Ammonia (NH3) is widely used in a wide range of fields because of its high energy density, and NH3 is simple to liquefy and transport. Nitrate is also a source of pollution of the environment and drinking water sources. Therefore, there is a pressing demand for the design and production of high-efficiency catalysts for the nitrate reduction reaction (NO3RR). Herein, two nickel-added polyoxometalates (NiAPs), namely, [Ni(en)2][Ni6(μ3-OH)3(en)3(H2O)6(B-α-SiW9O34)]2·6H2O (Ni6en) and [Ni(enMe)2(H2O)2][Ni6(μ3-OH)3(H2O)6(enMe)3(B-α-SiW9O34)]2·8H2O (Ni6enMe) (en = ethylenediamine, enMe = 1,2-diaminopropane), were effectively synthesized under hydrothermal conditions that contained several electrons and were used as electrocatalytic nitrate reduction reaction (e-NO3RR) catalysts. The structures of the compounds were characterized by using various instruments such as powder X-ray diffraction (PXRD) spectroscopy, infrared (IR) spectroscopy, thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) method, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). e-NO3RR tests were performed using electrochemical workstation. Results show that Ni6en and Ni6enMe have high-efficient electrochemical catalytic nitrogen reduction to NH3. The highest NH3 yield rate for Ni6en was 3.66 mg∙h−1∙mgcat.−1 with Faradaic efficiency (FE) of 89.32%, whereas that for Ni6enMe was 3.46 mg∙h−1∙mgcat.−1 with FE of 86.75% at a low voltage (−0.5 V vs. reversible hydrogen electrode (RHE)). This finding creates a novel path for manufacturing highly effective NO3RR electrocatalysts using metal-added polyoxometalate as the catalyst in ambient settings. Furthermore, the findings of this research provide practical advice for creating effective electrocatalytic catalysts.
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Single-atom catalysts with precise structure and tunable coordination nature provide opportunities for developing novel catalytic centers and understanding reaction mechanisms. Herein, hollow Co9S8 polyhedrons with lattice-confined Ru single atoms (Ru-Co9S8) are fabricated. Aberration-corrected scanning transmission electron microscopy and X-ray absorption spectroscopy verify the isolated Ru atoms are confined in Co9S8 to form Co-S-Ru catalytic centers. Theoretical calculations indicate that the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) energy barriers are extensively reduced, and the d-band center of Co9S8 downshifts from the Fermi level, therefore boosting the desorption of O-containing intermediates. Consequently, the Ru-Co9S8 exhibits an ultralow overpotential of 163 mV at 10 mA·cm−2 for OER and could catalyze a rechargeable Zn-air battery with a high-power density of 92.0 mW·cm−2. This work provides a promising approach for designing novel bifunctional catalytic active centers for energy conversion.
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The construction of heterojunctions in composite materials to optimize the electronic structures and active sites of energy materials is considered to be the promising strategy for the fabrication of high-performance electrochemical energy devices. In this paper, a one-step, easy processing and cost-effective technique for generating composite materials with heterojunctions was successfully developed. The composite containing Ni3S4, NiS, and N-doped amorphous carbon (abbreviated as Ni3S4/NiS/NC) with multiple heterojunction nanosheets are synthesized via the space-confined effect of molten salt interface of recrystallized NaCl. Several lattice matching forms of Ni3S4 with cubic structure and NiS with hexagonal structure are confirmed by the detailed characterization of heterogeneous interfaces. The C–S bonds are the key factor in realizing the chemical coupling between nickel sulfide and NC and constructing the stable heterojunction. Density functional theory calculations further revealed that the electronic interaction on the heterogeneous interface of Ni3S4/NiS can contribute to high electronic conductivity. The heterogeneous interfaces are identified to be the good electroactive region with excellent electrochemical performance. The synergistic effect of abundant active sites, the enhanced kinetic process and valid interface charge transfer channels of Ni3S4/NiS/NC multiple heterojunction can guarantee high reversible redox activity and high structural stability, resulting in both high specific capacitance and energy/power densities when it is used as the electrode for supercapacitors. This work offers a new avenue for the rational design of the heterojunction materials with improved electrochemical performance through space-confined effect of NaCl.
Low-cost and easily obtainable electrode materials are crucial for the application of supercapacitors. Nickel hydroxides have recently attracted intensive attention owning to their high theoretical specific capacitance, high redox activity, low cost, and eco-friendliness. In this study, novel three-dimensional (3D) interspersed flower-like nickel hydroxide was assembled under mild conditions. When ammonia was used as the precipitant and inhibitor and CTAB was used as an exfoliation agent, the obtained exfoliated ultrathin Ni(OH)2 nanosheets were assembled into 3D interspersed flower-like nickel hydroxide. In this novel 3D structure, the ultrathin Ni(OH)2 nanosheets not only provided a large contact area with the electrolyte, reducing the polarization of the electrochemical reaction and providing more active sites, but also reduced the concentration polarization in the electrode solution interface. Consequently, the utilization efficiency of the active material was improved, yielding a high capacitance. The electrochemical performance was improved via promoting the electrical conductivity by mixing the as-synthesized Ni(OH)2 with carbon tubes (N-4-CNT electrode), yielding excellent specific capacitances of 2, 225.1 F·g–1 at 0.5 A·g–1 in a three-electrode system and 722.0 F·g–1 at 0.2 A·g–1 in a two-electrode system. The N-4-CNT//active carbon (AC) device exhibited long-term cycling performance (capacitance-retention ratio of 111.4% after 10, 000 cycles at 5 A·g–1) and a high specific capacitance of 180.5 F·g–1 with a high energy density of 33.5 W·h·kg–1 and a power density of 2, 251.6 W·kg–1.
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