Flexible on-chip micro-supercapacitors: Efficient power units for wearable electronics

Journal Pre-proof Flexible on-chip micro-supercapacitors: efficient power units for wearable electronics Rui Jia, Guozhen Shen, Fengyu Qu, Di Chen PII:

S2405-8297(20)30037-4

DOI:

https://doi.org/10.1016/j.ensm.2020.01.030

Reference:

ENSM 1080

To appear in:

Energy Storage Materials

Received Date: 30 December 2019 Revised Date:

22 January 2020

Accepted Date: 27 January 2020

Please cite this article as: R. Jia, G. Shen, F. Qu, D. Chen, Flexible on-chip micro-supercapacitors: efficient power units for wearable electronics, Energy Storage Materials, https://doi.org/10.1016/ j.ensm.2020.01.030. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Elsevier B.V. All rights reserved.

CRediT author statement Rui Jia: Writing- Original draft preparation Guozhen Shen: Writing- Reviewing and Editing, Supervision. Fengyu Qu: Writing- Reviewing and Editing Di Chen: Writing- Reviewing and Editing, Supervision.

Flexible on-chip micro-supercapacitors: efficient power units for wearable electronics

Rui Jia1, Guozhen Shen2,* Fengyu Qu,3 Di Chen1*

1

School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected] 2 State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronic Engineering, University of Chinese Academy of Sciences, Beijing 100083, China. E-mail: [email protected] 3

College of Chemical and Chemistry, Harbin Normal University, Harbin, China

Keywords: Integration

Flexible,

On-chip,

Micro-supercapacitors,

Wearable

electronics,

Graphical Abstract:

Flexible on-chip micro-supercapacitors: efficient power units for wearable electronics

Keywords:

Flexible,

On-chip,

Micro-supercapacitors,

Integration

1

Wearable

electronics,

Abstract To meet the requirements of wearable and portable electronics, flexible and light-weight on-chip micro-supercapacitors (MSCs) have attracted tremendous attention due to the good mechanical and electrochemical performances as well as easy on-chip integration with flexible functional electronic devices. In this review, the recent progresses of flexible on-chip micro-supercapacitors are summarized. Typical strategies toward flexible on-chip MSCs were first discussed, followed with the highlight of multifunctional on-chip MSCs including stretchable MSCs, self-healable MSCs, electrochromic MSCs and thermoreversible self-protection MSCs. Following these parts, wearable electronic systems integrated with flexible on-chip MSCs were discussed in detail. Finally, the challenges and future research emphasis on the flexible on-chip MSCs are put forward.

2

1. Introduction With the rapid development of flexible, multifunctional and wearable electronics, the lightweight and deformable micro energy storage devices that can be integrated in circuit have become more and more important, from the view points of both basic research

and

practical

applications.

Currently,

flexible

batteries

and

micro-supercapacitors (MSCs) are the most attractive candidates for wearable electronics and have attracted much attention [1-6]. Compared to the flexible batteries (micro-batteries or thin-film batteries), MSCs possess much longer operating lifetime over 100000 cycles, faster charging/discharging rate and higher power density. On the other hand, MSCs have smaller volume and deliver higher energy density in comparison with conventional capacitors [7]. That is to say, MSCs can bridge the gap between the batteries and conventional capacitors. Among several types of MSCs, the flexible on-chip MSCs with patterned electrodes exhibit superiorities, which not only own narrow interspace between the adjacent fingers, short ions diffusion distance and the exposed edges of the electrode in the electrolyte to improve the electrochemical performance, but also have the potential to integrate with other electron devices without external metal wires [8-10]. Consequently, flexible on-chip MSCs can be used as the most promising energy storage devices in wearable electronics. In the past decade, the flexible planar MSCs have been well studied and Fig. 1 displays a brief timeline of the development of flexible on-chip MSCs. In 2006, Sung et al. reported the polypyrrole (PPy) based flexible MSCs on solidified hydrogel substrate by photolithography and electrochemical polymerization [11]. Since then, 3

more and more research was conducted on diversified electrode preparation methods, electrode patterns and stable all-solid-state electrolytes [12-15]. Besides simple MSCs, high-voltage devices, multi-functional devices as well as self-powered integrated systems have also been explored for practical applications in the recent years [16-25].

Fig.1 A brief timeline of the development of flexible on-chip MSCs. Reproduced with permissions [11-25].

This review summarizes the recent achievements of the flexible on-chip MSCs. We try to focus on the diverse strategies towards flexible on-chip MSCs. Besides common on-chip MSCs, several types of hybrid devices including Li-ion and Zn-ion MSCs are also introduced in this work. Following these parts, the multifunctional on-chip MSCs possessing

stretchable,

self-healable,

electrochromic

and

thermoreversible

self-protection properties are also discussed. With the succeed in preparation of high performance flexible on-chip MSCs, the integrated wearable systems powered by on-chip MSCs were recently demonstrated, which are also highlighted in this paper. At the end of this review, we discuss the existing problems of this interesting area and 4

correspondingly put forward our suggestions to better realize the wearable and portable purpose in the future.

