Abstract
Supercapacitors (SCs) have broad applications in wearable electronics (e.g., e-skin, robots). Recently, graphene-based supercapacitors (G-SCs) have attracted extensive attention for their excellent flexibility and electrochemical performance. Laser fabrication of G-SCs exhibits obvious superiority because of the simple procedures and integration compatibility with future electronics. Here, we comprehensively summarize the state-of-the-art advancements in laser-assisted preparation of G-SCs, including working mechanisms, fabrication procedures, and unique characteristics. In the working mechanism section, electric double-layer capacitors and pseudo-capacitors are introduced. The latest advancements in this field are comprehensively summarized, including laser reduction of graphene oxides, laser treatment of graphene prepared from chemical vapor deposition, and laser-induced graphene. In addition, the unique characteristics of laser-enabled G-SCs, such as structured graphene, graphene hybrids, and heteroatom doping graphene-related electrodes, are presented. Subsequently, laser-enabled miniaturized, stretchable, and integrated G-SCs are also discussed. It is anticipated that laser fabrication of G-SCs holds great promise for developing future energy storage devices.1. INTRODUCTION
Energy storage devices play important roles in smart electronics, such as e-skin [1–4], sensors [5,6], wearable units [7–11], and other applications [12–17]. Among various energy storage devices [18–25], supercapacitors (SCs) show obvious superiority, such as high power density, fast charge/discharge rates, and high cycle stability [26–30]. Typically, the device structure of an SC contains electrolytes and electrode materials [31–36]. Generally, electrolytes and electrode materials are assembled in a plane or sandwich structure. During charging and discharging processes of SCs, physical adsorption/desorption of electrolyte ions or a reversible redox reaction occurs on the electrode–electrolyte interface [37–40]. Nowadays, in order to improve SC performance, more and more effective materials have been developed to design and fabricate high-performance electrode materials [41,42].
From the point of view of materials, graphene has high mechanical flexibility, strength, electrical conductivity, and surface area [43–47]. Because of these excellent properties, graphene plays an important role in satisfying the requirements for high-performance electrode materials. In addition, graphene-based supercapacitors (G-SCs) are able to act as flexible and deformable energy storage devices in electronic skins, wearable displays, curved smartphones, etc. [48–50]. Initially, graphene was prepared by mechanical exfoliation. This kind of graphene shows ultrahigh conductivity; however, graphene prepared by mechanical exfoliation suffers from small size and irregular shape, which is not suitable for fabricating SC electrodes. In recent years, technological advances have been made in large-area graphene film preparation, such as reduction of graphene oxide (RGO) [51,52] and chemical vapor deposition growth graphene (CVDG) [53]. Therefore, tremendous preparation methods involving chemicals, high-temperature treatment, etc., have been investigated to fabricate G-SCs. Recently, laser technologies have been adopted: laser reduction of graphene oxides (LRGOs), laser treatment of CVDGs (LCVDGs), and laser-induced graphene (LIG) [54]. Compared with the graphene prepared via mechanical exfoliation, thermal/chemical RGO and CVDG, laser-enabled graphene can be prepared in large-area films with high electrical conductivity, flexibility, and high chemical/physical stability. Importantly, compared with these fabricating methods, laser is a powerful tool to fabricate G-SCs, which avoids the need for additional toxic-reducing agents, high temperature, and inert gas protection. Also, laser technologies allow high resolution for miniaturized SC design and fabrication. Moreover, laser technologies permit programmable patterning in arbitrary shapes and are also compatible with other functional units for integration [55]. Therefore, laser fabrication of G-SCs holds great potential for effective energy storage devices.
![Progress in laser fabrication of G-SCs. The structured graphene image, adapted from Ref. [56]; the graphene hybrid image, adapted from Ref. [57]; heteroatom doping graphene image, adapted from Ref. [58]; the miniaturized SC image, adapted from Ref. [59]; the stretchable SC image, adapted from Ref. [60]; the integrated SC image, adapted from Ref. [61].](/richHtml/prj/2020/8/4/04000577/img_001.jpg)
Figure 1.Progress in laser fabrication of G-SCs. The structured graphene image, adapted from Ref. [56]; the graphene hybrid image, adapted from Ref. [57]; heteroatom doping graphene image, adapted from Ref. [58]; the miniaturized SC image, adapted from Ref. [59]; the stretchable SC image, adapted from Ref. [60]; the integrated SC image, adapted from Ref. [61].
