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Laser transmission welding and surface modification of graphene film for flexible supercapacitor applications
Applied Surface Science 483 (2019) 481–488
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Laser transmission welding and surface modification of graphene film for flexible supercapacitor applications Hayelin Choi, Phuong Thi Nguyen, Jung Bin In
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Laser Thermal Nanoengineering Laboratory, School of Mechanical Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser transmission welding Graphene Flexible device Supercapacitor Pulsed laser
When a graphene film is coated onto a flexible polymer substrate, weak adhesion can cause delamination of the film under mechanical bending. Moreover, as each graphene layer restacks, the performance of the film as an electrode for a supercapacitor becomes limited. In this study, facile laser welding and surface modification processes are demonstrated to overcome these limitations. First, a continuous wave laser beam is applied to the interface between the coated graphene and the underlying transparent polycarbonate substrate. This welding process significantly improves their adhesion and enables excellent mechanical bendability. Second, surface modification of graphene is achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. Sandwich-type supercapacitors are fabricated using these surface-modified graphene electrodes with a PVA-H3PO4 electrolyte. The effect of the laser fluence on the performance of the supercapacitor is investigated. At an optimal laser power, an areal capacitance of 4.7 mF/cm2 is achieved.
1. Introduction Graphene is one of the promising materials considered for use as supercapacitor electrodes, owing to its outstanding features. The exclusive two-dimensional structure of graphene enables a large specific surface area, which is key to producing supercapacitors with a high energy density [1,2]. Graphene provides an excellent transport platform for electrolyte ions [3]. In addition, its excellent electrical properties reduce resistance to the transport of electrons, leading to a high specific power [4,5]. Moreover, graphene can be readily coated into a thin film, while retaining its functionality under mechanical deformation in flexible supercapacitors [6]. Thus, graphene has attracted vast interest among researchers regarding its use in this application. Nevertheless, the practical implementation of flexible thin film graphene supercapacitors has been hampered. To coat a flexible substrate with a graphene film on a large scale, several methods have been developed including layer-by-layer transfer [7], spin-coating [8], barcoating, and spray-coating [9]. Few-layer graphene adheres to the underlying substrate by Van der Waals forces strong enough to bond them even under severe mechanical strain [10]. However, for application as an energy storage medium, the thickness of the graphene film needs to be ~10 μm or higher [2,4]. Unfortunately, as the film thickness is increased, the mechanical strain becomes proportionally amplified, resulting in delamination.
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Another problem with using coated graphene films for supercapacitors is the low accessibility of electrolyte ions to the graphene particularly when it is not located close to the film surface [5]. When graphene slurry is coated as a film, each graphene layer agglomerates and restacks, which limits the performance of each layer of the film as an electrode for a supercapacitor [11]. Researchers have investigated various methods to overcome this problem, such as the use of vertical graphene nanosheets [12–14] or crumpled graphene [15,16]. However, these methods not only depend on the use of specific substrate materials but also are difficult to use for achieving relatively thick graphene films. Laser-induced reduction of graphene oxide (GO) is a plausible approach to developing graphene structures on a flexible polymer substrate with a reasonable thickness [2]. When irradiated with a laser, graphene oxide films are immediately reduced to graphene, which is called reduced graphene oxide (rGO) [17] or laser-scribed graphene (LSG) [2]. It forms a porous structure with mixed macro/meso/micropores. However, due to the limited penetration of light and heat into this material, the chemical reduction of GO is not uniform throughout the film, resulting in incomplete reduction below the surface [18]. Recently, it was revealed that incompletely reduced GO can actually contribute to an increase in the capacitance, due to the pseudo-capacitive behavior of the functional groups that remain in the rGO [18–20]. This suggests that the functionalized rGO should be placed near the surface, where electrolyte ions are directly introduced. However, this is
Corresponding author. E-mail address: [email protected] (J.B. In).
https://doi.org/10.1016/j.apsusc.2019.03.349 Received 25 December 2018; Received in revised form 12 February 2019; Accepted 31 March 2019 Available online 01 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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2× objective lens (S Plan Apo L 2×; NA: 0.055, OptoSigma). The beam diameters were measured using the knife-edge method. The diameters of the focused CW laser beam and the nanosecond laser beam were 90 μm and 350 μm, respectively. An attenuator was installed in the beam path to fine-tune the nanosecond laser output. A gas nozzle (NZAK1-0.6, Misumi) was installed to blow a nitrogen gas toward the beam spot and prevent laser processing byproducts from contaminating the focusing lens. The power and fluence of the laser beams were measured using a set of a laser power/energy meter (Nova II, P/N 7Z01550, Ophir) and a power/energy sensor (3A-PF-12, Ophir). For scanning with the laser beam, graphene samples were translated by a 2axis motorized linear stage (OSMS26-100(XY), Sigmakoki).
hard to achieve through LSG. In this study, facile laser welding and surface modification processes are demonstrated to address these issues. Our group has developed the laser transmission welding technique to improve the adhesion of carbon nanomaterials to flexible polymer substrates, with a minimal thermal effect on the substrate. Laser transmission welding is a non-contact adhesion-promoting process, where a transparent workpiece is scanned with a focused laser beam [21,22]. A light-absorbing material in contact with the transparent substrate is selectively heated close to the melting point of the substrate. The localized heating greatly improves adhesion between them, while thermal damage to the substrate is minimized. This method was adopted to weld graphene to a polycarbonate (PC) sheet and produce a flexible graphene film electrode. Moreover, for the surface modification of graphene, a nanosecond pulsed laser was applied to the graphene surface intended to be exposed to the electrolyte. This dry process induced physical and chemical modifications in the exposed graphene, resulting in the enhancement of its electrochemical properties.
2.2. Bending and twisting tests Bending and twisting tests were carried out using a home-built apparatus. For the bending test, both ends of a strip of graphene-coated electrode terminated with a copper tape were clamped to a fixed constraint and a moving structure, mounted on a motorized linear stage, respectively. To prevent the contact resistance from varying by deformation, compliant liquid metal (In-Ga eutectic, Sigma-Aldrich) was applied to the interface between the graphene film and the copper tape. The strip sample was bent in a linear motion with a 1-cm stroke at 40mm/s along its longitudinal direction. For the twisting test, one end of the graphene strip was twisted by an Arduino servo motor (MG 996R). To comply with the change in length of the strip from twisting, the other end was set free to translate along the longitudinal direction. The change in electrical resistance of the graphene strip during the test was measured using a multimeter (34450A, Keysight technologies).
