is of Ni / Ni ( OH ) 2 nano sheets ( NSs ) and their application in asymmetric supercapacitors †

Academy of Scientic and Innovative Resear Research Institute (CSIR-CECRI) Campus, Ne in; kundu.subrata@gmail.com; Fax: +91-45 Electrochemical Materials Science (ECMS) Research Institute (CECRI), Karaikudi-6300 Department of Materials Science and Mecha College Station, Texas, TX-77843, USA † Electronic supplementary information specications, sample preparation for va analysis, gures and tables related to ele DOI: 10.1039/c6ra26584g Cite this: RSC Adv., 2017, 7, 5898


Introduction
Supercapacitors and fuel cells are alternative energy storage systems to batteries.2][3][4][5] Nowadays, supercapacitors supersede batteries in an array of applications.However, supercapacitors are still behind batteries in energy storage applications, due to their relatively poor energy densities. 6ntense efforts need to be invested in improving the energy and power densities of supercapacitors to establish them as an alternate means of power supply. 7,8Fabricating advanced supercapacitor devices that would give comparable energy densities and at the same time retain high power density and cycle life is therefore quite important. 9This can be done either by increasing the capacitance or by extending the operating potential window. 10,11One of the most commonly practiced ways of augmenting cell voltage nowadays is the application of organic electrolytes that are known for wider potential windows with improved electrochemical stability, compared to aqueous systems.However, problems arise due to their lower conductivity, toxicity and cost effectiveness, as compared to aqueous electrolytes.The low conductivity greatly reduces specic capacitance and unfortunately, spikes up the equivalent series resistance (ESR), which in turn prevents high specic power density.Hence, aqueous electrolytes are among the best options to develop cost-effective and ecofriendly supercapacitors.
Low-cost transition-metal oxide supercapacitor electrode materials in aqueous electrolytes are applied as symmetric and asymmetric capacitors, owing to their enhanced specic capacitance, although they possess a narrow potential window of 1 V in symmetric supercapacitors. 12Recently, Yamauchi et al.
demonstrated high cyclic stability, high specic energy and power densities using metal organic frameworks (MOF) 13 and nano porous carbon derived from MOF 14 in symmetric supercapacitors.However, due to the narrow potential window of symmetric supercapacitors, energy and power densities cannot be optimized to higher values as required in industries; it is therefore desirable to fabricate supercapacitors with two different electrodes, where one could be the electrochemical double layer capacitor (EDLC) and the other could be a pseudocapacitor.The asymmetric set-up greatly increases the potential window and also the specic capacitance, energy and power densities, and the dual electrode set-up may be a combination of two distinct transition metal oxide electrodes, 12,15 or a combination of an activated carbon and metal oxide electrode, 11,12,[15][16][17][18] or that of a conducting polymer and an oxide electrode. 19Several electroactive materials, such as Ni(OH) 2 , 20 MnO 2 , 21 V 2 O 5 , 22 CoO, 23 Fe 2 O 3 , 24 MnWO 4 , 25 NiWO 4 , 26 ZnWO 4 27 and Nb 2 O 5 28 have been reported as supercapacitor materials, amongst which Ni(OH) 2 is gied with high theoretical specic capacitance (2082 F g À1 ) and is oen applied as the dischargedstate material in the electrodes, owing to its stability in strong alkaline electrolyte, which is due to the short diffusion path length of the electrolyte, as well as high reversibility and excellent rate capability when oxidized to NiOOH. 29Nevertheless, besides having high specic capacitance, it is a poor electronic conductor, a factor that limits its ability to reach the theoretical capacitance on its own.32][33] 32 and cobalt monoxide nickel hydroxide nitrate composite was synthesized on nickel foam by a two-step hydrothermal route by Guan et al. 33 All of the above methods employed more than one step to synthesize the desired active material.Moreover, the control over the loading of active materials involved in the electrochemical reactions by these methods was very poor.Another major pitfall is the conversion of the nickel foam surface into nickel oxide during hydrothermal or electrodeposition, thus increasing the resistance of the nickel foam, which will directly affect the capacitive performance of pristine material.In order to avoid all these pitfalls, we have synthesized Ni/Ni(OH) 2 NSs by a facile one-step hydrothermal synthesis with the aid of oleylamine and ethanol as a reducing agent in basic medium, for the rst time.The maximum specic capacitances of 536 F g À1 Ni/Ni(OH) 2 NSs by cyclic voltammetry (CV) at a scan rate of 5 mV s À1 and 450 F g À1 by chronopotentiometry at a current density of 1 mA cm À2 were achieved in 1 M KOH aqueous solution.Our optimized ASC showed a specic capacitance of 62 F g À1 at 2 mA cm À2 and a maximum energy density of 23.45 W h kg À1 , without sacricing the power density.Moreover, the fabricated material also showed a good long-term cycling stability as derived from galvanostatic charge-discharge (GCD) measurements, where only a 10% decrease from its initial specic capacitance was observed, even aer 6000 cycles.These outcomes suggest that such Ni/Ni(OH) 2 NSs will be promising materials for the next generation of high-performance ASCs.For comparison, we synthesized NiO/Ni(OH) 2 NSs by varying the concentration of KOH and studied the electrochemical properties along with Ni/ Ni(OH) 2 NSs for a three electrode system.For a typical synthesis of Ni/Ni(OH) 2 NSs, 0.248 g of nickel(II) acetate hexahydrate were taken with 20 mL of ethanol and 1.5 mL of oleylamine and subjected to mechanical stirring for 20 min.Further, 1 mL of oleylamine dispersed in 10 mL of ethanol was swily added, followed by 2 mL of KOH (0.1 M) and the stirring was continued for another 30 min in order to homogenize the whole reaction mixture.The entire contents were then transferred to a Teon-lined autoclave of 50 mL volume capacity.The sealed autoclave was subjected to constant heating at 160 C for 24 h.The blackish precipitate obtained aer the solvothermal treatment was separated and washed a couple of times with ethanol and cyclohexane to expel the excess organics in the synthesized material.Finally, the blackish powder obtained was dried in a vacuum oven at 60 C for 2 h.The same synthesis procedure was followed for the synthesis of NiO/Ni(OH) 2 NSs, with the exclusion of the addition of KOH.The obtained greenish precipitate was separated aer the solvothermal treatment and washed a couple of times with ethanol and cyclohexane.The details of the concentrations of the reagents used and pH of the synthesis of Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NSs are listed in Table 1.The overall synthesis and visible changes at various stages of the synthesis are schematically illustrated in Scheme 1.The synthesized Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NSs were further characterized using X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) analyses (Scheme 2).