2. LASER FABRICATION OF GRAPHENE-BASED SCs
In this section, we summarize the working mechanism of SCs, including EDLCs and pseudo-capacitors and review laser-enabled fabrication of G-SCs, including LRGO-SCs, LCVDG-SCs, and LIG-SCs.
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A. Working Mechanism of SCs
The working mechanism of laser fabrication of G-SCs is divided into two types: EDLCs and pseudo-capacitors [62–64].
![Working mechanism of SCs including (a) EDLCs and (b) pseudo-capacitors; adapted from Ref. [62].](/Images/icon/loading.gif)
Figure 2.Working mechanism of SCs including (a) EDLCs and (b) pseudo-capacitors; adapted from Ref. [62].
As shown in Fig. 2(b), the working mechanism of pseudo-capacitors is decided by a fast reversible Faradic process at the electrode–electrolyte interface. The fast reversible Faradic process also occurs near the surface of active electrode materials. Importantly, due to Faradic processes, pseudo-capacitors have higher pseudo-capacitance. Nevertheless, the existing of pseudo-capacitive materials suffers from lower power density because of the poor conductivity during redox processes, and redox reactions will reduce cycle lives during charging and discharging processes. At present, the most widely explored pseudo-capacitive materials include transition metal oxides or hydroxides, conducting polymers, and materials possessing surface functional groups. Recently, laser-enabled graphene hybrids and heteroatom doping graphene have been developed as pseudo-capacitive materials. In addition, laser-enabled graphene has porous or grating structures, which helps with intercalation and deintercalation.
B. LRGO-Based SCs
Graphene oxide (GO) is an important derivative of graphene, which bears plenty of oxygen-containing groups (OCGs) on the plane of graphene [65,66]. In general, due to existing OCGs, GO film is electric insulation and has low surface area. In order to remove the OCGs, thermal methods () and chemical methods have been widely used for producing graphene-related materials. After thermal and chemical processes, the obtained RGO usually possesses high conductivity, and the obtained devices show excellent electrochemical performance.
Figure 3.(a) Schematic illustration of LRGO, adapted from Ref. [67]; (b) schematics and photos of planar and sandwiched LRGO electrodes on GO paper; (c) CV plots of planar and sandwich SCs (scan rate, 40 mV/s); (d) impedance spectra for the in-plane and sandwich devices; (b)–(d) adapted from Ref. [87].
As a typical example, Zhang et al. successfully achieved LRGO [78]. Specifically, a femtosecond laser was used to reduce and pattern LRGOs with high resolutions (). In this manner, any desired RGO patterns were successfully fabricated, such as graphene microcircuits and spiderwebs [58]. Subsequently, various laser devices were used to reduce and pattern GOs, such as UV lasers, lasers, and infrared lasers [79–86]. As a benefit from any designable patterning, LRGOs can be used to design SC electrodes. For example, Gao et al. patterned both planar and sandwiched LRGO electrodes on GO paper [Fig. 3(b)] [87]. In this case, GO film with trapped water served as both solid electrolyte and separator because of the ionic conductivity and electrical insulation. After laser reduction, the LRGO region turned to black due to the deoxydation process. It is worth noting that the obtained in-plane SC delivers the highest specific capacitance of [Figs. 3(c) and 3(d)]. This work provides a very easy and useful device structure for LRGO-SCs. A similar design for electrolytes was reported based on LRGO/polyvinyl alcohol-GO/LRGO (LRGO/PVA-GO/LRGO) [88]. In addition, Qu et al. successfully achieved RGO–GO–RGO fiber SCs [89]. This fiber SC has wide prospects for wovening. Additionally, to satisfy the increasing demand for on-chip energy storage, Kaner and El-Kady used a LightScribe DVD burner to fabricate graphene microsupercapacitors (MSCs) to scale. Over 100 MSCs can be fabricated in 30 min or less [90]. Notably, These MSCs exhibit the highest power density of .
