Recent advances in designing and fabrication of planar micro-supercapacitors for on-chip energy storage

Energy Storage Materials 1 (2015) 82–102

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Energy Storage Materials journal homepage: www.elsevier.com/locate/ensm

Recent advances in designing and fabrication of planar micro-supercapacitors for on-chip energy storage Haibo Hu n, Zhibin Pei, Changhui Ye n Anhui Key Laboratory of Nanomaterials and Technology, and Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

art ic l e i nf o

a b s t r a c t

Article history: Received 21 July 2015 Received in revised form 27 August 2015 Accepted 27 August 2015 Available online 21 September 2015

Continuous development and miniaturization of electronic devices greatly stimulate the research for miniaturized energy storage devices. Supercapacitor, also called electrochemical capacitor or ultracapacitor, as one of the most promising emerging energy storage devices, is of great interest owing to its high power density, fast charge and discharge rates, and long cycle-lives. However, conventional supercapacitors are too large to be integrated with miniaturized electronic devices. Therefore, much research has been devoted to designing and fabricating high-performance miniaturized supercapacitors, also called micro-supercapacitors with typical device feature sizes in the range of centimeter or even millimeter that can be placed directly on a chip and integrated with other microelectronic devices for replacing or complementing thin film batteries or microbatteries for powering them. This paper briefly discusses main factors affecting the performance of microsupercapacitors and mainly focuses on the architectural consideration of a micro-supercapacitor. Latest advances in the designing and fabrication of planar micro-supercapacitors for on-chip energy storage and related electrode materials are highlighted. Moreover, prospects and challenges in this field are discussed that are critical for further development of high-performance micro-supercapacitors. & 2015 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Fundamentals of MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.1. Main components that affect the performance of MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.2. Architectural consideration for MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 2.3. Key parameters for evaluating the performance of MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3. Fabrication methods and electrochemical performance of advanced planar MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.1. Conventional photolithography method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 3.2. Screen printing method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 3.3. Ink-jet printing method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 3.4. Selective wetting-induced micro-patterning fabrication method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 3.5. Microfluidic etching assisted patterning fabrication method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 3.6. Laser-irradiation assisted patterning fabrication method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 4. Challenges and perspectives for MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.1. Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 4.2. Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

n

Corresponding authors. Tel.: þ 86 551 65595629; fax: þ 86 551 65591434. E-mail addresses: [email protected] (H. Hu), [email protected] (C. Ye).

http://dx.doi.org/10.1016/j.ensm.2015.08.005 2405-8297/& 2015 Elsevier B.V. All rights reserved.

H. Hu et al. / Energy Storage Materials 1 (2015) 82–102

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1. Introduction With the rapid advancement of nanotechnology, there is a development trend towards miniaturized autonomous electronic devices, such as active radio frequency identification tags, remote and mobile environmental sensors, microrobots and implantable medical devices [1–3], which further stimulated the research for rechargeable micropower sources composed of energy conversion devices, such as piezoelectric nanogenerators [4], solar cells [5], and thermoelectric cells [6] and small-scale energy storage units. Currently, mature and commercially available small-scale energy storage devices are mainly thin film batteries or microbatteries that provide required power and energy for miniaturized portable electronic devices for a long duration of time without maintenance [7–10]. Batteries are the best in terms of energy densities in smallscale energy storage devices that can reach tens to hundreds of watt hours per kilogram. Although they are costly, the market of batteries still shows rapid expansion. However, there are still some limitations in the performance of batteries, which greatly hinders their application for some situations [11]. The first drawback of batteries is their limited lifetime, usually only hundreds or thousands of cycles [12]. This issue is highlighted when they are applied under some extreme conditions such as in remote and mobile environmental sensors that require micropower sources to work for a long time. A low power density is another limitation associated with batteries, which hinders their application for situations that require high power over a short time. Although this problem is often addressed by utilizing parallel and series combinations of batteries, both of these methods will cause an increase in the volume of the micropower sources, which will lead to a deviation from the purpose of miniaturization of micropower sources for portable electronic devices. Therefore, there have been tremendous efforts being devoted to explore other micropower sources with long lifetime and high power density to replace or complement batteries. Supercapacitors (SCs), also called electrochemical capacitors or ultracapacitors, are attracting more and more attention as an important class of energy storage devices [13–25]. According to the chargestorage mechanism as well as active materials used, SCs are generally divided into three types: (i) electrochemical double layer capacitors (EDLCs) storing the charge by the surface charge separation at the electrode/electrolyte interface, (ii) pseudo-capacitors or redox SCs using fast and reversible surface faradic redox reactions for charge storage, and (iii) hybrid capacitors consisting of both EDLCs and pseudo-capacitors in a single device [26–29]. Because neither of these two surface charge-storage mechanisms involve diffusion of ions within the inner bulk region of electrode active materials, resulting in very fast charge-discharge rates, SCs usually possess a higher power density which is an order of magnitude larger (10,000 W kg  1) than that of batteries (e.g., lithium-ion batteries). In spite of the relatively lower energy density compared with batteries, SCs can still deliver an energy density that is two orders of magnitude higher (10 W h kg  1) than that of conventional capacitors. So they offer a balanced energy density and power density that bridge the gap between batteries and conventional capacitors as shown in Fig. 1 [30]. Furthermore, unlike batteries, due to their highly reversible charge storage process, SCs have longer cycle-lives that can often achieve up to millions of charge–discharge cycles without energy storage capacity loss. With these excellent performances, SCs hold much promise as an efficient energy storage device to replace or complement batteries for miniaturized portable electronic devices. However, conventional SCs are too large to be integrated with miniaturized portable electronic devices. In addition, the performance of conventional SCs such as energy density is still not satisfactory. Therefore, design and fabrication of high-performance miniaturized SCs, also called micro-supercapacitors (MSCs) with

Fig. 1. Ragone plot for different energy storage devices. Reproduced with permission of the Nature Publishing Group from Ref. 30.

typical device feature sizes in the range of centimeter or even millimeter that can be placed directly on a chip and integrated with other microelectronic devices, have attracted more and more attention in recent years for replacing or complementing thin film batteries or microbatteries for powering microelectronics [31–35].

2. Fundamentals of MSCs 2.1. Main components that affect the performance of MSCs Generally, a SC is composed of electrodes, an electrolyte, current collectors, and in many cases, a separator. Intrinsic properties of materials used for each component and how these components are designed, matched, and assembled determine the final performance of the device [36]. That is to say, the improvement of the properties of isolated components is not necessarily sufficient to optimize properties of SCs. For example, EDLCs store the electrical charge based on charge separation at the electrode–electrolyte interface, which is a physical process without involving faradaic reactions on the electrode surface [37]. The double layer capacitance C can be described as the following equation [38]: C¼

εr ε0A d

or C=A ¼

εr ε0 d

ð1Þ

where εr represents the electrolyte dielectric constant, ε0 represents the dielectric constant of the vacuum, d represents the effective thickness of the double layer (charge separation distance) and A represents the electrode surface area. As described in the equation, the capacitance of EDLCs is strongly dependent on the specific surface area of electrode materials and the electrolyte used. Therefore, electronically conducting electrode materials with high specific surface area are beneficial for fabricating EDLCs with high capacitance [16]. In this regard, carbon-based materials such as activated carbon [39,40], carbon nanotubes (CNTs) [41,42], and graphene [43,44] that are electrochemically stable and have high specific surface area, are considered as ideal electrode materials for EDLCs and have been widely used in devices. However, carbonbased materials with a high specific surface area do not ensure

