It is necessary to explore efficient and environmentally friendly energy storage devices due to the rapid consumption of fossil fuels and increasing environmental pollution. Among various energy storage devices, supercapacitors have some advantages of high power density, short charging and discharging time, good cycle stability, safety and reliability. The related studies focused on transition metal oxides and sulfides. However, their conductivity and cycle stability are rather poor. Compared with transition metal oxides and sulfides, transition metal selenides as an advanced multifunctional material dominate the field of energy storage and conversion by virtue of their high theoretical capacity, good electrical conductivity, and good cycling stability. Also, the coexistence of different metal cations with multiple valence leaps enables multiple redox reactions and enriches structural defects, and the coupling between the bimetals favors the regulation of the electronic structure and improved electrical conductivity, resulting in better electrochemical activity than monometallic selenides. Transition metal selenide electrodes for electrochemical studies are usually prepared with polymer binders by conventional slurry coating methods, which increases the dead volume of the active transition metal selenide surface and decreases the contact with the electrolyte. Therefore, transition metal selenides can be designed and prepared on a conductive substrate with an ideal porous network to obtain better charge storage performance. In this paper, NiMo precursor was firstly synthesized on a nickel foam substrate by a hydrothermal method, and then NiSe/Mo₁₅Se₁₉ electrode materials were prepared via in-situ selenation reaction of this precursor with Se powder. In addition, the effect of NiMo ratio on their electrochemical properties was also investigated.

A 2 cm × 4 cm nickel foam (NF) substrate was firstly pretreated with 2 mol/L HCl, acetone, anhydrous ethanol and deionized water, and then 6 mmol of Ni(NO₃)₂-6H₂O and 3 mmol of Na₂MoO₄-2H₂O were dissolved in 60 mL of deionized water. The dissolved mixture and 1 block of pre-treated NF were added into a PTFE-lined reactor, and the hydrothermal reaction was carried out at 150 ℃ for 5 h. The effect of nickel and molybdenum molar ratio (i.e., 1∶1 and 3∶1) on the electrochemical performance was investigated. Finally, the precursor and 0.13 g of Se powder were placed at the two ends of a quartz boat into a tube furnace in Ar gas as a protective gas at 350 ℃ for 2 h. After natural cooling, NiSe/Mo₁₅Se₁₉ was obtained. The crystal structure of the samples was determined by a model D8AA25 X-ray diffractometer (XRD, Bruker). The microstructure of the samples was observed using a model S-4800 scanning electron microscope (SEM, Hitachi Co., Japan). The morphology, lattice, and lattice spacing of the samples were analyzed by a model F30G2 transmission electron microscope (TEM, Tecnai). The molecular structure, elemental valence states and chemical bonding were analyzed by a model Xi+ X-ray photoelectron spectroscope (XPS, ESCALAB). The electrochemical properties of the samples in the three-electrode system and the two-electrode system were tested by a model CHI660E electrochemical workstation (Chenhua Co., China) under an electrolyte of 6 mol/L KOH. Three-electrode measurements were carried out using Pt as a counter electrode and saturated calomel as a reference electrode. The ASC devices were made with NiSe/Mo₁₅Se₁₉ as a positive electrode and activated carbon (AC) as a negative electrode in the two-electrode system.

The SEM images show that the Ni∶Mo molar ratio has an effect on the morphology of the samples. When the Ni∶Mo molar ratio is 1∶1, the sample morphology is the coexistence of flower-like microspheres composed of nanorods and nanosheets, and the disorganized molecular pore arrangement is not conducive to the flow of electrolyte ions. The diameter of the microspheres composed of nanosheets is approximately 1 μm, the thicknesses of the nanosheets are 30-40 nm, the diameters of the nanorods are 150-200 nm, and the length of the nanorods is approximately 2 μm. When the Ni∶Mo molar ratio is 2∶1, the morphology of the sample is nanorods, and they are grown vertically oriented on the NF substrate, with the lengths of the nanorods of 1.0-1.5 μm, and the diameters of the nanorods of 50-120 nm. At the Ni∶ Mo molar ratio of 3∶1, the morphology of the sample is flower-like microspheres composed of nanosheets, the diameters of the microspheres are 1.5-2.0 μm, and the thicknesses of the nanosheets are 20-30 nm. The electrochemical test shows that the sample has the optimum electrochemical performance when the molar ratio of Ni∶Mo is 2∶1. The specific capacitance of the sample is 940.9 F/g at the current density of 1 A/g, and the specific capacitance of the sample is 940.9 F/g when the current density increases from 1 A/g to 10 A/g. The electrode with n(Ni)∶ n(Mo) of 2∶1 still maintains a specific capacitance of 75.5% when the current density increases from 1 A/g to 10 A/g. This is since the rod-like nanostructure possesses a large specific surface area, thus providing more active sites for the electrochemical reaction and being beneficial to the energy storage device. From the TEM images, NiSe/Mo₁₅Se₁₉ nanorods have a porous structure due to the release of gases during the calcination process. This unique porous structure can provide more transport channels for electrolyte ions and a large number of active sites in the Faraday reaction.

NiSe/Mo₁₅Se₁₉ composites had an optimum electrochemical performance at a Ni∶Mo molar ratio of 2∶1. The sample-specific capacitance was 940.9 F·g^-1 at a current density of 1 A·g^-1. In addition, this electrode material exhibited good multiplication properties and cycling stability. The NiSe/Mo₁₅Se₁₉//AC hybrid supercapacitor (ASC) made from NiSe/Mo₁₅Se₁₉ and activated carbon (AC) had a high energy (i.e., 46.5 Wh·kg^-1) and a power density (i.e., 800 W·kg^-1), indicating that NiSe/Mo₁₅Se₁₉ composites have a promising application potential in energy storage devices.