Fabrication of NiO@Ni(OH)2/Graphene Nanosheets//Carbon Electrode as Asymmetric Configuration in Coin Cell Supercapacitor

Electrochemical energy storage devices have emerged as a significant concern for contemporary society global and becoming a devices with great performance are in high demand. Supercapacitors are an alternative that is very suitable for use because of their high-power density, environmentally friendly, long-term cycle, safety, and abundance in nature. NiO and Ni(OH)2 nanoparticles have attracted global attention because of their varied application possibilities and advantages such as low toxicity, low cost, high theoretical capacitance, and environmentally friendly properties. NiO was composited with Ni(OH)2 in this study to enhance the supercapacitor device’s specific capacitance, energy density, and power density. The synthesis of NiO@ Ni(OH)2/Graphene electrodes used a blending method with mass variations of NiO (5, 10, 15, 20, 25) % as active material, and then was characterized using XRD to determine phase constituents and crystal size, SEM to determine morphology, FTIR to test functional groups, Cyclic Voltammetry and Galvanotic Charge-Discharge to test electrochemical properties. The asymmetric coin cell with 15% NiO mass in Ni(OH)2@NiO/ Graphene//Activated Carbon composite has the optimum power density and energy density of 327.4 W/kg and 17.1 Wh/kg.


Introduction
The increasing use of fossil fuels to meet energy needs is causing energy crisis and an environmental emergency.This encourages researchers and industry to develop alternative renewable energy sources from wind, sun, and geothermal.The two most advanced renewable energy storage technologies at the moment are supercapacitors, which have a high power density, and lithium-ion batteries, which have a higher energy density than supercapacitor [1].Lithium-ion batteries are not environmentally friendly and pose safety risks due to their flammability and thermal instability [2].Because of wide range of electronic and industrial applications, there is strong need for more effective IOP Publishing doi:10.1088/1742-6596/2734/1/012021 2 energy storage technologies.Supercapacitors are an excellent substitute because has high power density, short charge-discharge duration, extended cycle life, and safety [3].Supercapacitor in general is composed of electrolytes, electrodes, and a separator that maintains the electrical distance between the two electrodes [4].In addition, asymmetric supercapacitors are those with two electrodes divided by an electrolyte, where the cathode (negative electrode) is typically carbon and the anode (positive electrode) is made of a material with a higher capacitance [5].Large specific surface area (SSA), high ionic conductivity, and excellent redox properties are needed to maximize the high-performance supercapacitor's electrochemical performance [6].Furthermore, it is imperative to use pseudocapacitive materials such as conducting polymers or metal oxides, which are both materials enhance capacitance and increase energy density through surface redox processes without causing the phase change of material [7].In recent years, some transition metal hydroxides and oxides, including RuO2, Co3O4, NiO, and Ni(OH)2, have been studied and explored as potential materials for pseudocapacitors [8].The advantages of NiO and Ni(OH)2 nanoparticles, including their low cost, low toxicity, high theoretical capacitance, and environmentally friendly characteristics, have drawn interest from all over the world [9].The benefits of graphene and activated carbon include their large surface areas, excellent chemical stability, high mechanical strength, and outstanding electrical conductivity [10] [11].
In a prior work, used hydrothermal approach to successfully synthesize GNS/Ni(OH)2 that contained graphene oxide.Electrochemical properties at current density of 0.3 A g -1 was reaching high specific capacitance of 2053 F g -1 .However, at 3.6 A g -1 , this performance only has 61% of capacitance retention from initial capacitance [12] [13].The composite in this study continued to exhibit low retention capacitance and low rate capacity.Time-dependent GNSs/NiO@Ni(OH)2 nanoplates were created in a different study to yield a retention capacitance of 92% at 3 A g -1 and reached high specific capacitance up to 653 F g -1 at 24 A g -1 [14][15].This indicates that GNSs/Ni(OH)2 performs worse than composites of GNSs/NiO@Ni(OH)2.Impact of incorporating NiO and the utilization of other NiO structures, however, have not been examined in this work.Two metal (hydro)oxides built on graphene were found to perform better in several articles.Research on the optimization of supercapacitor performance by mixing GNSs/NiO@Ni(OH)2 cathode materials with Activated Carbon (AC) anodes with varied compositions and synthesis processes is therefore needed to overcome the constraints of its implementation in electrochemical energy storage devices.The development of this material into a single device is expected to show huge potential in increasing the performance of the supercapacitor.

