Enhanced areal capacitance through potassium incorporation into the graphene framework of laser-induced graphene for flexible electronics using LiCl gel electrolyte
Kategoria artykułu: Research Article
Data publikacji: 31 mar 2025
Zakres stron: 67 - 79
Otrzymano: 26 sty 2025
Przyjęty: 05 mar 2025
DOI: https://doi.org/10.2478/msp-2025-0007
Słowa kluczowe
© 2025 Nagih M. Shaalan, published by Sciendo
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
With recent developments in wearable and flexible electronics, supercapacitors have gained significance due to their mechanical flexibility, high power density, and fast charging/discharging ability. The developments were mostly made to improve their performance and integration aspects for applications such as flexible supercapacitors [1]. These devices are ideal for integration into applications, such as smartwatches, health monitoring systems, and portable electronic gadgets, which will build a future for efficient energy storage toward sustainable flexible electronics [2]. These adaptable energy storage units are utilized across a wide array of fields, spanning from electronic circuits to energy storage systems and sensing devices [3]. Supercapacitors are typically divided into two primary categories: pseudocapacitors and electrolytic double-layer capacitors (EDLCs) [4,5]. EDLCs derive their capacitance from charge accumulation within a thin layer at the interface between the electrode and electrolyte. Enhanced capacitance is achieved through strategies aimed at increasing the electrode surface area and improving the electrical conductivity.
The modified graphene with atoms revealed extraordinary performances in the processes of metal-based catalysts. The graphene has been doped with various atoms or molecules to modify its physical and chemical properties. It was doped by single elements, such as nitrogen [6], boron, and potassium [7], and co-doped with more elements such as nitrogen–boron [8] and potassium bromide [9]. Graphene boasts noteworthy attributes such as exceptional electrical conductivity, high surface area, robust mechanical strength, and lightweight nature. The working capacitance of the original graphene is lower than the expected value despite its unique properties. Hence, optimizing the properties of graphene toward its electrochemical activity is a challenge. Therefore, there are many ways to improve the properties of graphene as one of the most promising materials in supercapacitors. Some of these ways are for doping graphene with foreign molecules or elements, which affect the electronic properties [10]. Recent years have witnessed a surge in comprehensive studies within this particular field, highlighting the growing interest and exploration in this area. Notably, investigations into modified graphene at the atomic level have shown remarkable performances, particularly in the context of metal-based catalyst processes [11,12]. However, despite these inherent advantages, the working capacitance of pristine graphene often falls short of expectations, presenting a notable challenge in optimizing its electrochemical properties. Consequently, researchers face the task of creating strategies to enhance graphene’s suitability for electrochemical applications, recognizing its status as one of the most promising materials in the space of supercapacitors.
In this work, we propose a straightforward method for
Polyimide sheets, with a thickness of 5 mil and a width of 5 inches, were purchased from TapeCase, USA. Potassium hydroxide (KOH) pellets (EMSURE® grade) were obtained from Sigma-Aldrich for analytical purposes. Lithium chloride (ACS reagent, purity ≥99%) from Sigma-Aldrich was used as the electrolyte. Polyvinyl alcohol (PVA) pellets, with an average molecular weight of 89,000–98,000 and a purity of 99+% (hydrolyzed), were also sourced from Sigma-Aldrich for preparing the gel electrolyte. Whatman grade B-2 weighing paper was used as the separator. A premium double-sided adhesive tape (strong adhesion, brand: ECO FUSED, KSA) was used.
For LIG fabrication, the CO2 laser technique was utilized, in which the laser was housed within a machine capable of delivering a maximum power of 40 W and a maximum speed of 700 mm s−1. Raw materials for LIG comprised polyimide sheets. Operating parameters for the laser system were 7.0 W of laser power and a head speed of 80 mm s−. The dimensions of the fabricated electrodes were 1 cm × 2.0 cm. Subsequently, a 40 mM KOH solution was carefully applied to the produced graphene, followed by drying at 85°C. To enhance the integration of K ions into the hexagonal graphene layer, the sheet underwent another round of laser treatment under identical conditions, exploiting the rapid thermal effects of the laser beam, as shown in Figure 1. Post-fabrication, it was thoroughly cleaned with DI water to eliminate any residual KOH.
Diagram illustrating the procedures for fabricating LIG and PLIG electrodes utilizing a CO2 laser machine.