Fabrication of electrodes and electrochemical calculations
The working electrode was prepared by mixing the electroactive material, acetylene black, and polyvinylidene uoride (PVDF) in a mass ratio of 80 : 15 : 5 with N-methyl-2-pyrrolidone (NMP).About 2.4 mg of active material (excluding the mass of acetylene black and PVDF) of the as prepared homogenous slurry was coated on the nickel foam taken as the current collector (area ¼ 1 cm Â 1 cm), and dried at 120 C for 12 h to evaporate the solvent.The conventional three electrode system with aqueous 1 M KOH electrolyte was used for obtaining electrochemical measurements.A platinum sheet and Ag/AgCl were used as the counter and reference electrodes, respectively.The electrochemical impedance spectroscopy (EIS) was carried out over the frequency range of 100 kHz to 0.01 Hz with an AC amplitude of 5 mV at open circuit potential.The specic capacitances of the electrodes were calculated from the CV and GCD curves of the three electrode system, according to the following equations: Specic capacitance from CV curves Specic capacitance from GCD curves Energy density Power density where, ð idV is the integrated area under the CV curve (A), v is the scan rate (mV s À1 ), I is the discharge current (A), Dt is the discharge time (s), DV is the potential window (V), m is the mass of the electroactive material (g), C sp is the specic capacitance (F g À1 ) and T is the discharge time (s).The specic capacity of the ASC was optimized by amending the mass ratio between Ni/Ni(OH) 2 NSs and activated carbon (AC) by using the following equation:    100), ( 002), ( 110), ( 111), ( 200), ( 103) and (202).Fig. 1A shows XRD patterns of Ni also with three characteristic peaks at 44.6 , 51.9 and 76.5 (JCPDS 70-0989), which correspond to the (111), ( 200) and ( 220) diffraction planes, respectively.This is in accordance with the reported value for nickel metal nanoparticles. 34From Fig. 1B