2. Strategies towards flexible on-chip MSCs Generally, the demands for wearable one-chip MSCs require low-cost, light-weight, miniaturization, compatibility, mechanical flexibility, rechargeable and stability of electrochemical performance under deformation. For example, Zhi et al. designed the interdigital Zn-MnOx/PPy based microbattery array with photoluminescent gelatin electrolyte, in which the colloidal CdTe quantum dots and borax were added [26]. This kind of battery not only delivered excellent energy storage performance, but also exhibited photoluminescent features during the charging/discharging process. Therefore, the fabricated battery array can be used to drive the ultra-violet backlight in the screen and function as color filter as well. The method of embedding battery-in-screen construction is worthwhile to be further developed in the future, which can effectively reduce the device size. In order to further improve the compatibility of the MSCs with other wearable devices, the same group developed the transferrable PPy nanowires-based MSCs by using the heat releasing tape (HRT) [27]. Through this process, the fabricated devices can be transferred to various substrates, such as textile, plastic tape, paper, china, window and leaf, etc. Simultaneously, the plug-and-play function was also realized. After the transfer process, more than 85% of the electrochemical performance was retained, indicating the little influence of the transfer process to the device. A flexible on-chip MSC is composed of several parts: the flexible substrate, the 5

current collector, the active electrode materials and the electrolyte. The flexible substrates are commonly plastics/polymers such as polyethylene terephthalate (PET), polyimide (PI), polyethylene naphthalate (PEN), paper, polyethylene (PE), polyethersulfone (PES), polydimethylsiloxane (PDMS), solidified PVA-H3PO4 electrolyte and conductive fabric tape, etc [28-37]. They not only possess excellent flexibility, good resistance to high temperature but also are very stable in acid or alkali solutions. Active electrode materials are the most important component of flexible on-chip MSCs. The most commonly used electrode materials are carbons with different forms, such as graphene, activated carbon (AC), carbon nanotubes (CNTs) and carbide derived carbons [38-44]. Devices based on carbon materials are governed by the electrochemical double layers capacitive (EDLC) behavior on the interface of electrode and electrolyte, resulting in low areal/volumetric capacitance and energy density. To improve the energy density, pseudocapacitive electrode materials that store charges by the reversible redox reactions are also developed, including transition metal oxides/carbides/nitrides/sulfides/hydroxides (MnO2, NiFe2O4, MXenes, MoS2, and Ni(OH)2, etc.) and conducting polymers (polyaniline (PANI), polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT)) [45-53]. To further boost the performance of flexible on-chip MSCs, devices with hybrid electrode materials (carbon and pseudocapacitive materials) are further developed [43]. For this type of MSCs, the carbon materials act as the conductive scaffolds for pseudocapacitive materials, while the hybrid materials combine the two kinds of charge storage 6

mechanisms to improve their electrochemical performance. Electrolyte is a crucial issue to decide the working potential of MSCs. The design strategies of the electrolyte should involve the following aspects. First, the safety is the vital problem to be considered for practical applications. Second, high ionic conductivity and potential range are closely related to electrochemical performance of the device. Third, the compatibility and flexibility of the electrolyte should also be satisfied. Finally, proper operating temperature of the electrolyte is significant to the usage of the devices. Under normal conditions, the electrolyte can be basically classified as aqueous, organic and ionic liquid systems. Aqueous electrolytes have the advantages of good safety, low-cost and high ionic conductivity (∼ 1 S/cm). Whilst, the voltage of aqueous electrolyte is limited to 1.23 V owing to the decomposition of water [54]. Compared with aqueous electrolytes, organic electrolytes can supply a much higher voltage about 2.8 V. However, the organic solvents are volatile, toxic and flammable, which may induce safety problems, especially in case of being used in wearable electronics [55, 56]. The ionic liquid-based electrolytes own the features of excellent stability, good safety, wide voltage window and suitable in high temperature condition despite their high viscosity in room temperature [57]. To make flexible on-chip MSCs, all-solid-state electrolytes based on all the above three types of electrolytes are used for the sake of avoiding leakage. Table 1 summarized the state-of-the-art developed gel or solid-state electrolytes and their corresponding operating voltage windows [21, 22, 24, 49, 57-74]. To fabricate flexible on-chip MSCs, proper microfabrication techniques should be 7

applied to precisely pattern the electrodes. The resolution of microfabrication is critical for the corresponding electrochemical performance. Till now, several fabrication

approaches

have

been

developed

and

well-studied,

such

as

photolithography, plasma etching, laser-lithography, microfluidic etching and printing. In the following part, we will provide details about these micro-manufacturing processes Table 1 Summary of gel or solid-state electrolytes and corresponding operating voltage windows. Types Aqueous-based

Organic-based

Ionic liquid-based

Electrolytes

Voltage (V)