The process began with the independent preparation of LiCl and PVA solutions to prepare a 1 M LiCl gel electrode based on 10% PVA (by water weight). First, a 1 M LiCl solution (423.9 mg of LiCl dissolved in 10 ml DI water) was prepared by dissolving the appropriate amount of LiCl in deionized water under stirring until it dissolved completely. Separately, a 10% PVA solution (1.0 g of PVA in 10 ml DI water) was prepared by dissolving PVA in deionized water while stirring continuously at 80°C until a homogeneous solution was obtained. Once the PVA solution was ready and it was cooled to 40°C, the LiCl solution was added dropwise to the PVA solution under continuous stirring to ensure uniform mixing. Upon the long stirring process at 80 and 40°C, the water evaporated, and the mixture was reduced to a total volume of 10 ml. This process resulted in the formation of a LiCl gel electrode with a consistent distribution of LiCl within the PVA matrix. The LiCl gel was selected as the electrolyte due to its high ionic conductivity, excellent gel stability, and ability to provide efficient ion transport, which are essential for enhancing the performance of the supercapacitor device.
As shown in Figure 2, the fabrication of the symmetric supercapacitor device can be divided into a few key steps. First, the Ag paste was applied to the LIG to enhance its electrical conductivity as well as to act as a current collector. Next, premium double-sided adhesive tape was stuck along the edges of the polyimide substrate for its structural integrity and proper alignment of the components. The separator was prepared by using butter paper, so that the electrodes would not touch each other directly, but the ions could pass through it. Butter paper is cost-effective, easily available, and has adequate mechanical strength to prevent direct contact between electrodes while allowing ion diffusion, making it a simple and practical choice for experimental setups. Then, the LiCl gel electrolyte was coated on the separator for coverage to allow the conduction of ions effectively. Finally, the symmetrical supercapacitor device was assembled by sandwiching different components to complete the fabrication process with aligned current-collecting electrodes for reliable performance.
Fabrication of the symmetrical supercapacitor device based on PLIG and LiCl electrolyte.
Product analysis involved the following characterization experiments. HRTEM was performed with a JEOL JEM-2100F, Tokyo, Japan, operated at 200 kV. Before TEM examination, samples underwent sonication in ethanol and were subsequently deposited onto carbon-coated copper grids. Raman spectra were acquired employing a Raman confocal microscope (LabRAM-HR800) coupled with a He–Cd laser emitting at a wavelength of 633 nm with an output power of 50 mW, all performed at room temperature. The structure was examined by using an X-ray powder diffractometer (Philips-PW-1710) equipped with a CuKα radiation source of 1.546 Å. The XRD patterns were recorded at 2
An in-depth analysis of the products was conducted using HRTEM to examine LIG, as depicted in Figure 3. Figure 3a presents TEM images, revealing the fabrication of multi-layer graphene, with contrasting areas, indicating the accumulation of layers. In Figure 3b, HRTEM lattice images display the structure of various graphene sheets within LIG. Notably, the wavy graphene structure exhibited significant length and width ranging from 3 to 9 nm, as illustrated in Figure 3b. Crystalline phases of graphene were discernible in the lattice fringes, with no evidence of accumulation observed in LIG through HRTEM analysis. The spacing between fringes elucidates the formation of the corresponding planes within the graphene sheets. Enhanced magnification reveals visible atomic columns of carbon within the lattice image. Analysis of the graphene lattice planes indicates a dominant (002) plane with a d-spacing of 0.33 nm, underscoring its significance in this graphene structure [13,14].
The investigation comprised two main aspects: TEM images focusing on few-layer graphene for (a) LIG and (d) PLIG. HRTEM imaging capturing the extensive lattice variation of (b) LIG and (e) PLIG. SAED and lattice profiles for (c) LIG and (f) PLIG.
The EDX mapping images illustrate the morphological and elemental distribution of LIG and PLIG, as shown in Figure 4. Figure 4a reveals the porous and interconnected structure characteristic of LIG, while the corresponding elemental map (C) confirms a homogeneous distribution of carbon, indicating the formation of a graphene-rich material. Figure 4b shows the morphology of PLIG, which retains the porous structure but exhibits a slightly denser texture, likely due to the incorporation of potassium. The EDX mapping confirms the uniform distribution of carbon, while K mapping highlights the even dispersion of potassium throughout the LIG matrix. This suggests the successful doping of potassium into the graphene framework, enhancing its electrochemical properties for applications such as energy storage or catalysis. The EDX spectrum shown in Figure 4(c) exhibited a rough content of 0.5 wt% of K incorporated in the LIG matrix. It is expected that the K content is higher than this percentage, where the carbon-coated mesh contributes to the total carbon content in the sample.
EDX mapping of (a) LIG and (b) PLIG for carbon and potassium distribution. (c) EDX spectrum of PLIG.