Topographical analysis and electron diffraction studies
The structure and morphology of the NSs were observed by transmission electron microscopy (TEM).Fig. 3A and B show the low and high magnication images of the as-prepared Ni/ Ni(OH) 2 NSs.From Fig. 3B, the morphology seems to be of sheet-like architecture.It can also be observed that the sheets are interwoven with each other (Fig. 3A and B).Fig. 3C shows the corresponding SAED pattern of Ni/Ni(OH) 2 .Careful observation of Fig. 3C shows the ring pattern of Ni(OH) 2 and the square pattern of Ni metal, and the diffraction spots that are identied in Fig. 3C are consistent with the XRD results (Fig. 1A).Fig. 3D and E show the low and high magnication images of the as-prepared NiO/Ni(OH) 2 NSs.Fig. 3F shows the corresponding SAED pattern of NiO/ Ni(OH) 2 , which is in accordance with the XRD results (Fig. 1A).The addition of a small amount of KOH not only adjusted the pH of the solution, but also played a crucial role in the partial reduction of the nickel precursor to nickel nanocomposite (Ni/Ni(OH) 2 ).This is in accordance with the previous reports by the Chen group and Roselina group. 40,41hen et al. observed that in the absence of NaOH, there was no formation of nickel NPs despite adjusting the pH (10.5) with a strong reducing agent like hydrazine hydrate.From this result, they concluded that the addition of NaOH might play a dual role in adjusting the pH as well as acting as a catalyst. 40Likewise, the Roselina group noticed that in the absence of NaOH, the nickel precursor resulted in the formation of a complex mixture with hydrazine hydrate and nickel hydroxide instead of various nickel nanostructures. 41It must be noted that there are several other reports for the reduction of metal salts to their corresponding metal nanoparticles (NPs) by the reducing activity of compounds that have hydroxyl groups, such as ethanol, 42 ascorbic acid, 43 PVA, 44 and 2,7-dihydroxynaphthalene (2,7-DHN) 45 that act as reducing agents.Ethanol has previously been used as a reducing agent for the formation of noble metal NPs such as Ag, Au, Pd, and Os.Ayyappan et al. reported ethanol as a reducing agent for the formation of metal NPs such as Ag, Au, Cu, Pd. 46 Likewise, Pal et al. reported the synthesis of Ag NPs using ethanol as a reducing agent and polyvinyl pyrrolidone (PVP) as a stabilizer. 47Recently, our group reported the formation of Os NPs by the reduction of OsO 4 with ethanol by microwave heating. 48Hence, we also believe that ethanol might acts as a reducing agent by the formation of potassium ethoxide at high pressure and temperature, which is responsible for the formation of Ni/Ni(OH) 2 NSs.

Three electrode system electrochemical studies
The capacitive behavior of an electrode material is generally characterized by cyclic voltammetry (CV).It was observed that all the CV curves exhibited a similar pattern of anodic and cathodic peaks with increasing scan rates, demonstrating the good reversibility of the redox reaction at the nanostructure interface, with excellent rate capability.Moreover, the anodic and cathodic peaks were shied towards the positive and negative potential with rising scan rates.This might be due to the diffusion effect of protons within the electrode or mass transfer limitations of electrolyte ions in order to neutralize the electronic charge on the electrode surface during the redox reaction. 50From Fig. 4C, we can infer that the specic capacitance and the scan rate share an inverse relationship.This can be attributed to the circulation effect, which prevents the diffusion and movement of the electrolyte ions within the electrode at elevated scan rates, due to their inner active sites that are not capable of fully withstanding the redox alterations at higher scan rate. 51,52 S1 and S2, † respectively.From the tables, we can see that the Ni/Ni(OH) 2 NS composite shows better rate capability than the NiO/Ni(OH) 2 NS composite.Further, we calculated the coulombic efficiency (h) of the sample to check the reversibility of the working electrode, which was calculated by using the formula given below:   First, the specic surface area (SSA) of the as-synthesized Ni/ Ni(OH) 2 NSs is higher than the NiO/Ni(OH) 2 , consequently promoting the diffusion of electrolyte on the surface of the electrode and electrical conductivity of the electrode.Second, the presence of conductive Ni metal as part of the nano sheets will promote the electric conductivity of the electrode and thus, it provides a better way to produce commercial ASC cells because the electrode size is independent of the resistance, owing to the conductive nature of nickel metal.The third aspect relates to the clean and irregular structures.The disorder in Ni/ Ni(OH) 2 can greatly improve electrochemical efficiency, and a low-crystalline material has the potential to exhibit excellent electrochemical performance because of its high structural disorder.Moreover, the Ni/Ni(OH) 2 NSs are well adapted to the surface of the nickel foam electrode.This adaptability not only maintains structural continuity, but also results in good electrical contact between the nano sheets and the electrode, which is vital for commendable electrochemical performance.Further, we proceeded to fabricate aqueous ASC, Ni/Ni(OH) 2 as positive electrode and activated carbon as the negative electrode, as discussed in detail in the following section.

