Zhengfen Wan, Xi Chen and Min Gu*

Keywords: laser; graphene; laser scribed graphene; supercapacitor

Introduction

Supercapacitors, also called electrochemical capacitors or ultracapacitors, have been intensively studied over the past few years to meet the rapidly growing demand for highly efficient energy storage devices1-3.Owing to their unique advantages including high power density (10 kW/kg), short charge/discharge duration (in seconds),and long cycle life (over 1 million cycles), supercapacitors were considered to be promising candidates in the applications of consumer electronics, hybrid electric vehicles and industrial power management4,5.Supercapacitors with properties such as high energy and power densities, small sizes, light weights and mechanical flexibility have been highly demanded6-8.

Graphene, a flat monolayer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice,has become a hot research topic since its discovery9,10.It shows great potential for supercapacitor applications due to its exceptional high theoretical surface area (2630 m2·g-1) and electrical conductivity11.Many approaches such as mechanical exfoliation12, chemical vapor deposition (CVD)13and the reduction of graphene oxide14,15have been developed to produce graphene materials.Recently, a laser direct writing method for graphene has drawn tremendous attention, because of its unique advantages including selective and localized reduction,flexible patterning and no requirement for additional chemicals16,17.Based on the laser scribed graphene (LSG),various applications have been demonstrated, including hologram18, energy storage19-21, strain sensor22,23, biosensor24,25and antennas26.Unlike conventional microfabrication methods such as lithography, this laser technique does not require the utilization of masks, expensive materials, post-processing and cleanroomoperations27-32.The LSG can be directly prepared by laser irradiation and simultaneously patterned for the electrodes of supercapacitors.This facile and cost-effective method of graphene fabrication has demonstrated great potential for commercial supercapacitor applications.

In this review, the recent developments of the LSG supercapacitors were summarized.Firstly, we concluded the fabrication and modification of LSG.Particular attention is paid to the application of electrochemical double-layer capacitors (EDLCs), pseudo-capacitors and hybrid supercapacitors, and the diverse strategies to achieve high-performance, flexible, ultrafine-structured and integrated supercapacitors.Current challenges and future advancements of LSG based supercapacitors were also discussed.

Different precursors: graphene oxide, commercial polyimides and other carbon resources were employed for laser scribed graphene production in the literature.The resulting graphene materials are referred differently in scientific reports as laser reduced graphene oxide33,laser-induced graphene34, laser-scribed graphene35, or laser carbonized nanomaterials36.The term ‘laser scribed graphene (LSG)’ is adopted throughout this paper.

Laser scribed graphene

Preparation of laser scribed graphene

Graphene oxide

Graphene oxide (GO), which has the skeleton of graphene decorated with oxygen components, is considered as an important precursor of LSG15.It can be produced in large scale by cost-effective chemical methods, forming stable aqueous colloids that are favored by industrial processes37.GO films were prepared by spincoating, drop-casting, blade, or freezing-drying method.Graphene devices on diverse substrates, including leaf,lens, fabrics etc.can be fabricated by laser technology38.Xiao et al., fabricated LSG microcircuits with the line widths of 500 nm by the laser irradiation on GO films33.Figure 1(a-c) shows the fabrication of GO films, the LSG microstructures, and the scanning electron microscopy(SEM) images of the microcircuits.After the laser writing, the thicknesses of film decrease and the color of film turns to black.An obvious removal of oxygen components can be observed as indicated in AFM and XPS results in Fig.1(d-f).The mechanism of laser reduction of GO was strongly related to the photochemical and photothermal effect of laser15.The threshold of GO photoreduction was 3.2 eV (390 nm)39.For the laser with wavelength ＜ 390 nm, the photochemical effect of laser can trigger the C-O bonding weakening and the oxygen removal.Meanwhile, it was reported that the exothermic reduction of GO occurs at a temperature between 200-230 °C40.The high temperature induced by laser could easily break the C-O and C=O bonds, leading to the reduction of GO.In this laser reduction process, two sub-processes, namely the direct conversion from sp3carbon to sp2carbon and removal of oxygen functional groups can occur, resulting in the reduced graphene oxide (rGO)41.The ultrafast thermal transferred process triggered by the laser spot can also induce the simultaneous exfoliation and reduction of GO, and thereby enhance its specific surface area42.Beside GO film, GO in solution, GO fiber and GO aerogel can also be reduced to rGO with laser treatment.Figure 1(g) shows the dramatic color change of the GO solution with ammonia before and after the pulsed laser irradiation43.Upon pulsed laser irradiation, the yellow-brown color instantaneously turned black, indicating the effective reduction of GO in solution.GO fiber were also region-specifically reduced by laser irradiation to fabricate a flexible fiber supercapacitor with reduced GO layers as electrodes and GO as the separator44.Figure 1(h) shows that a precursor of GO aerogel, after being exposed to a laser spot, was reduced in only tens of milliseconds and converted to graphene bulks45.In another report, the self-assembled GO liquid crystals on the surface of GO solutions can also be reduced by laser46.Ibrahim et al.introduced the production of reduced GO gels by focusing a femtosecond laser on air/GO solution interfaces47.

Fig.1 | Illustration of various GO precursors reduced by laser scrbing.(a) The preparative procedure of LSG microcircuit on GO film.(b–c) Optical microscopy images of laser patterned microcircuit; Scale bars, 10 μm.(d) Atomic force microscope (AFM) image of LSG microcircuit on GO film, the height profile along the white line (L2), and its 3D image.(e) Survey X-ray photoelectron spectra of GO and LSG.Inset is a photograph of a LSG square on a GO film.(f) C1s x-ray photoelectron spectroscopy (XPS) spectra of GO and LSG.(g) The experimental setup of pulsed laser reduction system.The inset is optical images of GO solution (15 mL 0,1 mg/mL) before (i) and after (ii) pulsed laser irradiation.(h)Schematic illustration of the GO aerogel treated by laser for the preparation of graphene bulks.Figures reproduced with permission from: (a–e)ref.33 and (g) ref.43, Elsevier; (h) ref.45, John Wiley and Sons.

