Progress and prospects of graphene for in-plane micro-supercapacitors

LI Hu-cheng, SHEN Hao-rui, SHI Ying, WEN Lei,*, LI Feng,3,*

（1. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;2. School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China;3. Department of Biochemistry and Molecular Biology, College of Life Science, China Medical University, Shenyang 110122, China）

Abstract： Micro-supercapacitors hold great promise for powering the Internet of Things devices owing to their high power density and long cycling life. However, the limited energy density hinders their practical use. Electrode materials play an important role in the performance of micro-supercapacitors. With the advantages of a large specific surface area and a high electrical conductivity,graphene has been considered a good candidate for the electrode material of micro-supercapacitors. The two-dimensional surface of graphene is parallel to the direction of transport of the electrolyte ions for micro-supercapacitors with an in-plane structure, which helps improve the ion accessibility of the electrodes. Therefore, the construction of graphene-based in-plane micro-supercapacitors has aroused great interest among researchers. Here, we summarize the recent advances in graphene and graphene-based materials for in-plane micro-supercapacitors from the perspective of electrode material design. The electrode materials include graphenes produced by chemical vapor deposition, liquid-phase exfoliation, reduction of graphene oxide, laser induction and heteroatom doping, as well as graphene-based composites, such as carbon nanotube/graphene, transition metal oxide/graphene, conducting polymer/graphene and two-dimensional material/graphene composites. Challenges and opportunities in graphene-based in-plane micro-supercapacitors are discussed, and future research directions and development trends are proposed.

Key words: Graphene；Micro-supercapacitor；Electrode material；Energy storage

1 Introduction

As a revolutionary technology, the Internet of Things (IoT) aims to build an ecosystem of interconnected devices to improve our daily lives in a variety of applications, such as industrial, environmental and health monitoring, radio frequency identification(RFID), wearable electronics and drug delivery[1-3].Next-generation IoT devices place high demands on miniature power supplies in terms of charge/discharge rate and energy density, in addition to mobility and small size[1,3,4]. As a promising candidate for this kind of miniature power supplies, Li-ion micro-batteries deliver high energy density (20-200 mWh cm-3), but suffer from the lack of power density(0.01-1 W cm-3) and lifetime (＜1 000 cycles)[5,6]. Micro-supercapacitors, with high power density (10-1 000 W cm-3) and long lifetime (＞100 000 cycles),are expected to complement or even replace microbatteries to power IoT devices[5,7]. Micro-supercapacitors also have the potential to replace electrolytic capacitors for alternating current line filtering in integrated circuits, due to their fast frequency response over kHz range[8]. However, micro-supercapacitors exhibit limited energy density (0.01-10 mWh cm-3)[5],which is affected by the intrinsic properties of electrode materials. Electrode materials for micro-supercapacitors can be divided into two categories according to the charge storage mechanism. In the first category, carbon materials store charges in the electric double layer at the electrode/electrolyte interface,through the physical adsorption/desorption of electrolyte ions[9]. In the second category, pseudocapacitive materials, including transition metal oxides, nitrides and carbides, and conducting polymers, use fast and reversible redox reactions at the surface of active materials[9,10].

Graphene is composed of sp2-hybridized carbon atoms, which are tightly packed into a two-dimensional (2D) honeycomb lattice. Graphene exhibits advantages of high room temperature electron mobility(2.5×105cm2V-1s-1)[11], large specific surface area(2 630 m2g-1)[12], high areal capacitance (21 μF cm-2)[13]and gravimetric capacitance (550 F g-1)[14], making it one of the most promising electrode materials. As a 2D material, graphene is particularly favored for the construction of micro-supercapacitors with the inplane architecture. Since the electric field is parallel to graphene sheets (Fig. 1), the transport of electrolyte ions is facilitated, which is beneficial to increase the available surface area of graphene[15,16]. Therefore,graphene has attracted extensive attention for in-plane micro-supercapacitors in recent years.

In this review, we summarize the recent progress of graphene and graphene-based materials for in-plane micro-supercapacitors (Fig. 2). First, the device architecture, fabrication technology and performance metric of micro-supercapacitors are introduced. Next,graphene materials fabricated by various methods,such as chemical vapor deposition, liquid-phase exfoliation, reduction of graphene oxide (GO), laser induction and heteroatom doping, are comprehensively discussed. Composites combining graphene with other materials, such as carbon nanotubes, transition metal oxides, conducting polymers and two-dimensional materials, are also summarized. Meanwhile, the design strategies and electrochemical performance of those materials as electrodes for in-plane micro-supercapacitors are discussed. Finally, the challenges and future development directions of graphene-based inplane micro-supercapacitors are proposed.

