Microstructure and Electrochemical Performance of Supercapacitor Based on Nickel/Activated Carbon Composite Electrode

Supercapacitors are promising electrochemical energy storage devices because they have higher energy than conventional capacitors and higher power than rechargeable batteries. Based on material for supercapacitors, activated Carbon (AC) has large specific surface area, high conductivity, and chemical stability, but still shows less than optimal specific capacitance and energy density. In this work, AC was composited with nickel (Ni) and Carbon Black (CB) to modify microstructure and optimize its electrochemical performance. Initially, synthesis of Ni-AC (cathode) and AC (anode) slurry was carried out by blending method in different mass percentages of Ni (5, 10, 15, 20, 25 %). The slurry was coated using doctor blade on aluminum foil. Then, Ni-AC//AC was fabricated and tested by Galvanostatic Charge-Discharge (GCD) instrument to analyze the electrochemical performance changes of supercapacitor. Based on XRD pattern, AC peak was found at 26.5° and Ni-AC composite had additional peaks in 44.4°, 51.7°, and 76.2°. The crystal size and porosity of electrode were in range of 23.2 – 45.4 nm and 66.6 – 75.7%, respectively. Based on the electrochemical evaluation, addition of 20% Ni mass in activated carbon electrode has the optimum performance, which increases active sites of the electrode and ion electrolyte adsorption capacity. Further, the GCD revealed that the prepared Ni-AC//AC electrode have excellent capacitive behavior with specific capacitance of 56.6 F/g, power density of 308.8 W/kg, energy density of 20.8 Wh/kg, and specific retention of 69.6% until 50 cycles.


Introduction
Improvements in electrochemical energy devices are considered necessary by the increasing demand for low-cost, high-performance, and flexible energy storage devices.Supercapacitors are energy storage devices that utilize the principle of electrochemical energy conversion, which uses adsorption or chemical reaction processes to store electric charge from the electrolyte [1].Supercapacitors are employed as a compelling alternative to batteries because of their high-power density and fast charging capability [2], [3].The electrochemical performance of electrodes is dependent on several parameters, including electrolyte permeability, electrical conductivity, and surface area [4]- [6].Supercapacitors are classified as Electrochemical Double Layer Capacitors (EDLC) and pseudocapacitors based on charge storage mechanism.In EDLC, charge is stored at electrode interface by forming an electrochemical double layer from adsorption process ion to electrode.In the other side, pseudocapacitor undergoes a fast redox reaction with electrolyte ions [7].
EDLC-type supercapacitors have high power and long cycle life (>500,000 cycles), but it has low energy density [8], [9].EDLCs generally have an energy density of 5-6 Wh/kg compared to the energy density of batteries up to 150 Wh/kg [10], hence encouraging to enhance the performance of supercapacitors with longer life cycles and high energy densities.Asymmetric arrangemnet is a desirable method to increase the energy density of supercapacitors, as it can enhance the device capacitance [11], [12].Each electrode and electrolyte interface in an asymmetric supercapacitor represents a double-layer capacitor, allowing the entire cell to be thought of as two capacitors in series.In comparison to EDLC, pseudocapacitor materials exhibit faster faradaic redox reactions, which lead to increased capacitance and energy density [13].In EDLC, conduction band electrons from metal or carbon electrodes are utilized [14], while in pseudocapacitors come from valence orbitals from redox reactions [15].Some materials used in pseudocapacitors are transition metals (Co, Ni, Mn, etc.), transition metal oxides (NiO, RuO, MnO2, CuO, etc.), conducting polymers, Metal-Organic Frameworks (MOF), MXenes, Transition Metal Dichalcogenides (TMDs) [16]- [20].Integrating EDLC-type materials with pseudocapacitor-type materials is expected to achieve balanced performance between high energy and power density in supercapacitor devices.
Due to their high theoretical capacitance, strong cycle stability, and ease of modification, transition metals including their oxides or hydroxides have a greater specific capacitance than other materials [21], [22].Nickel and cobalt-based materials are more suitable for applications in supercapacitors because of their high corrosive resistance to alkalis and high capacitance in alkaline media [23], [24].Materials based on nickel have garnered interest due to their environmental friendliness, low cost, and high redox activity [25]- [27].Nickel has the potential application as electrode in supercapacitors because it has high thermal and electrical conductivity, and shows magnetic properties below temperatures of 345 ˚C [28].Some of its distinctive properties include having a specific surface area of 40-60 m 2 /g, particle size ranging from 2 -50 nm, density of 8.9 g/cm 3 , and thermal conductivity in the temperature range 0-100 ˚C of 88.5 W/mK [29].
In previous research, ZnO-FC-NiCo MOF with different Ni-Co mass was successfully synthesized but still had low specific capacitance of 6.62 F/g [30].In other research, nickel nanoparticles embedded in nickel foam substrates were successfully synthesized and showed good specific capacitance and cycle.Nickel nanoparticles with activated carbon were also successfully synthesized and produced a specific capacitance of 154.2 F/g, but it is still unknown regarding the effect of adding Ni mass to the Ni-AC composite [31].Additionally, Park et al. succeeded in making a Ni/C asymmetric supercapacitor with high energy density of 35.7 Wh/kg [32].Research report by Zheng et al [33] reported a model that uses mass ratios of positive and negative electrodes as well as amount of electrolyte needed to predict the highest energy density that can be obtained in an asymmetric design.The model predicts a maximum of 41.7 Wh/kg for Ni/C.However, the theory's practical application has not been tested experimentally.Therefore, the purpose of this work is to examine how nickel particles affect activated carbon in an asymmetric supercapacitor.Some research examines the increasing capacitance that focuses on metal oxides and carbon complexes.However, there are still few research reports that explain the effect of nickel nanoparticles addition and pure carbon on its microstructure and its electrochemical performance.