Polymer and biomass

In 2014, Tour’s group reported the fabrication of porous LSG films from commercial polymer films using a CO2laser48.The polyimide (PI) and ployetherimide out of 15 different polymers were successfully converted to LSG under laser irradiation.The LSG with excellent electrical conductivity (5-25 S·cm-1) can be readily patterned to interdigitated electrodes for in-plane supercapacitors with specific capacitances of ＜ 44 mF·cm-2and power densities of ～9 mW·cm-2.Figure 2(a-c) shows the laser pattern on PI and the SEM images of LSG, exhibiting high surface area (～340 m2·g-1) with pore sizes ＜ 9 nm.The Raman spectrum of LSG (Fig.2(d)) demonstrates a 2D Raman band (centered at 2700 cm-1), typically found in that of 2D graphite.The XRD pattern of LSG (Fig.2(e)) exhibits an intense peak centered at 2θ=25.9°, indicating the high degree of graphitization.This LSGformation is attributed to the extremely high localized temperature (＞2500 °C) triggered by the CO2laser beam,which can break C-O, C=O and C-N bonds and rearrange the aromatic compounds to graphene structures.In a similar process, Zhang et al., converted phenolic resin into LSG with 3D porous structures with low resistance (～44 Ω·sq-1) and good mechanical properties in large scale by a laser scribing49.The polybenzoxazine resin poly (Ph-ddm) with good flexibility, high thermal stability and superior chemical resistance was also employed for the fabrication of LSG by straightforward CO2lasing50.The graphitization of sulfonated poly (ether ether ketone) (SPEEK) film was obtained for an all-SPEEK flexible supercapacitor using a pulsed CO2laser51.The resulting LSG can act as the binder-free electrode.The current collector and the SPEEK is employed as both separator and polymeric electrolyte.With laser treatment, natural precursors such as wood52, cloth, paper,potato skins, coconut shells, cork53and lignin54, which are inexpensive, abundant, and biodegradable, can also be transformed into graphene, as shown in Fig.2(f-j).Kaner et al.converted carbon nanodots (CNDs) into high-surface-area 3D graphene networks with excellent electrochemical properties by an irradiation with an infrared laser55.The fabricated 3D LSG electrodes show high specific volumetric capacitance of 27.5 mF·cm-3and extremely fast charging rates with a relaxation time of 3.44 ms.

Fig.2 | (a) Schematic illustration of laser patterning on polyimide (PI).(b–c) SEM image of LSG on the PI substrate (b) with scale bar 1 mm and its enlarged SEM image with scale bar 10 μm (c).Inset is the corresponding SEM image with higher magnification, scale bar 1 μm.(d–e) Raman spectrum (d) and X-ray diffraction (XRD) (e) of a LSG and the PI film.(f) A photo of LSG on pine wood.(g) The letter “R” in LSG induced from bread.(h) Picture of LSG patterned into an “R” on a coconut (2 cm tall).(i–j) LSG on cloth in the shape of an owl (60 mm in height) (i) and the wrapped cloth (j).Figure reproduced with permission from: (a–e) ref.48, Springer Nature; (f) ref.52, John Wiley and Sons; (g–j) ref.53, American Chemical Society.

Modification of laser scribed graphene

The chemical component, structure and morphology of LSG is strongly affected by the laser scribing process and can be modified by adjusting the laser parameters, laser process, laser system, precursors and environment.Laser systems with varied wavelengths, including CO2laser(10.6 μm)36, near-infrared (NIR) laser (1064 nm)56,780 nm femtosecond (fs) laser41and 405 nm semiconductor laser57were reported to fabricate LSG.The laserreduction process is attributed to the photochemical effect and photothermal effect induced by laser15.The surface modification of LSG, including modulating the surface morphologies, carbonization and wettability was demonstrated by adjusting laser powers, scanning speeds and pulse repetition frequencies36,58,59.Different morphologies, namely “sheet”, “needle” and “porous”, can be achieved with the optimized laser-writing recipes60.With repeated laser irradiations, the structure of PI-derived LSG can be transformed from its original macroporous foam to an intermediate concave corrugated tile structure, and finally a carbon nanotube structure61.Instead of a focused round beam, a wide line beam was employed to transform GO into LSG, improving the efficiency of large area laser reduction62.The resulting LSG electrode exhibits a high specific capacitance (～130 F·g-1) at a current density of 1 A·g-1.Sun et al., reported a hierarchicalstructuring and synchronous GO photoreduction using a nanosecond laser holography technology63.The laser beam was split into two with an equal intensity, and then interfered on the surface of a GO film.The periodic light field patterns generated by the interfered laser beams resulted in the periodic micro-nano structures of LSG.It was found that the laser reduction of PI resulted in selfnitrogen-doped porous LSG (sourcing from the N element of PI precursor), improving its conductivity and electrochemical performance24.Guan et al., fabricated the porous LSG without agglomeration by a laser treatment on GO sheets in liquid nitrogen64.Due to cryotemperature development and depressed thermal expansion,

the frozen LSG exhibits crack-free porous morphologies and shows a decrease of sheet resistance by a factor of 104to 105.By controlling the laser reduction atmosphere(O2, air, Ar, H2, and SF6), the water contact angle of LSG was modulated from 0° (O2or air) to ＞150° (Ar or H2) or＞160° (SF6)65.

Laser scribed graphene based supercapacitor

Based on the different energy storage mechanisms, supercapacitors (SCs) can be divided into two types: electrochemical double-layer capacitors (EDLCs) and pseudo-capacitors66.EDLCs, which are non-Faradaic capacitors, store energy by building up charges in the layers of the electrical double-layer formed at the interface of electrode/electrolyte67.Owing to the fast physical charging and discharging process, EDLCs show great advantages of short charging time, high power densities and long lifespans.However, the capacitance of EDLCs is relatively low due to the limited effective surfaces of electrodes.In this way, LSG electrodes with excellent conductivities and high surface areas are very promising to improve the energy density of EDLCs.Different from EDLCs, the capacitance of pseudo-capacitors is acquired from the storage of charge in the bulk of a redox material following a redox reaction66.This fast redox reactionacts like capacitance (hence the name pseudo-capacitance) and contributes to an enhanced capacitance68.The poor conductivity of pseudocapacitive materials limits the power density.Moreover, the material deterioration during the redox process shortens the cycle lives of devices.The porous LSG with good conductivity and chemical stability was considered as a good framework of pseudo-capacitors.The LSG has been widely studied to fabricate both EDLCs and pseudo-capacitors.The electrochemical performances of diverse LSG-based SCs in literature were listed in Table 1.