2 Overview of micro-supercapacitors

2.1 Device architecture

Micro-supercapacitors are mainly composed of electrodes, electrolytes, current collectors and substrates. The performance of micro-supercapacitors depends on both the intrinsic properties of the materials for each component and the architecture of these components to form the device[17]. Micro-supercapacitors exhibit two types of spatial configurations. Early micro-supercapacitors adopt a stacked configuration similar to thin-film micro-batteries, in which the cathode, solid electrolyte and anode are stacked in sequence (Fig. 3a)[18]. The stacked architecture is convenient for fabrication and scale-up, but incompatible with current fabrication processes for microelectronic applications. This configuration has high area utilization because the footprint of substrates can be fully utilized. However, it typically exhibits low power performance because ions have to diffuse vertically through the entire thickness of the cell.

Currently, in-plane design is the most widely used architecture for micro-supercapacitors (Fig. 3b),in which the interdigital cathode and anode are placed on the same plane of the substrate, separated by a gap to avoid shorting the two electrodes. Compared with the stacked device, the in-plane one has several advantages[3,5-7,17,19-22]. (1) The in-plane design is compatible with fabrication methods used in the semiconductor industry, which enables facile on-chip processing and integration. (2) Arbitrary pattern design is possible. (3) This configuration exhibits better power characteristics because ions diffuse parallel to the substrate. It is also expected to achieve very narrow gaps between electrodes to reduce ion transport resistance in the electrochemical process, by the utilization of advanced microfabrication techniques with high resolution, such as photolithography and laser scribing.(4) Since the sides of the microelectrodes are exposed to the electrolyte, the ion accessibility of electrodes is improved, especially for electrodes composed of 2D layered materials, such as graphene. (5) It is easier to realize ultra-thin and flexible devices due to the planar configuration. However, the space utilization of the in-plane interdigital design is somewhat lower due to the halved active area of each electrode and the inactive gap between the two electrodes. In this review, we focus on the discussion of micro-supercapacitors with the in-plane interdigital design.

2.2 Fabrication technology

The fabrication methods of micro-supercapacitors can be divided into two categories according to the starting forms of active materials or precursors(Fig. 4). The first category of technologies is based on the powder form of active materials or precursors. The active material powder can be mixed with surfactants,stabilizers, binders, conductive additives or charging agents, and dispersed in a solvent to obtain an ink or slurry. Active materials in the ink or slurry are further deposited onto substrates or patterned current collectors to obtain a micro-supercapacitor, by techniques such as inkjet printing[23,24], spray coating[25], screen printing[21,26], mask-assisted filtration[27,28]and electrophoretic deposition[29]. Alternatively, the precursor powder can be dissolved in a solvent, and active materials, such as transition metal oxides and conducting polymers, are synthesized onto patterned current collectors by electrolytic deposition[30,31].

The second category of technologies is based on the film form of active materials or precursors. For example, the synthesis of active materials (e.g.graphene) and the patterning of microelectrodes can be accomplished simultaneously on the film of precursors, such as GO and polyimide (PI), by the utilization of laser scribing technique[32,33]. Micro-supercapacitors can also be constructed on the film of active materials by techniques, such as laser scribing[34,35],photolithography[36]and plasma etching[37]. In those processes, partial areas of the film of active materials are etched to obtain a patterned microelectrode.

Of the two categories of technologies, techniques, such as laser scribing, photolithography,plasma etching and inkjet printing, can be used to fabricate high-resolution microelectrodes[6]. Techniques,such as inkjet printing, screen printing and laser scribing, hold the advantages in terms of simplicity and scalability[6].

2.3 Performance metrics

Generally, the evaluation of supercapacitor performance is based on the mass or volume of the device to normalize capacitance (F), energy (Wh) and power (W). However, the gravimetric performance(F g-1, Wh kg-1, W kg-1) is inappropriate for micro-supercapacitors, since the mass of active materials used in the electrode is negligible compared to total mass of the device. The volumetric performance (F cm-3,mWh cm-3, W cm-3) seems much more appropriate because it allows a comparison between micro-supercapacitors with electrodes of different thicknesses[7,17].However, it can be misleading when the difference in electrode thickness is too large, since the resistance of ion and electron transport increases with electrode thickness. Furthermore, the areal performance(mF cm-2, mWh cm-2, mW cm-2) is a reasonable metric to evaluate the performance of micro-supercapacitors, since the footprint area is a key factor to be considered in micro-system applications[7,17].

When micro-supercapacitors are used as the power source for IoT devices, they must provide the required power for operation. For example, the RFID tag[38]requires powers ranging from 1 to 100 μW, and the average powers required by the wireless sensor network node IMote2[39], the wireless measurement node MICAz[39]and the remote sensor Ecobee[2]are 12 mW, 2.8 mW and 18 μW, respectively. For practical applications, the energy that the micro-supercapacitor can provide should also be considered, which determines how long it can power the device.