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good performance of SCs. More importantly, typical pore sizes of the porous carbon-based materials should be matched with electrolyte ions [45]. In other words, these pores should be accessible to electrolyte ions. Therefore, the choice of a complemented electrolyte is also important for the improvement of the performance of SCs in addition to the increase of the specific surface area of electrode materials. This example highlights that it is important to comprehensively consider all of these components when designing a MSC, because the fabrication of MSCs with high-performance involves numerous design considerations related to properties of electrodes, electrolytes, separators, and current collectors and interfaces between them. There have already been many review papers addressing different aspects of designing advanced MSCs. Here, we will mainly discuss the architectural consideration of a MSC, which is important for MSCs. We will first briefly discuss key parameters for evaluating the performance of MSCs, and then give an overview of the latest advancements in MSCs, mainly focusing on various fabrication methods and electrode materials used for planar MSCs. Finally, we will briefly discuss future challenges and opportunities for the fabrication of MSCs. 2.2. Architectural consideration for MSCs MSCs in early stage were fabricated similar to the architecture of thin film capacitors that consist of two thin film electrodes that are stacked on top of each other with the solid electrolyte in between as shown in Fig. 2a. The electrode configuration with sandwich-like design is applicable to most of electroactive materials and costeffective for mass production. However, from the practical application viewpoint, it suffers from obvious drawbacks of the possibility of short circuit and undesired position dislocation of electrodes under various application conditions. Another limitation of the conventional stacking configuration is that it is impossible to accurately control small distances between electrodes due to sandwiched solid or geltype electrolytes, which normally results in increased ion transport resistance in cells and further leads to a big loss of power. In comparison to the conventional sandwich-like electrode configuration, the

Fig. 2. Schematic illustration of the electrode configuration of on-chip microsupercapacitors with (a) conventional 2D architecture and (b) in-plane interdigital architecture. Reproduced with permission of the Royal Society of Chemistry from Ref. 31.

in-plane interdigital design of electrodes (Fig. 2b) has more advantages in spite of the lower areal energy density due to the side-by-side design. The dominant advantage of the in-plane interdigital design is its easily available narrow interspaces between electronically isolated electrode fingers achieved by conventional microelectronic fabrication techniques, advanced printing techniques, or other advanced patterning fabrication techniques. The ion transport resistance is small because of the decreased ionic diffusion path between electrodes, which results in an ultrahigh power capability [46,47]. In addition, MSCs constructed in this configuration can effectively prevent electrode short circuit and undesired position dislocation of electrodes under various application conditions. Furthermore, because there is no usage of any organic binders and polymer separators, planar interdigitized MSCs possess excellent mechanical and electrical properties. Finally, the in-plane configuration of the electrodes could facilitate the fabrication of MSCs and the integration with other microelectronic devices mounted on a planar integrated circuit, which is beneficial for the miniaturization of the entire microelectronic system. All these merits enable planar MSCs to be prospective candidates for direct on-chip integration with miniaturized electronics on the same plane to be powered. Therefore, with the rapid development of MSCs, on-chip MSCs with in-plane interdigital design of electrodes have been dominant in this field and various fabrication methods and electrode materials have been developed to fabricate planar MSCs [31,32]. 2.3. Key parameters for evaluating the performance of MSCs Gravimetric capacitance Cm (F g  1), energy Em (W h g  1) and power densities Pm (W g  1) are key parameters for evaluating the performance of traditional SCs, which can be calculated according to the following formula [13,14,16]: Cm ¼ Q =mΔV

ð2Þ

Em ¼ 0:5C ΔV 2 =3600m

ð3Þ

Pm ¼ ΔV 2 =4 ESR  m

ð4Þ

where ΔC is the total capacitance of the device, ΔV is the operating voltage (the applied voltage minus the voltage caused by iR drop) of the device, ESR is the equivalent series resistance of the system, and m is the total mass of the two electrodes. However, as discussed by Gogotsi and Simon [48], the gravimetric performance of SCs is dependent not only on the total mass of the two electrodes, but also on the thickness and the density of electrodes as well as the weight of other components. Therefore, comparison of different SCs based on the gravimetric performance is not reliable, especially for devices such as planar MSCs, of which the weight of the electrode materials used for thin film electrodes is almost negligible. As mentioned above, for MSCs, the total footprint area of the device is only in the centimeter or even the millimeter scale. Compared to the mass of the electrode that is often a small fraction of the total mass of the device, the size of the device is the limiting factor, which is the key consideration for miniaturized power generators and energy storage units [10]. Therefore, the gravimetric capacitance, energy and power densities do not provide a suitable basis for comparison in performance metrics of MSCs. As an alternative, the areal capacitance, energy and power density are the most important performance metrics for MSCs, because the footprint area of the device is a more critical limiting factor than the weight of the electrode materials used for micrometer-thin electrodes and the energy and power delivered by a given MSC is dependent on the size of the device among other factors. Areal capacitance Cs (F cm  2), energy Es (W h cm  2) and power densities Ps (W cm  2) can be calculated according to the following formula: Cs ¼ Q =sΔV

ð5Þ

H. Hu et al. / Energy Storage Materials 1 (2015) 82–102

Es ¼ 0:5C ΔV 2 =3600s

ð6Þ

Ps ¼ ΔV 2 =4ESR  s

ð7Þ

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All the parameters (Q , C, ΔV ) are the same as above parameters for calculating the gravimetric capacitance Cm, energy Em and power densities Pm, except for s, which indicates the total area of the microelectrode array. In addition, volumetric energy and power densities are more appropriate performance metrics to compare the performance of different electrode materials and study intrinsic properties of electrode materials.

3. Fabrication methods and electrochemical performance of advanced planar MSCs So far, there have been various fabrication methods reported for planar MSCs such as conventional photolithography fabrication method [46,49–54,59–62,65–69], inkjet printing method [74,75], screen printing method [73], selective wetting-induced fabrication method [76], microfluidic etching assisted fabrication method [78], and laser-irradiation assisted fabrication method [47,83,87]. However, there has not been a fabrication method which is versatile for all active materials because different active materials have different properties. Therefore, it is important to choose the most suitable fabrication method for designed planar MSCs, of which the used active materials, the type of electrolytes, and the interface between electrodes and electrolyte need to be fully considered. 3.1. Conventional photolithography method From a historical viewpoint, the first prototype planar MSCs were originally reported by Sung et al. in 2003 [49]. In their method, gold or platinum microelectrode arrays were firstly fabricated on a silicon substrate by UV photolithography and a wet etching method. Then conducting polymers such as polypyrrole (PPy) and poly-(3-phenylthiophene) (PPT) were potentiostatically synthesized on these microelectrodes with electrochemical polymerization techniques to form conducting polymer interdigital MSCs that consist of 50 parallelconnected pairs of microelectrodes and the width of the microelectrodes and the distance between them are both 50 μm. Depending upon different electrolytes (0.1 M aqueous-based H3PO4 electrolytes and 0.5 M non-aqueous-based Et4NBF4/acetonitrile electrolytes) and different conducting polymers (PPy or PPT), the cell potential between 0.6 and 1.4 V and the cell capacitance between 1.6 and 14 mF can be obtained. Considering the possible leakage of the used liquid electrolytes in practical applications, Sung et al. subsequently made great efforts to improve the stability of MSCs [50]. They fabricated all-solidstate MSCs on SiO2/Si wafers using a new gel-polymer electrolyte. Because of the use of solid electrolyte (aqueous-based PVA/H3PO4 electrolyte and a non-aqueous-based PAN/LiCF3SO3-EC/PC), all-solidstate MSCs can be directly integrated into microdevices because they do not suffer the electrolyte-leakage problem. In addition, despite the use of solid electrolytes, the device performance of all-solid-state MSCs is comparable to that of MSCs with a liquid electrolyte. Cell capacitance can be controlled by the total synthesis charge of conducting polymers and a cell potential of 0.6 V can be obtained. Subsequently, the device was further improved by the same group, and flexible and all-solid-state MSCs were designed and fabricated by them, which could be further applied in flexible electronic devices [51]. The procedure for the preparation of flexible and all-solid-state MSCs is presented in Fig. 3. Fabrication steps of the devices were kept almost the same as that of the previous methods, except for the use of a PVA/H3PO4 gel-polymer electrolyte layer as the substrate. Because the adhesion between PPy electrodes and PVA film is stronger than

