rGO-ZnO Nanowire Deposited Filamentous Seaweed Nanofibrous Cellulose for Paper Supercapacitor

A nanosized architectural (a spider’s web) structure of cellulose (Iα) was extracted from green seaweed Chaetomorpha antennina through bleaching treatment. Furthermore, reduced graphene oxide (rGO) and zinc oxide (ZnO) nanowires were deposited over seaweed cellulose while using a simple hydrothermal method. A simple press method was used to prepare rGO-ZnO seaweed cellulose nanocomposite for the paper supercapacitor. This rGO-ZnO seaweed cellulose paper anode material was characterized by using various analytical techniques such as FT-IR, SEM, TGA, XRD, and tensile tests. XRD peaks reveal that graphene oxide powder when mixed with seaweed cellulose got reduced and gave XRD peak of reduced graphene oxide (rGO). In this paper, supercapacitors were tested in CV, GCD, and EIS. From GCD, the specific energy density of the ZnO-cellulose paper device is found to be 0.00066 Wh/kg whereas, for the rGO-ZnO cellulose, paper device gives a greater energy density of 5.21 Wh/kg. From EIS, the series resistance of ZnO-cellulose is found as 326 Ω and for ZnO-rGO-cellulose as 2.16 Ω. This marine resources based rGO-ZnO seaweed cellulose paper supercapacitor has application in various energy storage domains including electric vehicles and electronic industries as it is bio-degradable, cost-effective, thinnest, bearing high performance, and safe for getting used.


Introduction
For the past few decades, seaweeds have gained substantial attention all over the globe owing to their potential applications in biofuels, chemicals, hydrocolloids, cosmetics, feed, food supplements, pharmaceuticals, etc. As a result, seaweeds are getting harvested commercially all over the globe, bearing no land requirement for farming to the need for additional supplements. On one side, their production cost is very low and on the other side have a high commercial market of USD 11.8 billion [1]. Seaweeds are macroalgae which are found in coastal areas that are attached with rock or other substrata. It is classified according to its pigmentation such as chlorophyta (green), rhodophyta (red), and phaeophyta (brown). Among them, chlorophyta (green) holds much more potential components in the cell wall as carbohydrates, lipids, proteins, and bioactive compounds. Green seaweed Chetomorpha antennina is classified as filamentous algae that grows on rocks with filamentous erect and reaches up to 10 cm tall in shape. It has a high amount of I α cellulose in its cell wall naturally [2,3].
Cellulose is a linear homogeneous polysaccharide consisting of repeated units of D-glucopyranose linked by β-1,4 linkage. It is found in plant, algae, fungi, bacteria, etc. Cellulose has drawn much attention among the researchers due to the low cost and weight, nontoxic nature, high mechanical strength, low thermal expansion, superior biodegradability, and high crystallinity [4]. Generally, cellulose can be prepared by using harsh chemicals, enzymatic hydrolysis, or mechanical treatment from lignocellulosic biomass such as wood, cotton, and hemp, wherein such process may affect the cellulose structure as well as the environment [5]. Seaweed cellulose consists of only d-glucose monomers as well as the same X-ray diffraction (XRD) pattern from native cellulose derived from land sources. Especially, green filamentous algae such as Chaetomorpha or Cladophora is found consisting of native nanosized cellulose-I in its cell wall naturally [6]. Generally, cellulose derived from wood consists of I β and can be converted into nanofibers I α by using harsh treatment. Filamentous seaweed has I α cellulose in its cell having high crystallinity naturally hence when it is bonded with a metal oxide such as copper, zinc, and silver then it can effectively be used as a drug carrier, filtration media, antibacterial membrane, or electrical insulator purpose [7,8]. Many research works had been reported for cellulose-metal oxides nanocomposites for anti-oxidant, drug delivery, catalytic, antimicrobial activity, and molecular docking but there are few research work showing cellulose nanofibers based energy storage devices [35][36][37][38][39][40]. Recently, seaweed-derived components have been used for energy storage applications [9,10].
It will need time to develop a technology which is flexible, lightweight, or cost-effective as that can also work as a biodegradable energy storage device. For this purpose, cellulose is found as the most suitable biopolymer material for manufacturing paper-based electrode material as a paper supercapacitor or battery for energy storage application. Cellulose itself is an insulating material that requires to be coated with conductive material to make paper-based energy storage device. The supercapacitor is an electrochemical charge storage device with having a fast charging/ discharging cycle, high power density, and a longer lifecycle. Supercapacitors can be used in electronics, memory back-up system, industries, heavy machines, electric vehicles, etc. It is an electrochemical double-layer capacitor that can fill up the gap between the capacitors and batteries for high energy density and high power density needs though, and it has low energy density and appears difficult to replace the battery. Henceforth, continuous efforts are made to increase the specific capacitance. Anodic electrode materials such as carbon nanotubes, graphene oxide, metal oxides, and metal-deposited cellulose have been promising nanocomposite materials for supercapacitor applications [11]. Reduced graphene oxide (rGO) with transition metal oxides such as aluminum, zinc, manganese, lithium, and copper have high conductivity as well as high energy density [12]. There are many tested materials reported for metal oxides-cellulose nanocomposites and tested for the supercapacitor applications [13]. Among these, cellulose-ZnO nanoparticles-based nanocomposites have been reported recently for supercapacitor applications [14]. Here, only ZnO is not expected with good performance but when added with other transition metal oxides or polypyrrole, polyaniline, or graphene oxide can behave as a superior performance of super capacitor [15,16]. The rGO-polypyrrole-cellulose nanocomposite has been reported by Wan et al. for supercapacitor application [13]. They reported that the energy density of nanocomposite was 1.18 mWh cm −3 by using a three-electrode system and also polypyrrole decreases cellulose tensile strength. Algaal cellulose-metal organic framework-based materials have also been reported for supercapacitor application [17]. Cotton cloth-graphene oxide (GO) has also been reported for supercapacitor applications [18]. Carbon nanotubes-MnO 2 have also been reported for supercapacitors that have high energy density (16.7 kW/kg) [19]. Graphene oxide-ZnO powder nanocomposites have also been reported for supercapacitor applications with a specific capacitance as 122.4 F/g [20].
The present study deals with the thread-type nano-fibrillated cellulose (I α ) extracted from green filamentous seaweed Chaetomorpha antennina through a simple bleaching process. Furthermore, rGO-ZnO nanowires were grown over the sheet by using a single-step hydrothermal method, and the prepared sheet has been used as anode material in a paper supercapacitor. The developed electrode is used in an asymmetrical supercapacitor by sandwiching NaCl electrolyte-soaked paper separator between the rGO-ZnOseaweed cellulose nanocomposite and activated charcoal powder slurry coated nickel foil. This cost-effective, lightweight, eco-friendly seaweed-based paper supercapacitor bears excellent super capacitive behavior.

