The X-Ray diffraction (XRD) patterns of LIG obtained from commercial polyimide (PI) sheets is revealed in Fig-2(a). Curve (a) displays a prominent peak centered at 2θ = 25.44°, revealing LIG-like structures with significant degrees of graphitization, indicating an interlayer spacing of ~ 3.4 Å between the (002) layers. The increasing interlayer gap is attributed to defects formed on hexagonal graphene layers. Another distinguishing feature of the XRD pattern is the low-intensity peak at 2θ = 44°, which is associated with the reduction peak of PI into graphene-like LIG material. This peak is related to the in-plane structure of LIG and is a reflection from (100) planes. The laser-induced graphitization phenomenon may be caused by the presence of repeated aromatic and amide units in PI, as suggested by few previous studies 17,34−36. These units are responsible for the formation of graphene-like structures in the LIG material. Curve (b) represents the XRD pattern of a multi-walled carbon nanotube (MWCNT). The strongest and sharpest diffraction peak for MWCNT appears at 2θ = 25.7°, which is labeled as the (002) plane. This peak exhibits a general decline in intensity compared to conventional graphite, 2θ = 26.5°, suggesting an increase in the sp2, C = C layer distance37. Besides, there is also a small diffraction at 2θ = 43.0° for MWCNT. Curve (c) depicts the XRD pattern of multiwall carbon nanotube-coated laser-induced graphene. At 2θ = 25.4°, a modest increase in the peak labeled as 002 indicates an increase in the content of graphene-like structures with high levels of graphitization due to the coating of MWCNTs. The 002 peak is a characteristic peak of graphitic materials and is associated with the stacking of graphene layers along the c-axis. Therefore, the presence of this peak in LIG-MWCNT confirms the formation of graphene-like layered structures. Additionally, the low-intensity peak at 2θ = 44° represents the signature of the reduction peak of PI into graphene-like LIG-MWCNT material. This peak is a reflection from (100). This peak further affirms the reduction of PI into graphene-like LIG-MWCNT material. Eq. (1) was used to calculate the crystalline size along the c-axis (Lc).
$${L}_{c}=\frac{0.9 \lambda }{{B}_{1/2}\left(2\theta \right)\left(cos\theta \right)}$$
1
Where \({B}_{1/2}\)(radians) is the FWHM for peak (002) and λ = 1.54 A (Cu kα source of XRD). Lc for LIG and LIG-2mg MWCNT coated is 24 nm and 21 nm, respectively38,39. Overall, the XRD pattern of LIG-MWCNT confirms the successful formation of graphene-like structures and the coating of MWCNTs onto the LIG, which can have potential applications in various fields, such as energy storage, sensing, and catalysis.
Fig-2(b-c) show a 2D AFM scan of the PI and LIG, respectively, whereas Fig-2(d-e) show a 3D AFM scan. The analysis revealed that the PI film had a more compact structure and a smoother surface than LIG. On the other hand, LIG had a surface dominated by islands, and an overall increase in surface roughness is attributed to the laser treatment and subsequent polymer-to-graphitic carbon conversion38. The intensity of spikes on the surface increased after photoexcitation. The rough, graphene-like surface of LIG was beneficial for developing a double layer and the adherence of the electrolyte, which improved the capacitive performance of the device.