PVA/H3PO4 PVA/LiCl PVA/KOH/KI PVA/LiClO4 PVA/H2SO4 PVA/NaCl GO-PAA CMC/Na2SO4 H2SO4-MC-g-PEO SiO2-LiTFSI ADN/SN/LiTFSI/PMMA PC-PMMA-LiClO4 PC/Et4NBF4 PEGDA/[EMIM][TFSI] P(VDF-HFP)/[EMIM][TFSI] EMImNTF2 PS-PMMA-PS/[EMIM][TFSI] P(VDF-HFP)/[EMIMBF4] LiTFSI-P14TFSI-PVDF-HFP BMIMPF6 SiO2-[EMIM] [TFSI] SiO2-[BMIM][NTf2]

0-0.8 0-1.4 0-1 0-1.4 0-1 0-1 0-0.8 0-1.5 0-0.8 0-2 0-2 0-1.6 0-1 0-2.2 0-1.5 0-3 0-1 0-3 0-3 0-3 0-2.5 0-2.5

References [58] [49] [59] [64] [65] [67] [66] [68] [24] [70] [72] [21] [60] [61] [62] [63] [69] [57] [22] [71] [74] [73]

. 2.1 Photolithography Photolithography is a well-developed technique that is widely used in currently microelectronic industry. It is a technique that can transfer the graphics on the 8

photomask to the substrates under the ultraviolet (UV) irradiation. It is a relatively mature processing technology with high resolutions. Photolithography is also widely used to fabricate flexible on-chip MSCs with well patterned electrode structures. Generally, there are two major strategies to prepare flexible on-chip MSCs through the photolithography technique. In the first strategy, the interdigital metal electrodes were directly patterned on the soft substrates as current collectors via the photolithography technique. And then the active materials were coated on the current collectors by the electrodeposition, spray-coating, drop-casting or near-field electrospinning methods, etc [29, 75-77]. For example, in 2015, Alshareef et al. fabricated the PEDOT-based flexible on-chip MSCs with outstanding frequency characteristics by combining the conventional photolithography and electrochemical deposition techniques as shown in Fig. 2a [62]. In their method, the photoresist was first spun coated on the clean PEN substrate. Then, the UV exposure and developing were performed to get the interdigital patterns, which have the width and spacing between the fingers of 100 and 50 µm, respectively. A layer of 200 nm Au/20 nm Ti film was deposited on the patterns by sputtering technique. In the following step, PEDOT was electrodeposited on the Au/Ti film as the active electrode materials. After lift-off process, the gel electrolyte was coated on the interdigital electrodes to finish the fabrication of the flexible on-chip MSCs. Due to the electrochemical activity and stability of the PEDOT, the highest frequency of the fabricated device in 1 M H2SO4 electrolyte was up to 400 Hz at the phase angle of -45°, which was 16 times higher than the conventional PEDOT supercapacitors (25 9

Hz). The areal/volumetric capacitance of the flexible on-chip MSCs in PVA/H2SO4 electrolyte was 5 mF/cm2 and 33 mF/cm3, respectively. Moreover, only 1-2% of the original capacitance value decreased under bending conditions, illustrating the good flexibility of the device. In order to extend the potential window of the flexible on-chip MSCs, the P(VDF-HFP)/[EMIM][TFSI] ion gel was then applied (Fig. 2b and 2c). The cyclic voltammetry (CV) curves were measured in the voltage of 0-1.5 V for the single cell. The parallel and series configurations can effectively increase the capacitance and working voltage for practical applications. The ion gel-based device offered the maximum energy and power densities of 7.7 mWh/cm3 and 850 mW/cm3, respectively, which are much higher than those of carbon (0.15-6 mWh/cm3) and metal oxides (MnO2, RuO2) based devices (1-5 mWh/cm3) [12, 78-86].

Fig. 2 (a) Schematic illustration for the fabrication of PEDOT on-chip MSCs. (b) CV curves of two cells in parallel and (c) in series configurations. Reproduced with permission [62]. (d) Schematic diagram of the preparation of the flexible NiFe2O4 nanofiber-based on-chip MSCs. (e) CV and (f) GCD curves of the device after bending for different times. Reproduced with permission [52].

In the second strategy, the fabrication started from the coating of active electrode material on the flexible substrates and then the pattern of metal interdigital electrodes 10

via the photolithography technique. For instance, Shen et al. reported the flexible NiFe2O4 nanofibers based on-chip MSCs as energy storage devices to power a graphene pressure sensor and Fig. 2d showed the corresponding fabrication process [52]. Electrospun NiFe2O4 nanofibers were first dispersed in ethanol and then coated on the treated clean PET substrate. A layer of Ni film with the thickness of 35 nm as current collector was then sputtered on the NiFe2O4 nanofibers. In the following step, photolithography was carried out to get the patterned electrodes. After lift-off, the flexible on-chip MSCs were obtained by coating the PVA/KOH gel electrolyte. As-fabricated device displayed a volumetric capacitance of 2.33 F/cm3 and excellent cycling