2. Experimental section 2.1. Laser irradiation system A continuous-wave (CW) laser (MGL-FN-532-400 mW; wavelength: 532 nm, CNI) and a nanosecond laser (Nano L200-20; wavelength: 532 nm; pulse duration: 6–9 ns; repetition rate: 20 Hz, Litron) were used for laser transmission welding and surface modification process, respectively. The CW laser was installed in the middle of the nanosecond laser beam path to share the focusing optics. As shown in Fig. 1a, the CW laser was mounted on a z-axis stage so that the CW laser could be removed from the optical path when the system was used in surface modification mode with the nanosecond laser. The laser beam was delivered using multiple mirrors and focused on a graphene film via a
2.3. Fabrication of flexible supercapacitors Flexible supercapacitors were fabricated as a symmetric sandwich
Fig. 1. Illustration of (a) the laser irradiation setup and (b) the electrode fabrication process. (c) Photographic images of welded graphene-PC samples (top: hotplate welding, bottom: laser welding). 482
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performed with Al Kα, (spot size: 400 μm, pass energy: 40 eV, step size: 0.1 eV). The electrochemical properties of the graphene were measured using an electrochemical instrument (SP-150, Bio-Logic Science Instruments). Cyclic voltammetry (CV) curves, galvanostatic charging and discharging (CC) curves, and electrochemical impedance spectroscopy (EIS) profiles (frequency: 100 mHz to 200 kHz, amplitude: 10 mV) were obtained. The area-specific capacitance (CA, mF cm−2) was calculated from the CC curves using the equation: CA = I/(A × dV/ dt), in which A is the area of the electrode, I is the discharge current, dt is the discharge time, and dV is the working potential excluding its IR drop (ΔVdrop). The equivalent series resistance (ESR) was also calculated from the CC curves by the equation: ESR = ΔVdrop/2I, in which ΔVdrop is the voltage drop from the discharge curve and I is the employed current [23]. 3. Results and discussion 3.1. Laser transmission welding of graphene and PC Fig. 1a and b are schematic illustrations of the laser processing apparatus and the fabrication process of the flexible graphene electrodes, respectively. A graphene film was produced by bar-coating a graphene dispersion (graphene in water, 5 mg/ml, ACS material) onto a PC sheet (thickness: 0.38 mm, McMaster-Carr). For the bar-coating, a film applicator was used with a 0.45-mm coating gap at a speed of 4 mm/s. The film was then dried overnight under ambient conditions. The thickness and the mass loading of the coated graphene film were approximately 14 μm and 1.8 mg/cm2, respectively. The graphene-PC sample was flipped and then placed on a 2-axis motorized linear stage. For laser transmission welding, the contact between the graphene film and the PC sheet was raster-scanned with a CW laser beam to improve the adhesion between the PC and the graphene. As the PC sheet was transparent, the laser beam was absorbed locally at the interface, raising its temperature above the glass transition point of PC. The PC at the interface then became fluidized, improving the adhesion to the graphene [21,22]. Laser transmission welding enabled the heat-affected zone at the absorption interface to be confined to a small area. Fig. 1c shows the difference between hotplate and laser welding. Hotplate welding of a graphene film was performed by heating the entire graphene-PC sample at 200 °C for 10 s. This heating caused serious deformation of the PC sheet. In contrast, the localized heating of a PC sheet by the laser (welding power: 3.3 kW/cm2) enabled its structure to be retained. As will be discussed next, 3.3 kW/cm2 was the highest tested laser power, where defect formation by vaporization at the welding interface could be avoided. Thus, the laser welding process presumably increased the local temperature of the PC to at least 200 °C without causing deformation of the PC sheet. To determine the optimal laser power for welding, laser transmission welding was performed while varying the laser power (1.6, 2.0, 2.4, 2.8, 3.3, 3.7 kW/cm2). For raster scanning, the line-to-line pitch and scanning speed were set to 70 μm and 10 mm/s, respectively. Optical microscopy images of the graphene-PC interface after welding are shown in Fig. 2, with a lower laser power corresponding to a narrower welding line. As a result, incomplete welding was observed at low laser powers (Fig. 2a–d). Meanwhile, excessive laser heat generated at 3.7 kW/cm2 caused thermal damage at the welding interface, possibly due to the vaporization of the PC at the interface (Fig. 2f) [22]. Once the thermal damage was created, further scanning was apt to cause severe burning of the film because the absorption of laser light significantly increases in the damaged area. At a power of 3.3 kW/cm2, neither thermal damage nor incomplete welding was observed. Therefore, this power was selected for all laser welding experiments. The effect of laser transmission welding on the flexibility of graphene electrodes on PC was evaluated via bending and twisting tests. Two graphene-PC strips were prepared for comparison. Laser
Fig. 2. Optical microscopy images of the laser-welded interface; each surface was irradiated at (a) 1.6, (b) 2.0, (c) 2.4, (d) 2.8, (e) 3.3, (f) 3.7 kW/cm2. The schematic describes the imaging configuration.
type. A PVA-H3PO4 solid-state electrolyte was prepared by mixing 10 wt% of polyvinyl alcohol (PVA) powder and 10 wt% of H3PO4 in deionized water. As an electrical terminal, a strip of copper tape was attached to the end of the graphene electrode strip, and a tiny amount of silver paste was applied to the interface to reduce contact resistance. The electrical connection area was tightly covered by a Kapton tape to isolate it from the electrolyte. The electrolyte was applied between two identical 1-cm2 areas of graphene electrode, and the assembled supercapacitor was dried in a fume hood for 5 h. 2.4. Characterizations The morphology of the graphene electrodes was characterized using a field emission- scanning electron microscope (FE-SEM, SIGMA, Carl Zeiss) operated at 10.0 kV. Raman Spectroscopy of the sample was performed by using a laser (excitation wavelength: 785 nm) and a highsensitivity spectrometer (QE Pro-Raman, Ocean Optics). X-ray photoelectron spectroscopy (XPS, K-alpha+, ThermoFisher Scientific) was 483
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Fig. 3. Comparison of the graphene-PC strips before and after welding for (a) bending and (b) twisting tests. Resistance change during (c) 10,000 cycles of bending and (d) 2 cycles of twisting.