Laser carving is capable of being applied on RGO-based materials. Different from laser reduction and patterning, after laser carving processes, unwanted RGOs were removed [91–93]. For example, Wong et al. achieved MSCs via laser patterning and ablation [94]. First, the laser beam was used to pattern and reduce the GO films; after that, the laser beam with high power was applied to ablate the unwanted area. It is worth noting that this work overcomes the size/performance limitations for current MSCs and exhibits overwhelming volumetric energy density compared with commercial MSCs, even at a 1000 mV/s scan rate. Alternatively, due to the limited electrical conductivity of conventional LRGOs, Shi et al. laser-carved a low sheet resistance () and mechanically robust (330 MPa) RGO film [91]. After the laser carving process, the electrodes were shaped into designed in-plane structures. The devices achieved a high areal specific capacitance of . These methods hold promise for improving the performance of RGO-SCs.
C. LCVDG-Based SCs
Although GO can be reduced to recover conductivity by laser technologies, LRGO suffers from limited electrical conductivity. To produce graphene with high conductivity, CVDG is considered as an effective method for developing high-quality graphene. Typically, Cu or Ni foils under a gas mixture of and are used as a catalytic substrate for graphene growth [95–100]. In order to obtain high-quality CVDGs, the gas ratio of , growth time, and cooling rate need to be optimized. Nevertheless, CVDG needs to be transferred to target substrates for use and suffers low production.
Figure 4.(a) Scheme for the preparation of graphene-based microsupercapacitors (G-MSCs); (b) CV curves of G-MSCs on PET substrates and (c) corresponding areal and volumetric capacitances at different scan rates; adapted from Ref. [59].
D. LIG-Based SCs
Figure 5.(a) Diagram of LIG converted from PI; (b) scanning electron microscope (SEM) image of the as-prepared owl-shaped LIG pattern; scale bar, 1 mm; (c) Raman spectra and (d) XRD patterns of LIG and PI; (e) high-resolution transmission electron microscope (HRTEM) image of LIG; scale bar, 5 nm; (f) transmission electron microscope (TEM) image of selected area of LIG, scale bar, 5 Å; adapted from Ref. [101].
Particularly, based on the similar preparation mechanism, LIG process has been extended using diverse materials such as wood [103], food, cloth, and paper [104]. Recently, Lamberti et al. demonstrated an all-sulfonated poly(ether ketone) (SPEEK) flexible SC, in which SPEEK served as both separator and polymeric electrolyte [105]. The graphitization mechanism of SPEEK can be summarized as laser-induced reticulation, during which sulfonic groups degrade and oxidatively dehydrogenate. Importantly, in this work, they demonstrated that only one material was used for the whole device.
3. UNIQUE CHARACTERISTICS OF LASER-ENABLED G-SCs
In this section, the unique characteristics of laser-enabled G-SCs are summarized, including the ease of developing structured graphene and graphene hybrids, heteroatom doping of graphene-based electrode materials, and fabricating miniaturized, stretchable, integrated SCs.
A. Structured Graphene-Based Electrode Materials
Currently, to improve the surface area of graphene-based electrodes and the contact area between electrode and electrolyte, two kinds of structured graphene-based electrode materials have been well studied, including porous structures and grating structures. The porous structures are attributed to LRGO- or LIG-based electrode materials because of the gas generation during laser processing, whereas grating structures are mainly based on LRGO via the laser holography technique.
Figure 6.(a) Diagram of LSG-based electrochemical capacitors, adapted from Ref. [106]; (b) schematic illustration of fabricating LRGO films with 1D grating structures by the TBLI technique; (c) SEM image; (d) atomic force microscope (AFM) image of LRGO films with 1D grating-like structures; (b)–(d) adapted from Ref. [56].