stability

with

the

capacitance

retention

of

93.6%

after

10000

charging/discharging cycles. The corresponding energy density and power density were calculated to be 0.197 mWh/cm3 and 2.07 W/cm3, respectively. In addition, the fabricated on-chip MSCs exhibited outstanding flexibility and stability at different bending states (Fig. 2e and 2f). Interestingly, the optimized on-chip MSCs can be used to drive different wearable sensors, illustrating their promising applications in future wearable electronics. Via the similar fabrication process, they also prepared flexible on-chip MSCs based on two-dimensional (2D) Ni(OH)2 nanosheets, which delivered a specific capacitance of 8.8 mF/cm3 at the scan rate of 100 mV/s and 0.2% of the initial value loss after 10000 cycles [51]. The energy density and power density were 0.59 mWh/cm3 and 1.8 W/cm3, respectively. Reduced graphene oxide (rGO)/Fe2O3 hollow nanospheres based on-chip MSCs were also prepared by similar process, which can be used to power integrated photodetectors [87]. The corresponding 11

devices showed a specific capacitance of 11.57 F/cm3 at the scan rate of 200 mV/s and 92.08% of the capacitance retention after cycling for 32000 times. The maximum energy and power densities were 1.61 mW/cm3 and 9.82 W/cm3, respectively. The works mentioned above illustrated that the photolithography technique provided a powerful method to fabricate flexible on-chip MSCs. Unfortunately, burdensome preparation steps as well as cleaning room are required for this method. As a result, other fabrication methods are required to be developed to get flexible on-chip MSCs.

2.2 Plasma etching Plasma etching is a dry-etching technique that is often used to prepare the desired patterns with the assistance of masks. In the typical plasma etching process to flexible on-chip MSCs, the active electrode material was first coated (for example, dip-coating, spin-coating or other methods) on the flexible substrates. After that, the working gas is transformed into plasma, which is directly exposed to the electron region and reacted with the active electrode material film to form the patterned electrodes. The pattern is defined via a template or controlled by software program [40, 44, 88-90]. One typical example was demonstrated by Yay et al. In the work, they employed thermal evaporation method to make an interdigital gold pattern on the vacuum filtration prepared graphene oxide (GO)/manganese dioxide (MnO2)/silver nanowire (AgNW) ternary hybrid film [88]. O2 plasma was then used to etch the exposed GO, while the materials covered by Au were protected. The residual un-protected MnO2 and AgNW were removed by the HCl solution. After the thermal reduction process, 12

the patterned rGO/MnO2/AgNW (RGMA) film was obtained and used as binder-free electrode for the flexible on-chip MSCs, which showed high energy and power densities (2.3 mWh/cm3 and 162 mW/cm3), robust cycling stability (90.3% of the original capacitance after charging/discharging for 6000 times) and good flexibility. Via a similar process, Dai et al. reported a flexible on-chip MSC based on nanoporous gold/MnO2 [89]. The prepared device with the PVA/LiCl electrolyte exhibited the specific capacitance of 11.58 mF/cm2 at the scan rate of 10 mV/s, sound flexibility and quick response time of 1.25 ms. Besides, mask-free plasma-etching strategy was recently developed to obtain the planar interdigital electrodes for flexible on-chip MSCs. For instance, Wu et al. fabricated flexible on-chip MSCs based on patterned multi-walled carbon nanotubes (MWCNTs) by atmospheric pulsed micro-plasma-jet (APMPJ) [40]. The on-chip MSCs had twelve interdigital fingers with the interspace of 300 µm, delivering a volumetric capacitance of 2.02 F/cm3 at the scan rate of 10 mV s-1 and capacitance retention of 94.1% after 6000 cycles. They also exhibited outstanding mechanical properties that 98.2% of the pristine capacitance retained after 600 cycles under bending state. By control the number of the interdigital electrodes per unit area, the energy density and the power density could be optimized. In another work, the same group fabricated a flexible on-chip MSC with the AgNWs as the current collector and MWCNTs as the active materials using the similar APMPJ method [44]. The MWCNT/AgNW composite film was made by vacuum filtration and then transferred to the PET substrate (Fig. 3a). Fig. 3b displayed the schematic illustration of the 13

micro-plasma-jet generation. Mixed He and O2 was used as the working gas. Meanwhile, the pulsed high voltage as power supply was linked with the steel needle inside. As a result, the micro-plasma-jet was sprayed out from the nozzle outlet. The photograph of the micro-plasma-jet was shown in the inset of the Fig. 3b. Fig. 3c exhibited the fabrication flowchart of the on-chip MSCs. Before micro-plasma-jet scanning, the PET substrate coated with the MWCNT/AgNW composite film was fixed on the X-Y motorized platform controlled by computer. During the etching process, a lot of joule heating was produced, promoting the oxidation of MWCNT and formation of fragmented AgNW. Consequently, the interspaces between the interdigital electrodes appeared. The width and interspace of the electrodes were set to be 800 µm and 200 µm, respectively. The electrochemical performances of the fabricated device (Fig. 3d) were superior to the MWCNT-based MSC (C-MSC) because of the enhancement of electrical conductivity through introducing the AgNW as current collector. It delivered the areal capacitance of 274.8 µF/cm2, an energy density of 0.17 mWh/cm3 at the power density of 1.2 W/cm3 and a long cycle life of 92.3% of the initial capacitance value after 10000 times, respectively. Moreover, the excellent flexibility of the device under different bending radius was presented (Fig. 3e and 3f). This work supplied a novel approach of mask-free patterning the current collector and electrode materials in one step for flexible on-chip MSCs. However, this method was often applied in the given carbon-based materials with high-efficiency, such as graphene, conducting polymers, flexible polymer substrates, etc. Therefore, expanding the application of such a method to other materials deserves further 14

research in the future.