3 Graphene for micro-supercapacitors

3.1 Graphene materials

Typical graphene materials as the electrode for in-plane micro-supercapacitors as well as their electrochemical performance are summarized in Table 1.Graphene can be fabricated by chemical vapor deposition (CVD), liquid-phase exfoliation, reduction of GO and laser induction, et al. Among them, liquidphase exfoliated graphene (EG) is generally used in powder form for micro-supercapacitors based on the first category of fabrication technologies. Graphene by CVD, reduced graphene oxide (rGO) and laser-induced graphene (LIG), often in the film form, are used for micro-supercapacitors based on the second category of fabrication technologies.

3.1.1 Graphene by CVD

In the CVD method, graphene is grown on the surface of a metal or non-metal substrate by chemical reaction of the hydrocarbon gas such as methane in a high temperature environment. Large-area and highquality graphene films can be obtained by CVD[56,57].For instance, a monolayer graphene film was grown on copper foil by CVD process, and transferred to a target substrate with the help of a polyethylene terephthalate (PET) pressure-sensitive adhesive (PSA) film(Fig. 5a)[34]. Multilayer graphene films were obtained by repeated transfer. The laser was used to etch multilayer graphene films to obtain patterned electrodes for micro-supercapacitors. This process was expected to be used for the large-scale fabrication and integration of micro-supercapacitors, given the following advantages. (1) Graphene by CVD could be obtained in large quantities. (2) The laser scribing used to fabricate electrodes was very efficient. (3) The electrode pattern could be arbitrarily designed (Fig. 5b, c). The fast ion transport and high electronic conductivity of graphene electrodes endowed micro-supercapacitors with excellent electrochemical performance. Microsupercapacitors with the H2SO4/poly(vinyl alcohol)(PVA) gel electrolyte exhibited an operating voltage of 1.0 V, an energy density of 5.0 mWh cm-3and a power density of 1 714.0 W cm-3(Fig. 5d). Benefiting from the wide electrochemical stability window of fumed silica nanopowder/ionic liquid (FS/IL) ionogel electrolyte, micro-supercapacitors could deliver higher operating voltage (2.5 V) and energy density(23.0 mWh cm-3).

Graphene nanoribbon films can also be synthesized by CVD. Graphene nanoribbons refer to graphene strips with a width in the nanometer scale.The high-density edge structures of graphene nanoribbons facilitate ion transport and charge storage, endowing them with high specific capacitance[58,59]. Recently, Müllen et al.[41]reported the bottom-up CVD synthesis of armchair graphene nanoribbon (AGNR)films with different widths, namely n-AGNRs (n=5, 7 and 9, the number of carbon atoms used to indicate width). Micro-supercapacitors could be obtained by plasma etching on the AGNR film. The electrical properties of AGNRs played a key role in their electrochemical performance. 5-AGNRs had the highest charge carrier mobility, compared to 7-AGNRs and 9-AGNRs. Therefore, the micro-supercapacitors based on 5-AGNRs exhibited the highest electrochemical performance, with an energy density of 42.6 mWh cm-3and a power density of 2 000.0 W cm-3.

3.1.2 Graphene by liquid-phase exfoliation

EG with low oxygen content and integral structure can be obtained by liquid-phase ultrasonic exfoliation of graphite[60-62]. For instance, EG flakes with lateral dimensions of 100-500 nm were obtained by ultrasonic exfoliation of graphite in N,N-dimethylformamide (DMF)[42]. A small amount of ethyl cellulose was added to prevent the agglomeration of EG flakes, and EG ink (1 mg mL-1) was obtained by distillation, displacing DMF with terpineol. EG ink was further used to fabricate micro-supercapacitors by inkjet printing. Micro-supercapacitors with the EG electrode exhibited an areal capacitance of 0.6 mF cm-2. However, this process suffers from low yield and small size of graphene sheets. There is a need to develop new methods for the exfoliation of graphite.