Table 1 | The performances of laser scribed graphene based supercapacitors

LSG based electrochemical double-layer capacitors

In 2012, EI-Kady et al., fabricated the LSG based EDLCs by employing a standard LightScribe DVD optical drive,as shown in Fig.3(a)69.The GO films were deposited on the disk and then treated with laser.The resulting LSG shows a high electrical conductivity (1738 S·m-1) and a specific surface area (1520 m2·g-1).An ion porous separator was sandwiched between two identical LSG electrodes for SCs (total thickness ＜ 100 μm).The fabricated LSG SCs exhibit a energy density of 1.36 mWh·cm-3and a power density of ～20 W·cm-3.With the similar laser direct writing technology, this research group demonstrated a scalable fabrication of LSG in-plane supercapacitors, Fig.3(b)70.The GO irradiated by laser was converted into graphene and applied as the SC electrodes.The unirradiated GO served as the separator between the positive and negative interdigitated electrodes.More than 100 LSG SCs can be produced on a single disc in 30 minutes or less.These efficiently fabricated LSG supercapacitors exhibit an ultrahigh power of 200 W·cm-3and an excellent cycling stability retaining 96% of the initial performance after 10000 charge-discharge cycles.Gao et al.demonstrated laser reduction and patterning of hydrated GO films for all-carbon monolithic supercapacitors20.The LSG electrodes were fabricated for both inplane and conventional sandwiched supercapacitor.The hydrated GO, which contains substantial amounts of trapped water serves as both the electrolyte and the electrode separator.The resulting LSG SCs show a good cyclic stability (30% drop in the capacitance after 10000 cycles) and a high areal capacitance (0.51 mF·cm-2).By laser scribing, the amorphous carbon nanospheres (CNS)precursors are transformed into highly turbostratic graphitic carbon (CNS-LSG)82.The sandwiched supercapacitors based on the CNS-LSG electrodes exhibit a high volumetric power density of 28 W·cm-3.For high quality LSG SCs, a CO2laser beam was employed to fabricate LSG with high crystallinity and a low degree of defects.Then a successive ultraviolet (UV) pulsed laser direct carving was performed for high resolution electrode pattern83.The fabricated SCs with an electrode width of 50 μm exhibit a areal capacitance of 43.7 mF·cm-2and a 90% capacitance retention after 3000 cycles.In addition to the laser irradiation method, a successive electrochemical reduction was combined to fabricate highly conductive graphene networks for current collectors of supercapacitors84.The developed LSG supercapacitor shows notable improvement of the stability performances (100000 cycles).A large cell voltage of 10.8 V was realized by modularizing nine devices in series, exhibiting rectangular shapes of the cyclic voltammetry curves at high scan rates of 100 V·s-1.

Fig.3 | (a) Schematic illustration of the fabrication of LSG based supercapacitors with sandwiched structures.(b) Schematic diagram of the preparation process for an in-plane LSG supercapacitor.Figure reproduced with permission from: (a) ref.69 , AAAS; (b) ref.70 , Springer Nature.

Doped LSG for supercapacitors

Doping with heteroatoms (such as boron, nitrogen,phosphorus, and sulfur) has been regarded as an effective way to tailor the electrochemical properties of graphene-derived materials and to enhance their capacitive performances85-87.Heteroatom-doped graphene materials were intensely studied as active electrodes in energy storage devices.Tour et al., demonstrated that boron-doped porous graphene can be prepared from boric acid containing polyimide sheets using a facile laser induction process in ambient air, as shown in Fig.4(a)72.At the same time, the LSG was patterned for electrodes of flexible supercapacitors.Fig.4(b) shows that the B1s peak of X-ray photoelectron spectroscopy (XPS) shifts from 192.5 eV in B-PI down to 191.9 eV in B-LSG after laser induction, showing the effective boron doping in the LSG sheet.The boron-doped LSG supercapacitor demonstrates an enhanced performance compared to bare LSG supercapacitor, as shown in Fig.4(c-d).The areal capacitance of boron doped LSG supercapacitor can reach 16.5 mF·cm-2, 3 times higher than nondoped devices, with a concomitant energy density increase of 5-10 times.Fu et al., prepared N-doped LSG by direct laser writing on the mixture of the nitrogen-rich carbon nanoparticles and GO88.After laser treatment, the composites show N concentration as high as 7.78 atom %.With the N-doped LSG electrodes, the fabricated supercapacitors show a high capacity retention of 48.76%(18.17% for undoped LSG-SCs) at scan rate from 5 to 100 mV·s-1.Similarly, Wang et al., synthesized nitrogendoped and hierarchical porous graphene from GO/urea mixture using a picosecond laser89.This N-doped LSG SC reaches to a high areal capacitance of 60.7 mF·cm-2,which is about 3 times higher than that of the undoped LSG SC.Instead of mixing in solution, Liu et al., treated KOH-coated polyimide films for synchronous heteroatoms (nitrogen and oxygen) doping and a wettability improvement of graphene by direct laser writing90.The nitrogen content can reach 4.94% and the doped LSG in-plane supercapacitors present an areal capacitance of 32 mF·cm-2(4.27 μWh·cm-2), which is about 10 times higher than that of the bare LSG.

Fig.4 | (a) Synthetic scheme for the preparation of boron-doped LSG and its fabrication of supercapacitor.(b) The B1s spectrum of XPS spectra of PI/H3BO3 sheet and boron-doped LSG.(c–d) Cyclic voltammetry curves (c) and galvanostatic charge-discharge curves (d) of LSG SC and boron-doped LSG SC with different H3BO3 loadings.Figure reproduced with permission from ref.72 , American Chemical Society.

