Investigations and improvement of Nickel Sulde modied electrode material from single source precursor for energy storage application

High-performance energy storage electrode materials are emerging demand in near future for the construction of supercapacitor with high energy and power densities. Herein, Nickel (II) Diethyldithiocarbamate was used as single source precursor for Nickel Sulde (Ni 9 S 8 ) two dimensional (2D) nanosheets preparation and hexadecylamine as shape directing agent via simple solvothermal method. The orthorhombic structure of Ni 9 S 8 nanosheets was conrmed by X-ray diffraction (XRD) pattern. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) images revealed that as-prepared Ni 9 S 8 nanoparticles possess sheet-like morphology. Besides, the thermal stability of Ni(DTC) 2 complex was studied by Thermo-gravimetric/Derivative thermo gravimetric(TG/DTG) with Differential scanning calorimetric (DSC) analysis. The electrochemical properties of Ni 9 S 8 nanosheets was studied using galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) techniques. From the charge-discharge study of Ni 9 S 8 nanosheets, a high specic capacitance of 281 Fg − 1 was obtained at a current density of 1 Ag − 1 , and up to 82 % retentivity was achieved after 5000 cycles. Thus, the prepared Ni 9 S 8 nanosheets could be one of the attractive potential active electrode materials for the application of supercapacitor.


Introduction
Recently, the demands for energy storage devices are highly expanding due to its worldwide utility.
Present decades, the research area on energy storage devices development that uses nanoparticles (NPs) in it is interesting due to their size-dependent chemical and physical properties. The transition metal oxide (MO) and metal sul de (MS) nanomaterials are being concentrated broadly due to extraordinary properties of optical, electrical, electronics, thermal, mechanical, and catalytic behavior. These characteristics are attributed due to their unique structures and size. The area of MS nanoparticles has wide applications such as superconductors, conversational solar energy devices, at panel displays, uorescence devices, electroluminescence devices, semiconductor devices, and photo-catalyst, etc., [1][2][3].
At present, the two possible solutions for energy demands are available in the marketplace; they are supercapacitors and hybrid batteries. Supercapacitor gives high energy density, but storage capacitance is low; consequently, the SCs do not be utilized in gadgets that require more storage capacitance. By this way, in the current year supercapacitors have increased a lot of consideration instead of batteries. There are three kinds of supercapacitors such as (i) pseudocapacitors, (ii) EDLC (electrochemical double-layer capacitors), and (iii) hybrid capacitors [19]. Presently, SCs, as the best device for e cient power energy storage, have increased quickly expanding consideration for its powerful energy density, small size, more life cycles. SCs can be utilized in different elds, for example, hybrid storage device cars and other electric vehicles, the backup energy source in convenient electronic gadgets, and versatile hardware. Recently, Ni 9 S 8 has standard capable electrode material in SCs. Also, it has exclusive properties like abundant oxidation and reduction activities, changing the magnetic phase, and more electrical conductivity, ecofriendly in nature, good electrochemical stability. The nickel sul de materials are more excellent redox activity, high capacitance performance, and low cost, which are predictable to ful ll the expanding needs of the storage energy system [20][21][22].
Many researchers broadly concentrated on the utilization of single-source precursor-like metal complexes for the pure and perfect formation of MS with different architecture like 2D and 3D structured nanomaterials on a large scale. Moreover, single-source precursors are more stable, simple reaction conditions, and reducing by-products as well as less expensive. Arrangement of MS NPs has been broadly investigated using single-source precursors of the metal-dithiocarbamate complex [3,7,[23][24][25].
Additionally, the MS materials demonstrated the great speci c capacitance on the grounds, and the explanations de ne numerous redox states along with a variety of the progressive structure as well as pores nature of the material. The compositional MS materials with carbon-based complex give electrode material with high e ciency.
Here, Nickel dithiocarbamate [Ni(DTC) 2 ] complex has been utilized as single-source precursor for effective Ni 9 S 8 2D nano sheets synthesized, by using simple solvothermal technique. The Ni 9 S 8 NSs were analyzed and con rmed with properties like phase-purity, good crystalline structure. Additionally, the prepared nanomaterials were analyzed with their electrochemical nature. Such as, cyclic voltammetry (CV), Chronopotentiometry (GCD) and electrochemical impedance spectroscopic (EIS) investigation was performed to study oxidation, reduction properties, charge-discharge mechanism and conductivity, respectively of the prepared Ni 9 S 8 electrode materials. Here, nickel diethyldithiocarbamate [Ni(DTC) 2 ] complex and HDA were considered in weight ratio 1:1 quantity and dissolved in 100 ml of N, N-Dimethylformamide and the mixture was stirred about 10 minutes. These homogeneously mixed solutions were transferred to a double-necked round bottom ask and it was heated at 120 °C for 3 hrs by using solvothermal method. Subsequently, the PPT colour was observed with change from a bright green to a dark green colour. This colour change indicated the conversion of metal complex into metal sul de and the graphical synthesis scheme were shown in Fig. 1.