comparable to a previous study on the laser welding of vertically aligned carbon nanotubes (VACNTs). For example, most VACNTs could be welded to the underlying PC substrate because the vertical arrangement of 1-dimensional CNTs allows each CNT to contact the underlying PC surface [22]. However, in the case of the graphene film, only a small portion of 2D graphene layers placed at the bottom of the graphene film were in contact with the PC surface and could be welded to it. Nevertheless, the bending and twisting tests unambiguously revealed that the laser welding process significantly improved adhesion and enabled excellent mechanical bendability of the graphene-PC electrode. This suggests that the adhesion between the non-welded graphene film and the PC is weaker than the attraction between graphene layers in the film. Indeed, the inspection of the delaminated surface of the non-welded PC supports this idea as the bare PC surface was exposed by the delamination. In contrast, a larger amount of graphene was observed for the welded and delaminated PC. The optical microscopy and the SEM images of the delaminated surfaces are provided in Fig. S2 of the supplementary material.
transmission welding was conducted on one of the strips prior to the test; however, the other strip was tested as originally coated. A schematic illustration of the prepared sample is provided in Fig. S1 of the supplementary material. During the tests, the electrical resistance of the strip was measured. The resistance change (%) was calculated by the equation: (R − R0) / R0 × 100, in which R is the electrical resistance of the sample and R0 is the initial resistance of the sample. For the bending test (Fig. 3a), the graphene film of the non-welded sample became torn and delaminated in the middle of the strip after approximately 1000 cycles, whereas the welded graphene film sample maintained its original structure to at least 10,000 cycles. Fig. 3c shows the resistance change for two conditions (bent and flat), which is consistent with the observation in Fig. 3a. The resistance of the non-welded sample increased abruptly at approximately the 1000th cycle, but the laser-welded sample almost maintained its resistance change value below 10% for 10,000 cycles. Twisting tests were also performed to apply more severe strain to the strip samples than the bending test. One end of the strip was twisted from 0 to 180° at increments of 45°, while the other end was fixed. The twisting tests revealed the dramatic effect of laser transmission welding on the mechanical robustness of the graphene-PC sheets. Fig. 3b shows the sample state after two cycles of twisting tests. In the case of the nonwelded graphene electrode, the graphene film was delaminated from the PC substrate after just a single cycle. Fig. 3d shows that the resistance increased significantly and irreversibly for this sample. In contrast, the resistance change for the laser-welded graphene-PC strip was almost negligible for two cycles of twisting. The laser-welded graphene remained attached, only with a few small wrinkles in the center of the strip. The results for laser transmission welding of the graphene films is
3.2. Laser surface modification of graphene As described in Fig. 1b, the surface modification of graphene was achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. The graphene film prepared by the above method were irradiated with the nanosecond laser beam at pulse energies of 0, 5, 50, 150, 250, 350 and 500 μJ (0, 5.2, 52, 156, 260, 364 and 520 mJ/cm2 in terms of fluence), and are denoted as G1, G2, G3, G4, G5, G6 and G7, respectively. The scanning speed was 4 mm/s with a line-to-line distance of 70 μm. Laser transmission welding was necessary before the pulsed laser irradiation. Otherwise, 484
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Fig. 4. FE-SEM images of the surface of graphene films corresponding to (a) G1, (b) G2, (c) G3, (d) G5, (e) G6, and (f) G7.
Fig. 5. Raman spectroscopy and XPS of the graphene sample prepared by varying the nanosecond laser energy: (a) Raman spectra of G1 (0 mJ/cm2) and G3 (52 mJ/ cm2), (b) ID/IG ratios, and (c) oxygen and nitrogen atomic ratios measured via XPS.
Fig. 6. Electrochemical properties of the graphene electrodes irradiated by a pulsed laser beam at different laser fluences: (a) cyclic voltammetry curves, (b) galvanostatic charge/discharge curves at 1 mA/cm2, and (c) areal capacitance (mF/cm2) and equivalent series resistance calculated from IR drop values in (b).
the film would be easily delaminated because of the mechanical stress induced by the pulsed laser. The FE-SEM images in Fig. 4 show the microstructure of the
irradiated graphene surface. The surface became rough when irradiated with the laser pulses. In Fig. 4c, pronounced sharp edges of graphene flakes that stand out from the surface plane are observed. This surface 485
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Fig. 7. Electrochemical properties of the graphene electrode irradiated at 52 mJ/cm2 (G3): cyclic voltammetry with scan rates of (a) 5–100 mV/s and (b) 0.2–10 V/s, (c) galvanostatic charge/discharge curves with current densities of 0.2–2 mA/cm2, capacitance retention calculated from charge and discharge curves with (c) varying current, (d) cycle numbers and (e) mechanical bending conditions.
because the high-energy laser pulses induce oxidation of the graphene layer by adsorbed oxygen, water, or other oxygen-containing molecules that may already have existed in the graphene film as well as by molecular oxygen in the air [34,35]. Thermal damage to the graphitic structure induced by laser ablation may generate abundant graphene edge sites that are preferred to be terminated by oxygen-containing functional groups. Nitrogen was also detected in the non-welded graphene film (G1: 0 mJ/cm2) since it is a component of the dispersion surfactant [36]. The atomic ratio of nitrogen increased at a fluence of 52 mJ/cm2. Likewise, this increase can be ascribed to the defects and the edges in the graphene structure generated by the laser pulses, which are the preferred sites for the doping of nitrogen [37]. In contrast to oxygen, however, a higher fluence (364 mJ/cm2) reduced the nitrogen content, compared with the case of 52 mJ/cm2. This may result from the thermal dissociation of the CeN bonds at an elevated temperature induced by the high-intensity laser pulses [38–41]. As shown in Fig. S4b, a redshift in the N1s peak of XPS was observed with increasing laser fluence, which implies reconfiguration of nitrogen from pyrrolic N to pyridinic N [39].