Fig. 3 (a) Schematic diagram of the preparation of the MWCNT/AgNW composite film. (b) Schematic illustration of the experimental setup for micro-plasma-jet generation. (c) Fabrication flowchart of the on-chip MSC with interdigitated MWCNT/AgNW electrodes. (d) Digital photograph, (e) Bending test, and (f) CV curves of the device under different bend radius. Reproduced with permission [44].

2.3 Laser lithography Laser lithography technique is another method to fabricate flexible on-chip MSCs. It can be used to produce conductive carbon-based patterns by laser induction on one hand and the desired electrode gaps by laser engraving on the other hand. The mechanism is based on the photo-thermal effect. It is a fast, scalable and single-step method for micro-patterning without masks or complex clean environment. Currently, laser lithography is mainly focused on carbon-based materials and polymer films [91-94]. For example, in 2018, Lin et al. reported a high-voltage (2090 V) 15

graphene-based flexible on-chip MSC through laser induction for the first time [38]. Fig. 4a showed the schematic diagram of laser-induced graphene (LIG) on the Kapton (PI) substrate. 210 porous LIG electrodes separated by 209 gaps were first made through the CO2 infrared laser heating. Subsequently, PVA-H2SO4 electrolyte was coated over the gaps to obtain the MSC array device. Interestingly, this method can be used to produce the planar MSCs with different voltages, such as 1 V, 3 V and 6 V, etc. The digital photographs of the flexible 209 V on-chip MSC and the gaps of 500 µm between the LIG electrodes coated with electrolyte were shown in Fig. 4b and 4c. In addition, The GCD curves of the high-voltage device at various bending states were measured in Fig. 4d. A negligible degradation of electrochemical performance was observed, demonstrating the good flexibility and stability of the fabricated 209 V MSC. Besides, it delivered the maximum volumetric capacitance of 1.43 µF/cm3 and high energy density of 31.3 mWh/cm3 at the power density of 5.7 mWh/cm3. For practical application, the authors used the flexible on-chip MSCs in series as energy source to power the piezoresistive microsensor (6 V) and walking robot (> 2000 V), which paved the way for wearable electronics. In addition to polymer film that can be carbonized, GO can also be reduced to rGO under laser irradiation. Zhou et al. fabricated a flexible on-chip MSC based on rGO/MWCNT nanocomposites through infrared laser induction [95]. In the process, the brown insulating GO was transformed into black conductive rGO under laser reaction in 30 minute, resulting in the formation of the desired interdigital electrode fingers. The as-fabricated devices can offer a high volumetric capacitance of 46.6 F/cm3, a maximum energy density of 6.47 16

mWh/cm3 at the current density of 20 mA/cm3 and 88.6% of the capacitance retention after 10000 cycles at 50 mA/cm3, respectively.

Fig. 4 (a) Schematic diagram of the high-voltage device based on laser-induced graphene. (b) Photography of a fabricated flexible planar 209 V MSC. (c) Magnified optical image of the gaps between the graphene electrodes. (d) Charging and discharging curves at different bending angles of a 209 V MSC. Reproduced with permission [38]. (e) Schematic illustration of the fabrication of planar MGFs-based MSCs. (f) Fatigue resistance of MGF-60-2 MSC under the bending state of r = 0.51 cm for 4000 cycles. Reproduced with permission [43].

17

Laser engraving as an environmentally friendly and low-cost approach was also applied to fabricate flexible on-chip MSCs within submillimeter scale. This method is suitable for graphene, carbon nanotubes, MXenes, conductive fabric, and black phosphorus, etc [34, 43, 45, 47, 66, 96]. For example, Xie et al. fabricated ultraflexible MSCs based on MnO2@rGO films (MGFs) through layer-by-layer coating and laser engraving processes [43] and Fig. 4e showed the corresponding preparation flowchart. GO was first transformed into MGF via electrophoretic deposition and reduction by hydrohalic acids (HI) and hydrothermal reaction. The MGF layers were then stuck together using the quasi solid electrolyte as adhesive. Laser engraving method was finally employed to make the on-chip interdigital structure on the multilayered MGFs. It should be noted that the amount of MnO2 significantly affected the electrochemical performance of the fabricated device. The device with two layered MGFs (MGF-60-2) after the growth of MnO2 for 60 min showed the best electrochemical performance because the excessive MnO2 not only decreased the electrical conductivity but also impeded the ion transportation. The fabricated MGF-60-2 MSCs displayed the highest areal capacitance of 31.5 mF/cm2 at the current density of 0.2 mA/cm2. After 6000 charge/discharge cycles, the capacitance still remained 77.0% of the original value at the current density of 1.5 mA/cm2, revealing a good cycling stability of the devices. The flexibility and stability of the device for long-term use were tested in Fig. 4f. It revealed that no capacitance attenuation appeared after 4000 bending measurements with the bending radius of 0.51 cm. The shape of the CV curves also did not change obviously when cycled for 18