Intercalated LSG for supercapacitors

Owing to the intensive pi-pi interaction of graphene,graphene sheets exhibit the strong tendency to restack together.The restacking issue leads to a significant decrease of ion-accessible surface area and thus a low capacitance of the graphene-based SC91.To prevent the restacking of LSG layers, carbon nanotubes (CNTs) with a smaller diameter (1-2 nm) were employed to insert between GO sheets before laser treatment, as shown in Fig.5(a)73.The GO/CNTs hybrid material was patterned into LSG/CNTs supercapacitors, yielding increased ionaccessible surface area.Figure 5(b-d) presents the images of LSG/CNT supercapacitor and the improved electrochemical performances.The laser-scribed LSG/CNT SC exhibits a volumetric capacitance of 3.10 F·cm-3, a volumetric energy density of 0.84 mWh·cm-3and a power density of 1.0 W·cm-3.The SC retains 88.6% of the initial capacitance value after 5000 cycles.Similarly, ZnO nanoparticles with sizes ranging from 20 to 50 nm were inserted into the LSG sheets by laser scribing on the mixture of Zn(NO3)26H2O and GO92.Consequently, a 12 times increase of the specific capacitance was achieved,compared with that of the pristine LSG SC.Lee et al.,demonstrated that a tiny amount of Zn infiltrated into GO led to an explosive reduction of GO under laser irradiation93.Attributed to a larger specific surface area and lots of mesopores, the LSG/Zn SC exhibited a nearly 4 times increase in the energy density.Besides intercalated graphene, components with advanced properties were also combined with LSG for enhanced performances.Li et al., reduced the mixture GO and chloroauric acid(HAuCl4) nanocomposite with a laser irradiation, patterning of LSG electrodes and producing Au nanoparticles in a one-step process, as shown in Fig.5(f-i)71.The porous LSG/Au electrode demonstrates a high conductivity of 1.1×106S·m-1and an enhanced accessible surface area.The LSG/Au SC shows an maximum specific capacitance of 3.84 mF·cm-2.Sun et al., reported a laser fabrication of flexible planar supercapacitors from GO and black phosphorus quantum dots (BPQDs) nanocomposites94.The introduction of BPQDs with more active sites on GO edge boosted the ion transportation on the interface between the electrode and the electrolyte.The LSG-BPQDs based supercapacitor delivered an enhanced areal capacitance of 5.63 mF·cm-2(1.87 mF·cm-2for LSG).Li et al., deposited the LSG by the fabrication ofpolyimide onto 3D nickel foam, as a porous electrode with laser processing95.The LSG/Ni electrode shows a high electrical conductivity (359712 S·m-1) and the fabricated LSG/Ni supercapacitor demonstrated a large areal specific capacitance (995 mF·cm-2), a power densitiy (9.39 mW·cm-2) and over 98% capacitance retention after 10000 cycles.

Fig.5 | (a) Schematic showing the insertion of CNTs between GO layers to effectively inhibit the restacking and the fabrication process for the flexible supercapacitor (LSG/CNTs SC).(b) Digital photographs of an assembled SC.(d–e) Cyclic voltammetry curves (d), charge-discharge curves (e) and for LSG SCs, LSG/CNTs SCs with different diameters.(f) Schematic illustration and photos of fabrication of LSG/Au supercapacitors onto a paper substrate.(g) The SEM image of LSG/Au microelectrodes.(h–i) Comparison of electrochemical performances of both the LSG/Au SCs and LSG SCs: cyclic voltammetry curves (h) and galvanostatic charge/discharge curves (i).Figure reproduced with permission from: (a–e) ref.73, Elsevier; (f–i) ref.71, Royal Society of Chemistry.

Pattern and structure of LSG based supercapacitors

Besides the electrode materials, the structure design of SCs has been also intensively studied to further enhance the performance of devices.Li et al., fabricated a flexible high-voltage LSG-SCs ranging from a few to thousands of volts with a planar in-series architecture, shown in Fig.6(a)74.210 isolated porous LIG squares were firstly patterned on PI by a programmable CO2laser system.The electrolyte was then added by brush coating process.The 209 V SC could achieve a high capacitance of 0.43 μF at a low applied current of 0.2 μA, and a capacitance of 0.18 μF at a high applied current of 5.0 μA.Figure 6(b) shows that a new structural design inspired by the traditional Japanese paper-cutting craftwork (known as “Kirigami”),has been employed to manufacture highly deformable SCs by laser-assisted graphitic conversion and cutting96.There is less than 2% shift in the LSG capacitance when the device is elongated to 382.5% of its initial length97.Figure 6(c) shows that vertically stacked LSG supercapacitors were assemblyed to multiply its electrochemical performance by laser induction on both sides of PI sheets34.A solid-state polymeric electrolyte, poly(vinyl alcohol) (PVA) in H2SO4was applied for the assemblyingof stacked LSG SCs.A novel bamboo-like series of infiber graphene supercapacitor was made along the GO fiber with LSG electrode arrays alternated with GO regions, as shown in Fig.6(d)98.Thousands of supercapacitor units can be fabricated within minutes and reach a high capacitance of 14.3 mF·cm-2.Gu et al., designed new bio-inspired LSG electrodes including Hilbert fractal structures, Peano fractal structures and Sierpinski fractal structures, and thus compared the SC performances base on the different designs75.Electrodes with Hilbert fractal structures and the corresponding SC were demonstrated in Fig.6(e-f).Compared to the conventional planar supercapacitors, the Hilbert fractal designed SC increased the ratio of active surface area to volume of the electrodes and reduced the electrolyte ionic path.The energy density is thus significantly increased to ～10-1Wh·cm-3, more than 30 times higher than that achievable by the planar interdigital electrodes.

Fig.6 | (a) Schematic diagram of a high-voltage planar SC based on laser scribed graphene.(b) Schematic illustration and the related strain property of the kirigami-inspired electrodes with different geometric unit numbers (scale bar 1 cm).(c) Illustration of stacked LSG-SCs in series and parallel circuits and its structure.(d) Schematic diagram of the direct laser reduction of GO fiber for the bamboo-like series of GO-LSG fiber.(e) Bio-inspired fractal electrode design of Hilbert fractal structures.(f) Schematic structure of the Hilbert fractal electrode supercapacitor.Figure reproduced with permission from: (a) ref.74, American Chemical Society; (b) ref.96, Springer Nature; (c) ref.34, American Chemical Society; (d)ref.98, Royal Society of Chemistry; (e-f) ref.75, under a Creative Commons Attribution 4.0 International License.