Synthesis Electrode GNSs/NiO@Ni(OH)2
Material that used in synthesis are NiO, Ni(OH)2, NaOH, graphene, LA133, activated carbon, carbon black, propylene carbonate, nickel foam, ethanol, alcohol, and DI water.Initially, 9.8 grams of Ni(NO3)2.6H2Owas added into 60 mL of DI water, and was stirred for an hour at 50 ˚C.In separated solution, 3 grams of NaOH were dissolved in 150 mL of DI water and stirred magnetically for 1 hour at room temperature.After preparing the NaOH solution, it is dripped slowly into homogenous solution of Ni(NO3)2.6H2Osolution while being stirred magnetically at 50 ˚C for 30 minutes.After that, mixed soultion were annealed at 150 ˚C for 11 hours to remove solvent until found the green powder of Ni(OH)2.The next step was created film electrode, 35 mg of binder LA133 was dissolved in 2 ml of DI water for 30 minutes.Then, NiO and Ni(OH)2 powder were added in solution with mass variation of NiO (5;10;15;20;25)% of the total mass.After 24 hours of continuous magnetic stirring, 10 mg of graphene was added, and mixture continously stirred until homogeneous slurry was obtained.The slurry coated on Ni foam 20 µL in each side.

Synthesis Electrode AC/CB
Material that used to create the AC/CB composite slurry are 80 mg of Activated Carbon (AC), 35 mg of LA133 as binder, and 10 mg of Carbon Black (CB) as conductive material were combined with 5 mL of DI water solution to create a homogenous mixture.Using a micropipette, the GNS/NiO@Ni(OH)2 composite solution and the AC/CB composite were applied to a nickel foam substrate.Each was then dried for an hour at 100 ˚C in an oven.

Preparing Asymmetric Supercapacitor
The fabrication of coin cell asymmetric supercapacitor consists of 3 components are electrodes, electrolyte and separator.Electrodes from the GNSs/NiO@Ni(OH)2 film and 1 sheet of activated carbon film with a diameter of 1.5 cm were prepared.The electrolyte used is EtNBF4 (tetraethylammonium tetrafluoroborate) and the separator is polyethylene with the same size.Figure 1 is a schematic of a coin cell asymmetric supercapacitor.

XRD Pattern
The lattice parameters, material phase, and crystal size can all be determined using the X-ray diffraction method.Utilizing a diffraction pattern in the 20°-70° range and a Cu Kα wave number with a wavelength of 1.54 Å.The resulting X-ray diffraction pattern was shown in Figure 2  The crystal size can be determined by equation ( 1): (1) D is explaining crystal size (nm), K is form factor, XRD wavelength, L is FWHM, and cos θ is Bragg angle.Crystal size of Ni(OH)2@NiO/Graphene sample with NiO mass variation (5,10,15,20, 25) % of active mass was obtained at 44.9 nm, 45.2 nm, 49.6 nm, 52.1 nm and 53.9 nm.The addition of NiO causes a significant size change [16].

