Comparison of Laser-Synthetized Nanographene-Based Electrodes for Flexible Supercapacitors

micromachines Article Comparison of Laser-Synthetized Nanographene-Based Electrodes for Flexible Supercapacitors Francisco J. Romero 1,2,* , Denice Gerardo 3, Raul Romero 1,2, Inmaculada Ortiz-Gomez 4, Alfonso Salinas-Castillo 4, Carmen L. Moraila-Martinez 3, Noel Rodriguez 1,2 and Diego P. Morales 2,5,* 1Pervasive Electronics Advanced Research Laboratory, University of Granada, 18071 Granada, Spain; raulromer[email protected].es (R.R.); [email protected] (N.R.) 2Department of Electronics and Computer Technology, University of Granada, 18071 Granada, Spain 3Parque de Innovación Tecnológica, Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Sinaloa, 80040 Culiacán, Mexico; [email protected] (D.G.); [email protected] (C.L.M.-M.) 4Department of Analytical Chemistry, Faculty of Science, University of Granada, 18071 Granada, Spain; [email protected] (I.O.-G.); [email protected] (A.S.-C.) 5 Biochemistry and Electronics as Sensing Technologies Group, University of Granada, 18071 Granada, Spain *Correspondence: [email protected] (F.J.R.); [email protected] (D.P.M.); Tel.: +34-958-241-000 (ext. 20193) (D.P.M.) Received: 2 May 2020; Accepted: 29 May 2020; Published: 30 May 2020   Abstract: In this paper, we present a comparative study of a cost-effective method for the mass fabrication of electrodes to be used in thin-film flexible supercapacitors. This technique is based on the laser-synthesis of graphene-based nanomaterials, specifically, laser-induced graphene and reduced graphene oxide. The synthesis of these materials was performed using two different lasers: a CO 2 laser with an infrared wavelength of λ =10.6 µ m and a UV laser ( λ =405 nm). After the optimization of the parameters of both lasers for this purpose, the performance of these materials as bare electrodes for flexible supercapacitors was studied in a comparative way. The experiments showed that the electrodes synthetized with the low-cost UV laser compete well in terms of specific capacitance with those obtained with the CO 2 laser, while the best performance is provided by the rGO electrodes fabricated with the CO 2 laser. It has also been demonstrated that the degree of reduction achieved with the UV laser for the rGO patterns was not enough to provide a good interaction electrode-electrolyte. Finally, we proved that the specific capacitance achieved with the presented supercapacitors can be improved by modifying the in-planar structure, without compromising their performance, which, together with their compatibility with doping-techniques and surface treatments processes, shows the potential of this technology for the fabrication of future high-performance and inexpensive flexible supercapacitors. Keywords: flexible electronics; graphene oxide; laser-induced graphene; laser-scribing; supercapacitors 1. Introduction Flexible electronics are expected to bring out a revolution in diverse fields of technology, such as electronic skin [ 1 , 2 ], robotics [ 3 , 4 ] or health-monitoring devices [ 5 – 7 ], among others. Most of the recent advances in this context have been possible due to the emergence of new conductive and flexible materials, many of which have reported outstanding results in terms of electrical conduction and integration, such as the polycrystalline silicon (poly-Si) [ 8 , 9 ] or several semiconducting metal Micromachines 2020,11, 555; doi:10.3390/mi11060555 www.mdpi.com/journal/micromachines Micromachines 2020,11, 555 2 of 14 oxides (e.g., SnO 2 , TiO 2 , ZnO or ITO) [ 10 – 12 ]. However, due to the complexity of their synthesis or the expensive fabrication processes required to obtained large-area samples, research groups are still exploring different alternatives that enable an inexpensive and massive fabrication of flexible electronics devices. Therefore, the efforts in this direction have led to extensive investigations on the use of several classes of nanomaterials with different conductivity and sensing capabilities, including carbon nanotubes (CNTs), graphene-derived materials, metal nanowires or conductive polymers [ 13 ]. All these materials have in common being compatible with printing techniques, which enable their economical and efficient processing on diverse flexible substrates, thereby providing a commercially attractive possibility to obtain multifunctional electronics over large areas [ 14 ]. Thus, techniques such as screen-printing or inkjet printing have enabled large achievements in all areas involved in the development of flexible electronics devices, extending from transducers to the antennas for the wireless data transmission [15–19]. However, apart from the latter, novel flexible electronics applications are also demanding flexible energy storage devices that, together with energy harvesting technologies, contribute to the development of self-powered devices and, thus, to the paradigm of the ubiquitous sensing [ 20 ]. In this respect, supercapacitors are expected to play an important role, thanks to the higher power density induced by their fast charging/discharging rates when compared to conventional batteries. Supercapacitors can be classified mainly into two different groups, electrochemical double-layer capacitors (EDLCs) and pseudocapacitors, depending on the storage mechanism, i.e., the interaction between electrode-electrolyte [ 21 ]. EDLCs are those whose electrode material is not electrochemically active, and therefore the capacitance is associated with the pure physical charge accumulation at the electrode/electrolyte interface. On the contrary, the energy storage in pseudocapacitors relies on fast and reversible faradaic redox reactions occurring on the electrode surface [ 21 – 23 ]. For these reasons, in both cases, the electrode is a key element in the development of supercapacitors and then, many materials have been investigated for this purpose. In the case of pseudocapacitors, the most studied materials are transition metal oxides and conducting polymers, which promote the reversible faradaic-type charge transfers of these redox