Fig. 3. Schematic illustrations of the procedure for fabricating the flexible and allsolid-state MSCs. Reproduced with permission of the Elsevier from Ref. 51.

that between PPy electrodes and Au surface, PVA electrolyte film can detach the fabricated PPy microelectrode arrays from a silicon wafer to form the flexible and all-solid-state MSC, which play the role of an electrolyte as well as a substrate for the flexible MSC cell. Significantly, the small, lightweight, and flexible device is solely composed of polymeric materials and can be arbitrarily bent and rolled up without any deterioration of the electrochemical performance. After the initial exploration of the fabrication of polymer-based planar MSCs, more and more attention has been paid to fabricate planar MSCs based on various nanostructured carbon materials, such as activated carbon, CNTs, carbide-derived carbon (CDC), onion-like carbon (OLC), and graphene because of the competitive and attractive potential for obtaining MSCs with high-frequency response and rate capability. As a typical example, in 2009, Jiang et al. reported a planar MSC based on vertically aligned porous CNT forests with the design of interdigital electrodes [52]. In their method, conventional lithography was firstly utilized to pattern two comb-like interdigital conductive electrodes consisting of three layers: Mo base layer (50 nm), Al interlayer (10 nm), and Fe outer layer (5 nm). Then thermal chemical vapor deposition (CVD) technique was used to grow CNTs on the Fe outer layer serving as the common catalyst, forming aligned 3D porous CNT forest electrodes. The obtained planar MSCs acquired a

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specific capacitance of 428 mF cm  2 using 1-Butyl-3-methylimidazolium tetrafluoro-borate ([BMIM] [BF4]) ionic liquid as the electrolyte, which is about 1000 times higher than that of bare metal electrodes without CNT forests. Furthermore, the device exhibited an excellent charging/discharging efficiency of 92% and a robust cycling stability. Later, Lin et al. fabricated a planar MSC based on graphene and CNT electrodes [53]. In their method, interdigital-finger electrodes of Cr/Ni were firstly fabricated on a Si/SiO2 substrate with the conventional photolithography, in which nickel acted as both a current collector for the MSC and a catalyst to grow few-layer graphene (FLG) by CVD method. After that, Fe/Al2O3 catalyst particles were patterned on the

FLG. In the final step, dense CNT carpets were grown on the FLG by CVD again. The whole fabrication procedure is illustrated in Fig. 4a. By combining graphene layer with CNTs, the MSC exhibited significantly improved electrochemical performance, which showed excellent frequency response with an impedance phase angle as large as 81.5° at a frequency of 120 Hz, comparable to commercial aluminum electrolytic capacitors (AECs) for alternating current (ac) line filtering applications (Fig. 4d). In addition, the device acquired areal capacitances of 2.16 mF cm  2 in aqueous electrolytes and 3.93 mF cm  2 in ionic liquids, and could be charged and discharged at ultrahigh scan rate of 500 V s  1 (Fig. 4e).

Fig. 4. (a) Schematic illustrations of the procedure for fabricating MSCs. (b) Schematic of the structure of a MSC. Inset: enlarged scheme of a Ni-graphene-CNTs pillar structure. (c) Cross-sectioned SEM image of CNT carpet electrodes. (d) Impedance phase angle versus frequency of the device and AECs. (e) Cyclic voltammetry (CV) curve of a device at a scan rate of 500 V s  1. Reproduced with permission of the American Chemical Society from Ref. 53.

H. Hu et al. / Energy Storage Materials 1 (2015) 82–102

In addition to CNTs, Gogotsi's group reported a planar MSC integrated onto a Si chip based on a monolithic carbide-derived carbon (CDC) film fabricated by chlorination of thin films of carbides such as SiC and TiC [54]. As shown in Fig. 5, bulk TiC ceramic plates were firstly deposited on Si wafer by reactive DC magnetron sputtering using titanium target and acetylene (C2H2) gas as the carbon source [55,56]. Then Ti is extracted from TiC as TiCl4 with chlorination procedure similar to the well-established chlorine-containing plasma etching technique in semiconductor manufacturing, forming the continuous porous CDC film with a thickness of up to 3 mm. Finally, depending on standard photolithography techniques, planar MSCs with interdigital electrodes were fabricated based on the as-obtained porous CDC film. Significantly, the volumetric capacitance of as-obtained MSCs decreased remarkably with the increase of the thickness of CDC film, which can reach nearly  180 F/cm3 in tetraethylammonium tetrafluoroborate (TEABF4) electrolyte and  160 F/cm3 in 1 M aqueous sulfuric acid (H2SO4). During the same period, Pech et al. fabricated a planar MSC based on onion-like carbon (OLC) particles with diameters of 6–7 nm that were produced by annealing nanodiamond powder at 1800 °C (Fig. 6a) [46]. In their method, an interdigital Au current collector was first patterned on a Si wafer with conventional photolithography

87

techniques. Then OLC particles were deposited from colloidal suspensions on interdigital Au current collectors by electrophoretic deposition technique to form the final on-chip MSC. As a result of the hightemperature annealing treatment, OLCs were quasi-spherical nanoparticles consisting of concentric graphitic shells [57]. In addition, OLCs could offer a fully accessible external surface for ion adsorption/desorption and do not have narrow pores as in activated carbons or nanotubes that may cause the transport of ions as the rate-controlling factor limiting the charge/discharge rate for achieving ultrahigh power capability and fast frequency response. Hence, combined with the inplane electrode configuration and the binder-free deposition technique, the obtained MSCs with electrodes of high specific surface area can achieve an ultrahigh power capability of around 300 W cm  3, comparable to an electrolytic capacitor, and fast frequency response with a characteristic relaxation time constant of 26 ms which is much lower than that of an AC-based microdevice ( 700 ms) or OLC-based macroscopic device (t 40.1 s). Since capacitance of EDLCs is strongly dependent on the specific surface area of electrode materials, an effective way to increase the energy density of carbon-based MSCs is using carbon materials with high surface area. Therefore, by virtue of the large specific surface area and high in-plane electrical conductivity derived from the unique 2D

Fig. 5. Schematic illustrations of the procedure for fabricating TiC-CDC film based MSCs integrated onto a Si chip. Reproduced with permission of the American Association for the Advancement of Science from Ref. 54.

Fig. 6. (a) Transmission electron microscopy image of a carbon onion produced at 1800 °C; (b) Schematic of the MSC (25 mm2) with interdigital Au current collectors that were deposited by evaporation on an oxidized Si substrate and patterned using a conventional photolithography/etching process. OLC particles were then deposited by electrophoretic deposition onto Au current collectors; (c) Optical image of interdigital fingers with 100 μm spacing; (d) Scanning electron microscope image of the crosssection of the carbon onion electrode. Reproduced with permission of the Nature Publishing Group from Ref. [46].