Methods
In this work, Nickel (Ni), Activated Carbon (AC), Carbon Black (CB) as conductive material, Polyvinylidene fluoride (PVDF) as binder, and Dimethyl Acetamide (DMAC) as solvent were purchased from Merck Company (Germany) without any purification.In addition, tetraethylammonium tetrafluoroborate (Et4NBF4) as organic electrolyte was purchased from China.The ratio of active material, conductive material, and binder in electrode was 80%: 10%: 10%.Synthesis of Ni-AC slurry (cathode) was made by dissolving PVDF in DMAC at 300 rpm for 30 minutes (50˚C).Then, the temperature was lowered to ambient temperature (25 °C) and Ni was added with 5 -25% of active material.The mixture was stirred for 6 hours until the slurry was homogeneous.The slurry was coated on aluminum foil using doctor blade method with thickness set of 200 µm.The Ni-AC was annealed at 100˚C for 60 minutes.On the other side, AC-CB (anode) was prepared by the same method as cathode, but without Ni addition.The composition of Ni-AC//AC asymmetric supercapacitor electrodes is thoroughly explained in Figure 1.
The phase of powder and composite electrode were determined using a PANalytical Xpert Pro Diffractometer X-ray using Cu-Kα radiation (λ=1.54Å) and 40 kV.The morphology and porosity of the composites were investigated with a scanning electron microscope (FEI Nova NanoSEM450).The electrochemical performance was tested using Charge Discharge Battery Testing System NEWARE with sandwiching electrodes, 1 M Et4NBF4 (tetraethylammonium tetrafluoroborate) as electrolyte, and polyethylene as separator, as shown in Figure 1.Where D is the crystal size,  is the shape factor,  is the XRD wavelength,  is the FWHM,  the Bragg angle.Table 1 shows the result of crystal size and crystallinity of AC and Ni-AC.The addition of Ni caused a significant increasement in crystal size and crystallinity percentage.Crystal size and crystallinity can enhance conductivity and electroactive redox reaction sites [34], [35].Because of structural flaws and a disordered atomic arrangement, low crystallinity or amorphous structures can offer more electroactive sites for redox processes as well as improved electrolyte ion absorption and diffusion [34].