Afterwards, the electrochemical exfoliation of graphite has been developed to fabricate graphene with large sheets, high carbon/oxygen ratio and high electrical conductivity[63-66]. For instance, two graphite foil electrodes were inserted into a 0.1 mol L-1(NH4)2SO4aqueous solution, and a direct current voltage of 10 V was applied between the electrodes to exfoliate the graphite[23]. The fabricated EG could be used as both the current collector and the electrode material due to its good electrical conductivity. EG ink (2.3 mg mL-1) was obtained by adding ethyl cellulose as a stabilizer to the mixed solvent of cyclohexanone and terpineol (Fig. 6a, left). And the interdigital EG electrodes could be obtained on a variety of substrates by inkjet printing (Fig. 6b). The solid electrolyte was directly printed on the interdigital EG electrodes, by the utilization of H3PO4/poly (4-styrenesulfonic acid) (PSSH) ink (Fig. 6a, right). Owing to the large EG sheets with micron-scale lateral dimensions and their uniform distribution in the electrodes (Fig. 6c), the printed micro-supercapacitors exhibited an energy density of 1.3 mWh cm-3. Largescale integration of more than 100 micro-supercapacitors could be achieved by the printing process, with an operating voltage of 12 V (Fig. 6d,e).

3.1.3 Graphene by reduction of GO

In addition to the above-mentioned CVD and liquid phase exfoliation, graphene can be produced by reduction of GO to remove oxygen-containing functional groups on GO sheets. This approach holds promise for low-cost and large-scale production of graphene[67,68]. Ajayan et al.[32]used a laser to reduce GO films and obtained patterned rGO electrodes. GO between the rGO electrodes could act as both electrolyte and separator, since the large amount of residual water made GO a good ionic conductor and electronic insulator. Due to the photothermal effect of the laser, the functional groups and water on the GO sheets were removed, and the concomitant release of the generated gas resulted in a porous structure of rGO electrodes, favorable for ion transport. Micro-supercapacitors with the rGO electrode exhibited an energy density of 0.4 mWh cm-3. This work attracted extensive attention to the fabrication of micro-supercapacitors on GO film by utilizing a simple, low-cost and scalable laser scribing technique. For instance,Kaner et al.[45]used standard LightScribe DVD laser technology to reduce and pattern GO films, and obtained interdigital rGO electrodes for micro-supercapacitors (Fig. 7a). More than 100 micro-supercapacitors could be fabricated on a single disc in 30 min(Fig. 7b,c), which showed promise for large-scale production. Based on the large specific surface area and good electrical conductivity of the rGO electrode material, the micro-supercapacitor exhibited ideal capacitive behavior, as indicated by galvanostatic charge/discharge (GCD) curves with the isosceles triangular shape (Fig. 7d), with an energy density of 2.0 mWh cm-3and a power density of 200.0 W cm-3.

In addition, the reduction of GO films can be achieved by ultraviolet irradiation[69], chemical[70]and thermal[48]methods. For instance, Li et al.[48]performed heat treatment directly on the GO film by a heating pen, to simultaneously reduce and pattern the GO film. The reduction depth almost reached the bottom of the GO film at a heating temperature of 400 °C, and resulting micro-supercapacitors with the interdigital rGO electrode exhibited an energy density of 0.7 mWh cm-3.

3.1.4 Graphene by laser induction

LIG is a three-dimensional (3D) porous graphene material obtained by laser scribing of carbon precursors[71]. This fabrication process can be performed in ambient atmosphere and enables the synthesis and patterning of 3D graphene in one step, which is attractive for industrial applications[71]. Tour et al.[33]used a commercial polymer, PI, as the carbon precursor to realize the conversion of sp3-hybridized carbon atoms to sp2-hybridized carbon atoms by the photothermal effect of CO2laser irradiation, and obtained the patterned LIG film (Fig. 8a-c). LIG films were highly porous due to the release of gaseous products during the laser scribing process. These porous structures facilitated the penetration of electrolytes into the active materials and improved their available surface area. LIG films also exhibited high electronic conductivity (25 S cm-1). The LIG exhibited unusual ultra-polycrystalline feature with the lattice containing pentagon-heptagon structures, as shown in the aberration-corrected scanning transmission electron microscope (AC-STEM) image (Fig. 8d). This feature helped to enhance the electrochemical capacitance, as indicated by theoretical calculations. Therefore, LIG films could be used as high-performance electrodes for micro-supercapacitors. The resulting micro-supercapacitors exhibited ideal capacitive behavior, as indicated by CV curves with the quasi-rectangular shape and GCD curves with the isosceles triangular shape(Fig. 8e,f), with an energy density of 0.4 mWh cm-3.

Since the work of Tour et al., much progress has been made in the regulation of the LIG structure (e.g.morphology[72], pore structure[73,74], composition[52,75],surface hydrophilicity[76]) and the conversion of other carbon precursors to LIG[50,51,77]. For instance,Alshareef et al.[50]used lignin as a carbon precursor and converted it into LIG by CO2laser irradiation.With the hierarchical pore structure, high electrical conductivity (66 S cm-1) and high specific surface area (338 m2g-1) of the LIG electrode, the resulting micro-supercapacitors exhibited an energy density of 1.0 mWh cm-3.