In Fig-3(a), the red dotted line in the diagram represents the non-radiated area of the PI material, while the area beyond the red dashed line corresponds to the LIG region. This figure illustrates the effect of the laser beam on the PI sheet. The laser beam causes the termination of volatile C-N, C-O-C, and C = O bonds in the processed region of the PI sheet. This results in changes in the composition and shape of the material, which is reflected by the higher carbon content in the processed PI sheet compared to the unprocessed version40. It is possible that some nitrogen and oxygen atoms could be liberated as gases due to the laser-induced photo-thermal activity41. Fig-3(a) illustrates that the resultant content is transformed into porous carbon material. The irradiated region represents the LIG material on top of the PI sheet, as shown in Fig-3(b). This rough surface helps the electrolyte adhere well, which promotes diffusion and ultimately improves capacitance. Fig-3(c-d) show the magnified SEM image of the laser-ablated polyimide sheet. Fig-3(e) is an SEM image of the laser-carved merged area of LIG material coated with MWCNTs (multi-walled carbon nanotubes), which demonstrates strong links between the carbon flakes in the material and the meso-micropores on the surface. The Fig-3(f) is an SEM image at a different resolution, which shows the porous morphology of the LIG material after it has been coated with MWCNTs. Fig-3(g-h) are SEM images of LIG coated with MWCNTs. The results depict the formation of carbon flakes through the coating of MWCNTs, which are then organized in an aromatic graphitic structure to provide graphitic content42. The resulting honeycomb patterns offer a larger surface area and strong adhesion, making it highly beneficial for energy storage. The stacked flakes create high porosity, and their functional surfaces allow for the diffusion of electrolytes over the active material surface. The stacked flakes create high porosity and offer functional surfaces for the electrolyte diffusion over the active material surface. Creating laser-induced PI or laser-induced graphene involves a photo-thermal process that results in the formation of LIG porous layered flakes. These flakes further enhance the assisted diffusion process after the integration of MWCNTs. Overall, combining MWCNTs coating and laser-induced PI/LIG results in a highly beneficial material for energy storage applications, with enhanced surface area, porosity, and assisted diffusion capabilities.
In Energy Dispersive X-ray Spectroscopy (EDX), an electron beam strikes a sample, causing the ejection of inner shell electrons. An electron from an outer shell fills the resulting vacancy in the inner shell, and this transition releases energy in the form of an X-ray. By detecting the energy and intensity of these X-rays, it is possible to determine the elements present in the sample. Figure 4(a) and 4(b) show the elemental analysis of the samples with and without the coating of MWCNTs, respectively. In Laser-Induced Graphene (LIG), when the PI is reduced, it increases carbon content, which is evident in the LIG's dispersive energy X-ray (EDX) study, showing the highest carbon content. MWCNTs in the sample can also contribute to the increasing carbon content observed in the coated LIG sample. The size of the polymeric stub used in EDX to hold the substrate material is responsible for other visible aspects. The EDX analysis can detect other elements present in the sample. However, the element with the highest concentration typically indicates the target element that was successfully converted.
Raman Spectroscopy is an excellent method to characterize carbon nanomaterials. It provides details about the bands and the hybridization of the carbon structure. Fig-5(a) shows the Raman spectrum of the fabricated film with several lines: D (1326 cm–1), G (1579 cm–1), and G' (2690 cm–1). The D band indicates defects in the carbon structure, whereas the G band represents the sp2 hybridized carbon atoms. The G' band is related to the two-phonon scattering process in graphitic materials. The higher D band in LIG and LIG-MWCNTs indicates the breaking of sp2 bonds and the formation of more sp3 bonds (Fig-5(a)). This change in the hybridization of the carbon structure is likely due to the laser treatment used to fabricate the LIG and LIG-MWCNTs 33. However, a D band may also be present for several additional causes. Line G' is often referred to as the "2D peak" and is related to the second-order Raman scattering by Brillouin zone border phonons. The hexagonal structure of sp2 carbon atoms has defects that cause the first scattering line to appear. Line G, on the other hand, is related to the longitudinal oscillation mode of the carbon atoms.