LSG based pseudo-capacitors

Compared to the EDLCs, pseudo-capacitors can achieve much higher capacitances since they store energy through a Faradic process, involving fast and reversible redox reactions between electrolytes and electro-active materials on electrode surfaces68.Transition metal oxides, hydroxides and conducting polymers are usually used as the electrodes for pseudo-capacitors99.However,owing to the poor electrical conductivity and the unstable structure of materials during the redox process,pseudo-capacitors demonstrate relatively low power densities and cycling unstabilities, which hinder their practical applications66.To overcome these drawbacks,graphene materials with high electrical conductivity and large specific surface area are merged with these activematerials for pseudo-capacitor electrodes.Wu et al.combined a laser scribing technique with a subsequent lowtemperature solvothermal growth of pseudo-capacitive materials to fabricate symmetric SCs with significantly improved electrochemical behaviour, as shown in Fig.7(a-c)35.A CO2laser beam was firstly used to convert the PI into porous LIG with an interdigitated architecture.Then, the conductive Ni-catecholate-based metal-organic frameworks (Ni-CAT MOF) were selectively grown on 3D LSG with a low-temperature solvothermal method.Figure 7(d) depicts that the tiny uniform nanorods are anchored on the LSG sheets and the black-grey pattern turns into dark blue (inset of Fig.7(d)).Figure 7(e-f)demonstrates the cyclic voltammetry (CV) curves and the galvanostatic charge/discharge (GCD) curve of the bare LSG electrode and that of the LSG/Ni-CAT composite.The SCs with LSG/Ni-CAT MOF electrodes exhibit a wide operating voltage (1.4 V), a high areal capacitance (15.2 mF·cm-2), a high energy density (4.1 μWh·cm-2), a high power density (7 mW·cm-2), and a capacitance retaining of 87% after 5000 cycles.With electrodeposition method, Tour et al.reported that manganese dioxide (MnO2), ferric oxyhydroxide (FeOOH)or polyaniline (PANI), was deposited onto the LSG forming LSG-MnO2, LSG-FeOOH, and LSG-PANI composites for SCs76.Both the symmetric SCs of LSG-MnO2and LSG-PANI, and asymmetric SCs of LSG-FeOOH/LSG-MnO2were fabricated.For the LSG-MnO2SCs, the energy densities can reach 32.4 μWh·cm-2and 3.2 mWh·cm-3, which is an increase of ＞1200 and ＞ 290 times, respectively, compared with that of LSG SC.For LSG-PANI SCs, the energy densities are 8.0 μWh·cm-2and 1.1 mWh·cm-3, which are 41 and 15 times higher, respectively, than that of LSG SC.The ferrocene was also chemically bonded to the LSG by vapor deposition or wet adsorption.The graphene/ferrocene pseudo-capacitors was fabricated with an energy density of 6.19 Wh·kg-1while maintaining a power density of 26.0 kW·kg-1100.Yang et al., synthesized the Co3O4nanoparticles/graphene composites by irradiating the mixed solution of porous Co3O4nanorods and GO with laser101.The Co3O4/graphene supercapacitor reaches a high specific capacitance of 978.1 F·g-1(135.8 mAh·g-1) at the current densities of 1 A·g-1.It also shows a more than 93.7% capacitance retention at the current density up to 10 A·g-1with 20,000 cycles.In another report, the bisterpyridyl based molecular cobalt complexes (TPy-Co)inks were inkjet-printed onto the surface of LSG as pseudocapacitive additives102.The specific capacitance of the fabricated pseudo-supercapacitor was increased by 75 times without sacrificing the charging and discharging rates.The oxygen-deficient TiO2/LSG (ODTiO2/LSG) was prepared with a laser irradiation on mixed solution of TiO2nanocrystals and GO103.The ODTiO2/LSG supercapacitor delivered a maximum energy density of 14.1 Wh·kg-1and a maximum power density of 8.5 kW·kg-1.Moreover, polyaniline nanofibers were electrodeposited on LSG for extended surface area of the electrode, pseudocapacitance and prevention of the graphene restacking104.The composite electrode presents a specific capacitance of 442 F·g-1and a capacitance retention of 84% over 2000 cycles.The polymerized poly (3,4-ethylenedioxythiophene) (PEDOT) was spin-coated on the surface of LSG for SC application, with enhanced pathways for charge transport and the conductivity of the component films105.The fabricated supercapacitors displayed reversible capacities of 115.2, 97.0, and 78.4 F·g-1at rates of 0.5, 2, and 6 A·g-1,respectively.

Fig.7 | Schematic and structural illustration of LSG/Ni-CAT MOF.(a) An in-plane interdigital LSG pattern.(b) Solvothermal growth of Ni-CAT MOF nanorods.(c) Structure of LSG/Ni-CAT MOF.(d) SEM images of LSG and LSG/Ni-CAT MOFs, respectively.(e) Cyclic voltammetry comparison of bare LSG and LSG/Ni-CAT MOF.(f) Galvanostatic charge/discharge curves of bare LSG and LSG/Ni-CAT MOF.Figure reproduced with permission from ref.35, John Wiley and Sons.