supercapacitors [ 24 ], whereas EDLCs electrodes are fabricated from nanoscale materials with high porosity and high surface area. In this latter case, carbon-based materials are preferred to play this role, due to their exceptionally high surface area, relatively high electrical conductivity and acceptable cost. These properties make porous carbon-based materials, such as Laser-Induced Graphene (LIG) or reduced Graphene Oxide (rGO) [ 25 – 27 ], ideal candidates for the fabrication of the electrodes. Many researchers agree that these kind of electrodes will play an important role in the supercapacitor technology, and that is why a big effort is being made to further optimizing its properties through doping [28] or surface treatments [29]. In this context, this work is focused on the study of a cost-effective technique for the synthesis of raw graphene-based EDLCs electrodes and its optimization. Following a laser-photothermal process, we synthesized electrodes based on both LIG and Laser-rGO (LrGO) using two different kind of laser beams: CO 2 and UV. These electrodes were tested under the same conditions of layout and electrolyte to compare their performance as EDLCs electrodes. The work is structured as follows: after this introduction, Section 2summarizes the different materials used in our experiments, as well as the methodologies followed for the fabrication and characterization of the samples. Section 3presents the results obtained for the different electrodes together with a comparison of their performance. Finally, the main conclusions are drawn in Section 4. 2. Materials and Methods 2.1. Materials Two different flexibles substrates were used for the fabrication of the electrochemical capacitors, Kapton ® HN films with a thickness of 75 µ m from DuPont Corporation (Wilmington, DE, USA) Micromachines 2020,11, 555 3 of 14 and Polyethylene Terephthalate (PET) foils with a thickness of 160 µ m from ColorGATE Digital Output Solutions GmbH (Hannover, Germany). Graphene Oxide at a concentration of 0.4 wt% was acquired from Graphenea (San Sebasti á n, Spain). The electrolyte was prepared using poly(vinyl alcohol) (PVA, Mw 31,000–50,000, 98%–99% hydrolyzed) and phosphoric acid (H 3 PO 4 , product name: 1005731000), both acquired from Sigma-Aldrich (St. Louis, MO, USA). Electrical access to the capacitive devices was printed using a silver-based conductive paint from RS PRO (RS Components, Corby, UK). 2.2. Fabrication Processes Thegelelectrolyte waspreparedbydissolving1gofPVA (Mw31,000–50,000, 98%–99%hydrolyzed, from Sigma-Aldrich, St. Louis, MO, USA) in 10 mL of de-ionized water (10 wt%), stirring at 80 ◦ C for 2 h using a hot plate stirrer (Scilogex SCI280-Pro, from Scilogex, LLC, Rocky Hill, CT, USA). Once the PVA was completely dissolved, 1.5 mL of H3PO4was added to the solution and it was stirred for another hour [30–32]. The devices were fabricated following the schemas shown in Figure 1. In the case of the LIG-based supercapacitors (Figure 1a), the laser photothermal ablation was carried out directly on the polyimide film (Figure 1(a-1)), inducing the graphene-derived structures on its surface (Figure 1(a-2)) [ 33 , 34 ]. On the other hand, for the fabrication of LrGO-based electrodes, a PET foil (Figure 1(b-1)) was covered with GO at a concentration of 75 µ L/cm 2 (Figure 1(b-2)). After that, the sample was dried at ambient conditions during 48 h. Once the drying process was completed, the GO was turned into rGO through the laser photothermal process, as shown in Figure 1(b-3). Micromachines 2020, 11, x 4 of 14 Figure 1. Schematic representation of the fabrication process of the flexible electrochemical capacitors (ECs). (a) Laser‐Induced Graphene (LIG)‐based ECs: (1) Kapton® polyimide film (thickness: 75 μm), (2) laser‐scribing process to induce graphene‐derived pattern on the surface of the polyimide (this case shows the CO2 laser beam), (3) silver electrical contacts printed on each electrode, (4) poly(vinylalcohol) (PVA)/H3PO4 electrolyte drop‐casted on top of the InterDigital Electrodes (IDE) structure. (b) Laser‐reduced Graphene Oxide (rGO) (LrGO)‐based ECs: (1) PET film (thickness: 160 μm), (2) GO deposited onto the PET substrate (concentration: 75 μL/cm2), (3) laser‐scribing process to reduce the GO, (4) silver electrical contacts printed on each electrode, (5) PVA/H3PO4 electrolyte drop‐ casted on top of the IDE structure. 2.3. Characterization The sheet resistance of the graphene‐derived patterns was extracted through the Transmission Line Method (TLM) [34]. X‐ray Photoelectron Spectroscopy (XPS) analysis was performed using the Katros Axis Ultra‐DLD X‐ray photoelectron spectrometer (from Kratos Analytical Ltd, Manchester, UK). The samples were characterized in a vacuum chamber at a pressure of 10−10 Torr at an X‐ray power of 450 W. Raman spectra were obtained with a NRS‐5100 dispersive micro‐Raman spectrometer (from JASCO International Co. Ltd., Tokyo, Japan) using an excitation source with a wavelength of λ = 532nm (Elforlight G4‐30; Nd:YAG). The profilometry of the samples was acquired using a DekTak XT contact profilometer (from Bruker Corporation, Billerica, MA, USA) at a stylus force of 1 mg. Cyclic Voltammetry (CV) experiments were performed using the B2912A precision source‐measurement unit (SMU) from Keysight Technologies, Inc. (St. Rose, CA, USA). 