modification is attributed to not only ablative etching of the graphene at the surface of the film by the high-fluence laser pulses but also thermal exfoliation of the graphene layers by laser-induced rapid heating [24–27]. The graphene film in G7 (520 mJ/cm2) showed a ripple structure and was delaminated despite the laser welding due to the excessive laser fluence. Characterizations of the irradiated graphene surface were further performed via Raman spectroscopy and XPS. Fig. 5a shows the Raman spectra of G1 (0 mJ/cm2) and G3 (52 mJ/cm2). The other spectra are provided in Fig. S3 of the supplementary material. Two characteristic carbon peaks (G and D peaks) were observed in all samples (G1–G7). The G peak originates from the in-plane vibration of the graphitic lattice. The D peak indicates disordered carbon adjacent to a lattice defect or placed at the edge of a graphene layer [28]. The D peak may also have been generated from the remaining surfactant present inside the graphene dispersion [29]. In addition to the intrinsic D peak of the nonwelded graphene, the defect sites generated by the laser-induced ablative etching [26] may have amplified the D peak intensity. Fig. 5b shows the ID/IG ratio calculated from the Raman spectra of the samples, which is a quantitative indicator of disordered carbon containment. From G1 to G3, the ID/IG ratio increases with fluence. However, for fluences larger than 50 μJ (G5 and G6), the ID/IG ratio decreases. This is consistent with a recent report by Antonelou et al. [26]. This trend may result from the complicated and coupled phenomena that involve the effects of laser fluence on the exposure of abundant graphene edges (Fig. 4) [30], the thermal healing of the defects [31], and doping by heteroatoms [32,33], in addition to the aforementioned reasons for D peak generation. Regarding the heteroatoms and functional groups, the XPS inspection revealed the existence of other elements in the graphene. Fig. 5c shows the oxygen and nitrogen fractions calculated from the XPS data. The XPS spectra are provided in Fig. S4 of the supplementary material. It suggests that the graphene surface was oxidized and that the oxygen content increased with an increase in the laser fluence [34]. This is
3.3. Flexible supercapacitors To evaluate the electrochemical properties of graphene electrodes, sandwich-type supercapacitors were fabricated. A gel-type PVA-H3PO4 electrolyte was applied without the use of a separator to make the device compact and suitable for flexible and lightweight electronics [2,42]. Fig. 6 shows the electrochemical properties of the surfacemodified graphene supercapacitors prepared at different laser fluences. In Fig. 6a, the CV curves are shown as pseudo-rectangular, which is indicative of the ideal behavior of electrical double layer capacitors (EDLCs). The CC curves (Fig. 6b) obtained at 1 mA/cm2 (~1 A/g when converted using the density of the film) also exhibit ideal triangular shapes. Fig. 6c shows equivalent series resistance (ESR) values for the 486
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The data in Fig. 7(c–f) were obtained by galvanostatic charge/discharge (CC) tests. As shown in Fig. 7c, triangular CC curves were measured with small IR drops. Over a wide range of current density, from 0.2 to 25 mA/cm2, the capacitance was maintained at over 73% (Fig. 7d). This also indicates that the path for ion transport in the G3 electrode was well formed and that the transport was fast enough in a wide range of the current density. Fig. 7e shows the results of the capacitance retention test, evaluated for 10,000 cycles of galvanostatic charging/discharging at 5 mA/cm2. The supercapacitor maintained its capacitance over 96% of the initial value. As shown in Fig. 7f, the change of the capacitance was almost negligible up to a bending curvature of 0.8 cm−1. The EIS characteristics of the supercapacitor were also investigated. Fig. 8a and b show the Nyquist plot and the frequency-phase curve (Bode plot), respectively. A knee point in the Nyquist plot was estimated to be present at 24 Hz, to which a pure capacitive behavior was maintained. An ESR value of 4.85 Ohms was obtained at the intersection of the x-axis and the Nyquist curve. This value is even lower than the ESR (~16 Ohms) of the LSG supercapacitor fabricated with an aqueous H3PO4 electrolyte (1.0 M) [2]. This is presumably because the graphene used in our study is more electrically conductive than the LSG. Further analysis of the EIS result is provided in Fig. S5 of the supplementary material. The Bode plot shows a characteristic frequency (f0) at 5 Hz that corresponds to the −45° phase angle. This characteristic frequency indicates the point where the resistive and capacitive impedances are the same [2,45]. The time constant (τ0) can be simply calculated by using 1/f0, which is 200 ms in this study. This value lies between the time constant (33 ms) of LSG with the aqueous H3PO4 electrolyte and that (10 s) of the conventional activated carbon [2]. 4. Conclusions In summary, laser transmission welding and surface modification of graphene films were demonstrated. A flexible high-capacitance supercapacitor was successfully developed. Laser transmission welding significantly improved the adhesion between the graphene and the underlying PC sheet. Irradiation with a nanosecond laser modified the graphene films by partially exfoliating the surface graphene layers and chemically introducing nitrogen and oxygen. The effect of the laser fluence on the electrochemical properties of the surface-modified graphene electrode was investigated. Compared with LSG supercapacitors, the modified graphene supercapacitor exhibited an increased capacitance with a reduced ESR. Moreover, the fabricated supercapacitor was mechanically robust, enabling bending with a minimal change in capacitance.
Fig. 8. (a) Nyquist plot and (b) Bode plot of a graphene electrode irradiated at 52 mJ/cm2 (G3).
supercapacitors and their area-specific capacitance, calculated from IR drops and discharge slopes, respectively. It reveals that as the pulse energy increased, the capacitance first increased and then decreased with the largest capacitance of 4.7 mF/cm2 at an optimal fluence of 52 mJ/cm2, which is equivalent to 13.3 F/g in terms of gravimetric electrode capacitance. When calculated based on the total thickness of active graphene films, the volumetric energy and power densities are 169 μWh/cm3 and 155 mW/cm3, respectively. These values are comparable with those of other relatively thick graphene-based supercapacitors [2,43]. The ESR results showed a tendency opposite to the capacitance variation with laser fluence. A strong correlation can be found for ID/IG ratio, nitrogen content, and capacitance variations with fluence, which suggests that the role of active defect sites including Ndoped sites in the increased capacitance of graphene is critical [44]. The supercapacitor produced for G3 (52 mJ/cm2) was further investigated. To determine the electrochemical performance of this supercapacitor, CV scan rates from 0.02 to 10 V/s were applied as shown in Fig. 7(a, b). The pseudo-rectangular shape was maintained up to 5 V/ s, which suggests fast ion transport in the surface-modified graphene.
Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF2015R1C1A1A01052523). The financial support of the Chung-Ang University Graduate Research Scholarship in 2018 is also acknowledged. Appendix A. Supplementary data Sample preparation for bending and twisting tests, optical microscopy images of delaminated surface, Raman spectroscopy and XPS of graphene irradiated by varying laser fluence. Supplementary data to this article can be found online at doi:https://doi.org/10.1016/j. apsusc.2019.03.349. 487
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Full length article
Laser transmission welding and surface modification of graphene film for flexible supercapacitor applications Hayelin Choi, Phuong Thi Nguyen, Jung Bin In
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Laser Thermal Nanoengineering Laboratory, School of Mechanical Engineering, Chung-Ang University, Seoul 06974, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Laser transmission welding Graphene Flexible device Supercapacitor Pulsed laser
When a graphene film is coated onto a flexible polymer substrate, weak adhesion can cause delamination of the film under mechanical bending. Moreover, as each graphene layer restacks, the performance of the film as an electrode for a supercapacitor becomes limited. In this study, facile laser welding and surface modification processes are demonstrated to overcome these limitations. First, a continuous wave laser beam is applied to the interface between the coated graphene and the underlying transparent polycarbonate substrate. This welding process significantly improves their adhesion and enables excellent mechanical bendability. Second, surface modification of graphene is achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. Sandwich-type supercapacitors are fabricated using these surface-modified graphene electrodes with a PVA-H3PO4 electrolyte. The effect of the laser fluence on the performance of the supercapacitor is investigated. At an optimal laser power, an areal capacitance of 4.7 mF/cm2 is achieved.
1. Introduction Graphene is one of the promising materials considered for use as supercapacitor electrodes, owing to its outstanding features. The exclusive two-dimensional structure of graphene enables a large specific surface area, which is key to producing supercapacitors with a high energy density [1,2]. Graphene provides an excellent transport platform for electrolyte ions [3]. In addition, its excellent electrical properties reduce resistance to the transport of electrons, leading to a high specific power [4,5]. Moreover, graphene can be readily coated into a thin film, while retaining its functionality under mechanical deformation in flexible supercapacitors [6]. Thus, graphene has attracted vast interest among researchers regarding its use in this application. Nevertheless, the practical implementation of flexible thin film graphene supercapacitors has been hampered. To coat a flexible substrate with a graphene film on a large scale, several methods have been developed including layer-by-layer transfer [7], spin-coating [8], barcoating, and spray-coating [9]. Few-layer graphene adheres to the underlying substrate by Van der Waals forces strong enough to bond them even under severe mechanical strain [10]. However, for application as an energy storage medium, the thickness of the graphene film needs to be ~10 μm or higher [2,4]. Unfortunately, as the film thickness is increased, the mechanical strain becomes proportionally amplified, resulting in delamination.
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Another problem with using coated graphene films for supercapacitors is the low accessibility of electrolyte ions to the graphene particularly when it is not located close to the film surface [5]. When graphene slurry is coated as a film, each graphene layer agglomerates and restacks, which limits the performance of each layer of the film as an electrode for a supercapacitor [11]. Researchers have investigated various methods to overcome this problem, such as the use of vertical graphene nanosheets [12–14] or crumpled graphene [15,16]. However, these methods not only depend on the use of specific substrate materials but also are difficult to use for achieving relatively thick graphene films. Laser-induced reduction of graphene oxide (GO) is a plausible approach to developing graphene structures on a flexible polymer substrate with a reasonable thickness [2]. When irradiated with a laser, graphene oxide films are immediately reduced to graphene, which is called reduced graphene oxide (rGO) [17] or laser-scribed graphene (LSG) [2]. It forms a porous structure with mixed macro/meso/micropores. However, due to the limited penetration of light and heat into this material, the chemical reduction of GO is not uniform throughout the film, resulting in incomplete reduction below the surface [18]. Recently, it was revealed that incompletely reduced GO can actually contribute to an increase in the capacitance, due to the pseudo-capacitive behavior of the functional groups that remain in the rGO [18–20]. This suggests that the functionalized rGO should be placed near the surface, where electrolyte ions are directly introduced. However, this is
Corresponding author. E-mail address: [email protected] (J.B. In).
https://doi.org/10.1016/j.apsusc.2019.03.349 Received 25 December 2018; Received in revised form 12 February 2019; Accepted 31 March 2019 Available online 01 April 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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2× objective lens (S Plan Apo L 2×; NA: 0.055, OptoSigma). The beam diameters were measured using the knife-edge method. The diameters of the focused CW laser beam and the nanosecond laser beam were 90 μm and 350 μm, respectively. An attenuator was installed in the beam path to fine-tune the nanosecond laser output. A gas nozzle (NZAK1-0.6, Misumi) was installed to blow a nitrogen gas toward the beam spot and prevent laser processing byproducts from contaminating the focusing lens. The power and fluence of the laser beams were measured using a set of a laser power/energy meter (Nova II, P/N 7Z01550, Ophir) and a power/energy sensor (3A-PF-12, Ophir). For scanning with the laser beam, graphene samples were translated by a 2axis motorized linear stage (OSMS26-100(XY), Sigmakoki).
hard to achieve through LSG. In this study, facile laser welding and surface modification processes are demonstrated to address these issues. Our group has developed the laser transmission welding technique to improve the adhesion of carbon nanomaterials to flexible polymer substrates, with a minimal thermal effect on the substrate. Laser transmission welding is a non-contact adhesion-promoting process, where a transparent workpiece is scanned with a focused laser beam [21,22]. A light-absorbing material in contact with the transparent substrate is selectively heated close to the melting point of the substrate. The localized heating greatly improves adhesion between them, while thermal damage to the substrate is minimized. This method was adopted to weld graphene to a polycarbonate (PC) sheet and produce a flexible graphene film electrode. Moreover, for the surface modification of graphene, a nanosecond pulsed laser was applied to the graphene surface intended to be exposed to the electrolyte. This dry process induced physical and chemical modifications in the exposed graphene, resulting in the enhancement of its electrochemical properties.
2.2. Bending and twisting tests Bending and twisting tests were carried out using a home-built apparatus. For the bending test, both ends of a strip of graphene-coated electrode terminated with a copper tape were clamped to a fixed constraint and a moving structure, mounted on a motorized linear stage, respectively. To prevent the contact resistance from varying by deformation, compliant liquid metal (In-Ga eutectic, Sigma-Aldrich) was applied to the interface between the graphene film and the copper tape. The strip sample was bent in a linear motion with a 1-cm stroke at 40mm/s along its longitudinal direction. For the twisting test, one end of the graphene strip was twisted by an Arduino servo motor (MG 996R). To comply with the change in length of the strip from twisting, the other end was set free to translate along the longitudinal direction. The change in electrical resistance of the graphene strip during the test was measured using a multimeter (34450A, Keysight technologies).