B. Graphene Hybrid-Based Electrode Materials
Though graphene shows great superiority in developing high-performance SCs, the insufficient capacity of pristine graphene and graphene analogues-based SCs is prominent. Therefore, various studies have been carried out to fabricate graphene-based hybrid materials for performance improvement [108–110]. The advantages of graphene hybrid-based electrode materials can be summarized as follows. (1) The introduction of active materials effectively prevents the restacking of graphene layers after laser reduction processes, leading to much larger specific surface areas. (2) More active sites will also be introduced to acquire higher capacity. Graphene-based hybrid materials can be fabricated by laser-reducing GO/active materials composites into LRGO-based hybrid materials, laser-induced graphene/active hybrid materials, or depositing active materials after laser reduction of GO or LIG processing [111–114].
As for laser-reducing the GO/active materials composite into the RGO-based hybrid materials, for example, Liu et al. successfully demonstrated LRGO/carbon nanotubes (CNTs)-MSC by laser reduction and patterning GO/CNTs hybrid powders into LRGO/CNTs MSCs [111]. In this work, CNTs were used to prevent the restacking of laser-scribed graphene layers. The CNTs are also able to improve the ion-accessible surface area. What is more, the reduction in diameter of CNTs boosts the device performance. Therefore, the device based on laser-scribed GO/single-walled carbon nanotubes (GO/SWCNTs) with a diameter of 1–2 nm exhibits the best electrochemical performance. Other works have extended the investigation by fabricating [113], [114], and LRGO/Au nanoparticle [112] hybrid electrodes.
Figure 7.(a) Photo of molybdenum carbide-graphene (MCG) fabricated by DLW on paper substrate; (b) schematic illustration of the MCG fabrication process; (c) CV curves of interdigital SC; (d) CV curves of single, parallel, and series sandwich-structure devices measured at a scan rate of 100 mV/s; adapted from Ref. [57].
As for depositing active materials after laser reduction of GO or LIG processing, for example, Ghoniem et al. used a laser system to reduce and pattern GO [116]. After that, a Mn-oxide layer was electrodeposited on the resultant LRGO to form the hybrid electrodes. The hybrid electrodes show enhanced specific capacitance in the range of 172–368 F/g, which is much higher than that of pure LRGO electrodes (82–107 F/g). In addition, Tour et al. reported high-performance pseudo-capacitive MSCs from LIG, in which the laser induction process and subsequent electrodeposition of , FeOOH, and conductive polyaniline (PANI) were used to fabricate , LIG–FeOOH, or LIG–PANI electrodes [117].
C. Heteroatom Doping-Based Graphene Electrode Materials
Heteroatom doping has attracted much research interest in tailoring the characteristics of graphene for improving electrochemical properties because heteroatomic defects and functional groups will be introduced to graphene, leading to alteration of the electronic structure [118–124]. Compared with chemical reduction-, thermal annealing-heteroatom doping, laser-assisted heteroatom doping is a high-efficiency and chemical-free method that enables the formation of highly porous structures and operates at room temperature. Currently, various heteroatom doping methods have been adopted via solid, liquid, and gaseous state doping sources during laser reduction of GO and LIG processing.
Figure 8.(a) Illustration of the preparation of boron-doped LIG MSCs (B-LIG-MSCs); (b) B 1s spectrum and (c) N 1s spectrum of B-LIG; (d) CV curves of LIG-MSC and B-LIG-MSC; scan rate 0.1 V/s; (e) galvanostatic charge-discharge (GCD) curves of LIG-MSC and B-LIG-MSC; current density ; adapted from Ref. [127].
D. Miniaturized Graphene-Based SCs
As for laser-enabled miniaturized G-SCs, there are two functions of laser technologies. One of the most obvious advantages of laser-enabled G-SCs is the miniaturization of MSCs with the resolution of micrometers [128]. Because of the high resolution, femtosecond writing technique is commonly utilized for MSC fabrication. Therefore, laser can be used to shorten diffusion paths of electron/ion between two electrodes. The other advantage is the femtosecond laser-assisted controllable microdroplet transfer technology. This technology can be used to fabricate microelectrolyte droplets on the micro SCs.