3. Results and Discussion The feasibility of both CO2 and UV lasers to induce porous nanographene aggregates from commercial polyimides, and to reduce the GO, has been already proved in several works [6,28,33,34]. However, due to the different nature of the laser beams, not only is the power required to optimize the sheet resistance different, but so are the properties of the material synthetized [39]. Furthermore, the power required for the synthesis of these materials depends on the laser used, thus, we set the laser speed and studied the sheet resistance of the laser‐synthetized patterns as a function of the laser power, in order to minimize the sheet resistance while maintaining the integrity of the substrates. Following the TLM procedure, whose results are summarized in Supplementary Figure S1, the configuration of the laser to minimize the sheet resistance is presented in Table 1. Figure 1. Schematic representation of the fabrication process of the flexible electrochemical capacitors (ECs). ( a ) Laser-Induced Graphene (LIG)-based ECs: (1) Kapton ® polyimide film ( thickness: 75 µm ), (2) laser-scribing process to induce graphene-derived pattern on the surface of the polyimide (this case shows the CO 2 laser beam), (3) silver electrical contacts printed on each electrode, (4) poly(vinylalcohol) (PVA)/H 3 PO 4 electrolyte drop-casted on top of the InterDigital Electrodes (IDE) structure. ( b )Laser-reduced Graphene Oxide (rGO)(LrGO)-based ECs: (1) PET film( thickness: 160 µm ), (2) GO deposited onto the PET substrate (concentration: 75 µ L/cm 2 ), (3) laser-scribing process to reduce the GO, (4) silver electrical contacts printed on each electrode, (5) PVA/H 3 PO 4 electrolyte drop-casted on top of the IDE structure. Two different types of laser were used in this work for this purpose. On one side, we used a CO 2 laser with an infrared wavelength of 10.6 µ m (Rayjet 50, from Trotec Ltd., Marchtrenk, Austria). On the other side, we also used a UV laser with a wavelength of 450 nm (from KKmoon SA, Automatic K5). The use of this type of technology requires two main safety measures. Thus, the laser machines count on with an eye protection shielding, which filters the high intensity wavelength of their corresponding laser beams. In addition to that, a fume extractor was used during the laser photothermal treatment to avoid the inhalation of the gases released during this process. Micromachines 2020,11, 555 4 of 14 Both laser power and speed were set, in order to optimize the sheet resistance of the graphene-derived patterns, thus improving the effective electrons transport and the electrochemical property [ 35 ]. Therefore, we obtained a total of four different combinations laser-electrode (hereinafter referred to as LIG UV, LIG CO 2 , rGO UV, rGO CO 2 for each material and laser, respectively). The layout studied in this work consisted of planar InterDigital Electrodes (IDE) structure (Figure 1), given that this configuration allows to achieve lower thicknesses and a more accurate control of the distances between electrodes than its stack counterpart [ 36 ]. In addition, we considered two different patterns, firstly we characterized the different devices using the following layout: number of fingers N=20, width of the fingers W=1 mm, spacing between electrodes S=1 mm, interspacing between fingers i=1 mm and length of the fingers L=1 cm, which resulted in an effective area of ~4 cm 2 . Secondly, we also used a layout with the same effective area, but with different spacing between electrodes (S=500 µ m), interspacing between fingers (i=500 µ m), as well as the width of the fingers (W=500 µ m), to demonstrate the possibility of increasing the specific capacitance by changing the geometry of the IDE structure. After the laser scribing process, electrical contacts were printed on both sides using silver ink (Figure 1(a-3) and Figure 1(b-4)) with the objective of reducing the resistivity between the electrode and the current collector, and with that the Equivalent Series Resistance (ESR) of the capacitor [ 37 , 38 ]. Finally, ~1.5 mL of the gel electrolyte was drop-casted on top of the capacitive IDE structure covering all the effective area, as depicted in Figure 1(a-4,b-5), and the samples were left standing overnight to remove the excess of water before the characterization. 2.3. Characterization The sheet resistance of the graphene-derived patterns was extracted through the Transmission Line Method (TLM) [ 34 ]. X-ray Photoelectron Spectroscopy (XPS) analysis was performed using the Katros Axis Ultra-DLD X-ray photoelectron spectrometer (from Kratos Analytical Ltd., Manchester, UK). The samples were characterized in a vacuum chamber at a pressure of 10 −10 Torr at an X-ray power of 450 W. Raman spectra were obtained with a NRS-5100 dispersive micro-Raman spectrometer (from JASCO International Co. Ltd., Tokyo, Japan) using an excitation source with a wavelength of λ=532 nm (Elforlight G4-30; Nd:YAG). The profilometry of the samples was acquired using a DekTak XT contact profilometer (from Bruker Corporation, Billerica, MA, USA) at a stylus force of 1 mg. Cyclic Voltammetry (CV) experiments were performed using the B2912A precision source-measurement unit (SMU) from Keysight Technologies, Inc. (St. Rose, CA, USA). 3. Results and Discussion The feasibility of both CO 2 and UV lasers to induce porous nanographene aggregates from commercial polyimides, and to reduce the GO, has been already proved in several works [ 6 , 28 , 33 , 34 ]. However, due to the different nature of the laser beams, not only is the power required to optimize the sheet resistance different, but so are the properties of the material synthetized [ 39 ]. Furthermore, the power required for the synthesis of these materials depends on the laser used, thus, we set the laser speed and studied the sheet resistance of the laser-synthetized patterns as a function of the laser power, in order to minimize the sheet resistance while maintaining the integrity of the substrates. Following the TLM procedure, whose results are summarized in Supplementary Figure S1, the configuration of the laser to minimize the sheet resistance is presented in Table 1. As has been reported in previous works, one of the distinctive features of LIG, which is common for both UV and CO 2 laser exposition sources, is its highly porous structure, which is a consequence of the rapid liberation of gaseous products during the laser-scribing process, also associated with the drastic increase in the atomic percentage of carbon [ 34 , 40 – 42 ]. In the case of the LrGO, the laser treatment on the surface of the GO leads to a partial restoration of the crystallographic network of its graphitic structure, which was disrupted during the oxidation process [ 43 ]. However, this partial recovery makes the surface of the resulting material to present structural defects manifested