1000, 2000 and 4000 times, as shown in the inset of Fig. 4f, manifesting the strong interface adhesion between the MnO2 and rGO. Different with the above process, in some cases of the laser lithography process, the fabrication of flexible on-chip MSCs can start from the formation of designed patterns and then the coating of active electrodes. For example, Alshareef et al. developed MXene-based MSCs by combining spray-coating and direct laser cutting methods [97]. During the fabrication process, PET substrate was firstly cut into the desired pattern through CO2 laser and then MXene was spray-coated on the pattern as active materials. The prepared on-chip MSCs in PVA/H3PO4 electrolyte exhibited a capacitance of 23 mF/cm2 and 95% of the capacitance retention after 10000 charging/discharging cycles. As-fabricated MXene-based MSCs could be integrated with flexible triboelectric nanogenerator (TENG) to realize the self-charging function. 2.4 Microfluidic etching

Fig. 5 (a) Schematic diagram of the fabrication of MnO2/ITO-based MSCs through the microfluidic etching method. (b) Optical image of the MnO2/ITO interdigital fingers on a solid electrolyte film. (c) Schematic illustration of the material arrangement on one single finger. (d) SEM image and (e) magnified view of MnO2/ITO interdigital fingers. Reproduced with permission [53].

Microfluidic etching provides a facile, low-cost and universal method for the 19

preparation of micrometer-sized flexible on-chip MSCs. This method can be applied to most electrode materials of energy storage devices. Typically, microfluidic system consists of two layers, the substrate covered with the active materials and the moulage with micro-channels. PDMS is often used as the impression layer due to its stable chemical properties and excellent mechanical performance [98]. The designed patterns can be obtained when the etching solution flowed through the micro-channels. For example, Cao et al. prepared flexible MnO2-based on-chip MSCs by the microfluidic etching technique (Fig. 5a) [53]. MnO2 nanofiber film was first prepared by electrospinning, which was then transferred onto the PVA/H3PO4 electrolyte film and sputtered with a layer of indium tin oxide (ITO) as current collector. After that, the PDMS layer with micro-channels was attached to the MnO2/ITO film tightly. The etching solution and deionized water were injected into the micro-channel one after another to avoid the reaction-diffusion process. After drying, the PDMS moulage was peeled off to get the final MSC device with interdigital patterns. Fig. 5b displayed the width of the interdigital fingers around 20 µm. The schematic diagram of the interdigital fingers construction was presented in Fig. 5c, consisting the ITO current collector, MnO2 electrode materials and the solid electrolyte. Many pores were found on the surface of MnO2 nanofibers (Fig. 5d and 5e), benefiting the improvement of the electrochemical performance. The fabricated device showed a high capacitance of 338.1 F/g and good cycling stability of only 4% loss of the initial value after 500 charging/discharging times at the current density of 2 mA/cm2, superior to the common MnO2-based devices though directly casting and annealing process. 20

2.5 Printing Compared to the above fabrication methods, printing methods possess the features of universal, minimal material waste, low-cost and large-scale production. Currently, there are several printing approaches, including inkjet printing, screen printing, gravure printing, laser printing, 3D printing, mask-assisted spray deposition and stamp-assisted printing. Basically, most of the printing methods are suitable for producing flexible on-chip MSCs. In the following parts, we will give details about the fabrication of flexible on-chip MSCs with printing techniques. Inkjet printing is used to directly deposit the pre-designed patterns to the substrates with the superiorities of non-contact, no materials waste and easy to be realized on desktop printer. Despite of these advantages, preparation of stable ink suitable for smooth printing without blocking the nozzle is a significant problem. Generally, the value of inverse Ohnesorge number (Z) that relates to viscosity, density and surface tension of the fluid in the range from 4 to 14 suggests that the ink can produce the drops on demand in the process of printing [99]. Meanwhile, the maximum particle size in the ink is 1/50 of the diameter of the nozzle at most. Inkjet printing was widely used to produce flexible on-chip MSCs with various active electrode materials including carbon materials, transition metal oxides, conductive polymers and their composites [18, 50, 100-102]. For example, Elshof et al. demonstrated the flexible symmetric

on-chip

MSCs

based

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

on

δ-MnO2

and

(PEDOT:PSS)

through

inkjet printing process [50]. During this process, δ-MnO2 ink was prepared and first 21

printed on PI substrate, followed by annealing in nitrogen at proper temperature. PEDOT:PSS ink was then printed on top of the δ-MnO2 layer. The fabricated MSCs exhibited good electrochemical performance, including the high volumetric capacitance of 2.4 F/cm3 and the capacitance retention of 88% after 3600 cycles.

Fig. 6 Schematic illustrations of the preparation of flexible on-chip MSCs through (a) inkjet printing, (b) screen printing, (c) gravure printing and (d) laser printing. Reproduced with permission [39, 41, 46, 111].