At scan speeds ranging from 1 to 200 mV/s, cyclic voltammetry was conducted over a voltage window of -1 to + 1 V. From charge/discharge curves, Eq. (2)32,33,43 was used to determine the specific areal capacitance of SC:
$${C}_{device}=\frac{I}{\frac{dv}{dt}}$$
2
In Eq. (2), I stands for average discharge current and dv/dt for slope of Galvano static discharge curves. The following Eq. (3), is used to compute the areal capacitance:
$$C=\frac{1}{2\times s\times ({v}_{f-}{v}_{i) }\times \frac{dv}{dt}}{\int }_{{v}_{i}}^{{v}_{f}}I\left(V\right)dv$$
3
Where s is the electrodes' specific area (1 cm2), vf and vi are the final and starting voltages (in Volts), and dv/dt is the scan rate (in mV/s). C is the areal capacitance of the LIG and is measured in mFcm2. The Eq. 4 and Eq. 5 expressed below are used to obtain the real power density \({P}_{A}\) (µWcm− 2) and the specific areal energy density \({E}_{A}\) (µWhcm− 2)43.
\({E}_{A} = \frac{1}{2}\times C{\times \left(\varDelta V\right)}^{2}\times \frac{1}{\text{3,600}}\) | (4) |
P = \(\frac{{{E}}_{{A}}}{{t}}\) \(\times\) 3,600 | (5) |
PVA/H2SO4 gel electrolyte was used to examine the electrochemical characteristics of the developed electrodes in two electrode configurations44. Ionic gel electrolyte can provide a broader voltage window, enhancing the supercapacitor device's total energy density. Thus, a voltage window of -1 to 1 V was used to test the performance of the fabricated SC device.
In Fig-5(b), a symmetric fish-type CV curve from the CV analysis indicates that charge is stored through double-layer formation. An ideal CV curve for EDLCs is a perfect square; however, the CV curves for LIG deviate from this trend due to defects and surface roughness. In Fig-5(c), the specific areal capacitance of LIG is shown as a function of the scan rate. At a scan rate of 1 mV/s, the LIG electrode exhibited a high capacitance value of 30 mF/cm2, indicating its high energy storage capacity. Moreover, the absence of any significant change in the CV curve with increasing scan rate indicates that the electrode material has good electrochemical performance and rapid electrolyte diffusion37. This is important for the practical application of supercapacitor devices, as it indicates that the device can be charged and discharged quickly without significant loss of energy storage capacity. The large channels in the carbon electrode allow transient mass diffusion at higher scan rates while enabling its operation at a high scan rate42424242424244423630. With the increasing scan rate, a decreasing trend in capacitance was recorded. For example, capacitances of 19.47, 4.93, and 3.09 mFcm− 2 were obtained at scan rates of 2, 50, and 100 mV/s, respectively (Fig. 5(b)). Galvanostatic charge/discharge curves are commonly used to characterize the electrochemical behavior of energy storage devices, such as supercapacitors, by measuring the amount of charge that can be stored and released over time. Galvanostatic charge/discharge curves were obtained to better understand the charge storage process of the electrode. Galvanostatic charge/discharge curves for the LIG electrode displayed a symmetric pattern, as shown in Fig-5(d). This behavior further supports the theory of charge storage by double layer; however, the slight deviation can be attributed to the behavior of the electrolyte. Comparing the charging and discharging times from the GCD curves reveals a discrepancy between the predicted capacitance obtained from the GCD and CV curves. Specifically, the GCD curve yielded a capacitance of 6.09 mFcm− 2 at a current density of 0.2 mA/cm2, as illustrated in Fig-5(e). As shown in, EIS was used to analyze the impedance of the LIG electrode. Due to its insulating properties, the polyimide (PI) sheet exhibits a significantly high overall resistance. The insulating polyimide was converted into a conductive channel using CO2 laser ablation. The experimental data of the LIG electrode is shown by the black line in Fig. 5(f), while the red line represents a fitted model using an equivalent circuit.