thorough Micromachines 2020,11, 555 5 of 14 a large roughness and a 3D plate-shape structure with multiple craters, a phenomenon that has been reported for both UV [ 44 ] and CO 2 lasers [ 45 ], and that could be helpful to increase their double layer capacitance effect. Table 1. Values of the sheet resistance obtained after optimization for the different laser beams and materials. Laser Material Laser Power (W) Laser Speed (cm/s) Sheet Resistance (Ω/sq.) CO2rGO 1.5 15 196.8 LIG 6 15 43.3 UV rGO 0.39 1 305.7 LIG 1.5 1 240.1 Raman and XPS spectroscopies confirmed the graphene-derived nature of these materials. On one hand, as seen in Figure 2, all Raman spectra are composed mainly by three different peaks located at ~1345 cm −1 (D peak), ~1580 cm −1 (G peak) and ~2700 cm −1 (2D peak), respectively. In particular, the G peak is associated with the sp 2 -hybrized carbon networks of the graphitic materials, whereas the D peak reveals the presence of defects in this structure (it would be inexistent in single-layer pristine graphene). Moreover, the 2D peak and its full width at half maximum (FWHM 2D ) are of particular interest for the study of the multi-layer nature of these materials [46]. Micromachines 2020, 11, x 5 of 14 Table 1. Values of the sheet resistance obtained after optimization for the different laser beams and materials. Laser Material Laser Power (W) Laser Speed (cm/s) Sheet Resistance (Ω/sq.) CO2 rGO 1.5 15 196.8 LIG 6 15 43.3 UV rGO 0.39 1 305.7 LIG 1.5 1 240.1 As has been reported in previous works, one of the distinctive features of LIG, which is common for both UV and CO2 laser exposition sources, is its highly porous structure, which is a consequence of the rapid liberation of gaseous products during the laser‐scribing process, also associated with the drastic increase in the atomic percentage of carbon [34,40–42]. In the case of the LrGO, the laser treatment on the surface of the GO leads to a partial restoration of the crystallographic network of its graphitic structure, which was disrupted during the oxidation process [43]. However, this partial recovery makes the surface of the resulting material to present structural defects manifested thorough a large roughness and a 3D plate‐shape structure with multiple craters, a phenomenon that has been reported for both UV [44] and CO2 lasers [45], and that could be helpful to increase their double layer capacitance effect. Raman and XPS spectroscopies confirmed the graphene‐derived nature of these materials. On one hand, as seen in Figure 2, all Raman spectra are composed mainly by three different peaks located at ~1345 cm−1 (D peak), ~1580 cm−1 (G peak) and ~2700 cm−1 (2D peak), respectively. In particular, the G peak is associated with the sp2‐hybrized carbon networks of the graphitic materials, whereas the D peak reveals the presence of defects in this structure (it would be inexistent in single‐layer pristine graphene). Moreover, the 2D peak and its full width at half maximum (FWHM2D) are of particular interest for the study of the multi‐layer nature of these materials [46]. Figure 2. Raman spectra of the different electrodes, (a) rGO UV, (b) rGO CO2, (c) LIG UV (d) LIG CO2. Acquisition parameters: wavelength: 532 nm, data interval: 1 cm−1, exposure time: 15 s, accumulations: 5, center number: 1469.99 cm−1. GO spectrum can be found in supplementary Figure S2. In single‐layer pristine graphene, the ratio I2D/IG is ~2–3, and it decreases as the number of layers increase [47]. On this basis, the results of the rGO sheets showed that the rGO produced with the CO2 laser presents a lower number of layers (higher I2D/IG ratio) than the one induced with the UV laser, which might be related to its lower thickness (~12 μm, in comparison with ~18 μm, according to profilometry results) due to the higher irradiation power, as demonstrated in other works [48]. In addition, the lower FWHM2D also indicates that these layers have a better crystallographic structure [49]. The better recovery of the crystallographic structure of the rGO obtained with the CO2 laser is Figure 2. Raman spectra of the different electrodes, ( a ) rGO UV, ( b ) rGO CO 2 , ( c ) LIG UV ( d ) LIG CO 2. Acquisition parameters: wavelength: 532 nm, data interval: 1 cm −1 , exposure time: 15 s, accumulations: 5, center number: 1469.99 cm−1. GO spectrum can be found in supplementary Figure S2. In single-layer pristine graphene, the ratio I 2D /I G is ~2–3, and it decreases as the number of layers increase [ 47 ]. On this basis, the results of the rGO sheets showed that the rGO produced with the CO 2 laser presents a lower number of layers (higher I 2D /I G ratio) than the one induced with the UV laser, which might be related to its lower thickness (~12 µ m, in comparison with ~18 µ m, according to profilometry results) due to the higher irradiation power, as demonstrated in other works [ 48 ]. In addition, the lower FWHM 2D also indicates that these layers have a better crystallographic structure [ 49 ]. The better recovery of the crystallographic structure of the rGO obtained with the CO 2 laser is an indication of a more effective reduction process, which explains its smaller sheet resistance, in spite of having lower thickness. Moreover, the high I D /I G ratio indicates that the CO 2 laser rGO also presents a high defect density. In the case of the LIG electrodes, the higher I D /I G Micromachines 2020,11, 555 6 of 14 ratio and FWHM 2D indicate a higher defective and disordered structure (i.e., lower crystallographic quality) with respect to the rGO with no significant difference between lasers sources. On the other hand, the XPS analysis demonstrates that the ablated surfaces are mainly composed of carbon and oxygen (present as carbon–oxygen functional groups), and that the laser-treatment led to a drastic increase in the original C/O ratio of the raw materials, as summarized in Table 2. Table 2. Carbon and oxygen atomic concentrations their ratio obtained extracted from the X-ray Photoelectron Spectroscopy (XPS) spectra for the different samples. Material Carbon Content (%) Oxygen Content (%) C/O Ratio Kapton®78 