Using inkjet printing method, asymmetric devices can also be prepared [39, 103]. In one of the most recent work, Bhattacharya et al. described the flexible planar MSCs (PµSCs) via inkjet printing and Fig. 6a showed the corresponding fabrication process [39]. To produce the device, rGO, AC-Bi2O3, AC-MnO2 and PVA-KOH as the conducting layer, negative electrode, positive electrode and polymer electrolyte, were printed on the bond paper in sequence through the EPSON L130 printer. The Z values 22

of the all the inks were adjusted to 5.56-6.81 for better printing. In addition, the optimized width of the interspace and the electrode finger were both 150 µm. The fabricated asymmetric PµSCs were operated in the potential of 0-1.8 V and delivered the areal capacitance of 536.6 mF/cm2, short time constant of 0.09 ms, high energy density of 15 mWh/cm3 at the power density of 0.141 W/cm3 and a capacitance retention of 92.2% after 20000 cycles. Screen printing is a fast, shape diverse, cost-effective and mass production technique [104-106]. In a typical screen printing process to flexible on-chip MSCs, the stable and conductive ink of active electrode materials was first prepared, and then passed through the screen meshes with a squeegee to form the patterns on different substrates. This method is compatible with many functional inks and flexible substrates. For example, Bao et al. screen-printed flexible graphene-based on-chip MSCs with ultrahigh voltage [41]. Fig. 6b showed the schematic diagram of the fabrication process with various geometries, including interdigital finger, strip, concentric circle and circular interdigital patterns. Large-scale modularization of MSCs in series or in parallel can be manufactured in seconds through this micromachining method. The fabricated single interdigital MSC delivered a high areal capacitance of 1 mF/cm2 in PVA/H3PO4 electrolyte. Importantly, 130 cells in series were used to construct a modularization, displaying the highest voltage of 104 V among the reported printable MSCs. This work reveals the potential application of the screen printing in high voltage applications of micro-electronics, like micro-robots and health monitors. 23

Using the similar process, Liang et al. demonstrated the flexible planar RuO2-AgNWs-rGO based MSCs with high-resolution microelectrode fingers of 50 µm [35]. Due to the synergistic effect of the ternary composites, the electrical conductivity of the electrode can reach up to 5000 S/cm. As a result, the prepared MSCs exhibited excellent volumetric capacitance of 338 F/cm3, high energy density of 18.8 mWh/cm3 and power density of 40.9 W/cm3, respectively. Screen printing was also be used to prepare asymmetric MSCs. For instance, Guo et al. printed the glucose-derived AC nanospheres and MoO3-x nanorods on PI substrate with Au interdigital electrode to prepare the flexible MSCs [42]. The fabricated MSCs in sodium alginate bio-hydrogel electrolyte were operated in the voltage of 0-1.8 V and showed the areal capacitance of 47.2 mF/cm2 as well as superior capacitance retention of 95% cycling for 10000 times. Gravure printing is a technique that transfer patterns on plate cylinder to the substrate under pressure. Normally, this technique is suitable for high throughput, such as magazines, newspapers and packages, because it needs a long time to engrave the designable patterns on the plate cylinder. Such a method was also used to prepare flexible on-chip MSCs. For example, Shi et al. reported the flexible planar interdigital MSCs based on hybrid sulfonated reduced graphene oxide (S-rGO) and MoS2 nanoflowers (MoS2@S-rGO) on PI substrate via gravure printing (Fig. 6c) [46]. The MoS2@S-rGO electrode materials had porous structure and wrinkles, which were beneficial for increasing the contact area with electrolyte and thus improving the electrochemical performance of the devices. The devices gave a high areal 24

capacitance of 6.56 mF/cm2, energy density of 0.58 mWh/cm3 and power density of 13.4 mW/cm3, respectively. In order to further promote the application of the flexible planar MSCs in wearable electronics, Chen et al. constructed a self-driven system using the flexible interdigital MSCs based on crumpled graphene ink by gravure printing to power a liquid crystal display (LCD) [107]. During the preparation of crumpled graphene ink, Mg(OH)2 nanosheets were added as nanospacers, which can significantly avert the graphene restacking. The width of the finger and interdigital space are 0.8 mm and 0.7 mm, respectively. The specific capacitance of the fabricated MSCs with Mg(OH)2 template-assisted graphene nanosheets (TAGNs) was realized of 6.65 mF/cm2, 2.83 times higher than the value of devices free of spacers. The printed circuit board (PCB) with TAGNs-based MSCs array successfully powered the LCD for 35s after charging for 3s. This gravure-printed flexible on-chip MSCs supplied a good foundation for the energy storage devices in the future self-powered systems. Laser printing is a simple, efficient and commercially available printing method. Briefly, this technique adopts a laser beam to quickly drop the patterns to the photosensitive surface, on which the magnetic toner was attracted under electrostatic interaction to present the printing images. After that, the toner was transferred to the printing paper through a heating roller. It should go through six procedures of selenium drum charging, laser irradiation, development, transfer, fixing and cleaning. To prepare the interdigital electrode, the printed patterns were often used as sacrificial layer through laser printing [108-110]. For example, Hua et al. reported the on-chip symmetric MXene-based MSCs via the common laser printing, vacuum-assisted 25