Materials And
The product was washed several times using methanol to remove un-reactant and then the sample was dried in the hot oven at 70 °C for 12 hrs.

Instrumentation methods
The synthesized nickel sul de NSs were examined with different analytical methods. The crystalline pattern was studied using Powder X-ray diffraction spectrum (Bruker-D8 advance ECO) instrument with 1.5406 Å (Cu-Kα radiation). The surface morphological study of NSs was performed with Scanning Electron Microscope technique, provided by way of EDS analysis. Fourier transform infrared spectrometer be utilizing the spectrometer (IR Tracer-100) with the constant range from 4000-400 cm −1 utilizing the KBr pellet method for functional group study. The structural morphological study of the prepared material performed with TEM instrument (model Jeol/JEM 2100). TG/DTG with DSC analysis was carried out on a Perkin-Elmer Thermo-gravimetric analysis. The electrochemical analysis of Ni 9 S 8 was investigated by the electrochemical workstation (CHI 6008e, USA).

Fabrication of working electrode
The charge-discharge, redox performance, and impedance investigation of synthesized Ni 9 S 8 nanosheets were deliberated utilizing a three-electrode system at room temperature. A Ni 9 S 8 modi ed electrode material, platinum wire, and Ag/AgCl was used as a working electrode, counter electrode, and reference electrode, respectively. 1 M KOH solution was utilized as the electrolyte medium. The Ni 9 S 8 modi ed electrode material, activated carbon as well as polyvinylidene uoride binder ratio of 80:10:10 were considered and homogeneously mixed. Furthermore a drop of N-methyl-2-pyrrolidone (NMP) was added to the blend to make it as slurry and nally, it was applied as a small lm utilizing Nickel foil of 0.01mm thick with 1cm 2 . It was dried at 60 °C and used for electrochemical analysis.
The speci c capacitance values of the Ni 9 S 8 modi ed working electrode has been calculated from cyclic voltammetry and chronopotentiometry techniques by using the formulae (1) and (2), respectively.

Structural analysis
The powder X-ray diffraction patterns were performed for Ni 9 S 8 NSs as shown in Fig. 2 Nickel sul de nanosheets ts with the crystalline phases referring to the standard database of JCPDS card number 78-1886. All the XRD peaks con rmed the orthorhombic crystalline structure of nanosheets and the typical cell parameters, a=9.335, b=11.21, and c=9.430. The broad peaks indicate that the particle size is nano.

Thermal stability of Ni(DTC) 2 metal complex
Thermal stability and behavior of the prepared metal complex [Ni(DTC) 2 ] were examined by TG(DTA) and DSC were shown in Fig. 4 (a & b). The analysis was performed in temperature ranging from room temperature to 1000 °C in N 2 atmosphere and at the rate of 10 °C/min. Two-step decomposition patterns were noticed in Fig. 4 (a) TGA curve. The rst decomposition temperature range was observed between 163 °C to 205 °C which occurred in reason to the decomposition of unreacted sodium dithiocarbamate and weight loss of 2%. The second prominent weight loss was occurred between ~237 and 371 °C up to 85 % of weight loss was found and it was identi ed due to the single-source precursor [Ni(DTC) 2 ] completely converted into Nickel sul de (MS) and remaining ~15 % as pure nickel sul de residue.
Moreover, no one weight was found after the conversion of nickel sul de and it was con rmed the thermal stability of nickel sul de residue. The heat exchange was detected varying from 237 °C to 371.9°C , recognized by the peak in the DTG curve at 352.4 °C. The corresponding DSC curve of the Ni(DTC) 2 was shown in Fig. 4 (b). The second weight-loss region occurred between 237 °C to 371.9 °C and was accompanied by a signi cant endothermic heat ow, which corresponded to the decomposition of nickel complex to form nickel sul de (Ni 9 S 8 ). Hence, signi cant structural changes occurred at the range of the calcination temperature and it reviled that the thermal behaviour of Ni(DTC) 2 and stability of nickel sul des.

Morphological and elemental analysis
The surface morphological studies for the as-prepared nickel sul de sample were characterized by SEM micrograph as shown in Fig. 5 (a, b) with diverse intensi cations and it was look like two dimensional (2D) sheets morphology. The Ni 9 S 8 nano sheets are agglomerated in micrometer range and it is look feathers like structure. Also, the signi cant sheets like structure may occurred the in uence of hexadecylamine as shape directing agent. The purity and formation of the Ni 9 S 8 NSs were con rmed with EDX spectrum and shown in Fig. 5 (c). The EDX spectrum reviled that, Ni and S elements are equally (1:1 ratio) presence in the Ni 9 S 8 NSs and the non-existence of other elements existence in the prepared sample as shown in Fig. 5c (insert). The standard weight percentage of Ni and S were found at 55.6% and 44.4% correspondingly. Further con rmation, the elemental mappings are examined for the prepared Ni 9 S 8 NSs (Fig. (d)). It is a clear evident that, there is no impurity are additional elements were found the randomly selected area mappings and this result coincides with XRD and EDX spectrum. Furthermore, It is con rmed that, the usage of Ni(DTC) 2 complex as single source precursor for formation of high pure Nickel sul de nanomaterials with large scale.
The structural morphology of the prepared Ni 9 S 8 NSs was characterized by HRTEM. Fig. 6 (a-c) showed the sheet like morphology of Ni 9 S 8 NSs. The average size of the nanosheets observed between 30 nm to few micrometers (mm) this result coincided with SEM and XRD analysis. Hexadecylamine had the vital role for capping and shape directing agent. Fig. 6 (d) displayed the selected area electron diffraction (SAED) pattern of the Ni 9 S 8 NSs. The SAED pattern clearly showed concentrically diffraction rings of orthorhombic crystalline structure and Ni 9 S 8 NSs veri ed the polycrystalline nature preferably distinct single crystal.