Materials and Method
Sodium chlorite, hydrochloric acid, sodium hydroxide, sodium chloride, potassium hydroxide, and sodium carbonate were purchased from Merck, Germany. Zinc chloride was purchased from Sigma Aldrich, and graphene oxide was purchased from Shilpa Enterprise, Nagpur, India.

Collection of Seaweed
Chaetomorpha antennina was collected from Kucchadi, (21°40′17.8″N 69°32′35″E), Porbandar, Gujarat, India, in May 2022 and was first washed with tap water to remove salts and other contaminants. Furthermore, it was washed with distilled water and oven dried at 50 °C. After oven drying, seaweed was powdered using mortar and pestle.

Extraction of Nanofibreous Cellulose from Seaweed
A total of 20 g of dried seaweed powder has been mixed with 8 g of NaClO 2 (bleaching agent) with 200 mL sodium acetate buffer (pH-4.5) at 60 °C for 3 h. Afterwards, it was washed with deionized water till make it neutral by using a centrifuge. Then, 240 mL of 0.5 M NaOH was added to neutral biomass and took it to the oven at 60 °C overnight. Then, again it was washed with deionized water until neutrality followed by adding 480 mL of HCl (5%) to the biomass and heat till boiling and allow it to cool down to room temperature for overnight and then it was washed with deionized water till it became neutral. The resultant water-added cellulose was processed with ultra-sonication (40-s pulsing 70% amplitude for 20 min) treatment to break its cell wall. After that, it was dried [21].

Preparation of Reduced Graphene Oxide (rGO)-Zinc Oxide (ZnO) Nanowire Deposited Seaweed Cellulose Based Anodic Paper Supercapacitor
rGO-ZnO nanowire deposited seaweed cellulose-based paper supercapacitor was prepared by using single step hydrothermal method. In this method, added 0.2 gm ZnCl 2 and 20 g Na 2 CO 3 with 30 mL water and stirred it for 1 h. Later, 2.0 gm seaweed cellulose, 0.5 gm graphene oxide (GO) powder, and 30 mL water was added and stirred at room temperature for 1 h. Then, it was transferred to 150 mL teflon lined autoclave (hydrothermal reactor) and kept in to oven at 130 °C for 17 h [23]. After that, it was cooled at room temperature for further process. Then, opened the teflon lined autoclave and decanted the liquid layer and vacuum filtration for solid residue by using nylon filter membrane. This is followed by the simple press mechanism to prepare handmade nano seaweed cellulose paper supercapacitor and later dried at 50 °C [22].

