Interface Engineering of Nickel Selenide and Graphene Nanocomposite for Hybrid Supercapacitor

Nickel selenide is an emerging electrode material for high‐performance hybrid supercapacitors; however, poor electrical conductivity and sluggish ion kinetics limit its application. Herein, a unique architecture by decorating NiSe nanoparticles on reduced graphene oxides (rGO) is developed. The synergistic effect of NiSe and rGO facilitated by the optimized addition of rGO results in significant improvement in the electrochemical performance. The physicochemical characterizations suggest that the enhancement can be attributed to increased interfacial interaction and access to the electrochemically active sites. The NiSe/rGO hybrid delivers a specific capacity of 351 mAh g−1 at 1 A g−1, which is significantly higher than that for bare NiSe. Later, the hybrid supercapacitor based on NiSe/rGO hybrid as positive and activated carbon as negative electrode delivers a maximum energy density of 49.6 Wh kg−1 at a power density of 748.37 W kg−1. In addition, the device shows good cyclic stability of 83.3% over 5000 cycles. Thus, an innovative approach to the development of high‐performance hybrid supercapacitors is offered.

DOI: 10.1002/aesr.202300013 Nickel selenide is an emerging electrode material for high-performance hybrid supercapacitors; however, poor electrical conductivity and sluggish ion kinetics limit its application. Herein, a unique architecture by decorating NiSe nanoparticles on reduced graphene oxides (rGO) is developed. The synergistic effect of NiSe and rGO facilitated by the optimized addition of rGO results in significant improvement in the electrochemical performance. The physicochemical characterizations suggest that the enhancement can be attributed to increased interfacial interaction and access to the electrochemically active sites. The NiSe/rGO hybrid delivers a specific capacity of 351 mAh g À1 at 1 A g À1 , which is significantly higher than that for bare NiSe. Later, the hybrid supercapacitor based on NiSe/rGO hybrid as positive and activated carbon as negative electrode delivers a maximum energy density of 49. 6 Wh kg À1 at a power density of 748.37 W kg À1 . In addition, the device shows good cyclic stability of 83.3% over 5000 cycles. Thus, an innovative approach to the development of high-performance hybrid supercapacitors is offered.
(3d 8 4s 2 ), and diverse oxidation states are the origin of different phases such as NiSe 2 , NiSe, Ni 3 Se 2 , and Ni 1Àx Se. [1,7] NiSe-based nanoparticles are considered promising; however, they have sluggish charge transport kinetics, particle agglomeration, lower electrical conductivity, and rate capability. To improve their performance, they must be synergistically combined with other groups of materials. [1] Among different candidates, the incorporation of carbon-based materials can significantly enhance the overall conductivity of the composite. The improvement in charge storage capacity and charge transfer kinetics is expected to be observed. Different research groups have combined NiSe with carbon allotropes like graphene, [8] CNTs, [9] g-C 3 N 4 , [10] activated carbon (AC), etc. Among these, graphene has been utilized as a potential supporter for battery-type materials. [11] The intrinsic features of graphene are high specific surface area, good electrical conductivity, and electrochemically stable nature. These help in minimizing particle agglomeration and creation of more effective interconnected electron transport networks. [12] Also, the surface of rGO nanosheets possesses rich functional groups that help in designing composites with NiSe. Different research groups have synthesized nickel selenides with rGO. Recently, Bai et al. synthesized NiSe-Ni 0.85 Se nanoparticles and obtained a specific capacity of 669 C g À1 at current density of 1 A g À1 . [13] Other works include metal-organic frameworks-derived NiSe/rGO with a specific capacity of 781 C g À1 , [14] and rGO-decorated NiSe 2 nanoparticles having specific capacity of 467 C g À1 . [15] Encouraged by these findings, we assume that the synthesis and fabrication of nanostructured NiSe will result in significant improvement of electrochemical properties.