Two electrode (ASC) electrochemical studies
The detailed electrochemical study on Ni/Ni(OH) 2 and NiO/ Ni(OH) 2 NSs revealed the superior activity of Ni/Ni(OH) 2 .By using activated carbon (AC) as the negative electrode, an ASC with reasonable power and energy density can easily be fabricated along with our Ni/Ni(OH) 2 NS as a positive electrode, while eliminating the use of costly carbons (graphene, graphene oxides, etc.).The electrochemical performance of AC and Ni/ Ni(OH) 2 NSs were examined separately by running CV at a 5 mV s À1 scan rate in 1 M KOH electrolyte (Fig. S4 †).The rectangular shape of the CV acquired for AC without any redox peaks indicates the EDLC properties of AC, whereas the CV of Ni/Ni(OH) 2 does have a redox couple as expected, indicative of the pseudocapacitive behavior.From the careful evaluation of the obtained results on both Ni/Ni(OH) 2 and AC, the optimum voltage window was taken to be 1.65 V.The charge/discharge process of the ASC can be depicted as follows: The analysis of the specic capacitance values of Ni/Ni(OH) 2 and AC led to the optimal mass ratio of 1 : 4 in fabricating the ASC from eqn (6).The ASC cell was subjected to CV analysis in 1 M KOH solution at various scan rates ranging from 5-125 mV s À1 as shown in Fig. 6A.The shape of the CV curves looks slightly similar to that observed for the three electrode system of Ni/Ni(OH) 2 NSs.However, the combined effect of the EDLC properties from AC and redox characteristics from Ni/ Ni(OH) 2 can be seen as the scan rate increases.It is noteworthy here that although Ni(OH) 2 is known for being a better oxygen evolving catalyst in alkaline solutions, with the optimized mass ratio taken for the fabrication of the asymmetric capacitor, no such gas evolution was witnessed within the potential window of 1.65 V. Fig. 6B illustrates the GCD plots at different current densities (2-20 mA cm À2 ) with the potential window of 1.65 V.By making use of eqn (2), the maximum specic capacitance for the asymmetric device was achieved at current density of 2 mA cm À2 and found to be 62 F g À1 .We compared our results with other previous reports which are tabulated in Table 2. 7,18,[56][57][58][59][60][61] From Table 2, we can see that our ASC is highly competitive with NiO//RGO, nitrogen doped graphene//LiNi 0.5 Mn 1.5 O 4 and AC// V 2 O 5 $0.6H 2 O ASCs, and shows higher specic capacitance than most of the previous reports as tabulated.Further, we checked the columbic efficiency and kinetic irreversibility by making use of eqn (7) for the current densities of 2 and 20 mA cm À2 , which we found to be 80% and 100%, respectively (Fig. 6C), and hence it can be inferred that at high current density, our ASC also shows high reversibility and enhanced rate capability.Fig. 6C illustrates the plot of specic capacitance with respect to the current densities and it dictates that as the current density increases from 2-20 mA cm À2 , the specic capacitance decreases, which is attributed to their inner active sites that are not capable of completely withstanding the redox alterations at higher scan rates. 51,52he power density (PD) and energy density (ED) are generally used as important parameters to characterize the electrochemical performance of electrochemical cells.The ED at different PD was calculated for our ASC from the discharge curves at different current densities according to eqn (3) and ( 4).The plot of energy density versus power density (Ragone plot) of the fabricated asymmetric cell was constructed from the GCD measurements carried out at various current densities from 2-20 mA cm À2 and was compared with previous ASC reports as shown in Fig. 6D.The ED and PD of our ASC are higher than the previous reports as observed in Fig. 6D. 18,49,59,62,63Beyond the obvious reduction in the energy density upon increasing the power density, noticeable enhancements were noted in both.This might be due to the application of a comparatively high potential window of 0-1.65 V.The Ni/Ni(OH) 2 //AC asymmetric cell exhibited good retention ability in energy density from 23.45 W h kg À1 to 3.9 W h kg À1 , while increasing the power density from 530 W kg À1 to 4598 W kg À1 .5][66][67] The enhanced cycling stability was noticed even aer 6000 consecutive cycles of GCD at a current density of 15 mA cm À2 between 0 and 1.65 V. Fig. 7A shows the capacitance retention ratio of the asymmetric capacitor charged at 1.65 V as a function of the cycle number.It is worth noting that the specic capacitance sharply increases aer 2000 cycles.A substantial increase in the specic capacitance while extending the cycling might be due to surface wetting.The highly improved chronic cycling stability is depicted through the impressive 90.6% retention of specic capacitance of our asymmetric cell, even aer 6000 cycles.Initially, we observed a small decrease in capacitance in the range of 1000 to 2000 cycles, which might be due to some electro active material not being completely accessible for the diffusion of ions. 68,69Aer continuous cycling, all the inactive parts of the working electrodes were completely open for diffusion of electrolyte ions and consequently, capacitance retention improved and showed the highest capacitance retention in the range of 3000-4000 cycles. 70For better understanding the rst 20 cycles, 20 cycles in the middle and the last 20 cycles are provided in Fig. S6 and S7, † respectively.The reversibility and the rate capability were determined by checking the CV prole aer 6000 GCD cycles, shown in the inset of Fig. 7A.The two CV curves exhibit nearly similar proles that imply better reversibility and excellent rate capability of our ASC cell.Further, we checked the columbic efficiency of the ASC cell during 6000 cycles.The columbic efficiency observed for the rst cycle was 98%, and the efficiency retained aer 6000 cycles was 90% (Fig. S5 †).Moreover, the observed cycling stability and retention of specic capacitance is comparatively better than some previous reports of similar studies, such as NiMn 2 O 4 @CNT//AC showing 83% capacitance retention aer 3000 cycles, 71 NiO//RGO with 88% retention aer 2000 cycles, 56 NiCoS//AC showing 73.1% retention aer 3000 cycles, 72 Ni(OH) 2 /CNT//AC with 83% retention aer 3000 cycles, 73 AC//AC-NiO NFs with 88% retention aer 5000 cycles, 74 NiCoS//AC showing 79.1% retention aer 6000 cycles, 75 and Ni(OH) 2 /graphene//RuO 2 /graphene with 92% retention aer 5000 cycles. 76Further, we carried out the EIS to nd out the ESR and R CT of the ASC cell before and aer 6000 cycles.Fig. 7B shows Nyquist plots of the ASC cell before and aer cycling, tted with an equivalent circuit, as shown in the inset of Fig. 5B.A distorted semicircle in the high frequency region and a line with a slope in the low frequency region, are observed in the Nyquist plot.The semicircle signies the charge transfer process and the slope indicates the capacitive nature of the ASC cell.The ESR of the ASC cell is not varied before and aer cycling and remains at 1.14 ohm, but the R CT increased from 0.61 to 0.87 ohm aer cycling.This signies the greater stability and high rate capability of the ASC cell.The synergism between the negative AC electrode and positive Ni/Ni(OH) 2 electrode along with the wide potential window has substantially increased the overall electrochemical performance of the fabricated ASC cell (Ni/Ni(OH) 2 //AC).Further, the ASC cell shows 90% of coulombic efficiency, even aer 6000 cycles at high current density of 15 mA cm À2 (Fig. S5 †), which can be attributed to the low ohmic resistance of the positive Ni/Ni(OH) 2 NS electrode, due to the presence of a conductive network of nickel metal.8][79] In the future, we hope that this will open up a new avenue for fabricating other transition metal oxide/hydroxide composites without using conductive substrates, for application in different areas such as lithium ion batteries, fuel cells and biosensors.