Characterization
rGO-ZnO nanowire deposited seaweed cellulose-based anodic paper supercapacitor was analyzed by using various instrumental techniques such as Fourier transform infrared spectra (FT-IR), thermo gravimetric analysis (TGA), scanning electron microscope (FE-SEM), and tensile strength and X-ray diffraction (XRD). This prepared paper supercapacitor device was measured under cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS). Microscopy of collected marine seaweed was carried out by using a polarized microscope (AXIO Imager.A2m, Zeiss). The FT-IR spectra was carried out by using an FT-IR spectrophotometer (Perkin Elmer, Spectrum 2). The range was 400-4000 cm −1 and used ATR for the mode of operation.
XRD was carried out on a D8 Discover X-diffractometer (Bruker) using Cu-Kα radiation at an accelerating voltage of 40 kV and a current of 40 mA. The range of 2θ is from 10 to 80°. The crystallinity index (CrI %) of the prepared paper supercapacitor was calculated by following Segal's equation [21]: where, I max is the maximum intensity of the main peak and I min is the minimum intensity at a 2θ angle close to 18º for cellulose I.
The crystallite size (D) was calculated by using the Scherrer equation: where K is constant which a value was 0.94, λ is the wavelength of X-ray radiation (Cu-Kα = 0.15408 nm), β 1/2 is the full width at half maximum of deflection peak, and θ is the Bragg's angle [21].
The surface morphology of seaweed cellulose, ZnO nanorowire deposited seaweed cellulose, and rGO-ZnO seaweed cellulose nanocomposites based paper supercapacitor were characterized by using FE-SEM (Ultra-55, Zeiss). For this process, samples were placed over carbon tape on a 1 cm diameter and supper coated with gold for prior for imaging. The image was taken at 10.0 kV acceleration voltage, using a lens detector.
TGA of paper supercapacitor was analyzed by thermo gravimetric analyzer (Perkin Elmer,4000) from room temperature to 800 °C at a heating rate of 10 °C min −1 under nitrogen atmosphere. The tensile strength of the rGO-ZnO seaweed cellulose paper supercapacitor was analyzed by using 0.25 mm × 12 mm strips. The test was performed in UTM, Shimadzu by using 100 N load cells with 1 mm/ min strain rate.

Electrochemical Setup and Measurement
The electrochemical measurement of ZnO nanowirecellulose nanocomposites as well as rGO-ZnO nanowire deposited cellulose nanocomposites were assessed with the two-electrode assembly. The cyclic voltammetry (CV) has been performed over two electrode systems under galvanostat/potentiostat (Autolab PGSTAT 302 N). It is the fundamental analysis in which current flowing through an electrochemical cell is measured as the voltage is cycled over a specified range. It is typically measured at fix rate [24]. This two electrodes system was set up, and it is optimized under autolab galvanostat/potentiostat for cyclic voltammetry, galvanostatic charge-discharge, and electroimpedance spectroscopy.

Fabrication of Paper Supercapacitor
The rGO-ZnO nanowire deposited seaweed cellulose nanocomposite was optimized as electrodes for supercapacitor application. It is an asymmetric device with nickel foils to make the cell set up for further testing. An asymmetric supercapacitor was fabricated by sandwiching a piece of filter paper (Whatman paper, 125 mm, Cat No 1001 125) as a separator with one electrode of rGO-ZnO nanowire deposited seaweed cellulose sheet (1.5 cm × 2 cm × 0.005 cm, 80 mg), and the second electrode was activated charcoal with Ni metal connection strips. The filter paper roll was to make ionic transfer smoother between the two electrodes. Here, the sodium chloride (3 M) electrolyte was used. The electrodes were immersed in the sodium chloride solutions for 10 min. CV, GCD, and EIS measurements of the supercapacitor were conducted on Autolab (PGSTAT 302N) instruments [25]. Figure 1a shows the digital image (1.9 ×) of the collected seaweed highlighting several filaments with pores confirms the Chaetomorpha antennina [3]. The yield of cellulose from this filamentous seaweed was 32% by using a simple bleaching agent without any harsh treatment.