18 4.33 LIG-CO295.72 4.85 19.74 LIG-UV 87.72 9.28 9.45 GO 68.73 29.85 2.30 rGO-CO287.42 9.83 8.89 rGO-UV 84.45 10.70 7.89 The highest C/O ratios are those obtained from the LIG synthetized with the CO 2 (C/O=19.74) and UV (C/O=9.45) lasers, respectively, being the CO 2 –produced samples the ones reporting a higher level of reduction. A more detailed study of the nature of these changes can be obtained by means of the analysis of the high-resolution C1s XPS spectra, the results of which are shown in Figure 3. Micromachines 2020, 11, x 6 of 14 an indication of a more effective reduction process, which explains its smaller sheet resistance, in spite of having lower thickness. Moreover, the high ID/IG ratio indicates that the CO2 laser rGO also presents a high defect density. In the case of the LIG electrodes, the higher ID/IG ratio and FWHM2D indicate a higher defective and disordered structure (i.e., lower crystallographic quality) with respect to the rGO with no significant difference between lasers sources. On the other hand, the XPS analysis demonstrates that the ablated surfaces are mainly composed of carbon and oxygen (present as carbon–oxygen functional groups), and that the laser‐treatment led to a drastic increase in the original C/O ratio of the raw materials, as summarized in Table 2. Table 2. Carbon and oxygen atomic concentrations their ratio obtained extracted from the X‐ray Photoelectron Spectroscopy (XPS) spectra for the different samples. Material Carbon Content (%) Oxygen Content (%) C/O ratio Kapton® 78 18 4.33 LIG-CO2 95.72 4.85 19.74 LIG-UV 87.72 9.28 9.45 GO 68.73 29.85 2.30 rGO-CO2 87.42 9.83 8.89 rGO-UV 84.45 10.70 7.89 The highest C/O ratios are those obtained from the LIG synthetized with the CO2 (C/O = 19.74) and UV (C/O = 9.45) lasers, respectively, being the CO2–produced samples the ones reporting a higher level of reduction. A more detailed study of the nature of these changes can be obtained by means of the analysis of the high‐resolution C1s XPS spectra, the results of which are shown in Figure 3. As expected, the lasers are able to remove efficiently the oxygen‐containing functional groups of the raw GO material (see Figure S3), as well as most of the C–N, C–O–C and C=O bonds, which compose the Kapton® HN structure. For the rGO, the remaining non‐desirable bonds after the laser treatment are mainly associated with carbon–oxygen compounds in the case of the UV laser (Figure 3a), and with sp3 hybridized carbon bonds for the rGO reduced with the CO2 laser (which could explain its high ID/IG ratio). Finally, regarding the LIG, the intensity of the C=C peak, when compared with the intensity of rest of the compounds, indicates that the CO2 laser allowed a higher isolation of the sp2 hybridized atoms with respect to the LIG induced with the UV laser (Figure 3b,d, respectively), which is in accordance with its high carbon percentage. Figure 3. XPS C1s peaks of the different electrodes, (a) rGO UV, (b) LIG UV, (c) rGO CO2 (d) LIG CO2. Once all four different materials were analyzed from a structural point of view, we studied their electrochemical performance as electrodes for flexible supercapacitors. For that, we firstly performed the Cyclic Voltammetry (CV) experiments, considering a potential window of ΔV = 1 V (from −0.5 V to +0.5 V) at different scanning rates. The CV curves obtained for the different electrodes are shown Figure 3. XPS C1s peaks of the different electrodes, ( a ) rGO UV, ( b ) LIG UV, ( c ) rGO CO 2 ( d ) LIG CO 2 . As expected, the lasers are able to remove efficiently the oxygen-containing functional groups of the raw GO material (see Figure S3), as well as most of the C–N, C–O–C and C=O bonds, which compose the Kapton ® HN structure. For the rGO, the remaining non-desirable bonds after the laser treatment are mainly associated with carbon–oxygen compounds in the case of the UV laser (Figure 3a), and with sp 3 hybridized carbon bonds for the rGO reduced with the CO 2 laser (which could explain its high I D /I G ratio). Finally, regarding the LIG, the intensity of the C=C peak, when compared with the intensity of rest of the compounds, indicates that the CO 2 laser allowed a higher isolation of the sp 2 hybridized atoms with respect to the LIG induced with the UV laser (Figure 3b,d, respectively), which is in accordance with its high carbon percentage. Once all four different materials were analyzed from a structural point of view, we studied their electrochemical performance as electrodes for flexible supercapacitors. For that, we firstly performed the Cyclic Voltammetry (CV) experiments, considering a potential window of ∆ V=1 V (from − 0.5 V to +0.5 V) at different scanning rates. The CV curves obtained for the different electrodes are shown in Figure 4. In addition, the CV curves at a scan rate of 100 mV/s are plotted together in Figure 5a, Micromachines 2020,11, 555 7 of 14 whereas Figure 5b presents the specific capacitance as a function of the scan rate extracted according to Equation (1). CA=1 A·∆V·s· ZI(V)dV!(1) where Ais the area, ∆ Vthe potential window, sthe scan rate and I(V) the current response as a function of the voltage [27]. Micromachines 2020, 11, x 7 of 14 in Figure 4. In addition, the CV curves at a scan rate of 100 mV/s are plotted together in Figure 5a, whereas Figure 5b presents the specific capacitance as a function of the scan rate extracted according to Equation (1). = 1 ·∆··() (1) where A is the area, ∆V the potential window, s the scan rate and I(V) the current response as a function of the voltage [27]. As seen, apart from the rGO synthetized with the UV laser, the curves maintain a highly symmetric quasi‐rectangular shape over the increasing scan rates, indicating a good reversible EDLC behavior and a fast charge propagation within the electrode [50]. However, it can be noted that all these curves differ from each other, indicating that the interaction electrode‐electrolyte is different in each case. Thus, regarding the LIG electrodes, we can appreciate that the material synthetized with the different lasers presents an identical electrochemical behavior, but