Cyclic voltammetry analysis
The prepared Ni 9 S 8 NSs CV results were shown in Fig. 7 (a). The potential window of the Ni 9 S 8 was xed between 0 and 0.6 V. The electrochemical performance of Ni 9 S 8 was analyzed by the 1M KOH electrolyte solution. The shape of CV results revealed the oxidation and reduction behavior of the prepared Ni 9 S 8 sample. The redox reactions of Ni 9 S 8 were recorded in the scan rate of 10 mVs -1 to 100 mVs -1 . Besides, the CV all curves were a pair of redox peaks which indicated fast redox reaction [34]. While increasing the scan rates, the cyclic redox potential peak shifted which clearly represented the good electrochemical behavior of the Ni 9 S 8 compound. The NiS compound redox reaction becomes [21].
The outstanding electrochemical performance exposed due to Ni 2+ and Ni 3+ enabled the rich redox reactions of Ni 9 S 8 NSs [35]. The calculated speci c capacitance was shown in Fig. 7 (b) and also present in Table.1

Chronopotentiometry analysis
The clear identi cation of the GCD (or) chronopotentiometry analysis of Ni 9 S 8 was shown in Fig. 8 (a).
The measuring potential window of the Ni 9 S 8 was xed ranging 0-0.5 V to exhibit a higher speci c capacitance. The charge-discharge analysis of Ni 9 S 8 compound had a non-linear shape which clearly indicated the pseudocapacitance reactions. The anodic peak of Ni 9 S 8 GCD analysis was in good agreement with the CV results. The speci c capacitance was calculated with different current densities. The decreased speci c capacitance was owing to active electrolyte ions diffusion on the increment of current densities. The outer active surface area of the electrode material acted as a charge storage area of the nickel sul de compound [20]. Ni 9 S 8 exhibited the IR drop in the charging and discharging analysis between ~0.42 and 0.5 V. The calculated speci c capacitance of Ni 9 S 8 at various current densities was displayed in Fig. 8 (b) and present in the Table. 2.
The high speci c capacitance of Ni 9 S 8 was 281 Fg -1 due to provide a highly accessible area of the ion diffusion on the electrode species on the surface [20]. The cycle performance of Ni 9 S 8 NSs was calculated as shown in Fig. 8 (c). The retention of the Ni 9 S 8 electrode material was calculated from 10  Table 3.

Electrochemical Impedance spectroscopic analysis
The EIS spectrum of the Ni 9 S 8 compound was studied in the frequency range 1 Hz to 1 MHz. The impedance spectrum was analyzed before and after cycles of the charge-discharge analysis shown in Fig. 8 (d). The impedance spectrum can be separated into three different regions. The rst region was a high-frequency region, which re ected the internal resistance of the Ni 9 S 8 electrode material. The second mid-frequency region consisted of indicating capacitance and resistance. The third region revealed the existence of Warburg resistance in the Ni 9 S 8 electrode.

Conclusions
Nickel sul de (Ni 9 S 8 ) NSs were effectively prepared by simple solvothermal method with hexadecylamine as a capping agent. The Ni 9 S 8 NSs properties/characteristics were studied with various analytical methods; orthorhombic crystalline nature was a rmed with powder XRD pattern and closely matched with JCPDS Number 78-1886. The functional group stretching and bending vibrations con rmed the formation of Ni 9 S 8 NSs using the FTIR analysis. The sheets like morphology, the elemental composition of Ni 9 S 8 were con rmed by SEM images and EDS spectrum with mapping analysis, respectively. The obtained electrochemical result had good redox behavior as well as charge-discharge property. The chronopotentiometry result of Ni 9 S 8 NSs exhibited high speci c capacitance of 281 Fg − 1 at 1 Ag − 1 current density and the Ni 9 S 8 electrode materials achieved retentivity of 82% after 5000 cycles. Finally, Ni 9 S 8 nanosheets can be recommended for perfect electrode material for supercapacitor applications. Figure 1 Schematic representation of preparation of metal complex and Ni9S8 nanosheets Powder XRD pattern of prepared Ni9S8 nanosheets Figure 3 Representative FTIR spectra of Ni9S8 NSs