LSG based hybrid supercapacitor

To maximize the benefits of existing supercapacitors(high power density and stable cycle performance) and lithium-ion batteries (high energy density), hybrid supercapacitors were proposed by Naoi in 2009 by using an asymmetric electrode106,107.Asymmetric supercapacitors have been extensively explored by combining Faradic electrodes and capacitive electrodes to enhance energy density of high-power SCs108.Liu et al., reported a facile fabrication of an in-plane hybrid supercapacitor with the Fe3O4nanoparticle-anchored LSG (LSG/Fe3O4) as the anode and LSG as the cathode77.Figure 8(a) demonstrates the preparation of LSG/Fe3O4with laser irradiation on FeCl3crystal-coated PI film and subsequent laser annealing.Figure 8(b-c) shows the 3D porous LSG was well wrapped by the Fe3O4nanoparticles.Figure 8(d-e) demonstrates that the LSG/Fe3O4exhibits an enhanced performance compared to that of LSG, due to the Faradaic redox reaction of the electrochemically active Fe3O4nanoparticles.This hybrid supercapacitor demonstrates an ultrahigh areal capacitance of 719.28 mF·cm-2and an areal energy density of 60.2 μWh·cm-2.The superior performance is attributed to both the reversible H+ion (de)intercalation reaction with Fe3O4nanoparticles, significantly increasing the energy density and the unique 3D structures in LIG/Fe3O4, leading to superhydrophilic and capillary effects.Lee et al., designed novel compositions of LSG cathodes and AlPO4-carbon hybrid coated H2Ti12O25(H-HTO) anodes for coin-type hybrid supercapacitors109.The LSG/H-HTO combination exhibits a superior electrochemical activity with energy densities of 17.7-70.8 Wh·kg-1, power densities of 195.1-5191.9 W·kg-1, a cycling stability of 98% after 10000 cycles and a rate capability of 78% at a high current density of 3.0 A·g-1.A facile solution mixing and a subsequent laser reduction method were reported to fabricate LSG/LiNi1/3Mn1/3Co1/3O2(LSG/NMC) composite for high energy cathode materials in hybrid supercapacitors110.The LSG/NMC composites demosntrate a high capacitance of 141.5 F·g-1and an excellent capacitance retention of 98.1% after 1000 cycles.Beside asymmetric electrodes, MoS2decorated LSG (MoS2/LSG) was fabricated for hybrid supercapacitors by direct laser writing on PI coated with MoS2flakes111.The graphitization of PI and the MoS2decoration of the obtained LSG were achieved by one-step laser treatment.The porous LSG facilites the effective transportation of electrolyte ions and electrons throughout the electrode network, resulting in doublelayer capacitances.The decorated MoS2contributes to the pseudocapacitance, originating from faradaic charge-transfer mechanism.This combination of pseudo and double-layer capacitances enables thecomprehensive MoS2/LSG SCs with excellent electrochemical performances.Moreover, Liu et al.employed lithium containing polymer gel electrolyte for LSG supercapacitors fabricated via a direct semiconductor laser writing on PI sheets78.The LSG supercapacitors exhibit high areal specific capacitance up to 34.7 mF·cm-2, while that with the acid gel electrolyte is 8 mF·cm-2.This substantial enhancement is considered to be due to the combination of Faradaic intercalation and non-Faradaic absorption of the Li-ions at the LSG electrodes.

Fig.8 | (a) Schematic illustration of the deposition process of laser-oxidized Fe3O4 nanoparticles anchored on porous laser scribed graphene by direct laser writing technique.(b–c) SEM images of (b) 3D porous LSG and (c) 3D LSG/Fe3O4 nanoparticle composite.(d) CV curves of LSG/Fe3O4-X at 5 mV·s-1 (e) GCD profiles of LSG/Fe3O4-X at 1 mA·cm-2.Figure reproduced with permission from ref.77, Elsevier.

Featured LSG supercapacitor

Flexible LSG supercapacitor

With the increasing development of wearable devices,flexible energy storage units are highly demanded.Gu et al., reported large-scale flexible LSG supercapacitors with dimension 100 cm2fabricated on textiles in 3 minutes as shown in Fig.9(a)112.The fabrics were paint-coated with the GO/Matte binder solution to form thin films with thicknesses of 3 μm, which were then treated with a CO2laser.The fabricated SCs demonstrate an excellent water stability, an areal capacitance of 49 mF·cm-2, an energy density of 6.73 mWh·cm-2and a power density of 2.5 mW·cm-2.The LSG SCs show stable CV results for a maximum of 200% stretchability and a high capacitance retention of 88% under 500 cycles of 200% stretching condition, as shown in Fig.9(b-c).Xie et al., presented LSG SCs on the flexible substrate of Poly(ethylene terephthalate) (PET, 6 μm-thick)79.With electrostatic spray deposition method, the uniform GO film was deposited on a PET covered with Ni film (500 nm).By adjusting the laser power, the reduction and patterning of LSG electrode arrays can be fabricated in just one batch.These ultrathin (18 μm) SCs show a negligible change in the CV curves under 0-180° bending, suggesting their excellent flexibility.These flexible LSG SCs demonstrate high performances of outstanding scan rate (1000 mV·s-1), excellent cycle stability (20,000 cycles), and high volumetric energy density (0.98 mWh·cm-3in PVA/LiCl aqueous gel 5.7 mWh·cm-3in ionic liquid).To provide good mechanical properties for the stretchable devices,Lamberti et al., transferred porous LSG to elastomeric polydimethylsiloxane (PDMS) by vacuum infiltration and thermal curing113.The LSG/PMDS electrodes were then assembled and sealed into the sandwiched supercapacitors, which show good retention of energy storage performances under bending and stretching conditions.The CVs of the fabricated LSG/PDMS device remains almost unchanged even at the high scan rate of 10 V·s-1during the stretching of 0-50% and bending process of 0°-160°.The devices also show an excellent cyclingstability, retaining 84% of their initial capacitance after 1000 cycles in stretching condition and almost 90% in the bending condition.Furthermore, a PDMS/PI powder composite was directly treated by a CO2laser, resulting in the graphitization of polyimide for the application of flexible strain gauge and supercapacitor114.The laserwritten PDMS/PI substrates are sufficiently electrically conductive and mechanically stable for flexible electronics.A flexible melamine foam was employed as the skeleton to attach the GO sheets115.After laser irradiation,the sandwiched LSG-GO-LSG foam supercapacitor shows a high capacitance performance which can be easily regulated by adjusting the compressive state of electrodes.The supercapacitors show a volumetric energy density of 0.04 mWh·cm-3and 1 mWh·cm-3under 0%and 90% strain, respectively.

Fig.9 | (a) Schematic of the fabrication steps for the LSG supercapacitor on textile.(b) CV measurements on LSG supercapacitors without encapsulation under different stretchable conditions for a scan rate of 5 V·s-1.(c) Capacitance retention under the maximum stretchable condition of 200% along the uniaxial direction for a scan rate of 5 V·s-1.Figure reproduced with permission from ref.112, under a Creative Commons Attribution 4.0 International License.

