Supercapacitors are widely used due to their fast charging and discharging processes, high power density (i.e., kW·kg^-1), fast charging and discharging (i.e., minutes or even seconds), and long cycle term (i.e., up to 100 000 cycles theoretically). The electrode materials determine the electrochemical energy storage performance of the supercapacitor. It is thus important for the development of high-performance electrode materials to improve the performances of the supercapacitor. Many electrode materials are explored for supercapacitors, such as transition metal oxides, transition metal hydroxides and transition metal sulfide. The electrode materials show the promising practical applications. To further optimize the electrochemical performances of the electrode materials, the supercapacitors with the composites are developed as electrode materials. Among the composites, transition metal sulfide-based electrode materials have attracted much attention because of their outstanding performances for energy performances of supercapacitors. In this paper, Ni₃Se₂/Ni₃S₂ nanocomposites were synthesized on nickel foam by a solvothermal and selenization method, and their structures and electrochemical energy storage performances were systematically investigated.

4.4 g Na₂S·9H₂O was dissolved in 40 mL methanol under stirring vigorously for 30 min. The solution was moved to 2 Teflon-lined stainless steel autoclaves, equally, and two cleaned Ni foams were immersed in the solution, respectively, then the autoclaves were kept in an oven 140 ℃ for 16 h. After the reaction, Ni₃S₂ nanorods on Ni foam (Ni₃S₂/Ni) were obtained.

0.01　g Se powder was dissolved in 6 mL hydrazine hydrate (resolution A). 9 mL DI-water and 15 mL ethanol were added into resolution A, obtaining resolution B. Resolution B was transfered to 2 autoclaves. Two Ni₃S₂/Ni samples were put into two autoclaves with resolution B. The autoclaves were kept in an oven at 140 ℃ for 10 h. The samples were prepared at different growth temperatures and time.

The crystal structure and morphology of the samples were determined by X-ray diffraction (XRD, Bruker-D8, λ=0.154 05 nm), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The chemical elements and valence bond states were analyzed by X-ray photoelectron spectroscopy (XPS, Al Kα) and X-ray energy spectrometry (EDS). The electrochemical properties were examined on an electrochemcial system with Ni₃Se₂/Ni₃S₂ nanocomposites as working electrodes at room temperature.

The SEM images of Ni₃Se₂/Ni₃S₂ nanocomposites show that Ni₃Se₂ nanoparticles cover Ni₃S₂ nanorods. Besides, some Ni₃Se₂ clusters are on the top of Ni₃S₂ nanorods. Moreover, the HRTEM images of Ni₃Se₂/Ni₃S₂ nanocomposites reveal the interfaces between Ni₃Se₂ and Ni₃S₂.

Under the optimized growth conditions, the specific capacitance of Ni₃Se₂/Ni₃S₂ nanocomposites can reach 2 482.00 mF·cm^-2 (@1 mA·cm^-2), and the specific capacitance retention rate is still 68.45% after 2 000 charge-discharge cycles at 10 mA·cm^-2, and the coulombic efficiency is 97.20%-99.28%. The assembled liquid (6 mol/L KOH) Ni₃Se₂/Ni₃S₂//AC asymmetric supercapacitor demonstrates a specific capacitance of up to 768.75 mF·cm^-2 (@2.5 mA·cm^-2). The assembled Ni₃Se₂/Ni₃S₂//AC all solid-state supercapacitor (ASS) has a specific capacitance of 983.57 mF·cm^-2 (@7.5 mA·cm^-2). The outstanding electrochemical performances of Ni₃Se₂/Ni₃S₂ nanocomposites can be since: 1) Ni₃Se₂/Ni₃S₂ nanocomposites are fabricated on Ni foam, directly, reducing the contact resistance due to the good conductivity of Ni foam; 2) Ni₃Se₂/Ni₃S₂ nanocomposites on Ni foam have a three-dimensional structure, exhibiting the large specific surface area for the electrochemical reactions; and 3) the synergies of Ni₃Se₂ and Ni₃S₂ can improve the electrochemical performances. In addition, the integrated circuits with Ni₃Se₂/Ni₃S₂//AC solid-state capacitor are also analyzed. The series Ni₃Se₂/Ni₃S₂//AC ASSs demonstrate a typical increase in the operating voltage window, and the parallel ones show the improvement of the specific capacitance, indicating the promising practical applications for energy storage of Ni₃Se₂/Ni₃S₂ nanocomposites.

Ni₃Se₂/Ni₃S₂ nanocomposites were prepared as electrode materials for supercapacitors and their the electrochemical performances were investigated. As the electrode material, Ni₃Se₂/Ni₃S₂ nanocomposites showed the outstanding properties for energy storage. At 1 mA·cm^-2, the constructed supercapacitor in 6 mol/L KOH solution exhibited the specific capacitance of 2 482 mF·cm^-2. the ASS devices were also constructed with Ni₃Se₂/Ni₃S₂ nanocomposites, showing good electrochemical performances for energy storage. Furthermore, the ASS devices with Ni₃Se₂/Ni₃S₂ nanocomposites as electrode material showed the typical integrated features in series and parallel circuits. Singe ASS device demonstrated 0-1.4 V working voltage window, and two series devices could work well in 0-2.8 V, while the specific capacitance presented the obvious improvement for two parallel ASS devices. Therefore, the study could demonstrate a potential application of Ni₃Se₂/Ni₃S₂ nanocomposites, and provide an approach to design the new materials for energy storage.