Effect of electrolytes on the performance of graphene oxide anode material for ultracapacitor, Li-ion capacitor, and Li-ion battery: three-in-one approach

Graphene-based 2D nanomaterials are gaining much interest in energy storage systems, specifically in ultracapacitors. Various electrolytes increase the performance of ultracapacitor (UC), Li-Ion capacitor (LIC), and Li-Ion battery (LIB). In the present work, we have successfully designed a "three-in-one" artificial method to engineer anode from a single precursor for high-performance UC, LIC, and LIB. In the present investigation, graphene oxide (GO) slurry was developed using the modified Hummers’ method. The effect of KOH, H2SO4, and KCl electrolytes on electrochemical performance of UC was demonstrated. The LiPF6 organic electrolyte solution on electrochemical performance of LIC and LIB is demonstrated. The GO deposited on stainless steel electrode achieved its highest specific capacitance of 422 F/g, energy density of 45.50 kWh/kg, and power density of 10,000 W/kg in 3.0 M in KCl, whereas GO as an anode material delivered a first discharge capacity of 456 mAh/g at 0.05 A/g current density with the efficiency of 100%.


Introduction
With the quickly rising market in zero-emission rechargeable vehicles, gasoline, and electric vehicles, also called mixed electric automobiles (MEAs), and movable electronic gadgets, there is a constant need for environmentfriendly energy storage systems with the high-power and energy density and long cycle life. An ultracapacitor (UC), also known as a supercapacitor (SC), finds its widespread applications in energy storage systems [1].
Ultracapacitors (UCs) are classified into different categories depending on their charge storage mechanisms such as electrical double-layer capacitors (EDLCs), pseudocapacitor, and hybrid capacitors. Pseudocapacitors and EDLCs depend on the nature of the cathode and anode material to work simultaneously. In UCs, technologies are growing continuously to progress using new nano-sized electrode materials. Literature reveals new innovative ideas, scientific aspects, and technology of UC devices, anode and cathode materials [1][2][3][4][5][6]. Invention and development of high energy capacitors are essential to ensure good execution time duration of energy storage capacity and power distribution rate in the UC with appropriate pore size and specific electrode surface area of electrodes and aqueous and non-aqueous electrolytes [1][2][3][4][5][6].
Currently, Li-ion capacitor (LIC) is one of the multiple state-of-the-art power storehouse, bridging the interval between SC and LIB devices. The LIC is composed of three types of materials (i) cathode materials with its capacitive type, (ii) anode materials with its battery type; and (iii) organic electrolyte solution with lithium salt for a broad functional window [7] [7].
High-performance LICs have been constructed with high power and energy density properties. Poor capacity of cathode material used in combination with the anode materials reduces energy density of the LICs. Anodes with special storehouse tools include insertion, conversion, and alloy types. For example, Li 4 Ti 5 O 12 (LTO) [10] and graphite [9] belong to the anode material of insertion type, with adequate cyclic stability. However, its narrow specific potential degrades the energy density of LIC. On the other hand, the conversion type anodes such as manganese monoxide (MnO), ferric oxide (Fe 2 O 3 ) and alloy-type anodes such as lithiation anode tin (Sn) and silicon (Si) materials provide good specific capacity. LIC has an EDLC-type charge storage system as far as the cathode materials are concerned. In the LIC device, ions accumulate on the surface at the interface between the electrode and on the electrolyte. The charge is physically stored on the exterior of the electrode, resulting in poor specific capacitance and specific capacity. The LIC specific capacitance of cathode material can be enriched due to the electrochemical movement by different molecules in the form of doping [11].
Due to outstanding environmental compatibility, high energy density, low self-discharge, and total energy cycle rates, lithium-ion batteries (LIBs) are versatile tools for energy storage worldwide. The LIBs are on a large scale used in headphones, laptops, cell phones, iPods, smart watches, and nowadays, in zero emission electric vehicles [12][13][14][15]. Currently, LIBs require high capacity, long cycle life, low weight, and flexible design. Traditional LIBs use graphite material because of its theoretical specific capacity and increased current density retention [16][17][18][19]. The carbon-based materials became widely utilized in LIB as anode due to outstanding physical and electrochemical properties [20].
The conducting anode and cathode materials of graphene oxide (GO) also have higher lithium storage than graphite (G) and achieve outstanding chemical stability and dispersion in H 2 O. Similarly, the synthesis process of GO is feasible and straightforward, and costs much less than graphene. The GO possesses a higher capacity, excellent long-lasting life cycle performance and quickly forms its own-assembly layer/sheet of synthetic organic binder with the multiple active groups like hydroxyl, epoxide, and carbonyl, i.e., OH, C-O-C, C=O. Other active groups include sp 2 -hybridized, carboxylic acid, phenol, i.e., C=C, COOH, C 6 H 5 OH. However, not all functional groups can positively affect electrochemical performance; some have adverse outcomes, such as higher permanent Li-ion intake [21][22][23][24][25][26][27][28][29].
Graphene oxide, sometimes called graphitic oxide, was synthesized by treatment of ceylon graphene with an oxidation mixture containing potassium chlorate (KClO 3 ) and nitric acid (HNO 3 ) [30]. Recently, Staudenmaier-Hoffman-Hamdi technology incorporated concentrated H 2 SO 4 , and HNO 3 , and graphite into KClO 3 and gradually mixed in KClO 3 over a period of one week for cooling the prepared mixture. The chlorine dioxide was released with an inert gas such as nitrogen or carbon dioxide. The method needed more than 10 gm of potassium chlorate for per gram of treated graphite, and outbreak was a constant hazard [30].
In the current study, we have synthesized GO by modified Hummer's method and the effect of different electrolytes on performance of GO based UC, LIC, and LIB cell was studied.

