Junlei Qi

In situ encapsulated Fe3O4 nanosheet arrays with graphene layers as an anode for high-performance asymmetric supercapacitors

Energy density of asymmetric supercapacitors (ASCs) is greatly limited by the electrochemical performance, especially low specific capacitance and poor cycling stability, of anode materials. To achieve high-performance ASCs, herein, we designed and synthesized a new anode material of Fe3O4 nanosheet arrays, which were encapsulated in situ by graphene layers (G@Fe3O4) through plasma enhanced chemical vapor deposition. Vertical-standing G@Fe3O4 nanosheet arrays directly on the conductive substrates can facilitate electrolyte diffusion and reduce the internal resistance. Furthermore, the highly conductive graphene layers in situ encapsulating the Fe3O4 nanosheets not only could provide fast ion and electron transport pathways, but could also maintain a stable structure for G@Fe3O4. When used as electrodes, G@Fe3O4 exhibited highest capacitance (up to 732 F g−1), better rate capability, and cycling stability as compared to pristine Fe3O4. Furthermore, an asymmetric supercapacitor device synthesized using G@Fe3O4 as an anode and CuCo2O4 as the cathode showed a high energy density of up to 82.8 W h kg−1 at a power density of 2047 W kg−1 and good cycling stability (88.3% capacitance after 10u2006000 cycles).

Welding and Joining of Titanium Aluminides

Welding and joining of titanium aluminides is the key to making them more attractive in industrial fields. The purpose of this review is to provide a comprehensive overview of recent progress in welding and joining of titanium aluminides, as well as to introduce current research and application. The possible methods available for titanium aluminides involve brazing, diffusion bonding, fusion welding, friction welding and reactive joining. Of the numerous methods, solid-state diffusion bonding and vacuum brazing have been most heavily investigated for producing reliable joints. The current state of understanding and development of every welding and joining method for titanium aluminides is addressed respectively. The focus is on the fundamental understanding of microstructure characteristics and processing–microstructure–property relationships in the welding and joining of titanium aluminides to themselves and to other materials.

In Situ Synthesis of Vertical Standing Nanosized NiO Encapsulated in Graphene as Electrodes for High‐Performance Supercapacitors

Abstract NiO is a promising electrode material for supercapacitors. Herein, the novel vertically standing nanosized NiO encapsulated in graphene layers (G@NiO) are rationally designed and synthesized as nanosheet arrays. This unique vertical standing structure of G@NiO nanosheet arrays can enlarge the accessible surface area with electrolytes, and has the benefits of short ion diffusion path and good charge transport. Further, an interconnected graphene conductive network acts as binder to encapsulate the nanosized NiO particles as core–shell structure, which can promote the charge transport and maintain the structural stability. Consequently, the optimized G@NiO hybrid electrodes exhibit a remarkably enhanced specific capacity up to 1073 C g−1 and excellent cycling stability. This study provides a facial strategy to design and construct high‐performance metal oxides for energy storage.

Hierarchical CuCo2O4@NiMoO4 core–shell hybrid arrays as a battery-like electrode for supercapacitors

Herein, hierarchical CuCo2O4@NiMoO4 core–shell nanowire arrays were successfully synthesized on Ni foam via hydrothermal processes as a battery-like electrode. Owing to the unique core–shell structure and the synergetic effect from CuCo2O4 and NiMoO4, the resulting CuCo2O4@NiMoO4 core–shell arrays exhibit a high specific capacitance of 2207 F g−1 at 1.25 A g−1 and good rate capability with only about 4.4% capacitance loss after cycling tests. Furthermore, we assemble the corresponding asymmetric supercapacitor using CuCo2O4@NiMoO4 (positive electrode) and activated carbon (negative electrode), which delivers a high energy density (about 40 W h kg−1) and good cycle stability. Thereby, these electrochemical performances demonstrated that as-fabricated CuCo2O4@NiMoO4 core–shell arrays are promising candidates as electrodes for high-performance supercapacitors.

Modifying the electrochemical performance of vertically-oriented few-layered graphene through rotary plasma processing

Vertically-oriented few-layered graphene (VFG) has a unique three-dimensional morphology and exposed ultrathin edges, and shows great promise for high-performance supercapacitor applications. However, VFG shows limited capacitance owing to poor wettability with electrolytes, which has been a bottleneck for further applications. Herein, we designed and developed an effective strategy, based on rotary plasma etching, to create defects on the side surfaces while simultaneously maintaining the structural integrity of VFG. Rotary plasma etching decreased the contact angle (CA) of VFG from 123° to 34°, compared with conventional vertical etching, which only reduced the CA to 71°. Electrochemical studies demonstrated that VFG samples with a high density of surface defects, introduced by rotary plasma etching, exhibited a high areal capacitance of 1367 μF cm−2 (a volumetric specific capacitance of 137 F cm−3), which was approximately 4 times as large as for pristine VFG-based materials. Our study offers feasible insight into the use of an industrially viable method, rotary plasma processing, for modifying and enhancing the properties of VFG. Our findings may help to accelerate the development of more effective energy storage devices.

