A variety of clean and sustainable energy sources have been urged as fossil fuels are rapidly depleted and environmental concerns are growing[1, 2, 3]. This makes the storage of these various forms of energy a crucial problem. Supercapacitors, also referred to as electrochemical capacitors, are a distinct type of energy storage devices with a wide variety of applications because of their high-power capability, long cycle life, and quick charge/discharge rates[4, 5]. Carbon-based materials have been regarded as a key electrode component in supercapacitor devices due to their outstanding electrochemical properties[1, 2, 6]. These materials not only provide high electronic conductivity, cheap cost, and environmental friendliness, but they also alleviate cycle degradation difficulties caused by mechanical challenges[6]. Transition metal sulfides and oxides such as Ni​3S​2, WS​2, and NiO have been investigated widely as an active electrode material for supercapacitors[4, 5, 7]. They have a substantially greater specific capacitance because of their high energy density and fast kinetics of energy transfer reaction at the electrode surface. Furthermore, their capacitance is significantly higher than that of carbonaceous materials, which normally operate based on an electric double layer charge storage mechanism. The weak electrical conductivity of transition metal sulfides and oxides, on the other hand, limits their high-rate performance and cycle life[5, 7, 8]. In the development of new modified materials, this has been a challenging research problem.

Highly oriented pyrolytic graphite (HOPG) is a distinct form of graphite with a well-organized and layered structure[9, 10]. This structure is formed by the pyrolysis method, which involves heating a carbon precursor material to high temperatures under controlled conditions to make a single crystal structure of graphite[10]. HOPG has been attracting the interest of supercapacitor researchers due to its layered structure, which offers remarkable properties such as increased electrical and thermal conductivity, as well as mechanical properties. However, HOPG is also necessary to modified to increase specific capacitance as well as specific energy due to its basal plane graphite, surface functionalization edges sites and adhesive properties of surfaces. Only a small amount of research has been done on HOPG flakes to modify and develop few-layer graphene-like electrodes for use in energy storage devices[9, 10, 11]. Exfoliation of graphitic materials using plasma treatment methods has generated the interest of many researchers because of its simplicity and quick exposure time, as well as its low cost and ease of usage. Increased electrochemical surface area and improved surface modifications of HOPG could aid in the transport of electrolyte ions into graphite layers and contact between the electrolyte and graphite electrode, resulting in enhanced capacitance and energy density. Furthermore, over a long period of time, our lab has studied the effects of ambient plasma on various carbon-based materials and metal sulfides. Our research demonstrates that plasma treatment improves active surface area, surface adhesion, electrical conductivity, surface functionalization, and hydrophilicity/hydrophobicity properties[12, 13, 14].

This work presents a simple and effective plasma-treated HOPG is investigated in a coin-cell type symmetric supercapacitor with gel electrolyte for all-solid-state supercapacitor applications. The device fabricated with plasma treated HOPG for 90 min has a specific capacitance of 152.8 F/g at a current density of 1 A/g and capacitance retention of 84% after 3,000 cycles. Furthermore, with a power density of 802 W/kg, an energy density of 13.8 Wh/kg can be attained.

1. Material and reagents

Polyvinyl alcohol (PVA, 99%, hydrolyzed), sodium hydroxide (NaOH, ≥ 98%), nickel foil (Ni, 125 μm), and Whatman GF/C binder-free filter paper of 0.26 mm thickness was purchased from Merck. Highly oriented pyrolytic graphite (HOPG, 10 × 10 × 1 mm) was obtained from SPI supplies Division of STRUCTURE PROBE, Inc. West Chester, USA.