Conclusion
In summary, we have successfully synthesized Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NSs using ethanol as a reducing agent in the presence of KOH, and studied their electrochemical properties in three and two electrode systems.In a three electrode system Ni/Ni(OH) 2 shows better electrochemical properties in terms of specic capacitance and conductive nature, compared to NiO/ Ni(OH) 2 NSs.We have fabricated a hybrid supercapacitor using Ni/Ni(OH) 2 NSs and AC as the positive and negative electrodes, respectively.The ASC showed good specic capacitance, relatively high energy density, and a consistent cycling stability at an operating voltage of about 1.65 V in KOH aqueous electrolytes.It was found that coupling the Ni/Ni(OH) 2 NSs with AC to harvest supercapacitors yields a high specic capacitance of 62 F g À1 at a current density of 2 mA cm À2 .The systematic optimization of the mass of two electrodes resulted in the energy density of 23.45 W h kg À1 without sacricing the power density.The galvanostatic charge-discharge experiment showed excellent capacitance retention of $90%, even aer 6000 consecutive cycles, which is a remarkable achievement for the eld of ASCs.As a result of the interesting ndings obtained in our detailed study on asymmetric Ni/Ni(OH) 2 //AC supercapacitors, we state here that the Ni/Ni(OH) 2 NSs based ASCs could be the system of choice to meet the increasing demands for energy storage devices with high power and energy densities.The synthesis procedure can also be applied to other transition metals to synthesize their metal/metal hydroxide composites to enhance their conductive nature deprived of using conductive substrate.
All the chemicals used in this present work were analytical reagent (AR) grade.The nickel(II) acetate hexahydrate (Ni(Ac) 2 -$6H 2 O), oleylamine, potassium hydroxide (KOH) and cyclohexane were procured from Sigma-Aldrich and used without any further purication.Ethanol was purchased from SRL, India.Polyvinylidene uoride (PVDF), N-methyl-2-pyrrolidinone (NMP), carbon black, activated carbon and nickel foam were obtained from Alfa Aesar and used as received.De-ionized (DI) water was used for the entire synthesis and application purposes.The synthesized Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NSs were characterized using several techniques, such as XRD, TEM, and XPS, and electrochemical studies were done through an electrochemical work station, CHI-6084C.Instrument specications and sample preparation for various characterizations are provided in the online ESI.† Hydrothermal synthesis of Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NS arrays