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structure [58], graphene has become a robust and attractive electrode material for planar MSCs. For instance, by combining the conventional photolithography with a selective electrophoretic buildup, Niu et al. fabricated a flexible, compact, ultrathin, and all-solid-state MSC based on ultrathin reduced graphene oxide (rGO) interdigitated microelectrodes and phosphoric acid/polyvinyl alcohol (H3PO4/PVA) gel electrolyte [59]. In the typical fabrication process (Fig. 7), the interdigital Au conductive anode was firstly fabricated on PET film by thermal evaporation and photolithography, which was used as positive electrode to attract and deposit graphene oxide GO sheets on the interdigital electrodes. Subsequently, GO sheets were chemically reduced by hydrazine monohydrate to obtain conductive rGO interdigitated microelectrodes. Finally, H3PO4/PVA gel electrolyte was spread onto interdigitated microelectrodes. After the gel electrolyte was solidified, the planar all-solid-state MSC was obtained. The shortened ion diffusion path lengths in the normal and the parallel direction of the ultrathin rGO interdigital microelectrodes resulted in more effective utilization of the electrochemical surface area of rGO layers, consequently, the planar all-solid-state MSC acquired a high area capacitance of 462 μF cm  2 and a large coulombic efficiency (nearly 98%). Through micropatterning methane plasma reduced graphene films (denoted as MPG) on both rigid and flexible substrates, Wu et al. also fabricated graphene-based in-plane interdigital MSCs [60]. In a typical fabrication procedure as shown in Fig. 8a, a thin film of GO was firstly prepared by spin-coating GO dispersion (2 mg ml  1) on a modified Si wafer which was firstly treated with oxygen plasma and then rapidly reduced by methane (CH4) plasma treatment at 700 °C over a short time (20 s), as indicated by the color change of the film from yellow to gray. Subsequently, interdigital Au current collectors were fabricated onto MPG film with the conventional photolithography, followed by oxidative etching of the exposed graphene in an O2-plasma cleaner for several minutes to form final interdigitated MPG microelectrodes on Si wafer. Thereafter, 5 ml H2SO4/PVA gel electrolyte was drop-casted onto the surface of the interdigital microelectrode and solidified overnight to obtain on-chip all-solid-state MPG-MSCs with the inplane configuration. Due to cooperative effects of the high conductivity of MPG film ( 345 S cm  1) and the in-plane geometry of microelectrodes that facilitated the electron transport and maximize the use of the accessible surface area of graphene, in-plane MPG-MSCs acquired a high area capacitance of 80.7 μF cm  2, a power density of 495 W cm  3 (higher than that of electrolytic capacitors) and an energy density of 2.5 mW h cm  3 which is comparable to lithium thin-film batteries. In addition, the device exhibited an excellent rate capability, which still delivered an area capacitance of 4.5 μF cm  2 even at an ultrafast rate of 1000 V s  1, three orders of magnitude higher than that of conventional supercapacitors. Furthermore, the device possessed a superior cycling stability with  98.3% capacitance retention after 100,000 cycles at an ultrahigh scan rate of 50 V s  1. It

is worth noting that the device showed an extremely small time constant of  0.28 ms, which allowed it to work in ultrafast charge and discharge conditions for the instantaneous delivery of ultrahigh power and energy densities. Significantly, by further miniaturization of the finger width and the interspace between adjacent fingers, the electrochemical performance of the device could be greatly enhanced [61]. Graphene-based MSCs delivered an increased areal capacitance of 116 mF cm  2, offered a power density of 1270 W cm  3 which was much higher than that of electrolytic capacitors, and an energy density of  3.6 mW h cm  3 which was comparable to that of lithium thin-film batteries. The work highlighted the critical importance of adjusting the number and widths of fingers in the fabrication of high performance MSCs. Although these carbon-based planar MSCs can deliver high power density and achieve excellent charge–discharge cycling stability, they suffer from low energy density owing to the relatively low specific capacitance of carbon-based materials. In contrast, pseudocapacitors based on transition metal oxides and conducting polymers can achieve much higher specific capacitances because charges are stored on the electrode surface via fast and reversible faradaic reactions. Therefore, many pseudocapacitive materials were widely employed to fabricate planar pseudo-MSCs for improving the areal energy density. For example, through combining the top-down micro-fabrication technology and the bottom-up in-situ chemical polymerization approach, Wang et al. fabricated a planar, flexible all-solid-state pseudo-MSC based on a pattern of polyaniline (PANI) nanowire arrays microelectrode. [62] In their method, interdigital Au current collectors were firstly fabricated using conventional photolithography techniques on a flexible PET chip. Then PANI nanowire array was insitu deposited on the surface of Au layers by using a dilute polymerization process [63,64]. After removing the photoresist with acetone, the PANI nanowire array based interdigital microelectrode was produced. H2SO4/PVA gel electrolyte was drop-cast to the surface of Au/PANI nanowire array microelectrodes served as the solid electrolyte to fabricate the planar, flexible and all-solid-state MSC (Fig. 9a). Due to the relatively high theoretical capacity of polyaniline and the application of in-plane configuration, the as-obtained device acquired a superior volumetric capacitance (c.a. 588 F cm  3) and good rate capability. When the current density increased from 0.1 to 5 mA cm  2, the device could still retain more than 90% of the original capacitance. In addition, MSCs with different line-width of finger (100, 400, and 800 μm defined as MSC 100, MSC 400, and MSC 800, respectively) were designed to investigate the relation between electrochemical performance and the line-width. They found that shortening the line-width could reduce the transport path of ions and charges to improve the rate capability. Therefore, a high capacitance and a fast charge–discharge rate performance could be achieved as this line-width continuously decreased from 800 to 100 μm.

Fig. 7. Schematic illustrations of the procedure for fabricating all-solid-state flexible ultrathin MSCs based on graphene. Reproduced with permission of Wiley from Ref. [59].

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Fig 8. (a) Schematic illustrations of the procedure for fabricating MPG-MSCs integrated onto a Si water. (b–f) CV curves of MPG-MSCs at different scan rates from 1 to 1000 V s  1. (g) Dependence of the phase angle on the frequency for MPG-MSCs, TG-MSCs (in-plane MSCs based on graphene film), and MPG-SSCs (device with sandwich configuration). The characteristic frequency f0 at the phase angle of  45° was 3579 Hz for MPG-MSCs, much higher than that for TG-MSCs (2358 Hz) and MPG-SSCs (16 Hz). Reproduced with permission of the Nature Publishing Group from Ref. 60.

More recently, based on alternating stacked micrometer-thick graphene-conducting polymer compact hybrid films, Feng's group also reported a planar, flexible all-solid-state pseudo-MSC with wellestablished lithographical microfabrication technique as shown in Fig. 10 [65]. By taking advantages of the unique alternating stacked structure, with a strong coupling effect from the 2D pseudocapacitive polyaniline-functionalized graphene (PANI-G) and capacitive exfoliated graphene (EG) nanosheets, all-solid-state MSCs using H2SO4/polyvinyl alcohol (PVA) gel as electrolyte could simultaneously deliver an ultrahigh areal capacitance of 210 mF cm  2 and a landmark volumetric capacitance of 436 F cm  3 at 10 mV s  1. In addition, the device exhibited remarkable mechanical flexibility without any performance deterioration under bending, which held great promise as micropower

sources for direct integration into various electronic circuits for future portable, wearable, and implantable microelectronics. Furthermore, apart from using PANI-G nanosheets, the proposed fabrication strategy could be readily extended to other alternating stacked 2D hybrid films, such as nanohybrid films based on polypyrrole-functionalized graphene (PPY-G) nanosheets that provided a universal protocol for the construction of hybrid films for MSCs. Significantly, by combining the conventional photolithography and the nitrogen and boron co-doped graphene (BNG) films that were prepared using a layer-by-layer (LBL) assembly of anionic graphene oxide nanosheets and cationic poly-L-Lysine (PLL) as a nitrogencontaining precursor, followed by intercalation of H3BO3 within the layers and an annealing treatment, Feng's group fabricated

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Fig. 9. (a) Schematic illustrations of the procedure for fabricating PANI nanowire array based planar, flexible and all-solid-state pseudo-MSCs. (b) The photograph of the device unit array on a PET chip. (c) Enlarged photograph of a device. SEM images of PANI nanowire array (d) tilted view and (e) top view. Reproduced with permission of Wiley from Ref. [62].