XRD Analysis
X-ray peak of algal nanofiborous cellulose in Fig. 2a shows that the peaks between 13° and 22° with Chaetomorpha species consist predominantly of cellulose I especially, I α cellulose. This type of XRD peaks in algae is mainly shown in green filamentous algae such as cladophora and Chaetomorpha. Two peaks at 15.0° and 17.2°, which are not more common in other cellulose such as plant-based cellulose. These peaks are due to the specific planner direction of the  filamentous algal cell wall [2]. Generally, this cellulose has triclinic structure whereas higher plant and cotton cellulose consists monoclinic structure (I β ). This is a major difference in the structure of algal cellulose and higher plant cellulose. The peak at 2 theta value of 15.0º (001), 17.2º (011), and 23.0º (210) with h,k,l index plane, respectively. [21]. Higher plant cellulose does not have this type of XRD spectra hence, convert I α triclinic structure by using highly acidic treatment. Generally, cellulose I α triclinic has a lower moisture absorption capacity and is not easily brittle. Figure 2b shows the XRD peak of graphene oxide (GO) powder. Here, the peak at 2θ = 11.2° corresponds to the (002) indicating the graphene oxide was obtained by Hummer's method [26]. The peak at 2θ = 43.04º, 45.0º, and 73.96º are the peaks of the blank aluminum plate which is used as a holder in powder method XRD which we can be minimize. Figure 2c shows the XRD peak of rGO-ZnO nanowire deposited seaweed cellulose paper wherein the peaks at 15.0°, 17.2°, and 23.0° was alpha-cellulose peak as above discussed whereas peaks showed at 2θ value of 31.2º (203) [23,27]. Here, ZnO nanowires consist hexagonal wurtzite structure. The 2θ value at 26.02° is the weak diffraction peak which is for reduced graphene oxide (rGO) peak [26] that which clearly shows the amorphous in nature and also clearly proves that seaweed cellulose act as a reducing agent as shown in Fig. 2b that the peak of graphene oxide powder (GO) and when behaves as reduced and gave XRD peak of reduced graphene oxide (rGO). Hence, there is no need to add sodium borohydride for the reduction of GO which is mostly used in conventional methods.
The crystallinity index of rGO-ZnO deposited algal cellulose was 76.23% whereas extracted seaweed cellulose was 87.23% which also indicates that the high degree of crystallinity is due to the presence of thick nanofibrils structure and proves that the heating and deposition process decrease the crystallinity of seaweed cellulose. The crystallite size of seaweed cellulose, graphene oxide powder, and rGO-ZnO nanowire deposited seaweed cellulose were found to be 71.1 Å, 49.2 Å, and 58.0 Å, respectively. Figure 3a shows the FTIR spectrum of graphene oxide powder with the peaks around 3168 cm −1 and the lower intensity peaks at 1726 cm −1 due to the OH groups and stretching of -COOH groups, respectively. The peaks nearby 1586 cm −1 are due to the sp 2 -hybridized carbon groups after oxidation. The peak at 1489 cm −1 is due to the graphite carbon as well as the peak at 1398 cm −1 is due to the C-OH group which is in a tertiary position. The peak at 1039 cm −1 is due to the alkoxy group in the structure [28].  Figure 3b shows the FTIR spectrum of the rGO-ZnOseaweed cellulose paper supercapacitor. In this spectra, the peak at 3580 cm −1 is mainly due to O-H stretching. The peaks at 2998 cm −1 and 1271 cm −1 are due to C-H vibration and bending O-H peaks. The peaks at 1130 cm −1 and 1022 cm −1 are due to the glycosidic bond of C-O-C and C-OH, respectively [29]. The peak at 569 cm −1 is due to the Zn-O stretching vibration. In this FTIR spectra, the peak at 3580 cm −1 losses intensity due to the interaction of the -OH group with zinc ion as it is found to decrease with the -OH group in seaweed cellulose. This peak also reveals the decomposition of the carboxyl group in GO. The intense peak at 1439 cm −1 is due to the restoration of the sp 2 carbon which is a confirmed peak of rGO [30].