Scheme 2
Scheme 2 Schematic representation of ASC.
Fig. 3G shows the high resolution TEM image of Ni/ Ni(OH) 2 NSs, which reveals the corresponding crystal planes of Ni and Ni(OH) 2 .This is in accordance with the earlier reports by Chen et al. for their study on polypyrrole shell@3D-Ni metal core 38 and by Liu et al. on their study on layered Ni x Co 2x (OH) 6x @Ni material. 39Formation mechanism of Ni/Ni(OH) 2 NSs Ni/Ni(OH) 2 NSs have been synthesized by the partial reduction of nickel(II) salt by using ethanol as a reducing agent in the presence of KOH at high pressure and at a temperature of 160 C, via the hydrothermal method.We have monitored the role of various reaction parameters in the formation of Ni/ Ni(OH) 2 NSs.It was observed that when we used KOH, it formed Ni/Ni(OH) 2 NSs arrays, whereas on keeping all reaction parameters xed, except for the addition of KOH, it formed NiO/Ni(OH) 2 .On the other hand, when we applied the solvothermal procedure (stirring and heating at 160 C), keeping all other reaction parameters the same except pressure, we ended up with the formation of nickel hydroxide only, which strongly implies the role of KOH and pressure in the partial reduction of Ni(II) to Ni.Moreover, the appropriate combination of all reaction parameters is essential for the formation of Ni/Ni(OH) 2 NSs.Initially, ethanol reacts with KOH at high pressure and temperature and forms potassium ethoxide (CH 2 CH 3 O À K + ), which is a moderate reducing agent and could partially reduce the nickel(II) salt into Ni metal, and the remaining Ni(II) ions are converted to Ni(OH) 2 .A combination of these two simultaneous reactions lead to the formation of Ni/Ni(OH) 2 NSs.The corresponding chemical reactions for the formation of Ni/Ni(OH) 2 are given below.We have analyzed the role of KOH by measuring the pH of reaction mixtures at various stages during the synthesis of Ni/Ni(OH) 2 NSs.The pH of ethanol only was 6.4, 0.1 M KOH only was 13.0, oleylamine only was 11.0, the mixture of oleylamine and ethanol was 9.8, the mixture of ethanol and nickel(II) acetate was 6.4 and the mixture of all reagents was 9.5.
Fig. 4A shows the characteristic CV curves of the synthesized Ni/Ni(OH) 2 and NiO/ Ni(OH) 2 NSs at a 5 mV s À1 scan rate in 1 M KOH aqueous solution.In the case of Ni/Ni(OH) 2 , two anodic peaks were observed; one was related to the oxidation of b-Ni(OH) 2 to b-NiOOH and the other one was related to the phase transformation of a-Ni(OH) 2 to b-Ni(OH) 2 .Because of their unstable nature in alkaline conditions, only one cathode peak was observed, which is due to the reduction of b-NiOOH to b-Ni(OH) 2 within the cathodic region. 49The CV curves are comprised of strong redox peaks, suggesting that the capacitance characteristics are mainly controlled by faradaic redox reactions, which is very distinct from that of EDLCs that usually yield a CV curve close to an ideal rectangular shape.Reversible peaks are observed for the Ni/Ni(OH) 2 and NiO/Ni(OH) 2 NSs.Consequently, the CV curves demonstrate that the Ni/Ni(OH) 2 electrode exhibits much better electrochemical behavior in terms of specic capacitance than the NiO/Ni(OH) 2 electrode, owing to their high integral area.We calculated the specic capacitance values of both Ni/Ni(OH) 2 and NiO/Ni(OH) 2 from eqn (1), which are 536 and 440 F g À1 at 5 mV s À1 , respectively.