Synthesis of GO
The GO was synthesized using a slightly modified Hummers method [30]. The 5 gm GF, 2.5 gm NaNO 3, and 120 mL H 2 SO 4 were added in a 500-mL conical flask kept in ice bath. Then 15 gm of KMnO 4 was added slowly to maintain the temp below 20°C. The mixture was kept stirred with a magnetic stir for 24 h at room temperature. As the mixing time increases, the mixture becomes pasty, and the color of the mixture becomes light brown. After stirring for 32 h, 150 mL of water was added. The paste color changed to yellow. When the mixture turns yellow, 50 ml of 30% H 2 O 2 was added. The mixture was stirred for another 15 min, and finally, distilled water was added to the mixture for purification of GO. The resulting mixture was extracted in a glass jar (2000 mL). The mixture was turned into brownish slurry after cooling and centrifugation, which was used for further characterization.

Material characterization
The purity and crystallinity of the material were identified by X-ray diffraction (XRD) using a Rigaku miniflex-600 with CuKa (k = 1.54184 Å ) radiation in the 2h range of 10°-80°. Raman spectra of the sample were obtained using a Raman microscope (Renishaw inViaTM, laser excitation at 532 nm) in a frequency range of 50 to 3000 cm -1 . The field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDX) on Nova NanoSEM NPEP303 were used to analyze the surface morphology, size, and elemental mapping. The surface studies were performed on a JASCO FTIR 6100 instrument. Also, the contact angle of GO electrode was measured using a contact angle meter (HO-IAD-CAM-01, Holmarc OptoMechatronics Pvt. Ltd. India).