3.1.5 Graphene by heteroatom doping

The structure and surface properties of graphene materials can be effectively tuned by heteroatom doping to introduce pseudocapacitance and improve the electrolyte wettability for enhancing charge storage capacity[55,78]. Single or dual heteroatoms, such as B[52], N[79], O[55], P[80], S[37], F[53]and Cl[54], can be doped into graphene. For instance, S-doped graphene films with a thickness of about 10 nm were obtained by thermal annealing of trisulfur-annulated hexa-perihexabenzo-coronene films with the assistance of a thin gold protective layer (Fig. 9a)[37]. The catalytic effect and the 2D nanoconfinement effect of the gold layer facilitated the growth of high-quality S-doped graphene films. Micro-supercapacitors were further fabricated by plasma etching on S-doped graphene films. The obtained micro-supercapacitors exhibited pseudocapacitive behavior (Fig. 9b), with an energy density of 3.1 mWh cm-3.

Overall, the above graphene materials obtained by different methods have both advantages and disadvantages for micro-supercapacitors. Micro-supercapacitors based on films of graphene by CVD exhibit high power densities (Table 1)[15,34,40,41], due to the excellent electrical conductivity and ion transport properties. However, the areal loading of active materials in the electrode is limited by the film thickness ranging from a few nanometers to tens of nanometers,which results in low areal capacitance (≤0.2 mF cm-2)and areal energy density (≤0.1 μWh cm-2) of microsupercapacitors. The EG has a low content of lattice defects, favoring a high electrical conductivity. Main disadvantage of the EG by the liquid-phase ultrasonic exfoliation method is the low yield, with the use of toxic solvents such as DMF and N-methyl-2-pyrrolidone (NMP). The aqueous electrochemical exfoliation method is an alternative to obtain EG in terms of environmental friendliness and high exfoliation efficiency. Compared to the graphene produced by CVD and liquid-phase exfoliation, the rGO has more lattice defects and lower electrical conductivity,resulting in lower power performance for micro-supercapacitors. Nonetheless, the rGO offers more opportunities to enhance capacitance by tuning the chemical functionality, morphology and surface area exposed to the electrolyte[81]. The LIG has a 3D porous structure and high electronic conductivity, providing high capacitance and rate capability. However, the excessively high porosity of the LIG may degrade the volumetric performance of the electrode. The heteroatom-doped graphene can provide pseudocapacitance, but the electrical conductivity may be undermined due to the disruption of the graphene lattice[81].Therefore, graphene materials still need to be optimized to improve their electrochemical properties for micro-supercapacitors.

3.2 Graphene-based materials

When directly used as electrodes for micro-supercapacitors, graphene materials are usually difficult to achieve the satisfactory performance. On the one hand, the inevitable restacking of graphene occurs due to the π-π interaction between the sheets during electrode fabrication, resulting in a much lower specific surface area than the theoretical value. It is an effective strategy to introduce spacers such as carbon nanotubes between graphene sheets to suppress the restacking of graphene. On the other hand, graphene follows a non-Faraday electric double-layer capacitive mechanism, which limits the maximum charge storage. Pseudocapacitive materials, such as transition metal oxides, conducting polymers and other two-dimensional materials, can be added into graphene electrodes to improve the specific capacitance, due to the involvement of battery-like Faraday reactions. As a result of the synergistic effect between the graphene and other materials in the composite, the constructed in-plane micro-supercapacitors exhibit enhanced energy density (Table 2).

Table 2 Performance of typical graphene-based materials for micro-supercapacitorsa.

3.2.1 Carbon nanotube/graphene composites

Carbon nanotubes can improve the electrochemical performance of graphene electrodes from two aspects[82,83]. (1) Carbon nanotubes can act as spacers to suppress the restacking of graphene sheets, which is beneficial to increase the ion-accessible surface area of graphene. (2) Carbon nanotubes can build electron transport channels between graphene sheets, and enhance the electronic conductivity of electrodes. Recently, the wet-jet milling exfoliation process was proposed to obtain EG material by using a high-pressure jet stream to homogenize and exfoliate the graphite(Fig. 10a)[83]. EG was further dispersed in water/ethanol/terpineol solvent to obtain EG ink for screen printing of micro-supercapacitors. In order to increase the porosity of EG electrodes, SWCNTs were added as spacers for avoiding the restacking of the EG(Fig. 10b). SWCNT/EG electrodes exhibited a porous network structure, in which the dispersed SWCNTs could also act as linkers between the EG (Fig. 10c).The pyrolytic graphite paper was used as the current collector to reduce the equivalent series resistance of micro-supercapacitors. Owing to the above design,micro-supercapacitors with the SWCNT/EG electrode exhibited ideal capacitive behavior, with an energy density of 0.2 mWh cm-3. In another case, highly conductive MWCNT/rGO films were fabricated and could be simultaneously used as electrodes, metal-free current collectors and circuit interconnects for microsupercapacitors[84]. Micro-supercapacitors with the MWCNT/rGO electrode exhibited an energy density of 1.4 mWh cm-3.