Heterostructural Graphene Quantum Dot/MnO2 Nanosheets toward High‐Potential Window Electrodes for High‐Performance Supercapacitors

Abstract The potential window of aqueous supercapacitors is limited by the theoretical value (≈1.23 V) and is usually lower than ≈1 V, which hinders further improvements for energy density. Here, a simple and scalable method is developed to fabricate unique graphene quantum dot (GQD)/MnO2 heterostructural electrodes to extend the potential window to 0–1.3 V for high‐performance aqueous supercapacitor. The GQD/MnO2 heterostructural electrode is fabricated by GQDs in situ formed on the surface of MnO2 nanosheet arrays with good interface bonding by the formation of Mn—O—C bonds. Further, it is interesting to find that the potential window can be extended to 1.3 V by a potential drop in the built‐in electric field of the GQD/MnO2 heterostructural region. Additionally, the specific capacitance up to 1170 F g−1 at a scan rate of 5 mV s−1 (1094 F g−1 at 0–1 V) and cycle performance (92.7%@10 000 cycles) between 0 and 1.3 V are observed. A 2.3 V aqueous GQD/MnO2‐3//nitrogen‐doped graphene ASC is assembled, which exhibits the high energy density of 118 Wh kg−1 at the power density of 923 W kg−1. This work opens new opportunities for developing high‐voltage aqueous supercapacitors using in situ formed heterostructures to further increase energy density.

Rational construction of core–shell Ni3S2@Ni(OH)2 nanostructures as battery-like electrodes for supercapacitors

Rationally constructing hybrid nanostructure electrodes is a promising approach for the development of high-performance supercapacitors. Here, we propose the fabrication of Ni3S2@Ni(OH)2 nanostructures directly on Ni foam by employing hydrothermal and chemical bath processes. In this core–shell nanostructure, the design of conductive Ni3S2 nanorods directly on Ni foam without organic binders could ensure intimate contact and fast electron transport. Meanwhile, porous Ni(OH)2 nanosheets coated on conductive Ni3S2 nanorods can effectively supply large surface areas with electrolyte and provide fast ion diffusion. Consequently, the as-fabricated Ni3S2@Ni(OH)2 nanostructures showed good electrochemical performance, such as high capacitance up to 3.55 F cm−2 and a good capacity retained after 10u2006000 cycles of about 85%. Furthermore, an asymmetric device, based on Ni3S2@Ni(OH)2 and activated carbon, was also fabricated and achieved a maximum energy density of 60.5 W h kg−1 at 1159 W kg−1. These results suggest that the as-designed core–shell Ni3S2@Ni(OH)2 nanostructures demonstrated in this research are promising electrode materials for energy storage.

Effect of vertically oriented few-layer graphene on the wettability and interfacial reactions of the AgCuTi-SiO 2f /SiO 2 system

With the aim of expanding their applications, particularly when joining metals, a simple but effective method is reported whereby the surface chemical reactivity of SiO2f/SiO2 (SiO2f/SiO2 stands for silica fibre reinforced silica based composite materials and f is short for fibre) composites with vertically oriented few-layer graphene (VFG, 3–10 atomic layers of graphene vertically oriented to the substrate) can be tailored. VFG was uniformly grown on the surface of a SiO2f/SiO2 composite by using plasma enhanced chemical vapour deposition (PECVD). The wetting experiments were conducted by placing small pieces of AgCuTi alloy foil on SiO2f/SiO2 composites with and without VFG decoration. It was demonstrated that the contact angle dropped from 120° (without VFG decoration) to 50° (with VFG decoration) when the holding time was 10u2009min. The interfacial reaction layer in SiO2f/SiO2 composites with VFG decoration became continuous without any unfilled gaps compared with the composites without VFG decoration. High-resolution transmission electron microscopy (HRTEM) was employed to investigate the interaction between VFG and Ti from the AgCuTi alloy. The results showed that VFG possessed high chemical reactivity and could easily react with Ti even at room temperature. Finally, a mechanism of how VFG promoted the wetting of the SiO2f/SiO2 composite by the AgCuTi alloy is proposed and thoroughly discussed.

Hierarchical NiCo-LDH/NiCoP@NiMn-LDH hybrid electrodes on carbon cloth for excellent supercapacitors

To realize high-performance and long life span supercapacitors, highly electrochemically active materials and rational design of structure are highly desirable. Herein, a hierarchical NiCo-LDH/NiCoP@NiMn-LDH hybrid electrode (NCLP@NiMn-LDH) was synthesized on carbon cloth via a hydrothermal reaction and phosphorization treatment. Owing to the introduction of NiCoP and design of the core–shell structure, the hybrid electrode showed significantly improved electrochemical performance. The as-fabricated hybrid electrode exhibited a high specific capacitance of 2318 F g−1 at 1 A g−1, with a superior cyclic stability. Additionally, an asymmetric supercapacitor (NCLP@NiMn-LDH//AC) was assembled with a voltage window of 1.5 V. The ASC device delivered a maximum energy density of 42.2 W h kg−1 at a power density of 750 W kg−1.

Core-branched CoSe2/Ni0.85Se nanotube arrays on Ni foam with remarkable electrochemical performance for hybrid supercapacitors

Nanotube arrays have shown great potential in a variety of important applications, such as hybrid supercapacitors. In order to endow nanotubes with multifunctionality, rationally designed and fabricated nanotube arrays with controllable structures are highly desired, but still remain a great challenge. Herein, we report a facile synthesis strategy for core-branched CoSe2/Ni0.85Se nanotube arrays directly on Ni foam by simply selenizing Co-precursor nanowires for hybrid supercapacitors. This structure design could not only ensure the intimate contact of self-branched heterostructures, but also offer sufficient active sites for electrochemical reactions. Further, we propose an electrochemical activation strategy to fully boost the electrochemical performance of CoSe2/Ni0.85Se nanotube arrays. After electrochemical activation, CoSe2/Ni0.85Se nanotube arrays have been changed into porous CoOOH/NiOOH, accounting for the remarkable performance. Contributed to by short ion diffusion paths, large electroactive sites and low contact resistance, the CoSe2/Ni0.85Se electrode after electrochemical activation shows remarkable performance for hybrid supercapacitors.