Figure 5 depicts the Raman spectra of the prepared samples, revealing the presence of D-, G-, and 2D bands characteristic of graphene. The G band, associated with the graphitic structure, signifies E2g vibrations of the sp2 bond within the Brillouin zone center [15]. Concurrently, the D band at 1,358 cm⁻¹ indicates lattice disorders or defects within the graphene structure. The impact of K on the graphene lattice is evident from the shift in the D band position, signifying a modification in the fundamental graphene configuration, as demonstrated in Figure 5. Furthermore, the 2D band, representing two-phonon scattering processes, is another aspect of the D band [16,17]. Alterations in Raman parameters such as the peak position, full width at half-maximum (FWHM), and peak area are detailed in Table 1. In pristine graphene, the D-band, G-band, and 2D-band were observed at 1,357, 1,567, and 2,720 cm⁻¹, respectively. After K doping, all peak positions shifted to higher values, with an increase in FWHM indicating K incorporation into the graphene structure. The degree of crystallinity, assessed through Raman spectra, demonstrated increased disorder within graphene chains, as evidenced by the change in FWHM. The decrease in the D peak intensity and alteration in the peak area corroborate the FWHM results. The observed shifts in the D and G bands in the Raman spectra can be attributed to structural distortions and electronic modifications induced by potassium doping, which enhances the defect density and alters the electronic structure of the graphene lattice. Calculation of the intensity of 2D/G and D/G bands provided further insights into graphene characteristics, and the results are listed in Table 1. The
Raman spectra obtained from bare graphene and K-doped graphene.
Raman parameters for bare graphene and K-doped graphene.
| Sample | FWHM (cm−1) | D band (cm−1) | G band (cm−1) | 2D band (cm−1) |
|
|
||
|---|---|---|---|---|---|---|---|---|
| D band | G band | 2D band | ||||||
| LIG | 65.2 | 46.2 | 117.1 | 1,357 | 1,567 | 2,720 | 0.28 | 0.42 |
| PLIG | 169.2 | 76.4 | 242.7 | 1,369 | 1,580 | 2,743 | 0.54 | 0.38 |
The XRD results indicate significant structural changes in LIG upon potassium doping, as shown in Figure 6. The primary (002) diffraction peak of LIG, initially observed at 25.38° [13], shifts to a lower angle of 25.19° in the PLIG, suggesting an increase in the interlayer spacing. The shift can be attributed to the insertion of potassium atoms between the graphene layers, which expands the interlayer distance due to the larger atomic size of K compared to carbon. Additionally, the peak intensity decreases drastically, indicating a reduction in graphitization and an increase in disorder within the graphene structure. The same effect was observed on the (001) peak. The slight increase in the crystallite size from 5.5 nm in LIG to 5.6 nm in PLIG, calculated using the Scherrer equation [20,21], suggests that potassium incorporation does not significantly disrupt the crystalline domain size but affects the stacking order and structural integrity. Overall, these observations highlight that potassium doping induces structural modifications in LIG by introducing defects, reducing the graphitization, and increasing the interlayer spacing, which can influence its electrical, electrochemical, and adsorption properties in potential applications such as energy storage and catalysis.
XRD patterns of LIG and PLIG recorded in diffraction angles of 15–65o.
Figure 7 illustrates the electrochemical analysis of the supercapacitor involving the acquisition of cyclic voltammograms at sweeping rates ranging from 5 to 100 mV s⁻¹. The potential window spanned from 0 to 1.0 V, revealing no discernible redox peaks for either LIG or PLIG. Notably, square-like CV curve characteristics of carbon materials were observed for the current electrodes, indicating a behavior typical for carbon-based materials. The current density exhibited an increase for PLIG, reaching up to −2.2 mA cm⁻2, thereby yielding higher areal capacitance (AV) compared to LIG. The distinctive shape of these curves, alongside their alteration with the introduction of K, suggests a synergistic effect of doping on the electrochemical performance of the graphene electrode, potentially attributed to modifications in the electronic structure of graphene [22]. The enhanced CV profile of PLIG signifies an augmentation in the capacitance of the electrode. Additionally, Figure 8 illustrates the areal capacitance plotted against the scan rate for both bare and K-doped graphene. Bare graphene exhibited a median areal capacitance compared to the doped one. Specifically, at a scan rate of 5 mV s⁻¹, pristine graphene displayed an areal capacitance of 17 mF cm⁻2, decreasing to 6 mF cm⁻2 at 100 mV s⁻¹. In contrast, K-doped graphene demonstrated an areal capacitance of 26 mF cm⁻2 at 5 mV s⁻¹, decreasing to 7.5 mF cm⁻2 at 100 mV s⁻¹. Notably, even at higher scanning rates, the areal capacitance of doped graphene remained superior to that of bare graphene.