Here, we demonstrate a uniform growth of NiSe nanoparticles on rGO nanosheets using a simple one-step hydrothermal method. Furthermore, the interaction between NiSe and rGO results in high specific capacity, high coulombic efficiency, and outstanding electrochemical stability. When employed as a supercapacitive electrode, the NiSe/rGO delivered a specific capacity of 351 mAh g À1 at current density of 1 A g À1 . In comparison, the specific capacity of bare NiSe nanoparticles reached 221 mAh g À1 at current density of 1 A g À1 . HSC device is also constructed and tested using NiSe/rGO as positive electrode and AC as negative electrode. In the presence of 6 M KOH electrolytic solution, the potential window of the asymmetric arrangement can be increased to 1.5 V, and energy density of 49.6 Wh kg À1 at 748.37 W kg À1 power density is obtained. In addition, the device exhibits good cycling stability with a high retention rate of 83.3% at current density 5 A g À1 and 92% of coulombic efficiency after 5000 cycles.

Results and Discussion
The reaction conditions broadly affect the phase structure, composition, and morphology of the materials. The NiSe and NiSe/rGO samples were synthesized by a simple one-step hydrothermal method, as shown in Figure 1a. In this system, hydrazine hydrate was used for dissolving selenium powder and as a reducing agent to enhance the formation of NiSe nanoparticles. [16,17] The possible chemical reaction processes to form NiSe are as follows First, the Se undergoes a disproportionation reaction in an alkaline environment to become Se 2À and SeO 3 2À . Free Ni 2þ will then interact with Se 2À to directly create NiSe, as shown in Equation (2). Upon heating, SeO 3 2À gets reduced to Se, which is further converted to Se 2À . [13,18] At the same time, GO is reduced to rGO due to nucleophilic attack of hydrazine hydrate on oxygen groups of GO. [19,20] Thus, the in-situ formation of NiSe on rGO sheets takes place, as shown in Figure 1a. The defects on rGO matrix act as nucleation centers and help for the growth of NiSe nanostructures on rGO nanosheets. [21] The structural and chemical properties of the prepared materials were studied by XRD, Raman, and XPS measurements, as illustrated in Figure 1b . The absence of extra peaks confirms the structural purity of the prepared materials. The standard GO peak which is observed at 10.6°is not observed in the composite samples due to; stronger intensity peaks of NiSe compared to GO and, the transformation of GO sheets into rGO during synthesis shown in Figure S1a, Supporting Information. [22] It is also seen that with the increase in the GO concentration, full width at half maximum (FWHM) of the peaks increases which results in lower crystallite size. [23,24] The crystallite size was calculated by the Debye-Scherrer formula, [25] which was found to be 53.34, 37.83, 34.23, and 27.17 nm for NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO, respectively.
The Raman spectra of all the samples are depicted in Figure 1c. In the case of NiSe/rGO samples, the characteristic peaks at 1345 and 1578 cm À1 correlate to D and G bands, respectively. These two peaks are indicative of the structural disorder of graphene and sp 2 hybridization of carbon atoms. The intensity ratios of the D and G bands (i.e., I D /I G ratio) give information that helps to estimate the defects in the samples with higher ratio corresponding to greater defects. [17] In our case, the ratio values obtained for NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO were 1.37, 1.72, and 1.56, respectively. Additionally, the remaining GO, rGO hydrothermal, and rGO hydrazine hydrate ratio values 0.93, 1.02, and 1.05, respectively are shown in Figure S1b, Supporting Information. Thus, the intensity ratio of NiSe/20rGO was maximum implying presence of more defects on rGO surface which promotes successful integration of NiSe nanoparticles into rGO nanosheets. This conclusion supports the analysis from XRD, XPS, and HRTEM analyses. Also, in the Raman curves of Ni 0.85 Se, there are two main peaks at about 141 cm À1 (lower intensity peak) and 235 cm À1 (higher intensity peak) denoted by A 1g (out-of-plane) and E 2g (in-plane) phonon Raman modes, respectively. [26] The intensity of A 1g in NiSe is found to decrease for NiSe/xrGO samples. The redshift for A 1g phonon mode is observed for NiSe/10rGO and NiSe/30rGO while the blueshift is seen for the NiSe/20rGO compared to the bare NiSe sample. This indicates that for NiSe/20rGO, in-plane atomic interactions dominate while the coupling effect for out-of-plane interacting layers is