2. Experimental section 2.1. Laser irradiation system A continuous-wave (CW) laser (MGL-FN-532-400 mW; wavelength: 532 nm, CNI) and a nanosecond laser (Nano L200-20; wavelength: 532 nm; pulse duration: 6–9 ns; repetition rate: 20 Hz, Litron) were used for laser transmission welding and surface modification process, respectively. The CW laser was installed in the middle of the nanosecond laser beam path to share the focusing optics. As shown in Fig. 1a, the CW laser was mounted on a z-axis stage so that the CW laser could be removed from the optical path when the system was used in surface modification mode with the nanosecond laser. The laser beam was delivered using multiple mirrors and focused on a graphene film via a
2.3. Fabrication of flexible supercapacitors Flexible supercapacitors were fabricated as a symmetric sandwich
Fig. 1. Illustration of (a) the laser irradiation setup and (b) the electrode fabrication process. (c) Photographic images of welded graphene-PC samples (top: hotplate welding, bottom: laser welding). 482
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performed with Al Kα, (spot size: 400 μm, pass energy: 40 eV, step size: 0.1 eV). The electrochemical properties of the graphene were measured using an electrochemical instrument (SP-150, Bio-Logic Science Instruments). Cyclic voltammetry (CV) curves, galvanostatic charging and discharging (CC) curves, and electrochemical impedance spectroscopy (EIS) profiles (frequency: 100 mHz to 200 kHz, amplitude: 10 mV) were obtained. The area-specific capacitance (CA, mF cm−2) was calculated from the CC curves using the equation: CA = I/(A × dV/ dt), in which A is the area of the electrode, I is the discharge current, dt is the discharge time, and dV is the working potential excluding its IR drop (ΔVdrop). The equivalent series resistance (ESR) was also calculated from the CC curves by the equation: ESR = ΔVdrop/2I, in which ΔVdrop is the voltage drop from the discharge curve and I is the employed current [23]. 3. Results and discussion 3.1. Laser transmission welding of graphene and PC Fig. 1a and b are schematic illustrations of the laser processing apparatus and the fabrication process of the flexible graphene electrodes, respectively. A graphene film was produced by bar-coating a graphene dispersion (graphene in water, 5 mg/ml, ACS material) onto a PC sheet (thickness: 0.38 mm, McMaster-Carr). For the bar-coating, a film applicator was used with a 0.45-mm coating gap at a speed of 4 mm/s. The film was then dried overnight under ambient conditions. The thickness and the mass loading of the coated graphene film were approximately 14 μm and 1.8 mg/cm2, respectively. The graphene-PC sample was flipped and then placed on a 2-axis motorized linear stage. For laser transmission welding, the contact between the graphene film and the PC sheet was raster-scanned with a CW laser beam to improve the adhesion between the PC and the graphene. As the PC sheet was transparent, the laser beam was absorbed locally at the interface, raising its temperature above the glass transition point of PC. The PC at the interface then became fluidized, improving the adhesion to the graphene [21,22]. Laser transmission welding enabled the heat-affected zone at the absorption interface to be confined to a small area. Fig. 1c shows the difference between hotplate and laser welding. Hotplate welding of a graphene film was performed by heating the entire graphene-PC sample at 200 °C for 10 s. This heating caused serious deformation of the PC sheet. In contrast, the localized heating of a PC sheet by the laser (welding power: 3.3 kW/cm2) enabled its structure to be retained. As will be discussed next, 3.3 kW/cm2 was the highest tested laser power, where defect formation by vaporization at the welding interface could be avoided. Thus, the laser welding process presumably increased the local temperature of the PC to at least 200 °C without causing deformation of the PC sheet. To determine the optimal laser power for welding, laser transmission welding was performed while varying the laser power (1.6, 2.0, 2.4, 2.8, 3.3, 3.7 kW/cm2). For raster scanning, the line-to-line pitch and scanning speed were set to 70 μm and 10 mm/s, respectively. Optical microscopy images of the graphene-PC interface after welding are shown in Fig. 2, with a lower laser power corresponding to a narrower welding line. As a result, incomplete welding was observed at low laser powers (Fig. 2a–d). Meanwhile, excessive laser heat generated at 3.7 kW/cm2 caused thermal damage at the welding interface, possibly due to the vaporization of the PC at the interface (Fig. 2f) [22]. Once the thermal damage was created, further scanning was apt to cause severe burning of the film because the absorption of laser light significantly increases in the damaged area. At a power of 3.3 kW/cm2, neither thermal damage nor incomplete welding was observed. Therefore, this power was selected for all laser welding experiments. The effect of laser transmission welding on the flexibility of graphene electrodes on PC was evaluated via bending and twisting tests. Two graphene-PC strips were prepared for comparison. Laser
Fig. 2. Optical microscopy images of the laser-welded interface; each surface was irradiated at (a) 1.6, (b) 2.0, (c) 2.4, (d) 2.8, (e) 3.3, (f) 3.7 kW/cm2. The schematic describes the imaging configuration.
type. A PVA-H3PO4 solid-state electrolyte was prepared by mixing 10 wt% of polyvinyl alcohol (PVA) powder and 10 wt% of H3PO4 in deionized water. As an electrical terminal, a strip of copper tape was attached to the end of the graphene electrode strip, and a tiny amount of silver paste was applied to the interface to reduce contact resistance. The electrical connection area was tightly covered by a Kapton tape to isolate it from the electrolyte. The electrolyte was applied between two identical 1-cm2 areas of graphene electrode, and the assembled supercapacitor was dried in a fume hood for 5 h. 2.4. Characterizations The morphology of the graphene electrodes was characterized using a field emission- scanning electron microscope (FE-SEM, SIGMA, Carl Zeiss) operated at 10.0 kV. Raman Spectroscopy of the sample was performed by using a laser (excitation wavelength: 785 nm) and a highsensitivity spectrometer (QE Pro-Raman, Ocean Optics). X-ray photoelectron spectroscopy (XPS, K-alpha+, ThermoFisher Scientific) was 483
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Fig. 3. Comparison of the graphene-PC strips before and after welding for (a) bending and (b) twisting tests. Resistance change during (c) 10,000 cycles of bending and (d) 2 cycles of twisting.