heteroatom-doped graphene-based in-plane interdigital MSCs as shown in Fig. 11 [66]. Because the co-doping with dual heteroatoms could provide additional pseudocapacitance contributions and was favorable for improving the interface wettability of the electrode with the electrolyte, resulting in a thickened electrochemical double layer, the fabricated MSCs exhibited a remarkable pseudocapacitve behavior with an ultrahigh volumetric capacitance of  488 F cm  3, and an outstanding rate capability of up to 2000 V s  1, which indicated that graphene doping with heteroatoms (e.g., N, B, S, or P) was a promising strategy to enhance the performance of supercapacitors by the introduction of pseudocapacitance. Unlike some conducting polymers including polyaniline (PANI), polypyrrole (PPy), and poly[3,4]–ethylenedioxythiophene (PEDOT) and their derivatives for MSCs, the intrinsically poor electrical/ ionic conductivity and high impedances associated with surface intercalation redox reactions of oxides greatly limit the utilization of most transition metal oxides for planar pseudo-MSCs, especially in the case of a thick electrode which was commonly employed in increasing the areal capacitance and the energy density. Therefore, ability to overcome the congenital drawback of most transition metal oxide is the key for the fabrication of high-performance pseudo-MSCs based on transition metal oxides. To address this challenge, recently, Si et al. reported a symmetrical, all-solid-state and flexible planar pseudo-MSC, which was integrated on flexible

polyethylene terephthalate (PET) substrates [67]. In their method, MnOx/Au multilayer hybrid thin films served as electrodes were firstly fabricated on a flexible PET substrate by combining the conventional photolithography with electron beam lithography. Then H2SO4/PVA gel electrolyte was drop-cast to the surface of microelectrodes. After the gel was solidified, the preparation of planar pseudo-MSCs was completed. The novel multilayered electrode structure could effectively increase the contact area between MnOx active layer and Au conductive layer, significantly improved the electrical conductivity of MnO2 and the electron transport and diffusion of electrolyte ions of hybrid electrodes, resulting in enhanced pseudocapacitive behavior of MnOx (Fig. 12). Combined with the planar electrode configuration, planar pseudoMSCs based on MnOx/Au multilayer hybrid electrodes acquired a high volumetric capacitance of 32.8 F cm  3 at a high scan rate of 1 V s  1, and a maximum energy density of 1.75 mW h cm  3 and a maximum power density of 3.44 W cm  3, both being much higher than the values obtained for other MSCs. In addition, the device showed a good long-term cycling stability, with a capacitance retention rate of 74.1% after a large cycling number of 15,000 times, and a low relaxation time constant around 5 ms. More recently, by combining photolithographic process and chemical synthesis, Alshareef's group fabricated metal hydroxide-based planar pseudo-MSCs with impressive volumetric capacitance and

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Fig. 10. (a–c) Schematic illustrations of the procedure for fabricating a planar, flexible and all-solid-state pseudo-MSCs based on alternating stacked micrometer-thick graphene-conducting polymer compact hybrid films. (d) Cross-section SEM images of 2D hybrid film. (e) SEM image with a tilt angle of 30°. Reproduced with permission of Wiley from Ref. [65].

Fig. 11. Schematic illustrations of the procedure for fabricating planar MSCs integrated onto a Si water based on LBL-assembled BNG films. Reproduced with permission of Wiley from Ref. 66.

energy density as shown in Fig. 13 [68]. In a typical procedure, the conventional photolithography technique was firstly used to fabricate interdigital current collectors over a glass or a plastic flexible substrate. Then a chemical bath deposition technique was used to deposit uniform Ni(OH)2 nanoflakes over current collectors. Next, the lift-off process was performed in acetone to remove Ni(OH)2 and metal layers present on the unexposed regions of the photoresist layer to form final pseudo-MSCs. Due to contributions from both the electrode geometry and the pseudo active material, as-obtained planar pseudo-MSCs showed high rate redox activity up to 500 V s  1, with an area

capacitance of 16 mF cm  2, a volumetric capacitance of 325 F cm  3, and a maximum energy density of 21 mW h cm  3 in 1 M KOH electrolyte. In addition, PVA/KOH gel electrolyte was drop-cast onto the surface of planar interdigitated electrodes followed by drying at room temperature to fabricate all-solid-state planar pseudo-MSCs that showed an areal cell capacitance of 2 mF cm  2 at a current density of 12 mA cm  2. Furthermore, all-solid-state planar pseudo-MSCs also showed a good cycling stability up to 80% capacitance retention after 1000 cycles.

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It is worthy to mention that recently Xie's group has developed a unique ammonia-assisted strategy to exfoliate bulk VS2 flakes into ultrathin VS2 nanosheets stacked with less than five S-V-S single layers, which represented a brand new two-dimensional material having metallic behavior aside from graphene [69]. Based on the highly conductive VS2 thin films, they further fabricated a VS2 nanosheet-based in-plane MSC with a mechanical shaping process modified from the previous method adopted to fabricate graphenebased in-plane MSCs as shown in Fig. 14a. As was expected, the obtained planar device acquired a high area capacitance of 4760 μ F cm  2. In addition, no obvious degradation was observed even after 1000 charge–discharge cycles, while keeping a coulomb efficiency higher than 90%, revealing the excellent cycling behavior of the device, featuring a new class of in-plane pseudo-MSCs with high performance based on quasi-two-dimensional materials (Fig. 14f).

Although these reported advanced MSCs show excellent performance, they all share the common limitation that the used conventional microfabrication methods, involving lithographic techniques or using masks for making micropatterns on substrates, are cumbersome for fabricating low-cost MSCs for widespread applications, which spurs researchers to seek for other simple, low-cost and high throughput methods to fabricate planar MSCs with high performance. 3.2. Screen printing method Screen printing is a well established printing method for patterning on various substrates such as cloth and paper, which generally uses a woven mesh to support an ink-blocking stencil to obtain a desired image. When the ink is pressed through the

Fig. 12. (a) A photograph of the planar, flexible and all-solid-state pseudo-MSCs integrated on a PET chip. (b) Schematic of the MnOx/Au multilayered hybrid thin films deposited with electron beam evaporation. (c) CV curves of the device at low scan rates from 10 mV s  1 to 1 V s  1, and (d) at high scan rates from 5 V s  1 to 50 V s  1. (e) Volumetric capacitance of the device calculated from CV curves at scan rates from 10 to 1000 mV s  1. (f) A linear dependence of the discharge current density on the scan rate up to 7 V s  1. Reproduced with permission of the Royal Society of Chemistry from Ref. [67].

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openings of the mesh and adheres to the substrate, the pattern is obtained on the substrate after the ink dries [70]. Because the printing technique does not involve expensive equipment and complicated processes, the screen printing method is a simple and low-cost printing method, which has been used for electronic device fabrication, such as field effect emission devices [71] and transparent electrodes [72]. For example, Wang et al. reported an all-solid-state flexible MSC fabricated by using a simple method on the basis of screen printing technique [73]. As shown in Fig. 15, interdigital Ag current collectors were first printed by the screen printer on a PET substrate followed by drying and annealing in a vacuum oven at 200 °C for 1 h. Subsequently, MnO2/onion-like carbon (MnO2/OLC) composite active material served as the ink was in-situ printed on the surface of Ag current collectors to form electrodes, followed by drying at 110 °C in a vacuum oven. Finally, PVA/H3PO4 electrolyte sol was coated on the whole electrode. After the electrolyte sol was solidified, the preparation of the MSCs was finished. The obtained MSCs acquired a capacity of 7.04 mF cm  2 at a current density of 20 mA cm  2, and possessed an excellent cycling stability, with 80% retention of the specific capacity after 1000 cycles. In

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addition, the all-solid-state device exhibited remarkably high mechanical flexibility when the device was bent to a radius of 3.5 mm. Because the whole fabrication process can be done in ambient conditions without high cost metal deposition and the ink containing nanomaterials can be directly printed on flexible substrates through the openings of the mesh, the demonstrated fabrication method is versatile and suitable for a variety of materials, including materials that are incompatible with conventional fabrication methods. In addition, the demonstrated method provided a general route for the mass production of all-solid-state planar MSCs, which is compatible with the roll-to-roll process for various high performance active materials. 3.3. Ink-jet printing method Ink-jet printing technology is a kind of simple and highutilization method, which can accomplish the deposition and patterning in the same step, reducing material usage and process complexity. Because it can achieve computer printing which prints a pattern by propelling droplets of ink onto paper, plastic, or other substrates on demand, it has also been used in fabrication of

Fig. 13. (a) Schematic illustrations of the procedure for fabricating Ni(OH)2-based planar pseudo-MSCs, and photographs showing devices fabricated on glass and PEN substrates, respectively. (b–d) CV curves of devices at different scan rates in a two-electrode configuration in 1 M KOH electrolyte. The inset in (d) shows CV data collected at higher scan rate of 500 V s  1. Reproduced with permission of Wiley from Ref. 68.