2. Preparation of plasma treated HOPG electrode

A single layer of HOPG, measuring 10 × 10 mm, is carefully applied to the copper substrate, and the high-power supply (NTO-500, NT Ltd., Korea) is connected to the bottom copper tape, which acts as a negative electrode. However, the stainless-steel needle (Kovax-needle 22G, D=0.7 mm, length of 40 mm) acted as a positive electrode and was used described elsewhere[5, 12, 13]. The distance between the HOPG surface and positive electrode was maintained about 2 mm. The positive electrode moved from one place to the next with an interval distance of around 5 mm, and a square area of 0.5 × 0.5 cm was chosen to be scanned. This process was controlled by an automatically programmed control box until completely scanned. The discharge current of 30 mA was obtained by applying plasma directly to the HOPG at a discharge potential of 15 kV and a frequency of 25 kHz. We used a thermometer to determine the temperature at the end of the solution during the plasma treatment, which was 60 ​∘C. Figure 1 shows a schematic representation of the plasma treatment on HOPG. For the comparison, five samples were generated, each subjected to plasma treatment for duration of 0, 30, 60, 90, and 120 min. Consequently, their labels were changed to HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120, respectively.

Figure 1. (Color online) Schematic illustration of plasma treatment on HOPG and its application in a symmetrical coin cell device.

3. Characterization

Field emission scanning electron microscopy (FESEM, Hitachi, Japan) was employed to characterize the surface morphology and structural composition of the samples. Raman spectroscopy (NANOBASE 100X XperRAM C, Korea, with a laser excitation of 532 nm) was utilized to examine the phase and vibration modes of the samples.

The electrochemical performance of the samples was examined at room temperature using (Bio-Logic, SP-150) with a two-electrode system. For the two-electrode system, the HOPG films were punched to make circular electrodes, and the diameter of the electrodes was 8 mm. The mass loading of the HOPG and P-HOPG-90 electrodes was calculated as 36.2 and 31.7 mg/cm​2, respectively. The positive and negative electrodes in the two-electrode system were represented by two HOPG electrodes, with a cellulose membrane filter paper (Whatman GF/C binder-free membranes, 0.26 mm thick) serving as the separator. Figure 1 shows a symmetrical coin cell device with positive (+), and negative (-) terminals connected by Ni foil. The PVA/NaOH gel electrolyte was used for the electrochemical measurements and prepared as follows: 3 g of PVA was dispersed in 22 ml of DI water and magnetically stirred until the solution was clear at 70 ​∘C. Additionally, 3 g of NaOH was dissolved in 6 ml of DI water with stirring for 60 min. Finally, a PVA/NaOH gel electrolyte was prepared by mixing the two solutions under the magnetic stirring at 70 ​∘C for 30 min.

For the comparison of electrochemical performances, the other prepared samples after the plasma treatment HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 were used instead of HOPG in the same procedure. Samples were investigated electrochemically using cyclic voltammetry (CV), chronocoulometry, electrochemical impedance spectroscopy (EIS), and Galvanostatic cycling with potential limitation (GCPL). To explore the capacitive characteristics and assess the reversibility of an electrochemical process, CV analyses were conducted over a potential range of 0 to 0.8 V. Various scan rates, specifically 10, 20, 50, 100, and 200 mV/s, were employed for this investigation. In accordance with prior methodologies, chronocoulometry techniques were applied to compute the electrochemically active surface area[12, 13, 15]. Additionally, EIS was employed to scrutinize electron transfer, recombination movements, and electrochemical impedance properties. The frequency range examined during EIS ranged from 100 MHz to 500 kHz, providing valuable insights into the electrochemical behavior of the samples. The following formula was used to determine specific capacity (Cs) at F/ g based on the CV curve[12, 15]:

Using the integral area under the CV curve (A), the electrode mass is specified as m, the potential window as ΔV, and the scan rate as mV/s. Specific capacitances (Cs), coulombic efficiency, power density (P) in watts per kilogram (W/kg), and energy density (E) in watt-hours per kilogram (Wh/kg) were computed using the following equations. These computations were performed using the galvanostatic charge-discharge (GCD) curves obtained from GCPL[15, 16, 17]. The variables employed in the equations are a discharge time (Δt), measured in seconds, and a discharge current (I), measured in amperes.