weakened in contrast to the NiSe sample. Thus, the optimal amount of NiSe in the rGO matrix affects the peak intensity and is indicative of greater synergy between NiSe and rGO. [27] The chemical composition and bonding information were examined by XPS measurements for the optimized NiSe/20rGO sample, see Figure 1d-f and S1c,d, Supporting Information. The survey XPS spectrum reveals the presence of Ni, Se, C, and O elements. For the Ni-2p spectrum, curve fitting was carried out and it is seen that two spin-orbit doublets at locations 873.68 and 855.96 eV correspond to the Ni 2p 1/2 and Ni 2p 3/2 of Ni 2þ , while other two peaks at 861.01 and 879.62 eV are represented as shake-up satellites (sat) in Figure 1d. Similar to this, the peak values at binding energies of 54.51 and 53.69 eV correspond to the Se 3d 3/2 and Se 3d 5/2 of Se 2À , respectively based on curve fitting of the Se 3d spectrum. While in Figure 1e, the surface oxidation of Se as a result of exposure to the environment causes the binding energy peak value to be 59 eV. NiSe/20rGO's C1s core spectrum was deconvoluted into four components at 284.5, 285.09, 288.69, and 290.29 eV, which are each ascribed to a different bond type: C─C, C─O, C─O─C, and O-C═O (see Figure 1f ). [28] The O1s scan reveals two peaks, the strongest of which is at 530.9 eV and a shoulder at 532.8 eV, which corresponds to the C─O and C═O bands, respectively. These bands match oxygen-containing groups on the rGO sheets, partial reduction, and ambient moisture adsorption were thought to be the causes of these groups. The negative peak shifts of Ni 2p and a positive peak shift of C 1s for C═C bond, and peak broadening of O 1s for NiSe/20rGO hybrid confirm interaction between NiSe and rGO. All these values agree www.advancedsciencenews.com www.advenergysustres.com well with the previous reports of NiSe. [13,29] Thus, strong hybrid interaction between NiSe and rGO is beneficial to reduce the contact resistance and increase electrochemical activity and stability of NiSe. The growth of NiSe on rGO sheets and its morphology were investigated through FE-SEM analysis.  Figure 2b,e reveals that distance between NiSe nanoparticles improves in the latter due to the presence of GO sheets while the former has closely spaced NiSe nanoparticles. This is suggestive of the fact that particle agglomeration is subdued with the incorporation of GO and thus, more surface area of NiSe is exposed for electrochemical interactions. Also, Figure S2a-f, Supporting Information shows FE-SEM images of NiSe/10rGO and NiSe/30rGO samples, respectively. It observed that nonuniform growth of NiSe that takes place in both the cases implying the role of optimal concentration of rGO for synergistic interaction and composite formation. As shown in Figure 2g-i, HR-TEM and SAED pattern were used to investigate the structural analysis of NiSe/20rGO composite. Figure 2g shows an HR-TEM image of NiSe/20rGO with lattice interplanar spacing of 0.29 nm, which is indexed to (101) plane of hexagonal crystal structure. Direct growth of NiSe nanoparticles on the surface of rGO nanosheet is clearly confirmed from Figure 2h with the grain sizes ranging from 10 to 20 nm. The direct growth of NiSe nanoparticles on rGO could provide improved electronic transportation, ion accessibility, and chemically and mechanically stable structure, which can offer high rate and storage capability with electrochemical stability. [30,31] The indexed SAED pattern in Figure 2i is of the concentric ring type, indicating the polycrystalline hexagonal nature of NiSe nanoparticles (JCPDS card no. 00-018-0888). The EDS mapping shows the existence of C, O, Ni, and Se in the optimized sample, see Figure S3a, Supporting Information. The C and O elements peaks belong to rGO, while Ni and Se peaks correspond to NiSe. The corresponding functional groups of C and O on rGO sheets acted as nucleation center for the growth of NiSe nanoparticles. The atomic percentages of Ni and Se are 16.77% and 19.85%, respectively, and the ratio of Ni:Se is measured to be 0.85:1, which corresponds to Ni 0.8 Se crystalline phase. Figure S3b,c, Supporting Information, shows the BET-nitrogen adsorption and desorption comparative graph to understand pore size distribution and surface area. The pore diameter of the samples varies from 15 to 30 nm, while the surface area for NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO samples were estimated to be 21.9, 48.5, 73.9, and 65.4 m 2 g À1 , respectively. Thus, high surface area for NiSe/20rGO composite is consistent with the SEM images and improved electrochemical activity.