Thermal Stability
The thermo gravimetric analyses of ZnO-rGO seaweed cellulose paper and GO powder are shown in Fig. 4a and b, respectively. In Fig. 4a, the weight loss started nearby 100 °C due to the vaporization of moisture adsorbed by cellulose paper. The major thermal degradation in ZnO-rGO seaweed cellulose paper was between 130 °C and 320 °C which is due to the degradation of algal cellulose. The maximum mass loss at this step is nearly about 30%. In the third stage, the weight loss occurred between 320 °C and 720 °C, and this was due to the degradation of polymeric chains whereas above this temperature only carbonaceous residue was left. Here, the total weight loss of ZnO-rGO cellulose paper is 55% which shows that this composite has more thermal stability as compared to cellulose as reported elsewhere [23]. Figure 4b shows the thermal behavior of GO powder. Here, the initial slight degradation occurrs nearby 100 °C, which is due to the evaporation of water in GO powder whereas the major thermal degradation in GO powder lies between 140 °C and 240 °C. This might be due to the chemical decomposition of oxygen based functional groups and major weight loss of GO powder between this temperature as close 40% [31]. The total weight loss of GO powder at 800 °C is 76%. Generally, GO is less stable as compared to rGO as the weight loss between 10 °C and 800 °C only is 11%. This might be due to the vander waals force between the layers where oxygen functional groups removed [32]. Figure 5a shows the SEM image of nanofibrous seaweed cellulose sheet which clearly show brings highly porous membrane formed with each nanofibers twisted giving rise spider's web type network. Generally, in all higher plants, the cellulose structure is different as compared to the filamentous seaweed. This is due to the different terminal complexes in structure because in higher plant, terminal complexes have rosettes six hexagonally structure with random orientation whereas filamentous seaweed such as cladophora or chaetomorpha species has liner type of terminal complex with a high degree of orientation so it has I α type structure with naturally whereas, in the higher plant, it has I β type structure which can also prove in XRD peaks as mentioned by Moon et al. This is the major difference between the cellulose extracted from filamentous seaweed species and higher plants. Due to this difference, any harsh treatment such as enzymatic hydrolysis, mechanical treatment, or acid hydrolysis is not required for the extraction of nanofibrous cellulose from filamentous seaweed whereas, for extraction of cellulose from a higher plant, this treatment is required [21]. The average size of seaweed nanofibers from the SEM image is 36 nm. Figure 5b shows the SEM image of the ZnO nanowire deposited seaweed cellulose sheet. It clearly shows that nanowires were perfectly grown over a seaweed nanofibrous network. ZnO nanowire was grown over Ulva faciata seaweed as reported in our previous work but this sheet was brittle due to I β cellulose was in its cell wall [22] whereas in the present work, I α cellulose which has also proved in XRD spectra of cellulose. It have also tensile strength as well as a high yield [7].

SEM Analysis
Generally, when seaweed cellulose is mixed with zinc chloride and sodium carbonate, the hydroxyl group over the cellulose structure was interact with zinc ions to form the zinc-cellulose complex and nucleation is formed over the cellulose surface during the hydrothermal process. When the temperature and time increase, it formed nanowire from the nucleation on the seaweed cellulose surface [23]. Generally, the size, structure, and position of ZnO nanowire depend on temperature and sodium carbonate concentration as the lower concentration of zinc and sodium carbonate formed larger rods or wire type structure. In Fig. 5b, clusters of nanowires are formed in some places; this might be due to the lower concentration of sodium carbonate when increases with the concentration, and it became saturated and gives more ZnO nanowires separately and at higher temperature effects but in the present study, nearly about 200 °C, cellulose was started to degrade; hence, it is better to apply the hydrothermal method at a lower temperature [24].
Figure 5 c-f shows rGO-ZnO nanowire deposited seaweed cellulose paper supercapacitor. The uniform layer of rGO (reduced graphene oxide) and ZnO nanowires clearly appears in SEM images. Generally, rGO which mainly consists of polar oxygen-containing groups as well as have an excellent vander waals force with strong hydrogen bonding so it can easily bonded with the seaweed cellulose [33]. Figure 6 shows the tensile property of the developed rGO-ZnO seaweed cellulose paper supercapacitor. Wherein, the tensile strength (σ b ) was 7.2 MPa, and the tensile module is 0.6 GPa along with the maximum strain is 3.8%. The data shows that this paper's supercapacitor has high tensile property. Generally, this tensile property may slightly vary because it depends on seaweed climatic condition or when it grows but the Chaetomorpha antennina seaweed grows over rocks which consist of very high tensile strength by its nature [6].