Functional Group Bonds
Through group bonding, FTIR spectroscopy is utilized to detect the presence of foreign particles.The FTIR characterization results for the two samples Ni (OH)2 and NiO powder are shown in Figure 3. Ni(OH)2 FTIR spectra revealed distinctive peaks at 3429, 2986, 2465, 2357, 1952, 1323, and 837 cm -1 .The O-H strain vibration that is typical of Ni(OH)2 and other OH-containing compounds caused by broad absorption band of approximately 3429 cm -1 .The band at 1634 cm -1 was product of N-H bond remnants in sample or flexural vibration of water molecules [17].The existence of the CO 3 2− anion from Ni(OH)2 with air interaction caused by band at 1385 cm -1 .Additionally, the NiO nanoparticles spectra is shown to have distinct peaks from those of Ni(OH)2 nanoparticles, this suggests that Ni(OH)2 successfully converted to NiO during the calcination process [17].The absorption bands at 3640, 2785, 2417, 1763, 1634, 1385, 998, and 669 cm -1 contain the distinctive peaks.The vibration of Ni-O bond is reflected in emergence of a new band at 669 cm -1 .Majority transition of OH groups into oxides was further supported by the sharp drop in intensity from the OH peak at 3640 cm -1 in Ni(OH)2 to decreased peak at 3429 cm -1 in NiO [18].

Morphology
The morphology of Ni(OH)2@NiO/Graphene composite samples were characterized using a scanning electron microscope (SEM) instrument to analyse their shape and porosity.Composite of Ni(OH)2@NiO/Graphene that coated on surface of nickel foam substrate are shown in Figure 4. SEM result showed aggregation on top of nickel foam substrate and Ni(OH)2 nanostructures uniformly distributed on surface in squares shape or uneven sides [19].There are also spherical flakes that part of NiO.In addition, NiO mass also reduces agglomeration in composite and shows a slight change in pores.The porosity of a material can be determined by converting the SEM image into a gray scale matrix, as shown in Figure 4.The porosity of Ni(OH)2@NiO/Graphene composite can be measured from SEM image using Origin 2018 (OriginLab, Northampton, MA,USA), and using methodology described by Abdullah and Khairurrijal as in Equation ( 2

Galvanostatic Charge-Discharge
The measurement of electrochemical properties of Ni(OH)2@NiO/Graphene as an electrode was investigated using Galvanostatic Charge-Discharge (CD). Figure 5. shows the results of Charge Discharge electrodes with several variations in NiO mass (5,10,15,20,25)% of active material using 1.0 M EtNBF4 liquid electrolyte.From GCD graph, it can be processed to obtained specific capacitance values, energy density, and power density using Equations 3, 4, and 5: Cs is specific capacitance parameter (F/g), I is current in discharging process (A), t is time in discharging process (s), V is potential difference in discharging process (V), m is mass of active material in electrode (g), PD is power density (W/Kg), ED is energy density (Wh) /Kg) and multiplier by 4 is to match the ratio of a single electrode's capacitance to that of two electrode cells [21].Equations 3, 4, and 5 are used to process data from the CD graph and get values for specific capacitance, power density, and energy density,.
Supercapacitor performance is influenced by electrolyte and separator that chosen.Liquid electrolytes have promising potential because their non-flammable characteristic [22].One type of IOP Publishing doi:10.1088/1742-6596/2734/1/0120217 liquid electrolyte that is organic electrolyte such as Et4NBF4 (tetraethylammonium tetrafluoroborate) because can operate between 2.5 and 2.8 V in voltage [23].In order to prevent short circuits, separator keeps two electrodes apart.For transfer of electrolyte ions more easily, separator needs to be thin and porous.Polyethylene is a commonly utilized separator due to its thinness, that allowing ions to spread out without coming together when supercapacitor is charged with voltage [24].The Figure 5 illustrates how electrochemical performance of asymmetric supercapacitor Ni(OH)2@NiO/Graphene//Activated carbon can be determined through charge-discharge characterization using 1 M Et4NBF4 solution as electrolyte in a two-electrode system.GCD test have start with 2 Volts and current density of 0.1 Ag -1 .The Ni(OH)2@NiO/Graphene/Activated carbon composite has good reversibility, as demonstrated by almost symmetrical Charge-Discharge graph.The interaction between AC and NiO results in better electrochemical performance due to pseudocapacitance arising from NiO and contribution of EDLC by activated carbon [25].The curve in Figure 6.shows the relationship between voltage (V) and time (s) on Ni(OH)2@NiO/Graphene//Activated carbon supercapacity device which was most optimum at addition of 15% NiO mass.This curve has a slightly sloping triangular shape, this is caused by a redox process which indicates that the sample has good electrochemical reversibility.The presence of a sloping shape in this curve is caused by a voltage drop (IR drop).Based on the curve of Charge-Discharge data's as shown in Figure 7.This demonstrates how supercapacitor Ni(OH)2@NiO/Graphene/Activated carbon can have its energy density and power density increased by addition of NiO mass.The most optimal values of 17.1 Wh/kg for energy density and 327.4 W/kg for power density were obtained with the addition of 15% NiO mass.These findings suggest that the Ni(OH)2@NiO/Graphene composite has strong electrical conductivity when NiO is added.Additionally, NiO has demonstrated effective energy conversion and storage capabilities.NiO helps to increase capacitance and facilitates electron transmission in the electrolyte.Nevertheless, the energy density and power density dropped when 20% more NiO mass was added.The findings of Lili Ding et al. investigation demonstrate the limitations of adding NiO mass [25].As more NiO is added, capacitance will decrease because the NiO particles will clog pores and prevent ions from diffusing into composite pores.