with the difference of a lower capacitance, in the case of the LIG UV electrode. This difference in capacitance is practically constant along the different scan rates (e.g., 0.23 mF/cm2 against 0.16 mF/cm2 at 10 mV/s and 0.18 mF/cm2 against 0.12 mF/cm2 at 100 mV/s), indicating that this constant difference in capacitance might be associated with the difference in the effective area of the LIG electrodes, as a result of the lower mechanical resolution of the UV laser when compared to the CO2 laser, which yields a lower effective area of the electrodes [40]. In the case of the rGO CO2 electrodes, their electrochemical behavior agrees with the reported for equivalent devices presented in the literature, such as the ones presented by Ghoniem et al. [45] and Yoo et al. [51]. It can be also noted that, even though the sheet resistance of the rGO CO2 electrodes is higher than the obtained for the LIG patterns, they report the best performance in terms of specific capacitance (0.44 mF/cm2 at 10 mV/s) as a consequence of their higher specific surface area, when compared to the LIG, which could be up to 4.5 times higher, in the case of the laser‐synthetized rGO according to references [42] and [52]. Figure 4. Cyclic voltammetry curves at different scan rates for the different electrodes (a) LIG CO2, (b) rGO UV, (c) LIG UV, (d) rGO CO2. Figure 4. Cyclic voltammetry curves at different scan rates for the different electrodes ( a ) LIG CO 2 , (b) rGO UV, (c) LIG UV, (d) rGO CO2. Micromachines 2020, 11, x 8 of 14 Lastly, the tilted CV curves obtained with the rGO UV electrodes represent a high ESR, as a consequence of the sheet resistance of these patterns, as well as a larger internal resistance to penetrate into the pores of the electrode, as a result of their less reduced state with a high concentration of sp3 carbon bonds [53]. This is also reflected in the behavior of the capacitance as the scan rate increases. As seen, for low scan rates, the specific capacitance achieved with these electrodes is quite similar to that obtained with the rGO‐CO2, since, at these rates, the ions have enough time to penetrate deeply into pores; however, it decreases gradually as the frequency increases (2 μF·s/mV), due to the reduction of the penetration depth, hence, worsening their performance [54]. Figure 5. (a) Cyclic voltammetry curves at 100 mV/s. (b) Specific capacitance as a function of the scan rate. Furthermore, we tested the cyclability of the different electrodes over an increasing number of CV cycles (at 100 mV/s), extracting the specific capacitance for every single cycle (Figure 6). As observed, the capacitors are able to retain their capacitance with a variation less than 0.03 μF/cycle in all cases (being the LIG UV those with less retention capacity ~83% after 1000 cycles). As expected, the capacitance decreases as the cycles increase, with exception of the rGO UV electrodes, whose capacitance increases (up to ~700 cycles), and afterwards retains almost the same value, as a consequence of the electrochemical reduction of some of the remaining oxygen groups of these still highly‐oxidized electrodes, as has been demonstrated in other works [27,55]. Figure 6. Specific capacitance as a function of increasing cyclic voltammetry cycles at a scan rate of 100 mV/s obtained with the different electrodes. According to the previous results, the LIG electrodes synthetized with both lasers, as well as the rGO synthetized with the CO2 laser, show promising results for potentially being used as bare electrodes in supercapacitors. In particular, the LIG electrodes synthetized with the UV laser are of particular interest for the development of really inexpensive electrodes, since, using a low‐cost and low‐power laser, they achieve comparable results to that obtained with a CO2 laser, which could be further improved by increasing the mechanical resolution of CNC system driving the laser. In CA (mF/cm2) Figure 5. ( a ) Cyclic voltammetry curves at 100 mV/s. ( b ) Specific capacitance as a function of the scan rate. As seen, apart from the rGO synthetized with the UV laser, the curves maintain a highly symmetric quasi-rectangular shape over the increasing scan rates, indicating a good reversible EDLC behavior and a fast charge propagation within the electrode [ 50 ]. However, it can be noted that all these curves differ from each other, indicating that the interaction electrode-electrolyte is different in each case. Thus, regarding the LIG electrodes, we can appreciate that the material synthetized with the different Micromachines 2020,11, 555 8 of 14 lasers presents an identical electrochemical behavior, but with the difference of a lower capacitance, in the case of the LIG UV electrode. This difference in capacitance is practically constant along the different scan rates (e.g., 0.23 mF/cm 2 against 0.16 mF/cm 2 at 10 mV/s and 0.18 mF/cm 2 against 0.12 mF/cm 2 at 100 mV/s), indicating that this constant difference in capacitance might be associated with the difference in the effective area of the LIG electrodes, as a result of the lower mechanical resolution of the UV laser when compared to the CO 2 laser, which yields a lower effective area of the electrodes [ 40 ]. In the case of the rGO CO 2 electrodes, their electrochemical behavior agrees with the reported for equivalent devices presented in the literature, such as the ones presented by Ghoniem et al. [45] and Yoo et al. [ 51 ]. It can be also noted that, even though the sheet resistance of the rGO CO 2 electrodes is higher than the obtained for the LIG patterns, they report the best performance in terms of specific capacitance (0.44 mF/cm 2 at 10 mV/s) as a consequence of their higher specific surface area, when compared to the LIG, which could be up to 4.5 times higher, in the case of the laser-synthetized rGO according to references [42] and [52]. Lastly, the tilted CV curves obtained with the rGO UV electrodes represent a high ESR, as a consequence of the sheet resistance of these patterns, as well as a larger internal resistance to penetrate into the pores of the electrode, as a result of their less