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planar MSCs. For example, Pech et al. fabricated a carbon-based MSC with high surface area on Si based on the inkjet printing technology [74]. In their method, planar interdigital Au current collectors were firstly deposited on an oxidized Si substrate using a conventional photolithography/etching process. Then the printing ink composed of an activated carbon powder of high surface area (1700–1800 m2 g  1) with a PTFE polymer binder in ethylene glycol stabilized with a surfactant was deposited by inkjet on the patterned Au current collectors with the substrate heated at 140 °C to finally prepare the carbon-based planar MSC. Using 1 M Et4NBF4 propylene carbonate as the electrolyte, the as-obtained device showed an excellent capacitive behavior over a wide potential range of 2.5 V for a cell capacitance of 2.1 mF cm  2, which demonstrated the viability of such printing technique for the elaboration of integrated MSCs on Si substrate. However, this MSC showed relatively low power density and poor frequency response because of the limited ion transfer in the porous network of activated carbon and polymeric binders used in the electrodes. Recently, Ye's group also developed a novel, simple, and rapid fabrication method involving laser printing technique and in situ anodic electropolymerization for fabricating flexible, in-plane, and all-solid-state MSCs [75]. In this method, an interdigital-finger-like circuit template designed using a personal computer was firstly printed on a soft, bendable, and transparent PET film with a common office laser printer (Fig. 16a). Then a thin film of Au (80 nm) was deposited on the printed PET film by electron beam evaporation (Fig. 16b). After removal of the printed circuit template using tetrahydrofuran, symmetrical interdigital-finger-like Au electrodes (three fingers as anodes and three as cathodes, respectively) were finally produced (Fig. 16c). Subsequently, active materials (PANI nanowire networks) were in situ electrodeposited on the surface with anodic electropolymerization (AEP) to form interdigital Au/polyaniline network hybrid electrodes (Fig. 16e). Finally, a gel solution of polyvinyl alcohol (PVA) mixed with sulfuric acid (H2SO4) was drop-cast to the surface of Au/PANI network hybrid microelectrodes. After PVA/H2SO4 gel was solidified (served

as a solid state electrolyte), the preparation of the planar MSCs was finished. By this method, the fabricated MSCs acquired a maximum areal and volumetric capacitance of ca. 26.49 mF cm  2 and 67.06 F cm  3 at a current density of 0.1 mA cm  2. In addition, the device possessed remarkably high mechanical flexibility and showed a good cycling stability, with 72.7% retention of the specific capacity after 1000 cycles. Moreover, the devices can be optionally connected in series or in parallel to meet the voltage and capacity requirements for a given application. Significantly, by the combination of laser printing technology and in situ anodic electropolymerization, the method allowed for the fabrication of planar MSCs without the use of traditional time-consuming and labor-intensive lithography which is often used in the fabrication of planar MSCs with interdigital in-plane configuration. 3.4. Selective wetting-induced micro-patterning fabrication method Recently, Braun et al. reported a facile selective wetting-induced micro-patterning fabrication approach for MSCs [76]. The demonstrated method is versatile and can be applied to pattern many active materials into an interdigital arrangement, as long as electrode active materials can be dispersed in a hydrophilic solvent. Fig. 17a outlines the typical fabrication procedure of a MSC. Firstly, interdigital PDMS slabs containing interdigited finger-like micro-channels were constructed by replica molding [77]. The number, the width of the interdigital fingerlike micro-channels, and the interspace between the micro-channels could be changed to modify the performance of MSCs. Then the patterned side of the PDMS slabs was exposed to oxygen plasma, making it hydrophilic. Aqueous multi-walled carbon nanotubes (MWNTs) suspension was then carefully micropipetted into interdigital fingerlike micro-channels. After several times of drying-refilling-drying process, MWNT-patterned electrodes without electrical contact between two isolated micro-channels were prepared. Sufficient PVA/H3PO4 solution was drop-cast onto the MWNT-patterned side of PDMS. Once the mixture was solidified, PVA/H3PO4 film containing MWNT electrodes could be peeled off PDMS in one piece, serving as a solid

Fig. 14. (a) Schematic illustration of the as-fabricated in-plane MSC and (b) ion migration pathways in the in-plane electrode configuration. (C) CV curves of the device at different scanning rates from 20 to 200 mV s  1. (D) Galvanostatic cycling behavior of the device. (E) Galvanostatic charge–discharge curves of the as-fabricated in-plane MSC. (F) The long-term cycling stability investigation of the device, showing negligible degradations in the coulomb efficiency and specific capacitance. Reproduced with permission of American Chemical Society from Ref. [69].

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Fig. 15. (a) Schematic illustrations of the procedure for fabricating the planar, flexible, and all-solid-state MSC. (b) Photographs showing devices printed on PET, glass, and A4 printing paper substrates. (c) Photographs showing the flexibility of the printed device on a PET substrate. Reproduced with permission of Institute of Physics Publishing from Ref. [73].

Fig. 16. Schematic illustrations of the procedure for fabricating flexible MSCs with interdigital Au/PANI network hybrid electrodes on a chip. (a) Flexible PET film with the printed interdigital circuit template. (b) Thin layer of Au deposited on the printed interdigital circuit template. (c and d) Au interdigital-finger-like current collectors on the PET film after removal of the printed circuit template. (e) As-obtained interdigital Au/PANI network hybrid electrodes on a chip. (f) Schematic diagram of the symmetric MSC with 6-interdigital fingers. The inset in (c) shows an optical microscopy image of Au interdigital-finger-like current collectors. Reproduced with permission of the Royal Society of Chemistry from Ref. [75].

electrolyte as well as a flexible substrate. The obtained device exhibited excellent flexibility and long-term cycle stability as well as a reliable high power output, enabling it to be a promising candidate for high power on-chip energy storage applications. The fabrication method is distinctive in its ease of fabrication, avoiding harsh conditions, and realization of a binder-free electrode. However, the fabrication of PDMS interdigital PDMS slabs involves soft-lithography technique, and the described method still belongs to traditional lithography technology.

3.5. Microfluidic etching assisted patterning fabrication method Recently, Cao et al. reported a novel and universal method for fabricating planar pseudo-MSCs [78]. In the method as shown in Fig. 18, they firstly prepared nanofiber-based thin film of MnO2 on micro-Au-electrode collector deposited on a glass slide with electrospinning technique. Then MnO2 layer was transferred to H3PO4/polyvinyl alcohol gel film serving as the electrolyte as well as a

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Fig. 17. (a) Schematic illustration for the fabrication of MSCs. (b) A photograph of the flexible MSC with  100 mm of interspace between interdigital electrode fingers. (c) Top SEM image of microelectrodes and (d) cross-sectional SEM image of the electrolyte-filled MWNT layer and the overlying electrolyte layer of device with EDS elemental mapping of phosphorous shown as yellow dots. In the SEM image the overlying electrolyte layer is at the bottom. Reproduced with permission of Wiley from Ref. [76].

substrate for the MSC cell, on which a thin film of ITO (50–100 nm) was evaporated using a magnet sputtering. Finally, MnO2/ITO interdigital fingers on the solid electrolyte film were prepared with a microfluidic etching method, which is an facile soft lithography method [79] using a poly(dimethylsiloxane) (PDMS) elastomer stamp [80] by which micro- or submicro-sized patterns can be readily obtained with the etchant solution passing through micro-channels in seconds. The obtained device exhibited a high specific capacitance of 338.1 F g  1 in 1 M Na2SO4 (aq) at a current density of 0.5 mA cm  2 and a good cyclic stability. Even under conditions of repeated bending, it could still preserve most of its performance. Because the microfluidic etching can be extended to most active materials, this method is universal for fabricating MSCs, whereas it is still within the regime of traditional lithography technology. 3.6. Laser-irradiation assisted patterning fabrication method Although conventional microfabrication method is mature and compatible with fabrication methods currently employed in the semiconductor industry, it involves lithographic techniques or using masks for making micropatterns on substrates, which is cumbersome for fabricating low-cost micro-devices for commercial applications. Therefore, with the rapid development of MSCs, many efforts have been devoted to develop a simple, inexpensive, high-throughput technique which does not require masks, additional processing, or sophisticated operation while producing high-performance microdevices.