Cyclic Voltammetry
Ni(OH)2@NiO/Graphene composite electrodes electrochemical performance can also be assessed by cyclic voltammetry characterization utilizing a two-electrode system and Et4NBF4 solution.Figure 8. shown a quasi-rectangular (or semi-rectangular) CV curve, indicating that EDLC rather than a pseudocapacitor dominates capacity [26].This is inextricably linked to the supercapacitor device, which uses carbon material as a negative electrode and is part of EDLC material [27].The excellent curve that forms without being cut off demonstrates increasing in conduction of electron in the electrode, facilitating quick charge diffusion and resulting in excellent and stable capacitive performance.Although the CV curve displays the characteristics of EDLC, addition of NiO mass increases curve area.This finding indicates that Ni(OH)2 and NiO contribute to the enhancement of EDLC electrode performance [28].The increase in curve area also shows that a lot of charge is stored by electrode so that it can increase capacitance of the supercapacitor device.

Conclusion
The Ni(OH)2@NiO/Graphene electrode composite crystal size increased with the addition of NiO mass.The crystal sizes were 44.9 nm, 45.2 nm, 49.6 nm, 52.1 nm, and 53.9 nm for each incremental mass of NiO (5-25)%.The Ni(OH)2@NiO/Graphene composite has two prominent diffraction peaks, located at angles of 2θ = 44.5°and 51.8°.NiO mass addition also results in a modest alteration in the pores and decreases agglomeration in the composite.With inclusion of 15% NiO mass, the asymmetric coin cell composite Ni(OH)2@NiO/Graphene/Activated Carbon has the best power density and energy density of 327.4 W/kg and 17.1 Wh/kg, respectively.

Figure 1 .
Figure 1.Coin cell scheme of asymmetric supercapacitor , two dominating peaks representing the [003] and [012] planes form at 2 = 44.5°and 2 = 51.8°.Some diffraction peaks at 2θ 18.7˚, 33˚, 44.5˚, 51.8˚, and 76.3˚ from the XRD results it is known that the sample is in accordance with the NiO COD database number 1010093 and Ni(OH)2 COD number 1011134.

Figure 6 .
Figure 6.CD curve and cycle stability of supercapacitor performance at the addition of 15% NiO

Table 1
further demonstrates that porosity of Ni(OH)2@NiO/Graphene electrode decreased with each addition of NiO mass.Good electron density and high porosity influence mobility, which speeds up ion movement on the electrode surface and enhances supercapacitor performance[20].