Electrochemical characterization
The electrochemical measurement of GO-based UC was taken with a three-electrode (3E cell) in 3.0 M KOH, 0.5 M H 2 SO 4 , 3.0 M KCl electrolyte solutions. Herein, GO is the working (* 1 9 1 cm 2 ), a platinum (Pt) plate (2 cm 9 2 cm) as a counter, and a saturated calomel (SC) as a reference electrodes. The working electrodes were prepared by mixing GO with polyvinylidene fluoride binder (PVDF) in N-methyl-2-pyrrolidone (NMP) with a weight ratio of 90:10. Then, the above material (0.0002 gm) was coated onto a 1 cm 9 1 cm stainless steel (SS) substrate and dried at 45°C at 24 h in the laboratory oven. For UC, the electrochemical analysis was carried out using the ZIVE MP1 multichannel electrochemical workstation instrument. Furthermore, specific capacitance (Cs), energy density (ED), and power density (PD) of GO electrode were calculated using the following equations [31].
where 'm' is mass of deposited material on a working electrode. 's' is scan rate; V 2 max À V 2 min À Á is the potential window of CV analysis. 'I' is the current response. 'dT' is discharging time, and V max and V min are maximum and minimum potentials.
Furthermore, different electrochemical parameters were evaluated; the 'E' is energy density and 'P' is power density were calculated using following formula equation no (4) and (5), respectively [32].
The electrochemical performance of a graphene oxide (GO) working electrode was studied using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests in three different electrolyte solutions: 3.0 M KOH, 0.5 M H 2 SO 4 , and 3.0 M KCl. For the KOH electrolyte, the CV measurements were taken in the potential window of -1.2 to 0.0 V/SCE, with a stability potential window of -0.4 to 0.4 V/SCE. The GCD measurements were taken in the potential window of -1.2 to -0.8 V/SCE. For the H 2 SO 4 electrolyte, the CV measurements were taken in the potential window of -0.4 to 0.0 V/SCE. For the KCl electrolyte, the CV measurements were taken in the potential window of -1.0 to 0.0 V/SCE. Finally, for the LiPF 6 electrolyte, the GCD study for both LIC and LIB studies was performed in the potential window of 0.0 V to 3.0 V (vs Li/Li ? ). These tests help to evaluate the electrochemical behavior of the GO working electrode in different electrolyte solutions and provide important information on its electrochemical performance and stability. Graphical abstract of GO anode of half-coin cell as illustrated in (Fig. 1).

Structural analysis
The XRD pattern of GO electrode is illustrated in Fig. 2. The diffraction peak at 10.71°related to (001) plane corresponds to pure GO phase [30]. The crystallite size of GO was calculated using Scherrer's formula illustrated in equation no (6) [33].
where 'D' is the crystallite size of GO. '0.9' is the Scherrer constant. 'k' is the wavelength of the X-ray beam. 'b' is the full-width at half maximum (FWHM) of the plane (001). 'h' is Bragg's angle. The calculated crystallite size is 11.8 nm.

Fourier transform infrared spectroscopy (FTIR) analysis
The FTIR spectrum of GO anode in the wavenumber range of 400-4000 cm -1 is illustrated in Fig. 3. The spectrum displays the presence of signal associated with C-O stretching vibration signal at 1079 cm -1 , epoxy active group is C-O-C stretching vibration signal at 1192 cm -1 , phenolic C-OH phenolic OH stretching vibration signal at 1376 cm -1 , carboxyl active group C=O stretching signal at 1864 cm -1 in carbonyl moieties that are primarily present along sheet edges. In addition, Table 1 shows GO wavenumbers, peak origin, and vibration types [34].

Wettability analysis
In wettability analysis, a contact angles are formed between the interface of both liquid and vapor, and the liquid-solid interface of H 2 O [35]. Therefore, for controlling or modifying the working electrode surface, wettability is the main characteristic of the energy storage of UC [35]. The H 2 O contact angle measurement image of GO electrode is illustrated in Fig. 4. The GO electrode shows a contact angle of 71.6°. The wettability analysis shows that GO electrode is hydrophilic, which is suitable for UC, LIC, and LIB applications.  The Raman spectrum study was carried out to analyze structural aspects of GO sample. Figure 5 illustrates the Raman spectrum of GO. The characteristics peaks at 1350 and 1590 cm -1 represent D and G bands of GO, respectively [37]. Here in GO, it indicates no reduction in GO sheets [36].

Morphological analysis
The size of the particles and surface morphology of samples studied by field emission scanning electron microscopy (FE-SEM) are illustrated in Fig. 6(a-c). The GO morphology includes various sizes, which display a wrinkled and rippled sheet-like form, which may be caused by deformation upon the exfoliation and restacking procedure [37][38][39]. At 100KX magnification, pore dimensions of the network are in the range of few lm and walls of 5 nm thickness are clearly seen.