Electrochemical Performance
The electrochemical performances of NiSe and NiSe/xrGO electrodes were evaluated using a three-electrode system in 6 M KOH electrolyte. [32] The specific capacity of the electrode is related to the area under the CV curve at specific scan rate. The CV curves of NiSe, rGO, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO are www.advancedsciencenews.com www.advenergysustres.com illustrated in Figure 3a and S5a, Supporting Information, measured at the scan rate of 5 mV s À1 . The Faradaic sharp redox peaks reflected in the CV curves correspond to the redox-dominant/battery-type charge storage mechanism. Additionally, Figure 3a demonstrates higher CV area of NiSe/rGO composite electrodes over pristine NiSe. A plausible explanation for such an observation is that as rGO concentration increases, the overall content of NiSe decreases. This increases the likelihood of rGO layer restacking, and thus, the effective transportation of electrolytic electrons and ions to the interior of the electrode material is hindered, which in turn results in decreased capacity. [33] Now, among different electrodes, NiSe/20rGO shows the highest area under the curve, hence, demonstrating higher electrochemical activity due to synergistic effect between NiSe and rGO. From Figure 3a, identification of redox peaks was done and the values of 0.34 and 0.17 V were obtained. This can be ascribed to the redox reactions of Ni 2þ ↔ Ni 3þ and corresponding electrochemical equations are given by A detailed differential study was done by taking CV of the prepared electrode at different scan rates for NiSe/20rGO of 2, 5, 10, 15, 20, 30, 40, and 50 mV s À1 in the potential window of 0 V to 0.5 V versus Hg/HgO, as shown in Figure 3b. This procedure was repeated for rGO, NiSe, NiSe/10rGO, and NiSe/30rGO electrodes and Figure S4a-c, S5b, Supporting Information, were obtained. In the figure, it is seen that, as the scan rate is increased, the peak current density increases while maintaining the uniformity in the shape of the CV curves. Also, there is a positive and a negative gradual shift of oxidation and reduction peaks stipulating better reversibility of electrode redox reaction. [5,12,13,34,35] Furthermore, GCD curves of all electrodes were measured at a fixed current density of 1 A g À1 in the potential range 0-0.5 V versus Hg/HgO as shown in Figure 3c. The specific capacities at current density of 1 A g À1 for NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO, were 221, 269, 351, and 337 mAh g À1 , respectively. A nonlinearity is observed in the shape of all GCD profiles that confirms the dominating nature of Faradic reactions within the electrodes. Among all samples, NiSe/20rGO shows a maximum value of the discharging time. This shows that an optimized value of rGO integration into the NiSe matrix is necessary for higher performances.