Fig. 2 (
Fig. 2 (A) X-ray photoelectron spectra (XPS) of the survey of Ni/Ni(OH) 2 NS; (B) XPS of the Ni 2p state of Ni present in Ni/Ni(OH) 2 NSs.(C) XPS of O 1s state of O present in Ni/Ni(OH) 2 NSs.

Fig
Fig.4Band S2A (ESI †) display the CV curves of Ni/Ni(OH) 2 and NiO/Ni(OH) 2 electrodes at various scan rates such as 5, 10, 25, 50, 75, 100, and 125 mV s À1 .It was observed that all the CV curves exhibited a similar pattern of anodic and cathodic peaks with increasing scan rates, demonstrating the good reversibility of the redox reaction at the nanostructure interface, with excellent rate capability.Moreover, the anodic and cathodic peaks were shied towards the positive and negative potential with rising scan rates.This might be due to the diffusion effect of protons within the electrode or mass transfer limitations of electrolyte ions in order to neutralize the electronic charge on the electrode surface during the redox reaction.50From Fig.4C, we can infer that the specic capacitance and the scan rate share an inverse relationship.This can be attributed to the circulation effect, which prevents the diffusion and movement of the electrolyte ions within the electrode at elevated scan rates, due to their inner active sites that are not capable of fully withstanding the redox alterations at higher scan rate. 51,52g.4D and S2B (ESI †) illustrate the changes in the specic capacitance of the Ni/Ni(OH) 2 and NiO/Ni(OH) 2 electrodes at different current densities (1-8 mA cm À2 ).The symmetric and Fig.4Band S2A (ESI †) display the CV curves of Ni/Ni(OH) 2 and NiO/Ni(OH) 2 electrodes at various scan rates such as 5, 10, 25, 50, 75, 100, and 125 mV s À1 .It was observed that all the CV curves exhibited a similar pattern of anodic and cathodic peaks with increasing scan rates, demonstrating the good reversibility of the redox reaction at the nanostructure interface, with excellent rate capability.Moreover, the anodic and cathodic peaks were shied towards the positive and negative potential with rising scan rates.This might be due to the diffusion effect of protons within the electrode or mass transfer limitations of electrolyte ions in order to neutralize the electronic charge on the electrode surface during the redox reaction.50From Fig.4C, we can infer that the specic capacitance and the scan rate share an inverse relationship.This can be attributed to the circulation effect, which prevents the diffusion and movement of the electrolyte ions within the electrode at elevated scan rates, due to their inner active sites that are not capable of fully withstanding the redox alterations at higher scan rate.51,52Fig.4Dand S2B (ESI †) illustrate the changes in the specic capacitance of the Ni/Ni(OH) 2 and NiO/Ni(OH) 2 electrodes at different current densities (1-8 mA cm À2 ).The symmetric and non-linear characteristics of the GCD curves are shown at various current densities, which further corroborate the pseudo-capacitive behavior of our working electrodes.The voltage plateaus in the charge/discharge process are consistent with faradaic oxidation and reduction peaks in the CV.For comparison, the GCD curves of both electrodes at current density of 1 mA cm À2 are shown in Fig. 5A.The calculated specic capacitance of the Ni/Ni(OH) 2 and NiO/Ni(OH) 2 electrodes are 450 and 343 F g À1 , respectively, at a current density of 1 mA cm À2 .For better understanding, we compared the capacitance of two composites Ni/Ni(OH) 2 and NiO/Ni(OH) 2 at various scan rates (5-125 mV s À1 ) and at different current densities (1-8 mA cm À2 ), tabulated as TablesS1 and S2, † respectively.From the tables, we can see that the Ni/Ni(OH) 2 NS composite shows better rate capability than the NiO/Ni(OH) 2 NS composite.Further, we calculated the coulombic efficiency (h) of the sample to check the reversibility of the working electrode, which was calculated by using the formula given below:

Fig. 3 (
Fig. 3 (A and B) Transmission electron microscopic (TEM) images of Ni/Ni(OH) 2 NS at lower and higher magnification; (C) the corresponding SAED pattern; (D and E) TEM images of NiO/Ni(OH) 2 NS at lower and higher magnification; (F) the corresponding SAED pattern and (G) high resolution TEM image of Ni/Ni(OH) 2 NS.

Fig. 5C shows
Fig. 5C shows the nal 10 charge discharge cycles, which implies that our active material is showing excellent rate capability.Fig. 5D depicts the EIS behavior of Ni/Ni(OH) 2 NS before and aer cycling, tted with the same equivalent circuit (inset of Fig. 5B).There is not much increase in ESR and R CT values (before cycling 1.88 and 10.63 U and aer cycling 2.89 and 13.84 U, respectively), which indicates the better electrochemical stability and excellent rate capability of Ni/Ni(OH) 2 NS.Of the two different NSs, Ni/Ni(OH) 2 shows better electrochemical performance compared to NiO/Ni(OH) 2 .This is attributed to the nickel metal cores that are present in the Ni/Ni(OH) 2 NS, as well as the high specic surface area (SSA) of 58.13 m 2 g À1 , compared to the NiO/Ni(OH) 2 SSA of 27.26 m 2 g À1 using N 2 adsorption-desorption isotherms via the Brunauer-Emmett-Teller BET surface area measurement.Considering that the electrochemical performance of Ni/Ni(OH) 2 materials largely depends on their surface nano sheet structure and electronic conductivity, we propose three aspects that contribute to the superior electrochemical performance of the Ni/Ni(OH) 2 NSs.First, the specic surface area (SSA) of the as-synthesized Ni/ Ni(OH) 2 NSs is higher than the NiO/Ni(OH) 2 , consequently promoting the diffusion of electrolyte on the surface of the electrode and electrical conductivity of the electrode.Second,

Fig. 6 (
Fig. 6 (A) CV curves of ASC fabricated using Ni/Ni(OH) 2 //AC at various scan rates; (B) galvanostatic charge/discharge (GCD) curves of ASC at various current densities.(C) Specific capacitance and coulombic efficiency as function of current density for ASC; (D) Ragone plot comparison with previous reports.

Fig. 7 (
Fig. 7 (A) Cyclic performance of ASC at 15 mA cm À2 for 6000 cycles.Inset: CV curves at a scan rate of 5 mV s À1 before and after 6000 cycles.(B) Nyquist plots of the ASC cell before and after cycling; inset: high frequency region of Nyquist plot.
Some of the signicant reports are highlighted below.Yu et al. synthesized NiCo 2 O 4 nano needles on graphenenickel foam (GNF) by a two-step approach; 30 Ho et al. synthesized MnO 2 electrochemically deposited on stainless steel; 31 Gong et al. synthesized nickel cobalt hydroxide microspheres electrodeposited on nickel cobaltite nanowires grown on Ni foam by a two step method,

Table 2
Comparative specific capacitance of the Ni/Ni(OH) 2 //AC ASC cell and other earlier reports