After the LIG electrode was designed, we carried out the electrode fabrication process utilizing different MWCNT concentrations to understand the effect of various MWCNT coating concentrations on the properties of LIG electrodes. Specifically, we spray-coated LIG with 2 mg and 5 mg MWCNTs and then analyzed the resulting electrodes to optimize their design. The cyclic voltammetry curves for the LIG/2%CNTs sample are shown in Fig-6(a). An increase in capacitance from 30 to 71.313 mFcm− 2 at a scan rate of 1 mV/s was recorded, as shown in Fig-6(b). The trend indicates that charge is stored through double-layer formation, with a slight deviation from the ideal case, which could be due to factors such as the presence of impurities or imperfections in the electrode surface. The capacitance of 50.19, 2.36, and 0.993 mFcm− 2 were obtained at scan rates of 2, 50, and 100 mV/s, respectively, as shown in Fig-6(b). The Galvanostatic charge/discharge curves were obtained to better understand the charge storage process. The symmetric charge/discharge curves of the LIG-coated MWCNTs are shown in Fig-6(c). The electrolyte may cause the minor deviation from the symmetric behavior. These curves suggest that the charge storage process is dominated by the formation of a double layer, which is consistent with the CV data. There is a slight difference between the anticipated capacitance values obtained from the GCD and CV curves. Specifically, the GCD curve yielded a capacitance of 11.17 mFcm− 2 at a current density of 0.2 mAcm− 2 (Fig-6(d)), whereas the CV data suggested a capacitance of 32 mFcm− 2 at a scan rate of 1 mV/s.
The study reveals that capacitance can decrease when the coating is applied in excess of the surface pore size. This is because the excess coating will block the pores, and the surface area available for charge storage will be reduced. Additionally, the charging and discharging times from the GCD curves suggest that the device has a moderate coulombic efficiency. The GCD curves suggest that charge accumulation or storage is through the double-layer formation. The capacitance values computed using Galvanostatic charging and discharging curves and those derived using CV analysis differ. Eq. 4 and Eq. 5 are used to compute the relative power and energy densities. The energy density of the LIG/2%CNTs based SC device is measured as 6.5 Whcm− 2 at a corresponding power density of 0.219 mWcm− 2 which is relatively higher than what is mainly reported38\ in the literature. This suggests that the device has a high energy storage capacity per unit area. LIG/2%CNTs based electrodes with PVA/H2SO4 gel electrolyte improve the ionic conductivity of the SC device and increase its capacitance by facilitating the movement of ions. One reason for this increase in capacitance is the porous structure of the MWCNT-coated LIG, which provides a larger surface area for the electrolyte to diffuse and produce an electric double layer. A larger surface area and an abundance of wrinkles make it easier for the electrolytes to diffuse and cause electric double layer capacitive diffusion, as reflected by data shown in Fig. 6(c) and Fig. 6(d).
EIS was utilized, as depicted in Fig. 6(e) and Fig. 7(e), to investigate the LIG electrode that was coated with MWCNT at concentrations of 2 mg and 5 mg respectively. CO2 lasers are helpful in carbonizing carbon-based raw materials because of their operating wavelength within the medium- and far-infrared region of the electromagnetic spectrum, where most substrates exhibit strong absorptions. This facilitates rapid and efficient carbonization of the raw materials. Figure 5(f) demonstrate an increase in impedance following MWCNT coating. In addition, the impedance factor has increased with the increasing concentration of MWCNTs, which results in a pronounced hump in the EIS plot, showing a significant increase in the electrode impedance. To optimize the performance of the cell, it is crucial to minimize the impedance factor, which facilitates the efficient flow of electrolyte ions and electrode interactions, leading to a higher capacitance of the overall cell. The EIS plot in Fig. 5(f) depicts the actual model by the black line, while the red line represents a fitted model generated using an equivalent circuit.
