Xinhui Xia

Self-supported hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercapacitor capacitance

We report a facile hydrothermal synthesis method for the large-area growth of self-supported hollow Co3O4 nanowire arrays. The Co3O4 nanowires have an average diameter of 200 nm and grow vertically to the substrates forming aligned nanowire arrays. Interestingly, the as-prepared Co3O4 nanowire arrays combine properties of hollow structure and quasi-single crystallinity. A plausible formation mechanism of hollow Co3O4 nanowire arrays is proposed here. The Co3O4 nanowire arrays grown on the nickel foam are tested as a cathode electrode material for supercapacitor by cyclic voltammograms (CVs) and galvanostatic charge–discharge tests in 1 M KOH. The self-supported hollow Co3O4 nanowire arrays exhibit superior supercapacitor performances with high specific capacitances (599 F g−1 at 2 A g−1 and 439 F g−1 at 40 A g−1) as well as excellent cycle life, making them suitable for high-rate supercapacitor application. The enhanced supercapacitor performances are due to its unique porous structure providing fast ion and electron transfer, large reaction surface area and good strain accommodation.

A New Type of Porous Graphite Foams and Their Integrated Composites with Oxide/Polymer Core/Shell Nanowires for Supercapacitors: Structural Design, Fabrication, and Full Supercapacitor Demonstrations

We attempt to meet the general design requirements for high-performance supercapacitor electrodes by combining the strategies of lightweight substrate, porous nanostructure design, and conductivity modification. We fabricate a new type of 3D porous and thin graphite foams (GF) and use as the light and conductive substrates for the growth of metal oxide core/shell nanowire arrays to form integrated electrodes. The nanowire core is Co3O4, and the shell is a composite of conducting polymer (poly(3,4-ethylenedioxythiophene), PEDOT) and metal oxide (MnO2). To show the advantage of this integrated electrode design (viz., GF + Co3O4/PEDOT-MnO2 core/shell nanowire arrays), three other different less-integrated electrodes are also prepared for comparison. Full supercapacitor devices based on the GF + Co3O4/PEDOT-MnO2 as positive electrodes exhibit the best performance compared to other three counterparts due to an optimal design of structure and a synergistic effect.

A V2O5/Conductive‐Polymer Core/Shell Nanobelt Array on Three‐Dimensional Graphite Foam: A High‐Rate, Ultrastable, and Freestanding Cathode for Lithium‐Ion Batteries

A thin polymer shell helps V2O5 a lot. Short V2O5 nanobelts are grown directly on 3D graphite foam as a lithium-ion battery (LIB) cathode material. A further coating of a poly(3,4-ethylenedioxythiophene) (PEDOT) thin shell is the key to the high performance. An excellent high-rate capability and ultrastable cycling up to 1000 cycles are demonstrated.

Synthesis of Free‐Standing Metal Sulfide Nanoarrays via Anion Exchange Reaction and Their Electrochemical Energy Storage Application

Metal sulfides are an emerging class of high-performance electrode materials for solar cells and electrochemical energy storage devices. Here, a facile and powerful method based on anion exchange reactions is reported to achieve metal sulfide nanoarrays through a topotactical transformation from their metal oxide and hydroxide preforms. Demonstrations are made to CoS and NiS nanowires, nanowalls, and core-branch nanotrees on carbon cloth and nickel foam substrates. The sulfide nanoarrays exhibit superior redox reactivity for electrochemical energy storage. The self-supported CoS nanowire arrays are tested as the pseudo-capacitor cathode, which demonstrate enhanced high-rate specific capacities and better cycle life as compared to the powder counterparts. The outstanding electrochemical properties of the sulfide nanoarrays are a consequence of the preservation of the nanoarray architecture and rigid connection with the current collector after the anion exchange reactions.

Freestanding Co3O4 nanowire array for high performance supercapacitors

We report a single-crystalline Co3O4 nanowire array grown on a nickel foam prepared by a hydrothermal synthesis method for supercapacitor application. The Co3O4 nanowires show sharp tips and have an average diameter of 70 nm, and a length up to 25 μm. Impressively, the as-prepared single-crystalline Co3O4 nanowire array exhibits noticeable pseudocapacitive performance with a high capacitance of 754 F g−1 at 2 A g−1 and 610 F g−1 at 40 A g−1 as well as excellent cycling stability. The enhanced supercapacitor performance is due to the unique one-dimensional (1D) architecture, which provides fast diffusion paths for ions and facilitates the electron and ion transfer on the Co3O4/electrolyte interfaces. Moreover, the 1D nanowire array can accommodate the volume expansion and restrain the pulverization and deterioration of Co3O4 during the repeated cycling process, resulting in enhanced cycling stability.