3.2.2 Transition metal oxide/graphene composites

RuO2is the first reported pseudocapacitive material and is favored because of its much higher specific capacitance than that of electric double-layer capacitive materials[99-102]. Since then, other transition metal oxides, such as MnO2[103-106], V2O5[89]and Nb2O5[107], have also been reported for pseudocapacitors. However, transition metal oxides face the challenges of low electronic conductivity and poor cycling stability. It is a popular and effective solution to combine them with highly conductive nanocarbon materials, such as graphene. For instance, the 3D rGO framework with high conductivity and large specific surface area was obtained by laser scribing of GO films[86]. Subsequently, the rGO framework was coated with MnO2by electrolytic deposition, in which the loading amount of MnO2could be tuned by the current and time for MnO2deposition. The highly porous 3D rGO surface could serve as an excellent conductor for fast electron transfer, and the MnO2nanoflakes provided a large electrochemically active surface area for fast Faraday reaction. Owing to the unique electrode structure, micro-supercapacitors with the MnO2/rGO electrode exhibited an energy density of 30.0 mWh cm-3.

Besides transition metal oxides, nitrides (e.g.VN[36,108,109], TiN[110], Mo2N[111]) and carbides (e.g.V8C7[112]) can be used as pseudocapacitive materials.Recently, V8C7/rGO electrodes were designed for enhancing the electrochemical performance of micro-supercapacitors[91]. Fast synthesis and patterning of V8C7/rGO films could be achieved by laser scribing of NH4VO3/GO films (Fig. 11a). More than 20 micro-supercapacitors were obtained within half an hour(Fig. 11b). The V8C7/rGO electrode exhibited a highly porous micro/nanostructure with a large number of V8C7nanoparticles uniformly anchored on rGO nanosheets (Fig. 11c,d), which was beneficial to improve the charge carrier accessibility and active site utilization in the electrochemical process. Micro-supercapacitors with the V8C7/rGO electrode exhibited an energy density of 3.4 mWh cm-3.

3.2.3 Conducting polymer/graphene composites

As another class of pseudocapacitive materials,conducting polymers, such as polyaniline (PANI),polypyrrole (PPy), polythiophene (PTh) and their derivatives, are potential electrode materials with high specific capacitance for micro-supercapacitors[93,113-115]. However, conducting polymers have the similar issues as transition metal oxides, the poor rate performance and long-cycle stability. Those issues are expected to be well addressed by compositing conductive polymers with graphene. For instance, a general interfacial self-assembly method was proposed to construct 2D porous polymers, such as PPy, PANI and polydopamine (PDA), on graphene nanosheets[95].As shown in Fig. 12a, cylindrical micelles formed by P123 copolymer (poly(ethylene oxide)20-block-poly(phenylene oxide)70-block-poly(ethylene oxide)20) as the template, and pyrrole, aniline or dopamine as the monomer were mixed with GO in aqueous solution.Due to hydrogen bonding interactions, cylindrical micelles were closely packed on the surface of GO, and monomers were adsorbed to the poly(ethylene oxide)(PEO) domain, which confined the subsequent polymerization to form a polymer network. The polymer monolayer with cylindrical mesopores was constructed on the surface of rGO after removal of the P123 template and hydrothermal treatment. On the one hand, polymers distributed on rGO could provide high pseudocapacitance. On the other hand, cylindrical mesopores parallel to the rGO surface could provide in-plane channels that facilitated ion transport(Fig. 12b). Therefore, micro-supercapacitors with the PPy/rGO electrode exhibited a volumetric capacitance of 102.0 F cm-3(Fig. 12c), with an energy density of 2.3 mWh cm-3.

Recently, dual-mesoporous PPy arrays with pore diameters of approximately 7 nm and 18 nm were constructed on the surface of rGO by a soft and hard dual-template method, in which SiO2nanospheres served as the hard template and polystyrene-poly(ethylene oxide) (PS-PEO) served as the soft template[96]. PPy/rGO nanosheets could be used as bifunctional active materials, both as electrode materials for micro-supercapacitors and as sensing materials for NH3detection. As a result of the synergistic effect of PPy and rGO, micro-supercapacitors with the PPy/rGO electrode exhibited an energy density of 2.5 mWh cm-3, and sensors based on PPy/rGO showed enhanced NH3response.

