Synthesis of cellulose acetate (CA) from algae Gracilaria sp. composited with nickel oxide (NiO) as a supercapacitor base material

In this research, electrodes were made from cellulose acetate (CA) synthesized from algae Gracilaria sp. and then composited with nickel oxide (NiO), the concentration of which varied from 0, 0.2, 0.4, and 0.6 grams. Furthermore, FT-IR characterized cellulose acetate, and the CA-NiOn electrode was characterized by XRD, SEM, and cyclic voltammetry (CV). The results showed that CA was successfully synthesized from Gracilaria sp. Increasing the concentration of NiO added to CA as an electrode could increase the specific capacity, energy density, and power density of the electrode with the highest degree of 83.27 F/g, energy density of 4 Wh/kg, and a power density of 0.4 W/kg at a concentration of 0.6 gram NiO. The effect of the addition of NiO on the characteristics of the CA-NiOn electrode was also studied such as crystallinity, crystal size, and porosity. The presentation of CA doped with NiO has the promising prospects as a supercapacitor base material.


Introduction
Electrical energy in today's modern life has become a major necessity in human life [1]; this then drives the consumption of fossil fuels so that it has an impact on the increasing environmental pollution [2]. Batteries as an energy storage technology can reduce the use of fossil fuels, but storms have a relatively small power density, require quite a long time to charge, and are heavy, easily heated, and toxic [3]. Supercapacitors as an energy storage technology can be a solution for having advantages over batteries: high energy density, high-temperature resistance, fast charging, and environmental friendliness [4].
Cellulose acetate (CA) can be a promising alternative for manufacturing supercapacitors as efficient energy storage as it has a number of unique characteristics in terms of power and longevity and being environmentally friendly [5]. Gracilaria sp. as a source of cellulose can be synthesized into cellulose acetate [6]. Seaweed for industrial purposes is widely used to make semi-finished products such as agar powder and carrageenan flour [7]. One of the uses of seaweed in the industry is to produce solid waste, which still contains around 26.12% cellulose [8], which can later be further synthesized into cellulose acetate.
Research on supercapacitors based on the natural polymer cellulose acetate showed a potential as an energy storage material due to the ionic conductivity of the polar group, which had unpaired electrons but still exhibited a low specific capacitance as an electrode material [9].
Nickel oxide (NiO) is the most studied electrode due to its abundance, high capacitance, and low toxicity [10]. The addition of metal oxides to the cellulose acetate composite electrode film showed a change in structure and morphology. Meanwhile, the addition of metal oxides to the electrode film showed an increase of the surface area and the mobility of electrons enabling them to store energy [11].
Research on nickel oxide as an additional material for supercapacitor electrodes has also been studied and showed a high specific capacity. Al Kiey et al. [12] added nickel oxide to porous carbon fiber, increasing the specific capacity of 811 F/g at a current density of 1 A/g. Navale et al. [13] added nickel oxide to PANI and it increased the exact degree of 936 F/g at a current density of 1 A/g. Furthermore, Nunes et al. [14] added nickel oxide to carbon nanotubes and it increased the specific capacity of 1200 F/g at a current density of 5 A/g. Wang et al. [15] added nickel oxide to graphemes later increasing the specific capacity of 766 F/g at a current density of 1 A/g. All showed that a mixture of nickel oxide has the promising prospects as an additional material for supercapacitors.
Nickel oxide also has promising prospects as a base material for electrodes. However, research on cellulose acetate composited with nickel oxide as a supercapacitor electrode has never been carried out as an innovation. This research is expected to be an innovation in Blue Energy because it utilizes biomass waste from marine algae. It aims to study the potential of cellulose acetate, which is composited with nickel oxide as a base material for high-capacity supercapacitor electrodes.

Preparation sample
Sample preparation of Gracilaria sp. started by separating algae Gracilaria sp. from the unwanted contaminants. Gracilaria sp. was then washed with running water, and dried in the sun before being ground into powder [16].