Figure 2 shows the SEM images of HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120. HOPG has a smooth surface with a flat and layered structure, and no visible cracks (Fig. 2(a)). However, after plasma treatment for varying durations, the network of cracks, pores, and deformation appears on the rough surface of HOPG (Fig. 2(b–j)). Surface modification results in exfoliated layers with pores, particularly notable in the plasma-treated sample P-HOPG-90. This indicates that it could be beneficial for surface activation by improving surface energy and adhesion, as plasma can interact with the surface of HOPG[5, 12]. The 90 min treatment time exposes a consistent pore-like structure on the striking surface compared to the 60 min plasma treatment. Interestingly, extending the plasma treatment time by an additional 120 min seems to have no further effect. The regular pore-like structure collapses, and the flakes appear to be overlapped (Fig. 2(i–j)). The change in the physical structure of the top layer influences surface wettability and functionalization[18]. This modified structure may increase the surface area, implying improved supercapacitor performance or even specific capacitance.

Figure 2. SEM images of (a) before the plasma treatment, (b–c) 30 min plasma treatment, (d–e) 60 min plasma treatment, and (f–g) 90 min plasma, and (h–i) 120 min plasma treatment on HOPG.

The Raman spectra of HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 are shown in Fig. 3. The Raman spectra of untreated HOPG have two characteristic peaks, G and 2D; however, after the plasma treatment on HOPG, all samples exhibit a D band close to 1352 cm​-1, G band at 1580 cm​-1, and 2D band at approximately 2710 cm​-1[19, 20]. The D peak is generated by the A1g vibrational mode, representing the characteristic peaks of disordered carbon. The G peak corresponds to the vibrational mode E2g1 of carbon atoms bonded in a sp​2 configuration, while the 2D peak is a second-order characteristic attributed to disordered carbon within the samples. The ID/IG intensity ratio is commonly used to determine the degree of disorder in materials; the ID/IG intensity ratios of P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 are 0.61, 0.66, 0.79, and 0.88, respectively. After plasma treatment, there is minimal variation in the ID/IG intensity ratios, indicating a comparable chemical state of the carbon structures. The intensity of the D and 2D peak slightly increases after the 60 min plasma treatment, signifying an increase in disorder or defects within the carbon structure of HOPG. Furthermore, the decrease in the G peak is attributed to the plasma treatment process, resulting from the roughness and surface modification on the HOPG surface.

Figure 3. (Color online) Raman spectra results before and after the plasma treatment on HOPG.

The CV results of the samples are shown in Fig. 4. The scan rate-dependent CV results of HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 exhibit quasi-rectangular-shaped CV curves at different scan rates, ranging from 10 to 200 mV/s, as shown in Fig. 4(a–e). However, the P-HOPG-90 sample in Fig. 4(d) displays a large integrated area, demonstrating the high-rate capability. The corresponding specific capacitance, which is related to the surface area can be calculated from CV curve[8, 12, 15, 17, 21] using Eq. (1), is shown in Fig. 4(f), indicating that P-HOPG-90 has the highest specific capacitance across all scan rates. At a scan rate of 100 mV/s, P-HOPG-90 exhibited the highest capacitance of 164 F/g, nearly 2.2 times that of the HOPG sample (76 F/g). Additionally, the capacitances of P-HOPG-30, P-HOPG-60, and P-HOPG-120 were also found to be 99, 139, and 125 F/g, respectively, at the same scan rate. As a result, P-HOPG-90 may have a larger current response in comparison to the other samples, indicating a high specific surface area, low charge transfer resistance, and capacitance.

Figure 4. (Color online) (a–e) CV results of the samples and (f) corresponding specific capacitance.