GCD tests were carried out by varying current density values at 1, 2, 3, 4, 5, 8, 10, and 12 A g À1 as shown in Figure 3d for the NiSe/20rGO. A similar result is observed in the redox peaks of CV analysis coinciding with the charging and discharging voltage plateaus of the GCD curves. Initialization of the discharge phenomenon is accompanied by the internal resistance (IR) drop that is inherently dependent on the intrinsic resistances of the electrode material and the electrolytic solution. Similar behavior is seen for other samples, as shown in Figure S4d-f, Supporting Information. The major contribution to redox kinetics is provided by the rate of OH À ion diffusion and migration, therefore at higher current densities, specific capacities are severely subdued. From the GCD curves, the relationship between specific capacities at different current densities is obtained, as shown in Figure 3e. At current density of 12 A g À1 , the specific Figure 3. a) CV curves at scan rate 5 mV s À1 of NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO electrodes. b) CV curves of NiSe/20rGO at different scan rates (2-50 mV s À1 ) c) GCD at 1 A g À1 of NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO electrodes. d) GCD curves of NiSe/20rGO at various current densities (1-12 A g À1 ). e) Plot of specific capacity of NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO versus the current density, and f ) Nyquist plots of NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO electrodes.  [35] Thus, the results obtained from both the studies, i.e., CV and GCD show that NiSe/20rGO is better electrode material as compared to NiSe and other composites. EIS analysis was carried out in the range of 10 mHz-100 kHz to understand the charge transfer mechanism. The obtained Nyquist plots are demonstrated in the Figure 3f. From the Nyquist plot, diameter of semicircle provides with charge transfer resistance (R ct ) while the intercept on real axis gives solution resistance (R s ). The Rct for the electrodes is in order of NiSe/20rGO < NiSe/30rGO < NiSe < NiSe/10rGO while order for Rs is NiSe/30rGO < NiSe/20rGO < NiSe/ 10rGO < NiSe. At lower frequencies, the slope of the straight line reveals the electrode's capacitive behavior whereas, when the inclination changes from 45°to 90°, the capacitive performance becomes dominant. From Figure 3f, the inclination of the vertical line follows the sequence NiSe/20rGO < NiSe/ 30rGO < NiSe/10rGO < NiSe. The combined effect of all the three parameters reveals that NiSe/20rGO has superior charge kinetics compared to other electrodes. The synergy of nanostructurization of the electroactive material NiSe with the rGO results in the abundance of electrochemical reaction sites. The controlled and tuned different weight percentages of NiSe with rGO improved electrochemical performances over pristine NiSe.
From Figure 4a, the relationship between the peak current densities (oxidation and reduction) and the square root of scan rate is found to be approximately linear. This dependence suggests that interfacial redox reactions are diffusion controlled and quasireversible in nature. The CV test is further used to understand the extent of capacitive effect which can be derived from the power law relationship where i denotes peak current (A g À1 ), v denotes the scan rate in mV s À1 , a and b are both constants. The value of b determines the behavior of electrode material. When the b value is near to 0.5, it indicates diffusion-controlled process while b value of 1 signifies surface capacitive behavior. In our case, the value of b was found to be 0.43, 0.48, 0.55, and 0.65 for NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO, respectively, indicating the dominance of diffusion-controlled behavior of the samples.
To understand the nature of storage of charge, whether by the capacitive elements or insertion elements, the method devised by Dunn et al. [36,37] was used. The technique helps in quantitative separation of individual contributions of capacitive elements and diffusion-controlled insertion process. Thus where, i(V ) denotes current at given voltage, υ is the scan rate, k 1 and k 2 are constants. The first element in the equation is capacitive contribution while the second part is the diffusion-controlled contribution. This equation when divided by the square root of scan rate on both the sides becomes Figure 4. a) Log of peak current (Ipc) versus log scan rate (υ) plots for redox peaks of NiSe, NiSe/10rGO, NiSe/20rGO, and NiSe/30rGO nanocomposites. b) CV curves with the capacitive and diffusion-controlled currents for the NiSe/20rGO sample at 10 mV s À1 scan rate, and c,d) normalized contribution ratio of capacitive and diffusion-controlled capacities of NiSe, and NiSe/20rGO samples at different scan rates, respectively. www.advancedsciencenews.com www.advenergysustres.com iðVÞ υ On comparison with the straight line Equation y = mx þ c, we get that a plot ofi/υ 1/2 versus υ 1/2 at a given potential will give a straight line, where the slope of the line denotes k 1 while the intercept will give value of k 2 . Thus, at a given voltage, quantitatively we can separate the capacitive part k 1 υ and diffusioncontrolled part k 2 υ 1/2 .