3.2.4 Two-dimensional material/graphene

2D materials such as MXenes have large specific surface area, excellent electrical conductivity and high pseudocapacitance, and are expected to be used as high-performance electrode materials for micro-supercapacitors. MXenes, including carbides and nitrides, are 2D crystals obtained by exfoliating the MAX phase, with a general formula of Mn+1XnTx(n=1-4), where M represents the transition metal (e.g.Ti, V, Nb, Mo), X represents the C or N, and Txrepresents the surface terminal group (e.g. -F, -OH,＝O)[10]. Among these MXenes, Ti3C2Txis the most studied electrode material for micro-supercapacitors[116-119]. Similar to graphene, MXene nanosheets are easily restacked due to van der Waals interactions between adjacent nanosheets during electrode fabrication, which limits the ion transport and active site utilization in electrochemical process[120]. To overcome this issue, Gao et al.[98]designed a 3D Ti3C2Tx/rGO aerogel for self-healing micro-supercapacitors. The design of the 3D aerogel suppressed the restacking of the 2D Ti3C2Txand rGO nanosheets. In addition, the highly porous structure of the 3D aerogel facilitated ion transport and enhanced mechanical elasticity. Because of these advantages, micro-supercapacitors with the Ti3C2Tx/rGO electrode exhibited an areal capacitance of 34.6 mF cm-2, a capacitance retention of 91%after 15 000 charge/discharge cycles, and a capacitance retention of 81.7% after the fifth healing.

Another 2D material, phosphorene, obtained by liquid phase exfoliation of black phosphorus, exhibits good electronic conductivity, thermodynamic stability and high specific capacitance[27,121,122]. Wu et al.[27]composited phosphorene with EG to construct phosphorene/EG electrodes for micro-supercapacitors by mask-assisted filtration (Fig. 13a-c). With the help of the interdigital mask, EG and phosphorene nanosheets were filtered in sequence to obtain interdigital electrodes, where phosphorene and EG were alternately stacked. Due to the synergistic effect of phosphorene and EG, micro-supercapacitors with the phosphorene/EG electrode exhibited enhanced electrochemical performance, with an energy density of 11.6 mWh cm-3.

To sum up, the construction of graphene-based materials can enhance the electrochemical performance of micro-supercapacitors by the utilization of the synergistic effect of graphene and other materials.High-quality and highly conductive EG are the mostly used graphene material in the construction of carbon nanotube/graphene or 2D material/graphene composites for micro-supercapacitors. Usually, carbon nanotubes or 2D materials are directly mixed with EG,which can effectively suppress restacking of EG. To construct transition metal oxide/graphene or conducting polymer/graphene composites, GO is the mostly used one due to the easy functionalization and modification of the sheets. Transition metal oxides or conductive polymers can be grown in situ on graphene sheets in the fabrication process. As a 2D conductive framework, rGO can improve the poor electronic properties of oxides or conductive polymers, and the agglomeration of rGO is suppressed by oxides or conductive polymers on the sheets.

4 Summary and Outlook

In the past decade, many advances have been made in the research of graphene for in-plane microsupercapacitors. Benefiting from the unique 2D structure, large specific surface area and high electrical conductivity of graphene, in-plane micro-supercapacitors exhibit high power density. The energy density of micro-supercapacitors is improved by designing graphene-based materials with the synergistic effect,such as carbon nanotube/graphene, transition metal oxide/graphene, conducting polymer/graphene and 2D material/graphene composites. Nevertheless, the development of graphene-based micro-supercapacitors is still in its early stage. There is still a lot of space for research, from the design and optimization of electrode materials and architecture to the fabrication and integration of devices. The prospects of graphene for in-plane micro-supercapacitors are proposed as follows (Fig. 14), hoping to inspire the future work in this field and promote the practical applications of micro-supercapacitors.

(1) Material improvement

As mentioned earlier, the performance of microsupercapacitors is affected by the intrinsic properties of electrode materials. Graphene is an ideal electrode material for micro-supercapacitors, but its practical performance is limited by the restacking of sheets and the electric double-layer charge storage mechanism.The restacking of graphene can be suppressed by introducing spacers between sheets, which increases the available surface area and improves the ion transport in graphene electrodes. Pore structure engineering by strategies, such as activation of graphene[123]and tuning the interlayer spacing of graphene films[124], has also shown great potential in improving the electrochemical performance of graphene electrodes. The heteroatom doping of graphene can bring pseudocapacitance and improve the electrolyte wettability. It is still necessary to optimize the species, distribution and doping amount of heteroatoms, and improve the controllability by developing new doping methods. The introduction of pseudocapacitive materials into graphene electrodes can significantly improve the energy density of micro-supercapacitors, but this often leads to reduced rate performance due to poor electronic conductivity of pseudocapacitive materials such as transition metal oxides and conducting polymers.For future work, it is very important to optimize the structure and interface of the pseudocapacitive material/graphene composite to accelerate the electron transport in electrodes. Furthermore, the detailed mechanisms regarding the synergistic effect of graphene and pseudocapacitive materials need to be deeply studied to guide the design and development of next-generation graphene-based composites.