Figure 9.(a) Fabrication process for miniaturized SCs by using a femtosecond laser; (b) RGO electrode arrays with a spacing of 2 μm; (c) optical microscope image of microelectrolyte droplets covering the electrode; CV plots of RGO MSC with interelectrode spacing of (d) 2 μm and (e) 550 μm; adapted from Ref. [61].
E. Stretchable Graphene-Based SCs
Currently, in order to meet the huge requirements of wearable intelligent electronic devices, stretchable G-SCs have been achieved based on laser-enabled graphene [129].
Figure 10.(a) Schematic illustration of a highly stretchable SC using LIG electrode onto elastomeric substrate; (b) device structure; CV plots of SCs under (c) stretching and (d) bending tests; scan rate 10 V/s; adapted from Ref. [60].
F. Integrated with Other Devices
Figure 11.(a) Illustration of an integrated device including SCs and sensors; (b) photograph of the integrated device; (c) charging and discharging curve of the MG-MSCs and discharging curves of the MG-PANI MSCs; (d) leakage currents of MSCs; adapted from Ref. [59].
4. SUMMARY AND OUTLOOK
There has been much research progress since laser-enabled graphene has been successfully prepared. In this review, we summarized the working mechanism of SCs, laser fabrication of G-SCs, and the unique characteristics of laser-enabled G-SCs.
As for the working mechanism of SCs, there are two kinds of mechanism, including EDLCs and pseudo-capacitors. Because of the high conductivity and surface area, laser-enabled graphene, including LRGO, LCVDG, and LIG, has been adopted to fabricate electrodes of SCs. Compared with the graphene prepared via mechanical exfoliation, laser-based graphene is able to prepare large-area films with high electrical conductivity, flexibility, and high chemical/physical stability. They work based on the EDLCs. Lasers also have the ability to develop doped graphene and graphene hybrids. The heteroatom doping-based graphene and graphene hybrid-based SCs can be attributed to the pseudo-capacitors.
In order to obtain high device performance, lasers have been adopted to fabricate structured graphene to improve contact area between graphene and electrolytes. Porous and grating structures have been developed for graphene. Porous structures can be fabricated by LRGO and LIG because of gas generation. The grating structures can be fabricated via laser interference. Lasers can also be used to fabricate graphene hybrids and heteroatom doping graphene for higher device performance. Graphene hybrids have been developed via laser-reduced GO composite or depositing active materials after LRGO of LIG processing. The active materials contain abundant active sites, which is helpful in improving electrochemical performance. As for doping graphene, the heteroatom doping can increase the hole charge density.
In addition to developing high electrochemical performance, lasers play an important role in developing miniaturized, stretchable, and integrated devices. Thanks to its high-resolution ability, a femtosecond laser can be used to develop miniaturized SCs. As for stretchable G-SCs, they can be fabricated for stretching substrates or stretching structures via LIG or a laser cutting process. In addition, it is worth noting that graphene is a versatile material for developing electronic devices, which are able to develop integrated SCs. For example, fabricated SCs arrays improve energy storage ability and shed light on possible practical uses.
Though many successes have been achieved in this field, there are still urgent problems to be solved. First, to satisfy the huge demand for electrodes, mass production of high-quality graphene is still challenging. Then, introduction of essential functional groups and surface modification during the device fabrication still require long and arduous processes. In addition, developing easy and effective device fabrication methods towards miniaturized, integrated, and multifunctional devices is still a great challenge. What is more, exploring new materials and electrolyte factors should also be taken into account. Finally, laser processing efficiency should be considered for practical applications. To address this limitation, spatial light modulators and concurrent processing technologies have been widely used to improve processing efficiency. In short, the development of laser-enabled G-SCs may stimulate rapid progress in various wearable devices for future applications.
To summarize, laser-enabled G-SCs are still a promising method for developing high-performance energy storage devices. The method combines advanced fabrication technologies with unique material properties that will stimulate the rapid progress of wearable devices.
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