CV curves recorded at different scan rates for (a) LIG and (b) PLIG electrodes.
Relationship of areal capacitance of LIG and PLIG with the scan rate.
The GCD curves depicting the behavior of the electrodes are illustrated in Figure 9. Specifically, Figure 9(a) and (b) showcases the GCD profiles of LIG and PLIG electrodes, respectively, charged to a potential of 1.0 V. During discharge, the capacitors were subjected to current densities ranging from −0.75 to −2.5 mA cm⁻2, affirming the feasibility of charging these capacitors at both low and high current densities. As the discharging current varied from 0.75 to −2.5 mA cm⁻2, the discharging duration for both pristine graphene and PLIG electrodes decreased. Notably, the augmented discharging duration observed for PLIG is attributed to the beneficial influence of K on the electronic structure of graphene. Moreover, with increased current density, the charging and discharging processes accelerated across all electrodes, a characteristic feature indicative of EDLC behavior. As shown in Figure 10, the areal capacitance of the electrodes is plotted against the current density. K-doped graphene exhibits notably higher areal capacitance compared to pristine graphene. For instance, at a discharge current density of 0.75 mA cm⁻2, PLIG electrodes demonstrated an areal capacitance of 21 mF cm⁻2, in contrast to 11 mF cm⁻2 for pristine LIG. However, the areal capacitance dropped with increasing current density. At a high current density of 2.5 mA cm⁻2, the capacitance decreased to 5.0 and 7.5 mF cm⁻2 for LIG and PLIG electrodes, respectively.
Galvanostatic charging–discharging curves recorded at different current densities for (a) LIG and (b) PLIG.
Relationship between areal capacitance and current density for LIG and PLIG electrodes.
To delve further into the electrochemical processes happening at the interface between the electrode and electrolyte, EIS was performed over a frequency range extending from 0.1 Hz to 1.0 MHz. Figure 11a presents the EIS measurements, illustrating the Nyquist diagram for individual electrodes. Within this diagram,
(a) Nyquist diagrams and (b) cycling stability of the electrodes, evaluated at a discharge current density of 1.75 mA cm⁻2 over 2,000 cycles.
Cycling performance stands as a crucial metric for evaluating supercapacitor electrodes [26]. Figure 11b presents the areal capacitance values derived from 2,000 cycles of GCD curves, showcasing the electrodes’ stability. The cyclic charge–discharge curves were conducted at a current density of 1.75 mA cm⁻2. Notably, the electrodes demonstrated consistent cycling stability throughout all charge–discharge cycles. However, retention values exhibited a gradual decline with increasing cycle numbers for pristine graphene, with capacitance decreasing by 25%. Both LIG and PLIG underwent cycling performance evaluation over 2,000 cycles, revealing highly stable electrochemical performance. The stability of the electrode during electrochemical processes depends on several factors, such as surface corrosion, the dissolution of electrode material into the electrolyte, and secondary chemical reactions between the electrolyte and electrode [26]. These findings underscore that the current electrodes are robust electrode designs and materials that ensure the reliability and longevity of electrochemical devices.
Enhancing the carrier density through doping has emerged as a viable strategy for augmenting the electrical conductivity of graphene. Drawing inspiration from the framework of graphite intercalation compounds, doping at elevated concentrations or intercalation has been shown to enhance the conductivity of graphite fibers up to 1.3 × 107 S m⁻¹ [27]. Typically, nitrogen, boron, phosphorus, potassium, bromine, and other elements are utilized for graphene doping. These dopant atoms serve to widen the band gap, alter the electronic structure, boost the density of free carriers, and consequently enhance the conductivity and stability of graphene. Although potassium-doped graphene has received limited attention, Xue et al. [28] explored the impact of potassium on few-layer graphene utilizing a wet chemistry approach. Their findings revealed that by optimizing the potassium concentration, superconductivity could be achieved in K-doped graphene. In the realm of supercapacitors, electrical energy storage relies on electrostatic phenomena. Energy is stored based on the presence of negative and positive ions within the electrolyte. During the electric charging process, a positive voltage is applied to the electrode, attracting negative ions from the electrolyte and forming an electrostatic double layer on the electrode.