Cyclic Voltammetry
The specific capacitance value is identified from cyclic voltammetry (CV). The specific capacitance of the electrode was calculated from the area under the CV curves. The voltage range was − 1.0 V to + 1.0 V. The different scan rates were applied of 5,10,20,50,100 mV/s. The measurements were done in the two-electrode configuration by assembling the device, and the asymmetrical supercapacitor device was fabricated. The CV curves in Fig. 7a show rGO-ZnO nanowire deposited cellulosebased device cyclic voltammetry as determined from the 5 mV/s scan rate curve. The value of specific capacitance is 2.5 F/g, and the energy density is 1.38 Wh/kg. Results indicate that the electrical double layer capacitive behavior (EDLC) was shown wherein, the shape of CV curves at a lower scan rate between 5 and 20 mV/s is moderately rectangular without any faradic peaks. At higher scan rates such as 50 or 100 mV/s, the rectangular shape of the CV curve becomes slightly distorted due to the increased resistance of electrolyte ion transportation. Figure 7b shows the performance of electrochemical measurement of ZnO-cellulose nanocomposite material assessed in a two-electrode assembly. The value of specific capacitance is 0.28 F/g, and the energy density is 0.076 Wh/kg. Thus, from the calculated values, it can be stated that by integrating rGO in ZnO-cellulose nanocomposite, one can achieve better energy density as compared to ZnO-cellulose nanocomposite.

Galvanostatic Charging and Discharging (GCD) Test
Galvanostatic charging and discharging (GCD) measurements were used to calculate the energy density and power density of the supercapacitor device with the composite electrode material. Figure 8a shows the dependence of capacitance retention on a number of cycles, with an applied current density of 0.125 A/g. We have assembled two different devices using ZnO-cellulose nanocomposite and rGO-ZnO cellulose nanocomposite. The specific capacitance of ZnOcellulose nanocomposite is 0.118 F/g while the energy density is 6.6*10 −4 Wh/kg (Fig. 8b). However, when added the reduced graphene oxide (rGO) in the same composition, the specific capacitance values and energy density start increasing for rGO-ZnO cellulose device. The specific capacitance of rGO-ZnO cellulose nanocomposite is 37.5F/g while an energy density is 5.2 Wh/kg (Fig. 8c for the first ten cycles). It is proven from the above calculation as compared to ZnOcellulose nanocomposite that the addition of rGO in an equal proportion of ZnO resulted in superior performance. Specific capacitance has improved by almost nine times, and energy density has increased by almost eighteen times due to rGO. The rGO-ZnO cellulose nanocomposite supercapacitor device has been measured for cyclic stability till 5000 cycles (Fig. 8d) for the last cycles of GCD measurements).

Electrochemical Impedance Spectroscopy Test
Electrochemical impedance spectroscopy (EIS) refers to the measurement of frequency-dependent resistance to the current flow in an assembled device. EIS can identify the diffusion-limited reactions within the device. Here, the test was conducted for two different composite materials through the EIS technique. Figure 9a shows the EIS measurement of rGO-ZnO cellulose nanocomposite deposited cellulose material. Figure 9b shows the EIS measurement of the ZnO-cellulose nanocomposite. The equivalent circuitbased model is fitted to experimental data which evaluate the individual circuit elements of the supercapacitor device.
In general, the model predicts the value of series resistance (R S ), charge transfer resistance (R ct ), double-layer capacitance at the interface of electrodes (C dl ), and Warburg resistance (Z W ) [34]. For the rGO-ZnO-cellulose nanocomposite device, the value of R S is found to be 2.16 Ω (Fig. 9c) and for the ZnO-cellulose nanocomposite device, the value of R S is found to be 326 Ω (Fig. 9d). A significant reduction in R S in ZnO-cellulose nanocomposite as compared to rGO-ZnO cellulose nanocomposite is attributed to the addition of rGO and formation of an electrically conducting network, this assembled device can be used as energy storage device after adding reduced graphene oxide ( Fig. 10 and 11) [34].

Conclusion
Nanofibreous cellulose (I α ) extracted from seaweed green filamentous seaweed Chaetomorpha antennina and rGO and ZnO nanowire deposited over this seaweed cellulose for developing the paper supercapacitor. The developed device has high specific high capacitance (37.5 F/g) and high energy density (5.21 Wh/kg) and no air degradation that too after 5000 cycle. Finally, the study finds that the marine seaweed cellulose-based anode material helps in the development of the thinnest paper supercapacitor which is lightweight, biodegradable, and high tensile strength and it has a promising application in smart electronic devices. The developed device holds promising application in almost all types of electronic gadgets.