Vanadium Doped Nickel Sulfide@ Nickel Foam Electrode for Hybrid Supercapacitors

Abstract

With the depletion of energy sources, people are gradually deepening the development of new energy sources. However, energy storage devices are limiting the pace of the development of new energy sources.[1] Considering the potential of supercapacitors as a supplement or alternative to rechargeable batteries for fast energy harvesting and high power transfer has become a research focus recently. Hybrid supercapacitors (HSCs) can bridge the gap between supercapacitors and batteries. It is well known that the main components of HSC devices are anode and cathode. The anode mainly provides high electrochemical performance, while the cathode can supply a wide potential window and good stability.[2] The energy density calculation equation (E = 0.5 CV2 ) is known and deduced that can control the energy density of HSC by the operating voltage window (V) and specific capacitance (C). Obviously, constructing anode materials with high capacitance is one of the effective ways to achieve the high energy density of HSC. In addition, previous experimental results show that reasonable ion doping is beneficial to change the electronic structure of electrode materials and improving the energy storage performance of electrode materials.[3-5] One of the elements with more than one oxidation state can form two (or more) ions of different valence states under the action of a reducing agent, which will be called mixed-valence ions. Boosting the specific capacitance of the electrode, ions with mixed-valence states have higher charge storage capacity and more abundant redox reactions than most other transition metal ions. In this regard, vanadium has various stable oxidation states (+ 2, + 3, + 4, and + 5). In particular, its high oxidation states (+ 4 and + 5) can store charge in the positive potential range, thus providing a favorable pseudo-capacitance.Here, we chose Ni3S2 with high theoretical capacitance and prepared vanadium-doped nickel sulfide (V-Ni3S2, denoted as VNS) anode electrodes using vanadium ions as dopant ions (Figure 1). Using nickel foam as the nickel source, prepared the VNS electrode by a one-step hydrothermal method. Since the electrode is grown in situ on the surface of nickel foam, the electrode material can be employed as an electrode sheet directly after preparation without further fabrication. Figure 1. Schematic diagram of synthesis process of VNS electrode, and Electrochemical storage performance: (a) CV curves of the VNS and NS electrodes at 2 mV s-1 scan rate, (b) GCD curves of the VNS and NS electrodes at 1 A g-1, (c) GCD curves of VNS with different V doping amounts at 1 A g-1, (d) The Ragone plots, (e) Self-discharge of NS//AC device and VNS//AC device for five hours, and (f) Cycle performance of the VNS//AC hybrid supercapacitor with a voltage of 1.6 V at a current density of 2 A g-1. [6]The most direct effect of avoiding the use of binder and thus increasing the conductivity is that the specific capacitance of the prepared VNS electrodes is further enhanced (2072 F g− 1 at 1 A g− 1). In addition, the structure of the surface of the prepared VNS electrode material is nanoflower morphology. Integrating two-dimensional nanosheets into three-dimensional nanoflower morphologies increases the number of active sites while improving the structural stability (capacitance retention of 86.4% after 10,000 cycles, Figure 1a-f). Finally, using the VNS and activated carbon electrodes as anode and cathode to assemble the VNS//AC hybrid supercapacitors delivers an excellent energy density of 81.33 Wh kg− 1 at a power density of 160 W kg− 1. This simple preparation method and significantly enhanced performance of the electrode materials have far-reaching potential for application in HSC devices.