequently reduces the energy barrier for the HER. This work presents a convenient method for rational fabrication of carbon-metal heterostructures for highly efficient electrocatalysis.

Single-atom catalysts (SACs) have rapidly entered the field of nanomaterials and demonstrated great potential for energy devices in recent years. Of all types of SACs, porous carbon-based SACs are the most popular species because of their excellent conductivity, large specific surface area, and easily tunable heteroatom and metal components. However, most of the reported cases focus on the metal centers and their coordination environments, while they do not pay much attention to carbon precursors and carbon transformation during high-temperature treatment. In this work, we use a high-entropy aromatic molecule, azulene, for rational synthesis of azulene-enriched, sandwich-like polymer nanosheets and corresponding single-Fe-dispersed porous carbon nanosheets. The azulene-based metal-free polymer nanosheets exhibit a narrow band gap and temperature-dependent magnetism. As proof-of-concept electrocatalysts for CO2 reduction, the prepared carbon nanosheets exhibit high activity and stability. Operando X-ray absorption spectroscopy and density functional theory studies reveal the high activity of Fe-N coordination sites in the presence of 5/7-membered carbon ring-based topological defects in the carbon skeleton. Taken together, this work provides a new method of synthesizing high-entropy carbons using azulene-based high-entropy molecule as precursor and paves the way toward high-efficiency SACs with rich topological defects for energy conversion.