reduced state with a high concentration of sp 3 carbon bonds [ 53 ]. This is also reflected in the behavior of the capacitance as the scan rate increases. As seen, for low scan rates, the specific capacitance achieved with these electrodes is quite similar to that obtained with the rGO-CO 2 , since, at these rates, the ions have enough time to penetrate deeply into pores; however, it decreases gradually as the frequency increases (2 µ F · s/mV), due to the reduction of the penetration depth, hence, worsening their performance [54]. Furthermore, we tested the cyclability of the different electrodes over an increasing number of CV cycles (at 100 mV/s), extracting the specific capacitance for every single cycle (Figure 6). As observed, the capacitors are able to retain their capacitance with a variation less than 0.03 µF/cycle in all cases (beingthe LIGUV thosewithless retention capacity~83% after1000 cycles). Asexpected, the capacitance decreases as the cycles increase, with exception of the rGO UV electrodes, whose capacitance increases (upto ~700cycles), and afterwards retainsalmost thesame value, asa consequenceof theelectrochemical reduction of some of the remaining oxygen groups of these still highly-oxidized electrodes, as has been demonstrated in other works [27,55]. Micromachines 2020, 11, x 8 of 14 Lastly, the tilted CV curves obtained with the rGO UV electrodes represent a high ESR, as a consequence of the sheet resistance of these patterns, as well as a larger internal resistance to penetrate into the pores of the electrode, as a result of their less reduced state with a high concentration of sp3 carbon bonds [53]. This is also reflected in the behavior of the capacitance as the scan rate increases. As seen, for low scan rates, the specific capacitance achieved with these electrodes is quite similar to that obtained with the rGO‐CO2, since, at these rates, the ions have enough time to penetrate deeply into pores; however, it decreases gradually as the frequency increases (2 μF·s/mV), due to the reduction of the penetration depth, hence, worsening their performance [54]. Figure 5. (a) Cyclic voltammetry curves at 100 mV/s. (b) Specific capacitance as a function of the scan rate. Furthermore, we tested the cyclability of the different electrodes over an increasing number of CV cycles (at 100 mV/s), extracting the specific capacitance for every single cycle (Figure 6). As observed, the capacitors are able to retain their capacitance with a variation less than 0.03 μF/cycle in all cases (being the LIG UV those with less retention capacity ~83% after 1000 cycles). As expected, the capacitance decreases as the cycles increase, with exception of the rGO UV electrodes, whose capacitance increases (up to ~700 cycles), and afterwards retains almost the same value, as a consequence of the electrochemical reduction of some of the remaining oxygen groups of these still highly‐oxidized electrodes, as has been demonstrated in other works [27,55]. Figure 6. Specific capacitance as a function of increasing cyclic voltammetry cycles at a scan rate of 100 mV/s obtained with the different electrodes. According to the previous results, the LIG electrodes synthetized with both lasers, as well as the rGO synthetized with the CO2 laser, show promising results for potentially being used as bare electrodes in supercapacitors. In particular, the LIG electrodes synthetized with the UV laser are of particular interest for the development of really inexpensive electrodes, since, using a low‐cost and low‐power laser, they achieve comparable results to that obtained with a CO2 laser, which could be further improved by increasing the mechanical resolution of CNC system driving the laser. In CA (mF/cm2) Figure 6. Specific capacitance as a function of increasing cyclic voltammetry cycles at a scan rate of 100 mV/s obtained with the different electrodes. According to the previous results, the LIG electrodes synthetized with both lasers, as well as the rGO synthetized with the CO 2 laser, show promising results for potentially being used as bare electrodes in supercapacitors. In particular, the LIG electrodes synthetized with the UV laser are of particular interest for the development of really inexpensive electrodes, since, using a low-cost and low-power laser, they achieve comparable results to that obtained with a CO 2 laser, which could be further improved by increasing the mechanical resolution of CNC system driving the laser. In addition, Micromachines 2020,11, 555 9 of 14 both fabrication approaches are also compatible with the techniques of functional nanomaterials growth, as well as doping procedures in the case of the GO, which would help to improve their electrochemical activity [56–58]. However, it is also possible to obtain higher specific capacitances without resorting to any doping or additional treatment of the bare electrodes, simply by modifying the layout of these structures. Thus, as an example, we considered an alternative microstructure for the LIG UV and rGO CO 2 capacitors. This structure covers the same area than the previous one but with a distance between electrodes, spacing and width of the fingers of 500 µm, instead of 1 mm. As a result, this structure led to an increase of the capacitance in both cases, without compromising the electrochemical performance of the devices, as can be observed in Figure 7a. This increment arises from the increase of the capacitance between two consecutive fingers by reducing their interspacing, as well as from the increment of the density of fingers, according to the IDE analytical models [ 59 – 61 ]. In our case, the increment, depicted in Figure 7b, represents a factor of 1.42 ± 0.05 for the LIG UV electrodes and of 1.73 ± 0.03 for the rGO CO 2 , resulting in a specific capacitance at 10 mV/s of ~0.23 mF/cm 2 and ~0.7 mF/cm 2 , respectively. These values compare well to the ones reported for similar devices fabricated with other materials, such as activated carbon [ 62 ] or graphene-CNTs [ 63 ], and it is similar or even higher than the reported results for other laser-synthetized materials [64,65]. Micromachines 