comparable to a previous study on the laser welding of vertically aligned carbon nanotubes (VACNTs). For example, most VACNTs could be welded to the underlying PC substrate because the vertical arrangement of 1-dimensional CNTs allows each CNT to contact the underlying PC surface [22]. However, in the case of the graphene film, only a small portion of 2D graphene layers placed at the bottom of the graphene film were in contact with the PC surface and could be welded to it. Nevertheless, the bending and twisting tests unambiguously revealed that the laser welding process significantly improved adhesion and enabled excellent mechanical bendability of the graphene-PC electrode. This suggests that the adhesion between the non-welded graphene film and the PC is weaker than the attraction between graphene layers in the film. Indeed, the inspection of the delaminated surface of the non-welded PC supports this idea as the bare PC surface was exposed by the delamination. In contrast, a larger amount of graphene was observed for the welded and delaminated PC. The optical microscopy and the SEM images of the delaminated surfaces are provided in Fig. S2 of the supplementary material.
transmission welding was conducted on one of the strips prior to the test; however, the other strip was tested as originally coated. A schematic illustration of the prepared sample is provided in Fig. S1 of the supplementary material. During the tests, the electrical resistance of the strip was measured. The resistance change (%) was calculated by the equation: (R − R0) / R0 × 100, in which R is the electrical resistance of the sample and R0 is the initial resistance of the sample. For the bending test (Fig. 3a), the graphene film of the non-welded sample became torn and delaminated in the middle of the strip after approximately 1000 cycles, whereas the welded graphene film sample maintained its original structure to at least 10,000 cycles. Fig. 3c shows the resistance change for two conditions (bent and flat), which is consistent with the observation in Fig. 3a. The resistance of the non-welded sample increased abruptly at approximately the 1000th cycle, but the laser-welded sample almost maintained its resistance change value below 10% for 10,000 cycles. Twisting tests were also performed to apply more severe strain to the strip samples than the bending test. One end of the strip was twisted from 0 to 180° at increments of 45°, while the other end was fixed. The twisting tests revealed the dramatic effect of laser transmission welding on the mechanical robustness of the graphene-PC sheets. Fig. 3b shows the sample state after two cycles of twisting tests. In the case of the nonwelded graphene electrode, the graphene film was delaminated from the PC substrate after just a single cycle. Fig. 3d shows that the resistance increased significantly and irreversibly for this sample. In contrast, the resistance change for the laser-welded graphene-PC strip was almost negligible for two cycles of twisting. The laser-welded graphene remained attached, only with a few small wrinkles in the center of the strip. The results for laser transmission welding of the graphene films is
3.2. Laser surface modification of graphene As described in Fig. 1b, the surface modification of graphene was achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. The graphene film prepared by the above method were irradiated with the nanosecond laser beam at pulse energies of 0, 5, 50, 150, 250, 350 and 500 μJ (0, 5.2, 52, 156, 260, 364 and 520 mJ/cm2 in terms of fluence), and are denoted as G1, G2, G3, G4, G5, G6 and G7, respectively. The scanning speed was 4 mm/s with a line-to-line distance of 70 μm. Laser transmission welding was necessary before the pulsed laser irradiation. Otherwise, 484
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Fig. 4. FE-SEM images of the surface of graphene films corresponding to (a) G1, (b) G2, (c) G3, (d) G5, (e) G6, and (f) G7.
Fig. 5. Raman spectroscopy and XPS of the graphene sample prepared by varying the nanosecond laser energy: (a) Raman spectra of G1 (0 mJ/cm2) and G3 (52 mJ/ cm2), (b) ID/IG ratios, and (c) oxygen and nitrogen atomic ratios measured via XPS.
Fig. 6. Electrochemical properties of the graphene electrodes irradiated by a pulsed laser beam at different laser fluences: (a) cyclic voltammetry curves, (b) galvanostatic charge/discharge curves at 1 mA/cm2, and (c) areal capacitance (mF/cm2) and equivalent series resistance calculated from IR drop values in (b).
the film would be easily delaminated because of the mechanical stress induced by the pulsed laser. The FE-SEM images in Fig. 4 show the microstructure of the
irradiated graphene surface. The surface became rough when irradiated with the laser pulses. In Fig. 4c, pronounced sharp edges of graphene flakes that stand out from the surface plane are observed. This surface 485
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Fig. 7. Electrochemical properties of the graphene electrode irradiated at 52 mJ/cm2 (G3): cyclic voltammetry with scan rates of (a) 5–100 mV/s and (b) 0.2–10 V/s, (c) galvanostatic charge/discharge curves with current densities of 0.2–2 mA/cm2, capacitance retention calculated from charge and discharge curves with (c) varying current, (d) cycle numbers and (e) mechanical bending conditions.
because the high-energy laser pulses induce oxidation of the graphene layer by adsorbed oxygen, water, or other oxygen-containing molecules that may already have existed in the graphene film as well as by molecular oxygen in the air [34,35]. Thermal damage to the graphitic structure induced by laser ablation may generate abundant graphene edge sites that are preferred to be terminated by oxygen-containing functional groups. Nitrogen was also detected in the non-welded graphene film (G1: 0 mJ/cm2) since it is a component of the dispersion surfactant [36]. The atomic ratio of nitrogen increased at a fluence of 52 mJ/cm2. Likewise, this increase can be ascribed to the defects and the edges in the graphene structure generated by the laser pulses, which are the preferred sites for the doping of nitrogen [37]. In contrast to oxygen, however, a higher fluence (364 mJ/cm2) reduced the nitrogen content, compared with the case of 52 mJ/cm2. This may result from the thermal dissociation of the CeN bonds at an elevated temperature induced by the high-intensity laser pulses [38–41]. As shown in Fig. S4b, a redshift in the N1s peak of XPS was observed with increasing laser fluence, which implies reconfiguration of nitrogen from pyrrolic N to pyridinic N [39].