Transition Metal Carbides and Nitrides in Energy Storage and Conversion

High‐performance electrode materials are the key to advances in the areas of energy conversion and storage (e.g., fuel cells and batteries). In this Review, recent progress in the synthesis and electrochemical application of transition metal carbides (TMCs) and nitrides (TMNs) for energy storage and conversion is summarized. Their electrochemical properties in Li‐ion and Na‐ion batteries as well as in supercapacitors, and electrocatalytic reactions (oxygen evolution and reduction reactions, and hydrogen evolution reaction) are discussed in association with their crystal structure/morphology/composition. Advantages and benefits of nanostructuring (e.g., 2D MXenes) are highlighted. Prospects of future research trends in rational design of high‐performance TMCs and TMNs electrodes are provided at the end.

Hierarchically porous NiO film grown by chemical bath deposition via a colloidal crystal template as an electrochemical pseudocapacitor material

Hierarchically porous NiO film has been successfully prepared by chemical bath deposition through monolayer polystyrene sphere template. The film possesses an architecture with a substructure of NiO monolayer hollow-sphere array and a superstructure of porous net-like NiO nanoflakes. The pseudocapacitive behavior of the NiO film is investigated by cyclic voltammograms (CV) and galvanostatic charge-discharge tests in 1 M KOH. The hierarchically porous NiO film exhibits weaker polarization, better cycling performance and higher specific capacitance in comparison with the dense NiO film. The specific capacitance of the porous NiO film is 309 F g−1 at 1 A g−1 and 221 F g−1 at 40 A g−1, respectively, much higher than that of the dense NiO film (121 F g−1 at 1 A g−1 and 99 F g−1 at 40 A g−1). The hierarchically porous architecture is responsible for the enhancement of electrochemical properties.

Array of nanosheets render ultrafast and high-capacity Na-ion storage by tunable pseudocapacitance

Sodium-ion batteries are a potentially low-cost and safe alternative to the prevailing lithium-ion battery technology. However, it is a great challenge to achieve fast charging and high power density for most sodium-ion electrodes because of the sluggish sodiation kinetics. Here we demonstrate a high-capacity and high-rate sodium-ion anode based on ultrathin layered tin(II) sulfide nanostructures, in which a maximized extrinsic pseudocapacitance contribution is identified and verified by kinetics analysis. The graphene foam supported tin(II) sulfide nanoarray anode delivers a high reversible capacity of ∼1,100 mAh g−1 at 30 mA g−1 and ∼420 mAh g−1 at 30 A g−1, which even outperforms its lithium-ion storage performance. The surface-dominated redox reaction rendered by our tailored ultrathin tin(II) sulfide nanostructures may also work in other layered materials for high-performance sodium-ion storage.

Solution synthesis of metal oxides for electrochemical energy storage applications

This article provides an overview of solution-based methods for the controllable synthesis of metal oxides and their applications for electrochemical energy storage. Typical solution synthesis strategies are summarized and the detailed chemical reactions are elaborated for several common nanostructured transition metal oxides and their composites. The merits and demerits of these synthesis methods and some important considerations are discussed in association with their electrochemical performance. We also propose the basic guideline for designing advanced nanostructure electrode materials, and the future research trend in the development of high power and energy density electrochemical energy storage devices.

Graphene sheet/porous NiO hybrid film for supercapacitor applications.

We report the preparation of a nickel-foam-supported graphene sheet/porous NiO hybrid film by the combination of electrophoretic deposition and chemical-bath deposition. The obtained graphene-sheet film of about 19 layers was used as the nanoscale substrate for the formation of a highly porous NiO film made up of interconnected NiO flakes with a thickness of 10-20 nm. The graphene sheet/porous NiO hybrid film exhibits excellent pseudocapacitive behavior with pseudocapacitances of 400 and 324 F g(-1) at 2 and 40 A g(-1), respectively, which is higher than those of the porous NiO film (279 and 188 F g(-1) at 2 and 40 A g(-1)). The enhancement of the pseudocapacitive properties is due to reinforcement of the electrochemical activity of the graphene-sheet film.