Recently, Zhang et al reported a direct femtosecond laser reduction technique, with which any desired micrometer sized graphene circuits with complex patterns can be easily fabricated on GO films [81]. The synchronously reduced patterned graphene exhibited good conductivity for electrical applications. In addition, the resistivity of the obtained graphene microcircuits could be readily adjusted in a certain range by altering the output power of the laser. Wei et al reported a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip [82]. The rGO regions were up to four orders of magnitude more conductive than pristine GO. Both of these reports demonstrated the practicability of laser reduction conversion of GO into high-conductivity RGO with various geometries with micrometer resolution. Based on the direct laser reduction technique, Ajayan's group further developed a scalable fabrication method for a new type of all-carbon, monolithic MSC by laser reduction and patterning of graphite oxide films as shown in Fig. 19 [83]. Significantly, it was found that graphite oxide containing substantial amounts of trapped water was simultaneously a good ionic conductor and an electrical insulator, which enabled it to be both an electrolyte and an electrode separator with ion transport characteristics similar to that observed for Nafion membranes [84,85]. The obtained planar MSCs delivered an areal capacitance of  0.51 mF cm  2, nearly twice that of the sandwich supercapacitor. Furthermore, these MSCs (in a circular geometry) showed good cyclic stability with a  35% drop in capacitance after 10,000 cycles (Fig. 19c). In addition, in the presence of external electrolytes such as an aqueous

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electrolyte (1.0 M Na2SO4) and an organic electrolyte (1.0 M TEABF4), the performance of the devices could be further improved (Fig. 19d). Later, Kaner's group also developed a scalable fabrication method for graphene-based MSCs by direct laser writing on graphite oxide films using a standard LightScribe DVD burner [47]. By this method, more than 100 MSCs could be produced on a single disc in 30 min or less. Fig. 20 is the schematic illustration of the procedure for fabricating MSCs on a single disc. Firstly, the DVD disc was coated with a thin film of GO and then inserted into a LightScribe DVD drive. Then, with the Laser-irradiation from the DVD drive, circuits designed on a computer were patterned onto the GO film to produce graphene pattern. A copper tape was glued along edges of the electrode to improve electrical contact, and the whole interdigital area was covered by a polyimide tape. Finally, an electrolyte was added to obtain a planar MSC. Significantly, after receiving an ionogel electrolyte (ionic liquid 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide mixed with fumed silica) overcoat, the device could be operated at a larger potential window of 2.5 V, making them functional for more applications. The obtained devices demonstrated a power density of  200 W cm  3, which was among the highest values achieved for any MSC. The Laser-irradiation assisted patterning fabrication method is a simple, low-cost high-throughput lithographic technique which does not require masks, additional processing or complex operations, therefore, manifesting it as a promising method for fabrication of MSCs. In et al demonstrated a laser-assisted dry transfer technique for assembling patterns of vertically aligned carbon nanotube arrays on a flexible polymeric substrate [86]. In this technique, a laser beam was applied to the interface of a nanotube array and a polycarbonate sheet in contact with one another. The absorbed laser energy promoted the adhesion of nanotubes to the polymer in the irradiated regions and enabled selective pattern transfer. By taking advantage of the facile, maskless laser-assisted dry transfer technique, they further demonstrated the fabrication of a highly flexible planar MSC with interdigitated finger electrodes of vertically aligned carbon nanotubes (VACNTs) in a scalable and cost effective way as shown in Fig. 21a [87]. Firstly, a thin film of vertically aligned CNTs was prepared with CVD on a Si substrate [88]. A Ni layer was sputtered on the top of VACNTs to improve the in-plane electrical conductivity [89]. Then the laserassisted dry transfer process was used to pattern Ni-sputtered VACNTs (Ni-VACNTs) and transfer them to a flexible polycarbonate (PC) substrate. In the last step, an ionic liquid gel (ionogel) was applied onto

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patterned electrodes to serve as a solid state electrolyte. The asobtained MSC exhibited a specific capacitance of 430 μF cm  2 for a scan rate of 0.1 V s  1 and achieved rectangular cyclic voltammograms at high scan rates of up to 100 V s  1. The work demonstrated a novel fabrication technique that was different from conventional photolithography and printing technique, which showed promise for application in integrated energy storage for all-solid-state flexible microdevices.

4. Challenges and perspectives for MSCs We have summarized the recent development of on-chip planar interdigital MSCs that are expected to play an important role as standalone power sources or complementary microbatteries for applications in miniaturized electronics such as wireless microsensors, implantable medical devices, nanorobotics, and active radio frequency identification (RFID) tags. Significant advances have been made in developing the electrode configuration, electrode materials, and the electrolyte, and these are main components affecting the performance of a MSC device. Compared to conventional sandwich electrode configuration, the in-plane interdigital electrode configuration has shown promising advantages such as facilitating fabrication and integration of the MSCs, allowing the fast movement of ions in the same plane for enhanced power density and effectively preventing electrode short circuit and undesired position dislocation of electrodes under various application conditions, and becomes the preferable and widely used electrode configuration for on-chip MSCs. Therefore, various electrode fabrication methods such as screen printing method, ink-jet printing method, selective wetting-induced patterning fabrication method, microfluidic etching assisted patterning fabrication method, and laser-irradiation assisted patterning fabrication method, have been developed for fabricating planar interdigital electrodes (Table 1). As a result, by further combination of various electrode materials, multiple planar MSCs with outstanding electrochemical performance such as a large scan rate, fast frequency response, long-term cycling stability, and ultrahigh power and energy densities have been demonstrated, being superior to the classical sandwich-type supercapacitors. Despite many advances, there are still several challenges in the research and development of planar MSCs for on-chip energy storage at the current stage.

Fig. 18. (a) Schematic illustration of electrospinning set-up for producing aligned nanofibers deposited onto the micro-gold-electrode collector. (b) Schematic illustration of fabrication of the planar MSCs through the microfluidic etching. Reproduced with permission of the Royal Society of Chemistry from Ref. [78].

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Fig. 19. (a) Schematic illustration of the fabrication of rGO–GO–rGO MSCs with in-plane and sandwich geometries based on CO2 laser-patterning of free-standing hydrated GO films, and the photographs of patterned films. (b) Comparison of CV behavior of in-plane and sandwich devices at a scan rate of 40 mV s-1. (c) Comparison of long cyclability of as-prepared in-plane and sandwich devices. (d) Comparison of areal capacitance of an as-prepared sandwich device with excess deionized water, 1.0 M Na2SO4, and 1.0 M TEABF4 electrolytes. Inset: volumetric energy density versus power density data of the corresponding devices. Reproduced with permission of the Nature Publishing Group from Ref. [83].