modification is attributed to not only ablative etching of the graphene at the surface of the film by the high-fluence laser pulses but also thermal exfoliation of the graphene layers by laser-induced rapid heating [24–27]. The graphene film in G7 (520 mJ/cm2) showed a ripple structure and was delaminated despite the laser welding due to the excessive laser fluence. Characterizations of the irradiated graphene surface were further performed via Raman spectroscopy and XPS. Fig. 5a shows the Raman spectra of G1 (0 mJ/cm2) and G3 (52 mJ/cm2). The other spectra are provided in Fig. S3 of the supplementary material. Two characteristic carbon peaks (G and D peaks) were observed in all samples (G1–G7). The G peak originates from the in-plane vibration of the graphitic lattice. The D peak indicates disordered carbon adjacent to a lattice defect or placed at the edge of a graphene layer [28]. The D peak may also have been generated from the remaining surfactant present inside the graphene dispersion [29]. In addition to the intrinsic D peak of the nonwelded graphene, the defect sites generated by the laser-induced ablative etching [26] may have amplified the D peak intensity. Fig. 5b shows the ID/IG ratio calculated from the Raman spectra of the samples, which is a quantitative indicator of disordered carbon containment. From G1 to G3, the ID/IG ratio increases with fluence. However, for fluences larger than 50 μJ (G5 and G6), the ID/IG ratio decreases. This is consistent with a recent report by Antonelou et al. [26]. This trend may result from the complicated and coupled phenomena that involve the effects of laser fluence on the exposure of abundant graphene edges (Fig. 4) [30], the thermal healing of the defects [31], and doping by heteroatoms [32,33], in addition to the aforementioned reasons for D peak generation. Regarding the heteroatoms and functional groups, the XPS inspection revealed the existence of other elements in the graphene. Fig. 5c shows the oxygen and nitrogen fractions calculated from the XPS data. The XPS spectra are provided in Fig. S4 of the supplementary material. It suggests that the graphene surface was oxidized and that the oxygen content increased with an increase in the laser fluence [34]. This is
3.3. Flexible supercapacitors To evaluate the electrochemical properties of graphene electrodes, sandwich-type supercapacitors were fabricated. A gel-type PVA-H3PO4 electrolyte was applied without the use of a separator to make the device compact and suitable for flexible and lightweight electronics [2,42]. Fig. 6 shows the electrochemical properties of the surfacemodified graphene supercapacitors prepared at different laser fluences. In Fig. 6a, the CV curves are shown as pseudo-rectangular, which is indicative of the ideal behavior of electrical double layer capacitors (EDLCs). The CC curves (Fig. 6b) obtained at 1 mA/cm2 (~1 A/g when converted using the density of the film) also exhibit ideal triangular shapes. Fig. 6c shows equivalent series resistance (ESR) values for the 486
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The data in Fig. 7(c–f) were obtained by galvanostatic charge/discharge (CC) tests. As shown in Fig. 7c, triangular CC curves were measured with small IR drops. Over a wide range of current density, from 0.2 to 25 mA/cm2, the capacitance was maintained at over 73% (Fig. 7d). This also indicates that the path for ion transport in the G3 electrode was well formed and that the transport was fast enough in a wide range of the current density. Fig. 7e shows the results of the capacitance retention test, evaluated for 10,000 cycles of galvanostatic charging/discharging at 5 mA/cm2. The supercapacitor maintained its capacitance over 96% of the initial value. As shown in Fig. 7f, the change of the capacitance was almost negligible up to a bending curvature of 0.8 cm−1. The EIS characteristics of the supercapacitor were also investigated. Fig. 8a and b show the Nyquist plot and the frequency-phase curve (Bode plot), respectively. A knee point in the Nyquist plot was estimated to be present at 24 Hz, to which a pure capacitive behavior was maintained. An ESR value of 4.85 Ohms was obtained at the intersection of the x-axis and the Nyquist curve. This value is even lower than the ESR (~16 Ohms) of the LSG supercapacitor fabricated with an aqueous H3PO4 electrolyte (1.0 M) [2]. This is presumably because the graphene used in our study is more electrically conductive than the LSG. Further analysis of the EIS result is provided in Fig. S5 of the supplementary material. The Bode plot shows a characteristic frequency (f0) at 5 Hz that corresponds to the −45° phase angle. This characteristic frequency indicates the point where the resistive and capacitive impedances are the same [2,45]. The time constant (τ0) can be simply calculated by using 1/f0, which is 200 ms in this study. This value lies between the time constant (33 ms) of LSG with the aqueous H3PO4 electrolyte and that (10 s) of the conventional activated carbon [2]. 4. Conclusions In summary, laser transmission welding and surface modification of graphene films were demonstrated. A flexible high-capacitance supercapacitor was successfully developed. Laser transmission welding significantly improved the adhesion between the graphene and the underlying PC sheet. Irradiation with a nanosecond laser modified the graphene films by partially exfoliating the surface graphene layers and chemically introducing nitrogen and oxygen. The effect of the laser fluence on the electrochemical properties of the surface-modified graphene electrode was investigated. Compared with LSG supercapacitors, the modified graphene supercapacitor exhibited an increased capacitance with a reduced ESR. Moreover, the fabricated supercapacitor was mechanically robust, enabling bending with a minimal change in capacitance.
Fig. 8. (a) Nyquist plot and (b) Bode plot of a graphene electrode irradiated at 52 mJ/cm2 (G3).
supercapacitors and their area-specific capacitance, calculated from IR drops and discharge slopes, respectively. It reveals that as the pulse energy increased, the capacitance first increased and then decreased with the largest capacitance of 4.7 mF/cm2 at an optimal fluence of 52 mJ/cm2, which is equivalent to 13.3 F/g in terms of gravimetric electrode capacitance. When calculated based on the total thickness of active graphene films, the volumetric energy and power densities are 169 μWh/cm3 and 155 mW/cm3, respectively. These values are comparable with those of other relatively thick graphene-based supercapacitors [2,43]. The ESR results showed a tendency opposite to the capacitance variation with laser fluence. A strong correlation can be found for ID/IG ratio, nitrogen content, and capacitance variations with fluence, which suggests that the role of active defect sites including Ndoped sites in the increased capacitance of graphene is critical [44]. The supercapacitor produced for G3 (52 mJ/cm2) was further investigated. To determine the electrochemical performance of this supercapacitor, CV scan rates from 0.02 to 10 V/s were applied as shown in Fig. 7(a, b). The pseudo-rectangular shape was maintained up to 5 V/ s, which suggests fast ion transport in the surface-modified graphene.
Conflict of interest The authors declare no competing financial interest. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. NRF2015R1C1A1A01052523). The financial support of the Chung-Ang University Graduate Research Scholarship in 2018 is also acknowledged. Appendix A. Supplementary data Sample preparation for bending and twisting tests, optical microscopy images of delaminated surface, Raman spectroscopy and XPS of graphene irradiated by varying laser fluence. Supplementary data to this article can be found online at doi:https://doi.org/10.1016/j. apsusc.2019.03.349. 487
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