Based on the chronocoulometry measurement, the electrochemical active surface area was computed, as shown in Fig. 5(a), where the slope corresponds to the active surface area[15, 17, 21]. HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 surface areas were 2.4 × 10​-5, 2.9 × 10​-5, 3.6 × 10​-5, 4.4 × 10​-5, and 3.2 × 10​-5 cm​2, respectively. The P-HOPG-90 sample has the highest electroactive surface area when compared to the others, and it is 1.9 times larger than the HOPG sample. The P-HOPG-90 sample is expected to have better energy density and capacitance than the others due to its large surface area, which is a key characteristic of supercapacitors. Internal resistance and charge transfer resistance might be computed using Z-fitting software based on the previously published comparable Randle circuits[16, 17, 22]. Figure 5(b) illustrates the EIS results, displaying Nyquist plots where the negative imaginary part (-Im |Z|) is plotted against the real part (Re |Z|) for the various samples. An inset of Fig. 5(b) reveals an internal resistance (Rs), a charge-transfer resistance (RCT), a double-layer capacitance (CDL), and a Warburg component (W). The diameter of the semicircle indicates the resistance associated with charge transfer. Rs and RCT were calculated to be 11.9 and 33.3 Ω for HOPG, 9.3 and 27.1 Ω for P-HOPG-30, 5.6 and 19.1 Ω for P-HOPG-60, 2.9 and 13.7 Ω for P-HOPG-90, 7 and 24.7 Ω for P-HOPG-120, respectively. The observation indicated that P-HOPG-90 exhibits the least internal and charge transfer resistance, suggesting superior supercapacitor performance compared to the precursor HOPG.

Figure 5. (Color online) (a) Electroactive surface area and (b) EIS results of the samples.

Figure 6(a–e) illustrates the charge-discharge curve of HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120 at different current densities within a potential range of 0 to 0.8 V. When the results are compared, all HOPG-90 curves had a longer discharge time, showing good electrochemical reversibility and improved supercapacitor properties. The maximum specific capacitance of the samples was determined from the discharge curve using Eq. (2) at 0.25 A/g, yielding values of 76.2, 112.5, 155.9, 171.2, and 134.3 F/g for HOPG, P-HOPG-30, P-HOPG-60, P-HOPG-90, and P-HOPG-120, respectively. P-HOPG-90 has a high specific capacitance due to its large specific surface area, which ensures optimal electrode material consumption. Figure 7(a) depicts the corresponding Ragone plot obtained from the HOPG and P-HOPG-90 devices. The P-HOPG-90 device has an energy density of 13.8 Wh/kg, the HOPG device has an energy density of 5.28 Wh/kg, and the corresponding power density is 802 W/kg at a current density of 1 A/g[23, 24, 25], resulting that the energy density is increased nearly 2.6 times after the plasma treatment. Figure 7(b) illustrates the obtained capacitance retention of the devices following 3,000 cycles. Capacitance retentions of 90.2% and 79.4% were obtained for the P-HOPG-90 and HOPG devices, respectively. Following a 3,000-cycling stability test, the P-HOPG-90 device was found to be quite stable and trustworthy.

Figure 6. (Color online) (a–e) GCPL results of the samples and (f) corresponding specific capacitance.

Figure 7. (Color online) (a) Ragone plot and (b) capacitance retention after 3,000 cycles of HOPG and P- HOPG-90.

This research investigated the use of ambient plasma to improve Highly Oriented Pyrolytic Graphite (HOPG) for supercapacitor applications. The impact of time-dependent plasma treatment (ranging from 0 to 120 min) on HOPG is examined within a coin-cell type symmetric supercapacitor with gel electrolyte. The optimal treatment duration, determined to be 90 min, yielded superior performance with a high specific capacitance of 152.8 F/g, an energy density of 13.8 Wh/kg, and a power density of 802 W/kg at 1 A/g for supercapacitor applications. Through surface modification and functionalization, the ultimate objective is to advance the field of high-performance, low-cost energy storage devices.