4.1. Challenges The biggest challenge in the design and fabrication of planar MSCs is to miniaturize the MSCs as small as possible to be integrated with functional devices on a chip, and in the meanwhile, to improve the areal energy and power density as high as possible to meet the requirements of microelectronic devices. Although there are many factors that influence power and energy densities of MSCs, such as the configuration, electrode materials, and the type of electrolytes, the energy and the power delivered by a given on-chip MSC is mainly dependent on the size of the device. Hence, the reduction of the size of MSCs will inevitably lead to the decrease of the output energy and power. In addition, the enhancement of the output power is often accompanied by the deterioration of the output energy of the energy storage devices. Therefore, the challenge of miniaturization associated with the limited areal power density and energy density of MSCs

greatly hinders the functioning of systems that rely on these power sources. With the rapid development of portable/wearable electronics, another challenge which we are facing in the design and fabrication of MSCs is to make MSCs flexible enough to be integrated with flexible devices [90,91], which is essential for the further development and wider deployment of self-powered devices. Flexible on-chip MSCs generally consist of flexible electrodes, all-solidstate electrolyte, and a flexible packaging material, which is similar to that of conventional sandwich flexible MSCs except for a separator between electrodes. Although some flexible on-chip MSCs have been fabricated, it is still a major challenge to prepare flexible electrodes with robust mechanical properties and excellent electrochemical performance. Finally, developing simple, fast and low-cost electrode manufacturing technologies that can realize large-scale production of planar interdigital electrodes on arbitrary substrates is still a key

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99

Table 1 Comparison of the advantages and disadvantages of the reported fabrication methods for planar MSCs. Reported fabrication methods for planar MSCs

Fabrication efficiency

Cost

Complexity Manufacturing precision

Conventional photolithography

Low

High High

High

Yes

Screen printing Inkjet printing Selective wetting-induced fabrication method Microfluidic etching assisted fabrication Laser-irradiation assisted fabrication method

High High Low

Low Low Low Low High High

Low Medium Medium

Yes Yes No

[46,49–54,59– 62,65–69] [73] [74,75] [76]

Low

High High

Medium

No

[78]

High

Low

Medium

Yes

[47,83,87]

Low

Whether to meet the requirements of Ref. large-scale production

Fig. 20. (a–c) Schematic illustration for the fabrication process of laser scribed graphene (LSG) planar MSCs. (b–c) Photograph showing more than 100 MSCs fabricated on a flexible substrate in a single run. Reproduced with permission of the Nature Publishing Group from Ref. [47].

issue to obtain high-performance planar MSCs for further industrial applications. Although considerable progresses have been made in the electrode fabrication methods for planar MSCs, there are still some limitations in these existing methods. For example, conventional photolithography is an excellent fabricating method in controlling the inter-space between interdigital electrodes, being effective in improving the performance of MSCs because of the decreased ion transport resistance in cells as the inter-space between interdigital electrodes reduces. However, the lithography technique normally involves a complicated lithography process, toxic chemical treatments, and harsh fabrication conditions, thus resulting in high production cost, complicated production process, and a certain degree of environmental pollution. Screen printing technology is a kind of fast and simple electrode fabrication method which does not involve expensive equipment and complicated processes. However, as it is difficult to prepare stable precursor ink, the development of screen printing technology in MSC fabrication is slow and only a handful of active materials have been screen printed. In addition, the screen printing method needs the preparation of rigid template before each printing, of which the manufacturing precision can only be achieved at millimeter level in general, leading to a poor control in the printing precision

compared to conventional photolithography. Therefore, the limitations of these methods spur us to seek for other simple, lowcost and high-throughput methods to fabricate MSCs with high performance. 4.2. Perspectives Further improving the areal energy and power density of planar MSCs is a long-term challenging issue. Pseudocapacitive nanomaterials have shown promising advantages of higher specific capacitances depending on fast and reversible faradaic reactions on the electrode surface, and have been widely employed to fabricate planar pseudoMSCs, which achieved significantly increased areal energy density [92,93]. However, the intrinsically poor electrical/ionic conductivity of most pseudocapacitive nanomaterials and the high impedances associated with surface intercalation redox reactions of oxides greatly limit the utilization of pseudocapacitive nanomaterials for planar MSCs. Therefore, finding a way to overcome shortcomings of pseudocapacitive nanomaterials would be the next essential step for planar MSCs with further improved areal energy and power density. Apart from the selection of electrode materials, further optimization of the device architectures is also a performance growth point for MSCs. In

100

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Fig. 21. (a) Schematic illustration of planar MSCs fabrication (side-view) based on laser-assisted VACNTs transfer technique. (b) Side-view SEM image of transferred VACNTs on a PC substrate. (c) Side-view SEM image showing the entanglement of nanotubes. (d) Top-view SEM image of sputtered Ni-VACNTs before transfer. (e) Top-view SEM image of a transferred line. (f) Top-view photograph of interdigital lines. (g) Illustration of planar MSCs. Reproduced with permission of Institute of Physics Publishing from Ref. [87].

this regard, fabrication of hybrid MSCs composed of a carbon materials-based capacitor-like electrode (power source) and a pseudocapacitive materials-based battery-like electrode (energy source) with a capacitor-like electrode (power source) in the same cell may be a promising protocol. The hybrid electrode configuration can increase the cell voltage, further contributing to improvement in energy and power densities without sacrificing their power density. In addition, fabrication of 3D electrode architecture is also a promising method for improving the energy density of on-chip MSCs, which can effectively increase the energy density of the device in a limited footprint and decrease the transport path of electrolyte ions in the cell. To fabricate flexible electrodes, metal foils are usually deposited on flexible substrate as conductive substrates for the deposition of capacitive materials. However, these metal foil electrodes combined with capacitive materials are less flexible and break easily after several bending. Furthermore, some metal foils are easily corroded in aqueous electrolytes, which restricts the lifetime of the devices. Therefore, more research efforts should be focused on the design and fabrication of flexible electrodes using nonmetal materials. Considerable progress has been made in flexible planar MSCs by applying 1D and 2D carbon materials as electrode materials such as carbon nanotubes and graphene with excellent electrochemical and mechanical performance. These carbon materials provide an ideal platform for the fabrication of flexible electrodes that are vital for the construction of flexible planar MSCs and pave a new way for the construction of next-generation flexible MSCs with improved electrochemical and mechanical performance. The solid-state electrolyte is also a central element of flexible MSCs. Compared to liquid electrolytes, solid-state

electrolytes have more advantages such as easy operation, greater reliability, and wider range of operation temperature. In addition, the use of solid-state electrolyte can avoid a leakage problem, which further reduces the manufacturing cost of the device. The most widely used solid-state electrolytes in on-chip MSCs are gel polymers. However, the low ionic conductivity (10  8 to 10  7 S cm  1) and the presence of active functional groups such as OH in them greatly hinder the increase of the energy and power density of MSCs due to the limited cell voltage (o 2.5 V). Therefore, further development of solid-state electrolytes with high ionic conductivity, good mechanical strength, excellent stability, and a wide potential window is highly desired to fabricate planar flexible on-chip MSCs with high-performance. Ink-jet printing technology is a kind of simple and high-utilization method, which can accomplish the deposition and the patterning in the same step, reducing material usage and process complexity. Because it can achieve computer printing which prints a pattern by propelling droplets of ink onto paper, plastic, or other substrates on demand, it has good control in the printing precision. In addition, compared to traditional fabrication approaches for fabricating in-plane interdigital electrodes, this method does not involve a complicated lithography process, toxic chemical treatments, and expensive rigid template, which provides a simple route for fabrication of planar MSCs with high-practicality and high-performance. Therefore, it is a promising method that would be applicable to large-scale production of planar MSCs with high-performance for further industrial applications. Of course, as mentioned above, the performance of a MSC device is dependent not only on the intrinsic properties of

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materials used for each component, but also on how these components are designed, matched, fabricated, and combined to form a device. Therefore, further improvement in the performance of MSCs could be achieved through comprehensive and reasonable optimization of active electrode materials, the microelectrode configuration, the interfacial integrity of the main components (the electrode, the electrolyte, and the substrate), and the selection of suitable electrolytes and manufacturing techniques. We believe that with the rapid advancement of nanotechnology and progress in this field, the remaining challenges will be finally addressed by researchers working on the field of MSCs, and the advanced onchip MSCs will address the urgent need for micro-scale energy storage.

Acknowledgments The authors acknowledge the support from the National Basic Research Program of China (973 Program, Grant no. 2011CB302103), National Natural Science Foundation of China (Grant no. 11274308 and 21401202), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

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