Using this method and calculating contributions at different voltages, we obtain Figure 4b for NiSe/20rGO. The convoluted area inside is indicative of the capacitive charge storage reaction. Figure 4c,d gives the individual contribution in terms of percentage to the overall capacitance for NiSe and NiSe/20rGO. The same method was used to understand the behavior for remaining samples, as shown in Figure S5c,d, Supporting Information. It is seen that addition of rGO to NiSe enhances the capacitive nature of the electrode material. Also for all the samples, with an increase in scan rate, the percentage contribution due to capacitive contribution increases, which is consistent with the earlier reports. [38,39]

Hybrid Supercapacitor
Hybrid supercapacitors are assembled to study the chargedischarge characteristics and cyclic stability of the composites. The HSC device structure was designed with NiSe/20rGO as a positive electrode and AC as a negative electrode; and all the electrochemical tests are carried out in 6 M KOH aqueous solution. First, the CV and GCD performances of AC in the potential range of 0 to À1 V versus Hg/HgO were investigated. Figure S6a-c, Supporting Information, depicts the AC-GCD curve with energy densities ranging from 1 to 8 A g À1 . The symmetric isosceles triangular forms of the GCD curves can be used to deduce the EDLC feature in the AC. At current density of Figure 5. a) CV curves for AC and NiSe/20rGO at 10 mV s À1 . b) CV curves at different scan rates, and c) GCD curves at different current densities for NiSe/20rGO//AC. d) Ragone plots of energy versus power density of the NiSe/20rGO//AC compared with other works. e) EIS before and after 5000 cycles for NiSe/20rGO//AC. f ) Cycle charge-discharge performance and coulombic efficiency of the NiSe/20rGO//AC at 5 A g À1 for 5000 cycles. g) A small light bulb powered by the assembled ASC devices. 1 A g À1 , the specific capacity of the AC electrode is 283 F g À1 . At a similar scan rate of 10 mV s À1 , the CV curves of both the electrodes were analyzed. A difference in the nature of CV curves of NiSe/20rGO and AC is shown in Figure 5a. The NiSe/20rGO still maintains the redox peak curve but no such peak is observed in AC electrodes. Also, the working potential window of NiSe/20rGO electrode (0-0.5 V) and AC electrode (0 to À1 V) is different. Hence, the prospects of assembling this electrode to devise an HSC with AC and NiSe/20rGO serving as negative and positive electrodes respectively, and having wide working voltage are very high. To design HSC, the rule of charge balance needs to be followed. The mass ratio of NiSe/20rGO and AC was calculated to be 0.18. The as-assembled HSC results in the extension of the total operating voltage window of NiSe/20rGO //AC HSC in the range from À1 to 0.5 V (1.5 V). As shown in Figure S6c, Supporting Information, the CV tests were carried out in the potential range 0-1, 0-1.1, 0-1.2, 0-1.3, 0-1.4, and 0-1.5 V, maintained at scan rate 5 mV s À1 . As expected, the HSC device maintains a steady CV curve in the potential range of 0-1.5 V with no evident polarization. Figure 5b displays the CV nature of NiSe/20rGO//AC HSC at different scan rates in the voltage window 0-1.5 V. The energy storage mechanism for negative electrode is EDLC type while for positive electrode it's battery type. This difference in the energy storage mechanism leads to a quasi-rectangular shape of the CV. Also, from Figure 5b, it can be seen that the CV nature and shape remain similar at different scan rates demonstrating exemplary electrochemical stability of the assembled HSC device.
Similarly, GCD curves of NiSe/20rGO//AC HSC were investigated at different current densities as shown in Figure 5c. At the current density values of 1, 2, 3, 5, 8, and 10 A g À1 , specific capacity was calculated to be 110, 112, 117, 124, 132, and 157 mAh g À1 , respectively. A nonlinearity is observed in the GCD curve while charge/discharge behavior is not seen. This can be mainly due to the different capacitive storage mechanisms as discussed earlier.
In addition, it is seen that with the increase in the current density, the GCD curve remains similar in shape, implying excellent current response.
