Yinyan Gong

In situ growth of NiCo2S4@Ni3V2O8 on Ni foam as a binder-free electrode for asymmetric supercapacitors

A hierarchical NiCo2S4@Ni3V2O8 core/shell hybrid was in situ grown on nickel foam through a facile hydrothermal process combined with a simple co-precipitation method. Further characterization shows that the hybrid consists of highly conductive NiCo2S4 as the core and uniformly distributed Ni3V2O8 nanoparticles as the shell. When used as the binder-free electrode, the hybrid material takes advantage of both components and exhibits excellent electrochemical activity, demonstrating a higher specific capacity of 512 C g−1 at a current density of 1 A g−1 and a better rate capability of 396 C g−1 at 10 A g−1. The enhanced pseudocapacitive performance of the NiCo2S4@Ni3V2O8 hybrid is mainly attributed to its unique core/shell structure, which provides fast ion and electron transport. Finally, a NiCo2S4@Ni3V2O8//activated carbon asymmetric supercapacitor is successfully assembled and it can deliver a maximum energy density of 42.7 W h kg−1 at a power density of 200 W kg−1, making it a promising candidate for superior electrodes for supercapacitor applications.

Hydrothermal synthesis of CuCo2O4/CuO nanowire arrays and RGO/Fe2O3 composites for high-performance aqueous asymmetric supercapacitors

The applications of traditional asymmetric supercapacitors are restricted due to the low specific capacitance of carbon negative materials. The rational design of positive and negative electrodes that afford high-performance asymmetric devices is particularly important. In this paper, we fabricate a cost-effective, environmental-friendly aqueous asymmetric supercapacitor by using CuCo2O4/CuO nanowire arrays as the positive electrode and RGO/Fe2O3 composites as the negative electrode. The assembled device exhibits a high energy density of 33.0 W h kg−1 at a power density of 200 W kg−1, and it still operates at a high power density of 8.0 kW kg−1 with an energy density of 9.1 W h kg−1. The current strategy will provide a fresh route for the design and fabrication of novel asymmetric supercapacitors with high energy density and high power density.

Adjustable electronic performances and redox ability of a g-C3N4 monolayer by adsorbing nonmetal solute ions: a first principles study

During the process of hydrogen generation via photocatalytic water splitting, solute ions may be adsorbed on the surface of the graphitic carbon nitride (g-C3N4) monolayer, modifying its electronic and optical performances, as well as its redox ability due to chemical bond relaxation. With the aid of first principles calculations, we investigated the properties of a g-C3N4 monolayer with a series of nonmetal (NM) ions adsorbed on its surface. The obtained results revealed that the adsorbed solute ions can form NM–N or NM–C bonds with the g-C3N4 monolayer and result in bond relaxation, altering the valence band maximum and conduction band minimum synchronously. The small coverage rate of Br, Cl and I ions enhances the redox ability of g-C3N4 synchronously, while the adsorption of the other solute ions enhances the oxidizability and weakens the reducibility. In addition, the adsorption of solute ions can alter the active sites by impacting the distribution of the highest occupied molecular orbital and the lowest unoccupied molecular orbital. Therefore, we can adjust the electronic, optical performances and redox ability of a g-C3N4 monolayer by selecting the suitable type and quantity of solute ions, e.g., the photocatalytic efficiency of g-C3N4 can be enhanced by H ions plus B, N, Si, O, P and As ions with high coverage rates plus halogen ions with low coverage rates while it is suppressed by C, S, Se and Te ions.

The effects of nonmetal dopants on the electronic, optical, and catalytic performances of monolayer WSe2 by a first-principles study

Doping modifies the electronic, optical, and catalytic behavior of materials through the newly formed chemical bonds and the localized electrons. With the aid of first-principles calculations, the electronic, optical, and catalytic performances of the nonmetal (NM = H, B, C, N, O, F, Si, P, S, Cl, As, Br, Te, or I)-doped monolayer WSe2 were investigated. The results showed that the NM dopants substitute preferentially for Se under a W-rich condition and H, F, Cl, Br, and I atoms are willing to locate at the interstitial site. The electron-clouds around the dopants and nearby W or Se atoms were altered by the newly formed W–NM or Se–NM bonds, with the differences determined by the bonding strength between them. The band gap, optical absorption edge, and intensities were altered or shifted by less than 0.08 eV, 32 nm, and 9.5%, respectively. The H, F, P, Cl, As, Br, and I dopants were conductive to separating the photogenerated e−/h+ pairs, whereas the B, C, Si, and Te dopants became recombination centers for the photogenerated e−/h+ pairs. Compared with pristine monolayer WSe2, NM atoms with odd free electrons reduced the reduction potential by 0.39–0.71 eV and enhanced the oxidation potential by 0.45–0.75 eV. Thus, we can adjust the redox potentials of monolayer WSe2 by introducing different kinds of NM impurities for various catalytic reactions, and the H-, F-, P-, Cl-, As-, Br-, and I-doped specimens have excellent photocatalysis capability.

The common effects of surface and internal bonds on the electronic structure of Bi2Te3 nano-films by first-principles calculation

The effects of external stress on Bi2Te3 nano-films have been investigated by first-principles calculation, including stability, electronic structure, crystal structure, and bond order. It is found that the critical thickness of nano-film is sensitive to the stress in Bi2Te3 nano-film while the band gap is near constant. The critical thickness decreases under tensile stress, whereas it increases under compressive stress. The band gap and band order of Bi2Te3 film has been affected collectively by the surface and internal crystal structures, the contraction ratio between surface bond length of nano-film and the corresponding bond length of bulk decides the band order of Bi2Te3 film.

Hydrothermal synthesis of Fe-based negative materials for asymmetric supercapacitors with enhanced performance

The development of hybrid supercapacitors is limited due to the low specific capacity of traditional carbon-negative materials. Herein, we synthesized two different Fe-based negative electrodes (FeOOH and Fe2O3), and the relationships between structures and capacitive properties of electrodes are systematically studied. Results demonstrate that the Fe2O3 material exhibits higher electrochemical activity than their FeOOH counterparts obtained under the same conditions. Then, a high energy density asymmetric supercapacitor is fabricated by using nanostructured NiCo2S4 and Fe2O3–RGO composite as the positive and negative electrodes, respectively. The assembled device with an extended operation voltage of 1.6xa0V achieves a maximum energy density of 53.0xa0Whxa0kg−1 at a power density of 716xa0Wxa0kg−1 and can still operate at a high power density of 11.5xa0KWxa0kg−1 with an energy density of 14.2xa0Whxa0kg−1, thus holding great potentials for future energy storage devices.