(2) Architecture design

The performance of micro-supercapacitors is not only related to the intrinsic properties of the electrode materials, but also to the architecture used in the devices. The in-plane interdigital design is an ideal architecture for micro-supercapacitors, in terms of easy device fabrication and integration, and fast ion transport, which is particularly important when 2D materials such as graphene are used as the electrode. In order to further reduce ion/electron transport resistance and improve the power density of micro-supercapacitors, it is helpful to reduce the gap between the interdigital electrodes and reduce the device size. It involves the development of high-resolution fabrication methods for micro-supercapacitors. State-of-the-art fabrication techniques such as laser scribing and lithography can achieve micrometer-scale resolution[125].The realization of submicron-scale resolution is still challenging, although a recent report claimed an electrode gap of 500 nm by focused-ion-beam process[118].

The energy a micro-supercapacitor can deliver is limited by the small footprint area of typically less than one square centimeter. A strategy for enhancing the areal energy density of micro-supercapacitors is to increase the areal loading of active materials, which inevitably leads to an increase in electrode thickness.The problem is that the electron/ion transport resistance increases significantly with the electrode thickness, resulting in reduced utilization of active material in thick electrodes. 3D electrode design is expected to enable efficient utilization of active materials in thick electrodes by building fast charge transport channels. Researches on micro-supercapacitors with the 3D electrode architecture follow the concept of 3D micro-batteries, such as building current collector arrays[3,17,126]. Some studies on 3D micro-supercapacitors have achieved initial success[106,127-129].However, challenges remain in both the fabrication of 3D architectures with high area enhancement factors(i.e. the projected area per footprint area) and the conformal deposition of active materials on the 3D scaffold[3]. More works should be devoted to the design and development of high-energy-density 3D electrodes in the future.

(3) Fabrication technology

One of the trends for micro-supercapacitors is to develop ultrafast and high-throughput fabrication techniques. In most cases, the fabrication of micro-supercapacitors involves two essential steps, including the fabrication of the film of electrode materials and subsequent patterning processes. The development of large-scale, efficient, controllable and low-cost fabrication techniques, such as continuous centrifugal coating[130], for graphene films is favored for promoting the practical use of graphene in micro-supercapacitors. In some cases, such as laser scribing of GO[45]or PI[49]films, the fabrication of electrode materials and the patterning are performed simultaneously. The simplification of the process can improve the fabrication efficiency. Recently, the spatially shaped femtosecond laser technique[87]shows promise for ultrafast fabrication of micro-supercapacitors, in which the phase modulation is used to design the beam shape to achieve the synthesis and patterning of MnO2/rGO films in a single pulse laser. As a result, more than 30 000 micro-supercapacitors are produced within 10 min.

Another challenge in realizing practical microsupercapacitors is addressing the safety issues associated with the electrolyte leakage. When liquid electrolytes are used in micro-supercapacitors, it is essential to develop the wafer-level packaging process compatible with semiconductor microfabrication techniques.Another approach is to use solid-state electrolytes, but their lower ionic conductivity than liquid electrolytes may limit the power density of micro-supercapacitors.In the future, it is necessary to develop solid-state electrolytes with high ionic conductivity and fabrication processes for solid-state micro-supercapacitors.

(4) Device integration

In order to meet the requirements of voltage, current and capacitance for practical applications, microsupercapacitors can be connected in series or in parallel. Series/parallel designs that do not require the use of external metallic connectors are favored, as in some reported cases[21,25,49]. More efforts also need to focus on the on-chip integration of micro-supercapacitors with high areal number density (i.e. the number of unit cells integrated on a chip per area, in cells cm-2)[131]to lay the foundation for their practical applications in the field of semiconductor microelectronic devices. There is also a growing trend to develop self-powered integrated micro-systems[84]by the utilization of micro-supercapacitors as energy storage units, in combination with energy harvesting units(e.g. solar cells, nanogenerators) and energy consuming units (e.g. sensors). Furthermore, it is interesting to introduce smart functions into self-powered integrated micro-systems, such as stimulus-responsive and self-healing functions. This will drive the development of intelligent electronic devices.

Acknowledgements

The authors acknowledge financial support from National Natural Science Foundation of China(51927803 and 51902316), National Key R&D Program of China (2016YFA0200102 and 2016YFB0100100) and LiaoNing Revitalization Talents Program (XLYC1908015).