Miniaturized LSG supercapacitor

The large dimension of LSG significantly limits the density of graphene electrodes of the supercapacitor, decreases the effective surface area and thus severely deteriorates the energy densities of supercapacitors116.Consequently, the fabrication of LSG with a high spatial resolution is a promising approach to enhance energy densities of supercapacitors.Due to the diffraction limit and the heat diffusion generated during the laser reduction,the linewidth of LSG electrodes in a supercapacitor is strongly related to the laser direct writing system117.The fs laser fabrication is mainly considered as a non-thermal fabrication process, which involves the multiphoton absorption within the time scales of less than a picosecond18.By avoiding the undesired heat diffusion,this fabrication method can lead to high-resolution structures.Kumar et al., fabricated graphene-based inplane micro-supercapacitor (MSCs) devices with 100 μm width electrodes and a spacing of 400 μm between electrodes by 355 nm ns laser direct writing onto free-standing GO films (thickness 2.2 μm)118.The fabricated MSCs with liquid electrolyte have a capacitance of 288.7 mF·cm-3and a good cycling stability.Similarly, In et al.,fabricated flexible LSG MSCs with the electrode width of 120 μm and the gap distance of 90 μm with 522 nm fs laser direct irradiation on PI sheets.The areal capacitance of the fabricated SCs reaches 800 μF·cm-2at a voltage scan rate of 10 mV·s-1119.Tour et al.used visible 405 nm laser to directly convert PI into LSG with a spatial resolution of ～12 μm and a thickness of ＜ 5 μm, as shown in Fig.10(a)117.Furthermore, Shen et al., produced ultraminiature MSC electrodes with 100 μm long,8 μm wide and 2 μm spacing, based on fs laser reduced graphene oxide80.With fs laser-induced forward transfer,an electrolyte microdroplet was accurately transferred on top of each electrode, avoiding any interference of the electrolyte with other electronic components, as shown in Fig.10(b).Figure 10(c-d) shows the image of LSG electrodes for SCs.The CV curves of MSCs with electrodes width of 2 μm and 550 μm are compared in Fig.10(e-f), indicating that the miniaturization of MSC electrodes enhances the specific capacitance density.TheseMSCs exhibit a high specific capacitance (6.3 mF·cm-2and 105 F·cm-3) and ～100% retention after 1000 cycles.In addition, multi-layer 3D stacked MSCs based on laser carbonization of polyimide (PI) sheets were achieved with 3D femtosecond laser direct writing techniques120.For the fabrication of 3-layer-stacked SCs, fs laser is first focused at the 80 μm, then 60 μm and 0 μm below the surface of PI sheets.The total thicknesses of 2-layer and 3-layer MSC are around 115 μm and 140 μm, respectively.The 2-layer and 3-layer stacked MSCs show an improved areal capacitance as high as 37.2 mF·cm-2and 42.6 mF·cm-2at a current density of 0.1 mA·cm-2, respectively.With two-photon direct fs laser writing technology, Gu et al., developed high-performance 3D LSG MSCs with the fractal electrode distance down to 1 μm,as shown in Fig.10(g)81.The charge transfer capability is enhanced by order of 102and the fabricated SCs show an energy storage density of 10-1Wh·cm-3and a volumetric capacitance of 86 mF·cm-3with stretchability of 150%.This performance enhancement is attributed to the growth of defects within the increase of layers.Yuan et al., reported a spatially shaped femtosecond laser (SSFL)method for patterning of LSG/MnO2SCs121.Different from previous laser direct writing method, the initial Gaussian beam were modulated to various beam shapes using phase modulations, which can pattern the LSG MSCs with designed shapes.It greatly enhances the fabrication efficiency and over 30,000 LSG MSCs can be produced within 10 minutes.The fabricated MSCs were dozens of microns with gaps of 500 nm and exhibit a high energy density (0.23 Wh·cm-3) and outstanding specific capacitance (128 mF·cm-2and 426.7 F·cm-3).

Fig.10 | (a) Schematic of experimental setup using a 405 nm laser in SEM.(b) Fabrication of integrated micro-supercapacitors on a GO film using fs laser processing.(c–d) LSG electrode arrays maintain high resolution with a spacing of ～2 μm.(e–f) CV profiles of fs MSC with the interelectrode spacing of (e) 2 μm and (f) 550 μm.(g) Two-photon-induced 3D graphene micro-supercapacitor using a fs laser.Figure reproduced with permission from: (a) ref.117 and (b–f) ref.80, American Chemical Society; (g) ref.81, under a Creative Commons Attribution 4.0 International License.

Integrated LSG supercapacitor

With the recent rapid growth of portable and multifunctional electronic devices, the studies of integrated energy devices have attracted enormous attention.Gu et al.integrated the in-plane supercapacitors with commercial c-Si solar cells by using a CO2laser to scribe the GO film on the reverse side of solar cells, as shown in Fig.11(a)122.Under light illumination, electron-hole pairs are generated in the silicon solar cell and eventually collected bythe LSG energy storage unit.The integrated devices lead to the miniaturization in size without sacrificing the performance.Similarly, Liu et al.combined a silicon solar cell with a LSG sandwiched supercapacitor for energy harvesting and energy storage individually123.Besides the energy unit, Watanabe et al.developed a self-powered UV light detection system with LSG SCs as energy storage units, shown in Fig.11(b-d)124.The LSG SC and a ZnO nanoparticles-based photodetector were prepared by a one-step laser direct writing process and were integrated with commercial solar panels.It was demonstrated that the SC can be easily charged within 1 minute by the solar panel and remain above 0.55 V after 2 h, which is sufficient to drive the UV photodetector.Based on the laser direct writing on a PI film, the same research group also combined micro-circuits for wireless charging with LSG SC for energy storage, Fig.11(e-f).This integrated device can be wirelessly charged by a commercial wireless charger57.Kim et al., reported a novel thermally chargeable supercapacitor that can convert thermal energy to electricity and then store charge simultaneously,Fig.11(g-h)125.These devices were fabricated with laser irradiation on GO films intercalated by sulfate ions.With a temperature gradient of 10.5 K, a thermally charged voltage of 58 mV can be generated.This supercapacitor can perform as long as a temperature gradient exist.Therefore, any heat dissipating objects including the human body and power-consuming devices can be utilized as power source for charging.This laser direct writing for graphene shows a facile and versatile process, can be compatible with various devices and indicates great potential for integrated multifunctional units.