Given highly ionic nature of KOH, it readily dissolves in water, dissociating into K⁺ and OH⁻ ions. Consequently, KOH is anticipated to serve as a source of K⁺ in the graphene lattice. Few studies have investigated the impact of potassium on both graphene and graphite oxide [29–31]. Through K-doping, the electrical conductivity of doped graphene fibers has been reported to reach 2.24 × 107 S m⁻¹ [30]. Potassium doping markedly elevated the carrier concentration of graphene paper to 2.068 × 1021 cm⁻³, representing a two order-of-magnitude increase compared to pristine graphene. The doping mechanism for doped graphene is believed to involve an intercalation process, which may introduce defects into graphene crystallites. In the present study, the results indicate that potassium atoms were intercalated as individual entities. The sheet resistance measured for LIG and PLIG was determined to be 45 and 29 Ω, respectively. Furthermore, the impact of potassium was evident in both the CV curve and the electric current density, which exhibited a twofold enhancement for the PLIG electrode. These findings underscored the role of potassium in enhancing the electrical performance of graphene. Consequently, the intercalation of potassium at various sites within graphene layers emerges as the most plausible doping mechanism [37]. It is anticipated that potassium occupies diverse sites within the graphene layer, given its atomic radius of 0.22 nm per atom, facilitating its accommodation between graphene layers.
Table 2 lists previously reported promising capacitance values for LIG, highlighting its potential as an effective electrode material for energy storage applications. Through innovative fabrication techniques utilizing laser irradiation on various carbon sources, LIG has demonstrated remarkable capacitance performance. These studies have showcased the ability of LIG to achieve high specific capacitance values, making it a compelling candidate for supercapacitor applications. The table also includes areal capacitance values for both untreated LIG electrodes and those that have been doped. Additionally, it presents supercapacitor metrics such as capacitance, power density, and energy for comparable electrodes.
Comparison between supercapacitor electrodes made from laser-induced graphene.
| Electrode | Areal capacitance | Areal energy | Areal power | References |
|---|---|---|---|---|
| Graphene | 0.8 mF/cm2 at 10 mV/s | — | — | [32] |
| Graphene | 34 mF/cm2 at 0.1 mA/cm2 | 1.0 mWh/cm3 | 11 mW/cm3 | [33] |
| Graphene + MoS2 + MnS | 58.3 mF/m2 at 50 mA/cm2 | 7 µWh/cm2 | 49.9 µW/cm2 | [34] |
| NiO/Co3O4/graphene | 29.5 mF/cm2 at 0.05 mA/cm2 | — | — | [35] |
| Graphene | 6.1 mF/cm2 at 20 mV/s | 0.96 µWh/cm2 | 0.25 mW/cm2 | [36] |
| LIG | 11 mF/cm2 at 0.75 mA/cm2 | 1.5 µWh/cm2 | 186 µW/cm2 | Present work |
| PLIG | 21 mF/cm2 at 0.75 mA/cm2 | 2.8 µWh/cm2 |
In summary, a novel approach was introduced to dope graphene, thereby enhancing its electrical performance for supercapacitor applications. HRTEM images validated the formation of multilayer graphene sheets via a CO2 laser, with lattice fringes predominantly indicating the (002) plane. Raman spectra exhibited notable shifts in peaks, indicative of K intercalation within the graphene lattice, significantly affecting the D, G, and 2D peaks of pristine graphene. Electrodes, both doped and undoped, were evaluated as supercapacitors without additional treatment. Doping improved the electrochemical performance of LIG, as evidenced by an increase in areal capacitance from 17.5 to 27 mF cm⁻2 at 5 mV s⁻¹. The supercapacitor’s performance demonstrated robust stability over a 2,000 cycle lifespan. The PLIG capacitor exhibited an areal energy storage of 2.8 µWh cm−2 at 0.75 mA cm⁻2, with an areal power of 186 µW cm⁻2. This study opens avenues for cost-effective, high-performance supercapacitors suitable for wearable or flexible electronics. This work provides an advanced state-of-the-art technology by presenting a scalable laser-based doping technique that simplifies electrode fabrication while improving the performance. The results demonstrate that potassium-doped LIG is a promising candidate for high-performance, low-cost, and flexible energy storage applications, paving the way for further development in wearable and portable electronics.
This work was supported by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Saudi Arabia [GRANT No. KFU250550].
Conceptualization, N.M.S., S.K., and F.A.; methodology, M.M.A., O.S., and N.M.S.; formal analysis, M.M.A. and N.M.S.; investigation, O.S.; resources, F.A. and N.M.S.; data curation, N.M.S., S.K.; writing–original draft preparation, N.M.S.; writing–review and editing, N.M.S., S.K., O.S.; project administration, N.M.S.; funding acquisition, N.M.S.
Authors state no conflict of interest.