Fig-7(a) depicts the cyclic voltammetry curves for the LIG/5%CNTs sample. We obtain a capacitance of 32 mFcm− 2 at a scan rate of 1mV/s. Fig-7(b) illustrate the capacitance measured at scan rates of 2, 50, and 100 mV/s, respectively. We obtained capacitances of 28, 2.7, and 1.39 mFcm− 2 at scan rates of 2, 50, and 100 mV/s, respectively. These results suggest that the capacitance decreases as the scan rate increases, which is consistent with the behavior of a double-layer capacitor. The symmetric charge/discharge curves shown in Fig-7(c) of the LIG-coated MWCNTs suggest that the formation of a double layer still dominates the charge storage mechanism, and any deviations from the ideal behavior are likely due to the electrolyte. However, there is a slight discrepancy between the anticipated capacitance values obtained from the GCD and CV curves. As shown in Fig-7(c), the GCD curve offered a capacitance of 11.17 mF/cm2 at a current density of 0.2 mAcm− 2. Fig-6(b) and Fig-7(b) illustrate that the addition of minute amounts of MWCNTs to LIG can significantly enhance the electrical characteristics, capacitance, and energy density of the resulting electrode material. The capacitance of a supercapacitor can be increased by coating it with MWCNTs with optimized pore size. Fabricated electrode LIG/2%CNTs exhibited 71 mFcm− 2 from CV curves at a scan rate of 1 mV/s and 11.17 mFcm− 2 from GCD at a current density of 0.2 mAcm− 2 as illustrated in Fig-6(a) and Fig-6(d). As shown in Fig-7(a), the capacitance of the LIG/5%CNTs coated electrode is lower than that of the LIG coated with 2 mg MWCNTs, but it is still higher than that of the simple LIG electrode. The LIG/5%CNTs exhibits a capacitance of 30 mFcm− 2 at a scan rate of 1mV/s and a capacitance of 9.03 mFcm− 2 at a current density of 0.2 mAcm− 2 as shown in Fig-7(b) & (d).
Supercapacitors are becoming increasingly popular due to their high durability and fast charging capabilities. By carefully controlling the pore size and coating the electrode with MWCNTs, the capacitance of the supercapacitor can be significantly increased. Our experimental results, obtained from galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) measurements at a current density of 0.2 mA/cm2 and a scan rate of 2 mV/s, show a capacitance of 11.17 and 51 mF/cm2, respectively. These values are higher than those reported in previous studies25,26,38, and are summarized in Table 1. Coating multi-walled carbon nanotubes (MWCNTs) on electrodes reduces the charge transfer resistance between the electrolyte ions and the electrode, resulting in an enhanced energy density. Specifically, the measured energy density of MWCNT-coated electrodes was found to be 6.5 µWh/cm2, which is significantly higher than that of non-coated electrodes. Additionally, even small amounts of MWCNTs mixed into normally insulating materials can confer significant conductivity, making them attractive for use in the development of lightweight, high-strength components. Such components have potential applications in the field of smart electronics, including portable and wearable devices. The stability test results of the prepared LIG-2mg MWCNTs are shown in Fig. 6(f), and at a current density of 0.6 mAcm− 2 as depicted in Fig. 6(g)The test was not very stable due to the lack of a vacuum-controlled environment and the adverse effect of the gel electrolyte deposited on the electrode for 10,000 cycles over three days. As the process takes several days, the device's durability and stability were evaluated at a minimum current density of 0.6 mAcm− 2. Despite minor aberrations, the electrode exhibited 90% coulombic efficiency, which is the ratio between the charges removed from a supercapacitor compared with the charge used to restore the original capacity.
Table 1: Comparative analysis of the fabricated electrode with varying concentrations of MWCNT.
Sample
|
MWCNTs
Coating % on LIG electrode
|
Capacitance From CV at scan rate of 2 mV/s
|
Capacitance From GCD
|
Current Density
|
Energy Density
|
Power Density
|
#
|
mg
|
mFc
|
mFc
|
mFc
|
μWhc
|
mWc
|
1st
|
LIG-0mg
|
19.47
|
6.09
|
0.2
|
3.38
|
0.199
|
2nd
|
LIG-2mg
|
51.975
|
11.17
|
0.2
|
6.5
|
0.219
|
3rd
|
LIG-5mg
|
28.375
|
9.03
|
0.2
|
5.01
|
0.199
|