Fig.11 | (a) Schematic of the integrated energy storage with silicon solar cells.(b) Schematic illustration of the self-powered photodetection system including a commercial solar panel, a SC, and a ZnO-based photodetector.(c) Self-discharge curve of the SC after being charged by the solar panel for 1 minute.(d) Photocurrent curves of the photodetector driven by the SC.(e) Schematic illustration for the fabrication of a wireless charging and storage integrated device.(f) Potential change of the integrated SC charged by the wireless circuit placed on a commercial wireless charger.(g) Serially connected thermally chargeable SC modules whose ends are colored in black and silvery-white to create temperature differences under solar radiation.(h) The steady-state voltage of 8 thermally chargeable SC modules as a function of ΔT.Figure reproduced with permission from: (a) ref.122, AIP Publishing; (b–d) ref.124, Elsevier; (e, f) ref.57, American Chemical Society; (g, h) ref.125, Elsevier.

Conclusions and outlook

As concluded in this review, great progress has been made in the research field of LSG SCs.LSG can be fabricated from various precursors including GO, polymer and biomass with different laser systems.And its modifications were achieved by adjusting the laser parameters,fabrication processing and environment.The laser direct writing technology can simply induce graphene and simultaneously pattern the graphene electrodes for supercapacitors.Based on LSG, the fabrication of EDLCs,pseudo-capacitors and hybrid supercapacitors and their performances were discussed.Numerous studies have devoted to developing LSG SCs with enhanced performances by element doping, intercalation and pattern optimization.Diverse supercapacitors with advanced features such as being flexible, high power density, miniature, high voltage and new pattern design were also illustrated.The LSG SCs hold great potential for energy storage in the future.

However, there is still room for further improvements.Due to the increasing demand for the portable devices,the miniaturization of energy units is highly expected126.Currently, the resolution of laser direct writing, which is typically several micrometres or even millimetres has severally restricted the size of LSG SCs.Meanwhile, the number of LSG electrodes on devices was decreased due to the low resolution, which hampers the energy density of LSG SCs.It was highly expected to further improve the resolution of LSG for fabricating the nano-supercapacitors and enhancing their performances.For the fabrication of asymmetric SCs, the laser direct writing process is always followed with electrodeposition process to deposit the pseudo-capacitive materials on one of the electrodes of the in-plane supercapacitors.However, this process may not be suitable for the fabrication of asymmetric MSCs considering the size.Other leading methods or technologies, such as printing or lithography,should be explored and combined with laser direct writing method.

Moreover, a broad range of precursors should be explored for LSG, considering not only the properties of LSG but also their impacts on environment and potential for massive production.Various carbon sources,which were traditionally considered as waste, could be reused as precursors of LSG fabrication.The development of potential precursors can effectively utilize resource, reduce pollution and promote the massive production of LSG.In addition, the integration of LSG SCs with energy-generating and energy-consuming components should be further developed.Various energy devices, including thermal or mechanical energy generator, can be investigated as future energy suppliers of LSG SCs.The energy-consuming devices including diverse LSG sensors and LSG transistors can also be integrated.Importantly, these integrated all-LSG devices entirely engraved by laser will facilitate scalable manufacture and the industrialization of LSG SCs.The integration of supercapacitors, solar cells and radiative coolers can also be very attractive.Radiative cooling is a promising cooling method without external energy consumption.The radiative coolers can simultaneously possess a high solar reflection up to 97% and strong infrared emission, cooling the object below ambient temperature127,128.The reflectedsolar can be redirected to solar cell for higher absorption efficiency.The decreased temperature can improve the efficiency of the integrated devices since the rising operation temperature deteriorates the performance and reliability of solar cells129,130.

Furthermore, the laser technology should be combined with other advanced technologies.As a significant technique, artificial intelligence (AI) is emerging as an effective approach to solving complicated problems in various fields and is becoming more and more important nowadays131,132.AI system can adapt its parameters and generate desired outputs from given inputs.It has been applied to emulate the human cognitions, including autonomous decision, deduction, adaption and interpretation133.In the research of LSG SCs, AI can be very favorable in several key areas, including pattern designs,fabrications and applications.The AI-driven inverse design has demonstrated great potential in the demanded design of structures and devices134,135.Giving certain conditions and constraints, a sequence of patterns can be inversed designed by AI for modelling of the LSG SCs and prediction of their performances.Through the“training” phase, the unseen internal nonlinear relationships between the layout of electrodes and the performances of the corresponding LSG SCs can be statistically acquired.To meet the requirements of various applications, the LSG SCs can be custom-designed to achieve different energy densities, power densities and reliabilities.Avoiding conventional regulatory and constrains, AI offers tremendous potential for the novel patterning designs of LSG SCs.Meanwhile, the self-learning ability of AI gives it great advantage of biomimetic design.The bioinspired structures can largely enhance the variation of the electrode layout.With the assistance of the AI, diverse biomimetic designs for LSG SCs can be further optimized to improve the accessible active area of the electrodes and reduce the electrolyte ionic path.AI can also be utilized for accurate fabrication of LSG SCs.In the LSG production, laser spot sizes, laser powers and laser scan speeds strongly affect the reduction degree and the morphology of LSG, which largely determine the performance of fabricated LSG SCs.A variety of algorithms can be developed, combining AI with domain knowledge in laser parameters and properties of the resulting LSG.AI facilitates the production of LSG with targeted properties.Ultra-narrow LSG electrodes with accurately controlled gaps can be realized to reduce the size of devices and increase the density of electrodes.With the assistance of AI, 3D LSG SCs with complex configurations will also be precisely manufactured.AI can play a significant and decisive role to improve the fabrication capability of LSG SCs and achieve their desired performances.For the applications of LSG SCs, the implement of AI is very promising for developing intelligent energy management systems.With the properties of LSG SCs,energy production units and energy consuming components in the system, AI can forecast the power demanding, tune the output of individual device and balance the power supplies within the energy system.Especially for the highly integrated SCs systems, this overall predication and evaluation of the device performances can effectively enhance the efficiencies and reliabilities of the whole energy systems.