The energy and power densities at different current densities were calculated. From the GCD curves of NiSe/20rGO//AC, energy and power densities were obtained and used to plot the Ragone plots, as shown in Figure 5d. Compared to the previous literature reports, the NiSe/20rGO//AC HSC shows a higher energy density of 49.6 Wh kg À1 at 748.37 W kg À1 power density. This is comparable to earlier reported works, N-rGO/NiSe 2 (40.5 Wh kg À1 at 841.5 W kg À1 ), [35] NiSe@MoSe 2 nanosheets (32.6 Wh kg À1 at 415 W kg À1 ), [40] Ni 0 . 6 Co 0 . 4 Se 2 (42.1 Wh kg À1 at 716 W kg À1 ), [41] Ni 3 Se 2 NSs@CF (32.8 Wh kg À1 at 677.03 W kg À1 ), [42] NiSe/rGO (50.1 Wh kg À1 at 816 W kg À1 ), [7] NiSe 2 /rGO (33.13 Wh kg À1 at 850 W kg À1 ), [43] and NiSe-Ni 0 . 85 Se (38 Wh kg À1 at 939 W kg À1) . [13] Also, the energy density of a supercapacitor is directly dependent on the square of voltage. Hence, any increase in the voltage window favors the energy storage of HSC. There are several reasons for the optimized sample NiSe/20rGO//AC to show high energy density. First, the nanostructurization of the NiSe particles improves the interfacial electrochemistry by significantly lowering the diffusion path for OH À diffusion. Second, rGO integration improves the charge transfer rate and electrical conductivity thereby favoring the movement of ions. Third, introduction of rGO prevents the agglomeration of the NiSe nanoparticles. Thus, lowering of charge transfer resistance and improved electrode stability was seen by the incorporation of rGO in NiSe.
The EIS studies of the device before and post the electrochemical tests were also carried out, as shown in Figure 5e. It can be seen that there is a slight change in the curve. The change corresponds to the increase in the internal resistance and charge transfer resistance. The cycling stability tests are also shown in Figure 5f and a high retention rate of 83.3% at current density 5 A g À1 is observed for HSC devices after 5000 cycles with 92% coulombic efficiency. Also, two NiSe/20rGO //AC HSC devices were coupled in series to operate, as shown in Figure 5g. [44] It was seen that the combination could light a tiny LED bulb for 5 min, thus, demonstrating the excellency of the optimized sample shows for long run use and practical applications.

Conclusion
In this study, we developed a simple one-step hydrothermal process to create hybrid NiSe/rGO nanocomposites. This was done to solve the issues of particle agglomeration, sluggish charge kinetics, and lower electrical conductivity in NiSe. In comparison to bare NiSe, the supercapacitive performances were significantly improved by the right amount of rGO and the synergistic impact between NiSe and rGO. The GO content was varied from 10 to 30 mg to form different hybrids. Among these, in an aqueous 6 M KOH electrolyte, the NiSe and optimized sample NiSe/20rGO demonstrated a specific capacity of 221 and 351 mAh g À1 at 1 A g À1 , respectively. The rGO provided the structural and kinetic stability while tortuosity and the interfacial resistance of rGO interlayers lowered the rate capability of NiSe/20rGO compared to bare NiSe. The HSC was set up using an AC negative electrode and a NiSe/20rGO hybrid positive electrode to broaden the aqueous system's potential window. This HSC delivered high energy and power densities of 49.6 Wh kg À1 and 748.37 W kg À1 while exhibiting good cyclic stability of 83.3% at current density 5 A g À1 and 92% of coulombic efficiency over 5,000 cycles. Thus, the properties of rGO enlarged the potential window, improved coulombic efficiency, interfacial charge kinetics, energy density and cycling stability of NiSe.
Preparation of NiSe/rGO Nanocomposites: Graphene oxide (GO) powder was prepared by an improved Hummers method. [45] The NiSe/rGO nanocomposites were prepared by a simple hydrothermal method. In a typical preparation procedure, 10 mL of GO-suspension (1 mg mL À1 ), nickel acetate (0.523 g), and selenium (0.263 g) powders were dissolved in 150 mL www.advancedsciencenews.com www.advenergysustres.com