Morphology of Ni-AC//AC Composite
Figure 3 showed the SEM result of AC and Ni-AC composite.AC was in irreguler form and there was visible agglomeration.In Figure 3(b-f) there were also round flakes that were part of nickel, addition Ni reduces agglomeration in the composite.This is in correlate with research conducted by Lili Ding et al that reported nickel has weak agglomerated particles, which is caused by heterogeneous deposition [31].The porosity of a material can be determined by changing the SEM image into a gray scale matrix, as in Figure 4. Porosity and pore size of each sample, the porosity of the Ni-AC//AC composite can be measured from SEM images using Origin2018 (OriginLab, Northampton, MA, USA) and showed in Table 2 [36] :  Based on Table 2, it also shows that each additional nickel mass can increase the porosity of the resulting Ni-AC//AC electrode.High porosity indicates good electron mobility and density that can accelerate ion movement on electrode surface and improve supercapacitor performance.Pore size in the composite is in range of 39 -42 nm (39.2 nm average).Wider pore size is also related to ion accessibility and charge storage on the electrodes [37].

Electrochemical Properties of Ni-AC//AC Composite
Electrochemical performance of Ni-AC//AC as electrode was investigated using Galvanotic Charge-Discharge (CD) and was showed in Figure 5 using 1.0 M Et4NBF4 organic electrolyte.From the CD curve, some parameters can be obtained i.e. specific capacitance, energy density, and power density using Equations 2, 3, and 4 [38], [39]: () 2 With Cs is specific capacitance of supercapacitor (F/g), I is the discharging current (A), t is the discharging time (s), ∆V is potential difference in discharging process (V), m is mass of active material in electrode (mg), Ed is energy density (Wh/Kg), Pd is power density (W/Kg), and the Multiplier 4 is to adjust the capacitance of two electrode cells with single electrode capacitance [40].The recapitulation of specific capacitance values, energy density, and power density is shown in Table 3.The addition of Ni in composite increased the specific capacitance of supercapacitor because Ni enlarges specific surface area, porosity, and electroactive surfaces for faradaic redox reaction [7].Ni also demonstrated efficient energy conversion and storage performance, where Ni contributes to electron transmission in the electrolyte [31].Figure 5 shows that 20% Ni has a longer discharge time, which indicates it has better electrochemical performance than others.The interaction between AC and Nickel nanoparticles resulted in better electrochemical performance due to the pseudocapacitance arising from Ni and the EDLC contribution by AC.

3 .
Results and Discussion 3.1 Structure and Crystal Size X-ray diffraction with the Cu K and wavelength of 1.54 Ǻ was used for testing diffraction pattern with the intensity range of 10°-90°.The XRD pattern of AC and Ni-AC composite on aluminum substrate is shown in Figure2.Two main diffraction peak angles at 65˚ and 78˚ were clearly for aluminum substrate which matches COD database number 4313214.The diffraction also shows AC peak at 26.5˚ with a distance between planes of 0.34 nm and Ni-AC has the highest peak at 44.4˚ with a distance between planes of 0.20 nm.Where the composite diffraction peaks are at 2 26.5˚, 44.4˚, 51.7˚, 59.8˚, 76.2˚

Figure 2 .
Figure 2. The XRD pattern of AC-Nickel

Figure 5 .
Figure 5. shows that increasing the mass of Ni also increases the value of specific capacitance, energy density and power density which reaches the optimum value when adding a mass of 20% Ni.Fast redox reaction of Ni properties can increase the mobility and transmission of electrons in electrolyte.The stability of the supercapacitor or specific retention of the Ni-AC//AC electrode increases in direct proportion to the addition of Ni mass, as shown in Figure 6.This was corresponding with research conducted by Yu Cheng, et al where Ni-based electrodes showed excellent supercapacitor performance in long-term charge-discharge cycles [7], indicating that nickel nanoparticles are a promising electrode material for supercapacitor applications.The amount of specific retention for each additional mass of Ni (5, 10, 15, 20, 25)% were 42.2, 56.7, 63.3 , 69.6 , 59.2 % , respectively after reaching 50 cycles.

Figure 6 .
Figure 6.Energy density, power density, and capacitance retention of supercapacitor with variations Ni mass

Table 1 .
Recapitulation of Crystal Size and Crystallinity Percentage

Table 2 .
Porosity and Pore Diameter of Composite Electrode

Table 3 .
Electrochemical Performance of Supercapacitor Electrode