2020, 11, x 9 of 14 addition, both fabrication approaches are also compatible with the techniques of functional nanomaterials growth, as well as doping procedures in the case of the GO, which would help to improve their electrochemical activity [56–58]. However, it is also possible to obtain higher specific capacitances without resorting to any doping or additional treatment of the bare electrodes, simply by modifying the layout of these structures. Thus, as an example, we considered an alternative microstructure for the LIG UV and rGO CO2 capacitors. This structure covers the same area than the previous one but with a distance between electrodes, spacing and width of the fingers of 500 μm, instead of 1 mm. As a result, this structure led to an increase of the capacitance in both cases, without compromising the electrochemical performance of the devices, as can be observed in Figure 7a. This increment arises from the increase of the capacitance between two consecutive fingers by reducing their interspacing, as well as from the increment of the density of fingers, according to the IDE analytical models [59–61]. In our case, the increment, depicted in Figure 7b, represents a factor of 1.42 ± 0.05 for the LIG UV electrodes and of 1.73 ± 0.03 for the rGO CO2, resulting in a specific capacitance at 10 mV/s of ~0.23 mF/cm2 and ~0.7 mF/cm2, respectively. These values compare well to the ones reported for similar devices fabricated with other materials, such as activated carbon [62] or graphene‐CNTs [63], and it is similar or even higher than the reported results for other laser‐ synthetized materials [64,65]. In addition, it is shown in Figure 8 how these devices are capable of maintaining their electrochemical performance under different bend conditions, without significant changes to their specific capacitance (being the maximum deviation with respect to the flat state ∆C/C0 (%) = 1.87 in the case of LIG electrodes and ∆C/C0 (%) = 5.45 in the case of the rGO electrodes). This maximum deviation was reported for the outer bend state in both cases. Figure 7. (a) Comparison of the cyclic voltammetry curves at 100 mV/s for two different configuration of electrodes. (b) Specific capacitance as a function of the scan rate for these configurations. Figure 8. Cyclic voltammetry curves at a scan rate of 100 mV/s at different bend conditions (bend radius of 1.25 cm) obtained for the (a) LIG UV and (b) the rGO CO2 supercapacitors. Inset of (a) shows an inner bent LIG‐based supercapacitor, while inset of (b) shows an outer bent rGO‐based supercapacitor. Figure 7. ( a ) Comparison of the cyclic voltammetry curves at 100 mV/s for two different configuration of electrodes. (b) Specific capacitance as a function of the scan rate for these configurations. In addition, it is shown in Figure 8how these devices are capable of maintaining their electrochemical performance under different bend conditions, without significant changes to their specific capacitance (being the maximum deviation with respect to the flat state ∆ C/C 0 (%) =1.87 in the case of LIG electrodes and ∆ C/C 0 (%) =5.45 in the case of the rGO electrodes). This maximum deviation was reported for the outer bend state in both cases. Micromachines 2020, 11, x 9 of 14 addition, both fabrication approaches are also compatible with the techniques of functional nanomaterials growth, as well as doping procedures in the case of the GO, which would help to improve their electrochemical activity [56–58]. However, it is also possible to obtain higher specific capacitances without resorting to any doping or additional treatment of the bare electrodes, simply by modifying the layout of these structures. Thus, as an example, we considered an alternative microstructure for the LIG UV and rGO CO2 capacitors. This structure covers the same area than the previous one but with a distance between electrodes, spacing and width of the fingers of 500 μm, instead of 1 mm. As a result, this structure led to an increase of the capacitance in both cases, without compromising the electrochemical performance of the devices, as can be observed in Figure 7a. This increment arises from the increase of the capacitance between two consecutive fingers by reducing their interspacing, as well as from the increment of the density of fingers, according to the IDE analytical models [59–61]. In our case, the increment, depicted in Figure 7b, represents a factor of 1.42 ± 0.05 for the LIG UV electrodes and of 1.73 ± 0.03 for the rGO CO2, resulting in a specific capacitance at 10 mV/s of ~0.23 mF/cm2 and ~0.7 mF/cm2, respectively. These values compare well to the ones reported for similar devices fabricated with other materials, such as activated carbon [62] or graphene‐CNTs [63], and it is similar or even higher than the reported results for other laser‐ synthetized materials [64,65]. In addition, it is shown in Figure 8 how these devices are capable of maintaining their electrochemical performance under different bend conditions, without significant changes to their specific capacitance (being the maximum deviation with respect to the flat state ∆C/C0 (%) = 1.87 in the case of LIG electrodes and ∆C/C0 (%) = 5.45 in the case of the rGO electrodes). This maximum deviation was reported for the outer bend state in both cases. Figure 7. (a) Comparison of the cyclic voltammetry curves at 100 mV/s for two different configuration of electrodes. (b) Specific capacitance as a function of the scan rate for these configurations. Figure 8. Cyclic voltammetry curves at a scan rate of 100 mV/s at different bend conditions (bend radius of 1.25 cm) obtained for the (a) LIG UV and (b) the rGO CO2 supercapacitors. Inset of (a) shows an inner bent LIG‐based supercapacitor, while inset of (b) shows an outer bent rGO‐based supercapacitor. Figure 8. Cyclic voltammetry curves at a scan rate of 100 mV/s at different bend conditions (bend radius of 1.25 cm) obtained for the ( a ) LIG UV and ( b ) the rGO CO 2 supercapacitors. Inset of ( a ) shows an inner bent LIG-based supercapacitor, while inset of ( b ) shows an outer bent rGO-based supercapacitor.