Electrochemical analysis
The KOH, H 2 SO 4 , and KCl aqueous electrolytes have significantly electrical conductivity up to 1 S/cm as well as high dielectric stability [40]. Aqueous electrolytes have a higher concentration of ions than organic electrolytes and lower resistance. The ability to store charge in a power storehouse strategy relies on the permeability of the ion to its spongy texture area, so its ion dimensions and pore dimensions must be optimal. The distribution of pore dimension relies on the dimensions of ions in the electrolyte it contains, so selection of electrode and electrolyte must be done concurrently so that the movement of the ion is fast [41]. The UC using aqueous electrolytes shows  higher capacity and power than organic electrolytes. Another essential advantage of aqueous electrolytes is that they can be produced in the open air much simpler than organic electrolytes. Nevertheless, the disadvantage of aqueous electrolytes is smaller voltage window compared to organic electrolytes. Due to this limitation, these electrolytes exhibit low energy and energy density values.   Fig. 8(c). The GCD plots indicate pseudocapacitive Faradic-type materials behaviour [42]. The pseudocapacitive Faradic reduction and oxidation reaction with C=O and C 6 H 5 O 6 groups takes place as:   shown in Fig. 9(c). The Faradaic redox reactions with -C=O and C 6 H group take place as: The capacitance of GO/PANI composite electrode with 0.5 M H 2 SO 4 and 2 M H 2 SO 4 electrolyte showed 448, 320, 480 F/g at 0.5, 0.1, 0.1 A/g current densities [43]. The capacitance of GO/polypyrrole (PPy) composite electrode with 2 M H 2 SO 4 was 417 F/g from CV at a scan rate of 100 mV/s [43].   [42]. The GCD curves at current densities between 2 and 4 mA/cm are shown in Fig. 10(c). The CV curves of GO indicate two redox peaks attributed to the redox transition of GO and indicate the presence of pseudocapacitance. The faradic redox reaction of C-OH, C-O-C, C=O, and C 6 H 5 O 6 groups takes place as:

Electrochemical study in 3.0 M KCl electrolyte
Here, in three different electrolytes, the vertical shape of CV shows pure capacitive behaviour as the ideal UC. The rate of intercalation/deintercalation of electrolyte ions in the active electrode material depends on the morphology of the surface, as it has to increase the use of the active electrode material by giving an adequate period for intercalation/deintercalation of the electrolyte ions, resulting in enhanced electrochemical performance [25]. These three electrolytes (KOH, H 2 SO 4 , and KCl) show different capacitors as number of ions increases the SC, PD, and ED due to the smaller ionic size (Table 1). Table 2 represents name of the sample, electrolytes, scan rate, molar concentration, specific capacitance, and specific discharging/charging capacity of the GO electrode. Table 3 represents electrolytes, current density, discharge time, potential window, energy density, power density, and charging/discharging efficiency for GCD of the GO electrode. This present study reveals on the effects of different electrolytes as illustrated in Table 4. Electrochemical performance was tested using NEWARE battery testing system. ( Table 5) represents use of LiPF6 organic electrolyte in LIB.
To investigate the electrochemical performance of GO anode electrode, the GCD measurements were taken at an applied current density (ACD) of 0.05 A/g in the voltage range of 0.0050 to 3.0 V for 200 cycles. The typical charge-discharge profiles of GO electrode at the 1st to 200th cycles with ACD are 0.05 A/g and are shown in Fig. 11(a). During 200th cycle the discharge capacity was changed from 456 to 74 mAh/g. During Li insertion and exertion in GO electrode, the unavoidable ripple and flex may damage and break off particles [81].    Fig. 11(c). The specific capacity of GO anode half-coin cell is higher than the first discharging and charging cycle. Specific capacity is a critical parameter in the evaluation of electrochemical energy storage systems, as it determines the amount of charge that can be stored per unit mass or unit area of the electrode material. Low specific capacity can limit the overall performance and energy density of the system. Additionally, capacity retention is also an important factor to consider, as it reflects the stability of the electrode material over multiple charge/discharge cycles. Low capacity retention can lead to rapid degradation of the electrode, which can result in decreased performance over time. The discharging-charging rate performance studies of GO electrodes were conducted at different current densities of 0.05 to 1 A/g as shown in Fig. 11(d). The significantly increased CD may boost the corrosion of LiPF 6 OE in GO anode LIB. Consequently, the reduction in the cycle power of GO anode at a high current density, i.e., 1 A/g, is likely due to the utilization of Li ? generated by electrolyte corrosion and the action of metallic lithium on the anode surface. The GCD stability of energy (mWh) discharging-charging studies of GO anode at current densities of 0.05A/g number at discharging/charging cycles of 200 is provided in Fig. 11(e). The GCD rate capability of energy (mWh) studies of GO electrodes conducted at different current densities for 10 discharging/charging is shown in Fig. 11(f).
For LIC half-coin cell, Fig. 12(a) shows galvanostatic charge/discharge (GCD) curves at 0.05 A/g for stability of 200 cycles. This shows that after 200 cycles, the specific capacitance of the cell decreased from 369.1 to 231.8 F/g. The GCDs at various ACD, i.e., Fig. 12(b) 0.05 A/g, Fig. 12(c) 0.1 A/g, Fig. 12(d) 0.2 A/g, Fig. 12(e) 0.5 A/g, Fig. 12(f) 1 A/g, were carried out in 1 M LiPF 6 electrolyte. The GCDs at various current densities were studied with no of cycles ( Fig. 12(g)). Figure 12(h) Ragone plot with power density (W/kg) and energy density (Wh/ kg) at different current densities starting from 0.05 A/g up to the 1 A/g, for LIC of the GO anode electrode.
The GCD curves are nonlinear in nature, which is attributed to the pseudocapacitive mechanism of half-cell GO anode as a LIC. The graphite (G) and graphene oxide (GO) show a stable voltage plateau of around 0.1 V (vs. Li/ Li ? ) through the entire charge/discharge process and can offer a high working potential when paired with activated carbon (AC), making it attractive for LICs anode for using as a half-coin cell. It is seen that the potential falls rapidly during the first discharge and a plateau at 0.19 V, which might correspond to the solid-electrolyte interphase (SEI) layer formation in GO anode. The SEI layer acts as a protective layer on the surface of the anode and helps prevent the lithium ions from reacting with the electrolyte, allowing for efficient charge/discharge cycles. Additionally, this GCD information was used to calculate the specific capacity and capacitance of the half-coin cell as a Table 3 Name of the electrolytes, current density, discharge time, potential window, energy density, power density and charging/discharging efficiency for GCD of the GO electrode  Effect of electrolytes on the performance of graphene oxide anode material for ultracapacitor… lithium-ion capacitor. The specific capacitance has decreased with increasing CD, because at increased current, exclusively the exterior covers of GO anode are concerned in the electrochemical reaction that shows lower values of specific capacitance [82]. This excellent GO performance might be contributed to the partial decrease of GO, which raises electrical conductivity while keeping a significant excess of C=O groups in the oxidation and reduction process. In addition, GO anode partially shows increased charge storage capacities due to its  12 Aqueous zinc-ion capacitors (AZICs) [80] pseudocapacitive behaviour, affecting oxidation and reduction of peak voltage active groups (C=O/C-O) with the significant Li ? uptake. The GCD rate capability is valuable and property of GO anode materials for LIC and LIB, particularly when envisaging high power density applications such as automotive applications. To date, efforts to improve rates by reducing carbon coating and particle size improve electronic and ionic conductivity. However, some different opinions also appeared [83] to believe that the extra capacity attributed to OH, but the COOH, lactone, and C=O result in irreversible discharging/ charging process [84]. Hence, it has suggested the functions of additional active groups of GO in the electrochemical study of GCD for LIB and LIC. The GO was prepared by straightforward, scalable, and cost-effective methods, as explained above.

Conclusions
In summary, a straightforward, cost-effective, and scalable modified Hummers method to produce GO anode for the fabrication of a half-coin cell was described. The GO sheets showed a wrinkled and rippled sheet-like form by deformation upon the restacking and exfoliation process. Out of three aqueous electrolytes studied (KOH, H 2 SO 4 , and KCl) outstanding performance was achieved in KCl electrolyte. The GO electrode exhibits higher specific capacitance of 422 F/g, in 3.0 M KCl, 246 F/g in 3.0 M KOH and 23 F/g in 0.5 M H 2 SO 4 electrolyte. The GO sheets C-C edge in the middle of the stainless steel substrate resulted in low interfacial resistance. High power and energy density values observed for GO electrodes in KCl electrolyte. The prepared GO/PVDF characteristics demonstrate its potential application to design high-performance, low-cost UC. High-performance LIC and LIB based on GO as a anode and lithium metal chip as a cathode half-coin cells in an organic LiPF 6 electrolyte were prepared. The electrochemical measurements of the organic electrolyte LiPF 6 prove long-lasting specific capacity, specific capacitance consistent at increased current rates, and adequate cyclic stability for GO anode.