Cellulose insulation
Algae Gracilaria sp. has been made into dry powder in its cellulose isolation. Afterwards, 100 grams of 10% (w/v) sodium hydroxide (NaOH) was added to 1000 ml, and then heated at 90℃ to 100℃ for 3 hours before being filtered. The residue obtained was washed until making the pH of the filtrate neutral. The residue was then bleached by sequentially adding 50 ml of 30% (v/v) H2O2, heating it at 60℃ for 1 hour, and filtering. Subsequently, the filtered residue was dried in an oven at 60℃ [17]. FT-IR was then analyzed the dry solids were obtained.

Cellulose acetate synthesis
The isolated cellulose was further synthesized into cellulose acetate. Five grams of cellulose was added to 50 ml of the glacial acetic acid solution, and heated at 40°C for 60 minutes while stirring. 0.5 ml of 2% H2SO4 solution was again added and heated at 40°C for 60 minutes while stirring. 15 ml of acetic anhydride was added, and heated at 40°C for 30 minutes while stirring. Furthermore, 4 ml of distilled water and 7 ml of glacial acetic acid were added and heated at 40°C for 30 minutes while stirring. Another 160 ml of distilled water was added and allowed to stand for 2 hours prior to be filtered. The filtered residue was washed with distilled water until the sour smell disappeared and the pH became neutral. The residue was then dried in an oven at 55°C [18]. FT-IR then analyzed the dry solids obtained.

Electrode fabrication
In this study, variations in the concentration of nickel oxide were carried out on the electrode materials as shown in Table  1. 4 mL of dibutyl phthalate (DBP) was added. The mixture was then stirred at 250 rpm until being homogeneous at 80°C. The mixed solution was then poured into the mold [11]. SEM then analyzed the printouts to see the morphological structure, XRD to see the crystal structure, and tested with the cyclic voltammetry (CV) method to see the electrochemical properties of the electrodes.  Isolation aims to separate lignin from cellulose [19]. Lignin is undesirable because it can affect the acetylation reaction and the formation of the degree of substitution (DS) during further synthesis of cellulose into cellulose acetate [20]. As shown in Figure 1(a), a decrease in the intensity of the lignin band in the band in the range of 1500 cm -1 to 1,200 cm -1 , which was bound to the lignin carbonyl group occurred [21]. cm -1 and 2900 cm -1 were cellulose ring regions [22] Several bands absorption at wave numbers 1300-1400 cm -1 indicated the presence of (-O-) connected to the carbon chain in cellulose [23]. Figure 1(b) shows the formation of an acetate group, which was confirmed by increasing intensity bands in 1753 cm -1 (C=O), 1236 cm -1 (C-O-O), and 1375 cm -1 (C-H) regions of the methyl group in acetate [9]. In the FT-IR spectrum, the hydroxyl groups (O-H) detected at an intensity of 3491 cm -1 had a lower level of peak state than the hydroxyl groups (O-H) in Figure 1(a). This indicated that most of the hydroxyl groups (O-H) originating from cellulose were replaced by acetate groups [24]. The lack of absorption in the 1840 cm -1 region in Figure 1(b) indicated that the product was free from acetic acid [25]. The success of the acetylation reaction was also proven in the formation of a carbonyl group (C=O) at an intensity of 1753 cm -1 and an acetyl group (C-O) at a power of 1043 cm -1 with a fairly high level of peak state.

XRD analysis
Electrode samples were characterized using XRD with a Cu Kα radiation source (λ = 1.5406 Å) at a voltage of 40 kV and a current of 30 mA. As shown in Figure 2, the mass variation diffraction of NiO added to CA showed the formation of CA-NiOn composites. It was established in the new peak of the CA-NiOn film when NiO was added. It was called as composite because there was a peak indicating 2 phases. These phases were CA as a matrix and NiO as a dopant. This is by research conducted by Diantoro et al. [11], where cellulose acetate doped with metal oxide will form composites. All can be seen in Table 2 that the addition of NiO has increased the crystallinity of NiO and has decreased the crystallinity of CA, also seen in Figure 3 where the CA phase began to experience a decrease in peak intensity at position 2θ = 20°, 2θ = 32° and 2θ = 65° with the addition of NiO. The crystallinity of the CA-NiOn film was obtained using the following equation [26], where AC is the crystalline area and AA is the amorphous area.  Table 3 shows the The crystal sizes of CA and NiO at the CA-NiOn electrodes are shown in Table 3. The crystal sizes could can be obtained using Equation 2 [26]. The crystal sizes of CA and NiO were about are around 3.82 -30.8 nm and 7.93 -31.22 nm, respectively. The data obtained based on the equation showeds a decrease in the size of CA crystals along with the addition of NiO. These results are the same as previous studies conducted by [27], where CA combined with metal oxides could can reduce the crystal size of CA. In Figure 4, it can be seen that there was a spike in the size of the NiO crystals . This is probably due to the higher concentration of NiO compared to CA.
where D is the crystal size, k is the crystal form factor, λ is the wavelength of Cu (1.54056 Å), β is the FWHM (rad), and θ is the diffraction angle (°).

