(Digital Presentation) Mo Doped Nickel Sulfide with Enhanced Electrochemical Activity for Hybrid Supercapacitors

Abstract

Global energy depletion has become an irreversible fact. Researchers have vigorously pursued the development of renewable energy sources. It is well known that once renewable energy is produced, energy storage devices are needed to store the energy. Supercapacitor (SC) is considered to be a very promising energy storage device because of its long cycle life, high performance, and cost-effectiveness. As emerging SC devices, a battery-type electrode and a double-layer capacitor-type electrode as the anode and the cathode assembles a hybrid supercapacitor (HSC) well [1, 2]. Using anode materials to provide high capacitance, the purpose of the high energy density of HSC devices is achieved [3]. Nickel sulfide (Ni3S2) has tremendous potential for energy storage and conversion due to its impressive theoretical capacitance and high redox rate. Previous studies have demonstrated that optimizing the electronic structure can enhance the electrical conductivity of transition metal sulfides [4]. Aggrandizing surface defects and introducing impurities are effective strategies to modulate the electronic structure. Interestingly, the generation of sulfur vacancies on metal sulfides can modulate their electronic structure and elevate electrical conductivity. On the other hand, introducing foreign metals into Ni3S2 materials is another promising strategy to modulate the electronic structure [5]. The doping of molybdenum ions can optimize the electronic structure configuration. Mo doping works in concert with the vacancies to achieve a dual regulation of the electronic structure. However, few reports have been devoted to modulating the electronic structure by combining the two effective strategies mentioned above. To our knowledge, studies on the optimal amount of Mo doping to achieve the best electrochemical properties in the synthesis of Ni3S2 have not been adequately considered so far [6].[Figure insert] Figure. 1 (a) Crystal structures of NS and MNS, (b) ESR spectra of 0.75-MNS, (c) Comparative plots of the split peaks of the Ni 2p curves of 0.5-MNS, 0.75-MNS and 1.0-MNS, (d) High resolution XPS spectra of Mo 3d, (e) CV curves at 2 mV s−1, (f) GCD curves at 1 A g− 1, and (g) Ragone plot.Here, we propose that prepared Mo-doped Ni3S2 (denoted as MNS) by a one-pot hydrothermal method, which achieves the simultaneous two strategies of introducing impurities and increasing surface defects and attains a double optimization of the electronic structure of Ni3S2 materials (Fig. 1). To further shorten the charge transfer distance, the MNS is grown on nickel foam while avoiding the need for large amounts of conductive additives and adhesives. The prepared MNS microscopic features exhibit coral-like nanoclusters with a specific capacitance of 1531.2 C g−1 at 1 A g−1 when the Mo salt is added at 0.75 mmol. Furthermore, an HSC device was fabricated by utilizing the activated carbon (AC) electrode and MNS electrode, showing satisfying energy density (32.85 Wh kg−1 at 800 W kg−1). This work demonstrates the potential of MNS electrodes as anodes for HSC.