SEM analysis
The SEM image showed that the CA-NiOn electrode was a porous film. The pores of the CA-NiOn electrode could be seen more clearly in 3D in Figure 6. Porosity is related to the capacity of the CA-NiOn electrode, which can decrease or increase, as shown in Figures 5 and 6 where the specific capacitance of the electrode increased with the increasing pore volume [28], and the porosity of the CA-NiOn electrode can be seen in Table 4. The porosity could be calculated using the following equation 3 [29], Where A is the area of the SEM results.

Electrochemical properties analysis of electrodes
It was tested in the voltage range from 0.0 to 1.0 V at a scanning speed of 10 mV/s. Specific capacitance (Csp), energy density (Esp), and power density (Psp) were calculated using standard equations based on cyclic voltammograms [30].
The addition of NiO decreased the crystallinity of the CA electrode film, as shown in Figure 2. The decrease in CA crystallinity significantly increased the specific capacity, energy density, and power density of the CA-NiOn electrode. This resulted from its molecules being more polarized under the influence of external fields [31]. Specific capacity, energy density, and power density were calculated using the following equation [32], 2 () where Csp is the specific capacity (F/g), Esp is the energy density (Wh/kg), Psp is the power density (kW/kg), A is the area of the curve, m is the mass of the electrode (grams), k is the scan rate ( mV/s), V is voltage (V), and t is time (s).   Gradually, the CV curve widened as the concentration of NiO in CA increased. This occurred since because the expanding the NiO engagement increased the electrode's capacity [33]. The specific capacitance could be evaluated based on the CV curve through the standard equation 4, which has been presented previously. Based on the equation, the specific capacitance was 14.25 F/g, 15.78 F/g, 26.24 F/g, and 83.27 F/g in which the CA-NiO3 electrode had the highest specific capacity, followed by CA-NiO2, CA-NiO1, and CA-NiO0, and the increase in particular capacity was followed by the increase in NiO concentration.
An increase in the specific capacity of the electrode affected the energy density and power density of the CA-NiOn electrode [34] where this value could be evaluated through standard equations 5 and 6. The data showed that an increase in the specific capacity could increase the electrodes' energy density and power density with energy density and power density values for each electrode CA-NiO0, CA-NiO1, CA-NiO2, and CA-NiO3, respectively 0.69 Wh/kg, 0.76 Wh/kg, 1.27 Wh/kg, and 4 Wh/ kg for energy density and 0.06 W/kg, 0.07 W/kg, 0.12 W/kg, and 0.4 W/kg for electrode power density. Based on the specific capacity value equation, the CV curve's power density and energy density can be seen more clearly in Table 4.
In the CV profile, the comparison between voltage and time in Figure 8 showed the shape of a symmetrical triangle curve, indicating the presence of promising electrical doublelayer electrochemical capacitor properties in the sample [30]. The increase in specific capacity as the concentration of NiO increases indicated that NiO had an important role in increasing the power of the electrode.

Conclusion
Cellulose acetate has been successfully synthesized from the isolation of cellulose from the algae Gracilaria sp. as evidenced by FTIR spectral analysis. From the analysis of XRD data, the CA-NiOn electrode film was a composite film with CA and NiO crystal sizes of 3.82 -30.8 nm and 7.93 -31.22 nm, respectively. The results of the SEM analysis showed that the morphology of the CA-NiOn electrode included a porous film. In addition, increasing the concentration of NiO doped on the CA film could increase the specific capacity of the electrode with the highest capacity value of 83.27 F/g.