deposition and physical sputtering (Fig. 6d) [111]. The desired interdigital patterns were first designed on computer and printed on an A4 paper by the laser printing. Then, the few-layered Ti3C2Tx flakes suspension was filtered onto the printed paper through vacuum-assisted deposition. Afterwards, the conductive gold layer of 80 nm was sputtered on the Ti3C2Tx. Finally, the lift-off process was conducted with tetrahydrofuran to remove the printed layer. The average width of the finger and interdigital space were 745 and 305 µm due to the limited printing accuracy of the laser printer. The maximal areal capacitance of the fabricated MSCs reached up to 27.29 mF/cm2 at the current density of 0.25 mA/cm2, much higher than the carbon-based devices (0.1-6 mF/cm2) [79, 80, 82, 84, 112]. Additionally, the MXene-based device delivered the volumetric energy density of 6.1 mWh/cm3, which was comparable to the thin film batteries (<10 mWh/cm3). Like other printing method, laser printing method can also be used to fabricate asymmetric on-chip MSCs. Fu et al. demonstrated flexible on-chip MSCs from symmetric to asymmetric constructions by laser printing [113]. Different from the second step of the vacuum-assisted deposition in the above process, the printed paper and the PI film with electrode materials were sent to hot laminating together. The fused patterns on the paper can selectively adhere the active materials to form the desired electrode patterns. The laser-printed symmetric MSCs with rGO/Ag-NW hybrid materials offered the highest areal capacitance of 5.5 mF/cm2 among the graphene-based MSCs [114]. While for the AMSCs with the rGO/Ag-NW anode and α-Ni(OH)2/Ag-NW cathode, the devices achieved a high areal capacitance of 8.6 26

mF/cm2 with the extended voltage of 1.5 V. 3D printing is a process of direct ink writing, in which the continuous droplets of printing ink were extruded from the nozzle, forming a liquid bridge across the needle and the substrate. This method is different from the traditional extrusion-based 3D printing with long filiform ejection and inkjet printing with discrete droplets [39, 115]. Shear-thinning behavior and moderate solid contents are required for 3D printing ink. It is a low-cost, efficient and versatile way to fabricate flexible on-chip MSCs [15, 116]. For example, Liu et al. prepared interdigital CNT-based on-chip MSCs via heat-assisted 3D printing (Fig. 7a) [117]. The CNT ink was first extruded and deposited on a pre-heated glass plate through a 3D printing system. Subsequently, PVA/H3PO4 electrolyte was coated after the printed interdigital electrodes dried. Finally, the device was peeled off from the glass plate to form the flexible on-chip MSCs. The heated substrate can not only facilitate the solvent removal in the ink but also remain the structure integrity for fear of delamination or distortion. During the process, the temperature of the substrate, working distance, extrusion speed and the moving speed of the nozzle affected the patterns and the electrochemical performance of the devices. As a result, the fabricated MSCs owning interdigital electrodes with the height of 124 µm and interspace of 141 µm achieved good electrochemical performance, including the area capacitance of 5.9 mF/cm2, energy density of 0.82 µWh/cm2 and power density of 10 µWh/cm2. To satisfy the required energy and realize the wearable purpose, the in-situ 3D preparation of energy storage devices was investigated by Xu et al [48]. Two in-situ 27

strategies were developed for the fabrication of PPy/PAni coaxial nanotube and rGO (PPCANT/rGO) based on-chip MSCs. The prepared flexible on-chip MSCs with four layers exhibited a high areal capacitance of 151.2 mF/cm2 at the scan rate of 5 mV/s. The outstanding energy density and power density can reach up to 19.6 mWh/cm3 and 0.91 mW/cm3, respectively. The in-situ fabrication method provides a new idea for the portable and wearable electronics.

Fig. 7 Schematic illustrations of the fabrication of flexible planar MSCs by (a) 3D printing, (b) mask-assisted spray deposition and (c) stamp-assisted printing. Reproduced with permission [117, 70, 124].

28

Mask-assisted spray deposition is another direct printing method [114, 118-122]. The selected ink should have the features of stable, low-cost, good electrical conductivity and easily printable on various substrates. In the process, the deposition and patterning of the desired electrodes are achieved in one step. It is an efficient method for scalable production and integration with printed electronics on the same substrate. The resolution of the patterns made by this technique depends on size of the masks. For example, Wu et al. reported an on-chip asymmetric MSC with 2D mesoporous MnO2 nanosheets (m-MnO2) as positive electrode and porous VN nanosheets as negative electrode [70]. In order to improve the electrical conductivity of the electrodes, the electrochemical exploited graphene was used as conductive agent. The schematic diagram of the fabrication process was shown in Fig. 7b. The width of the electrode fingers and interspace of customized interdigital mask are 1 mm and 0.5 mm, respectively. The maximum energy density of the fabricated devices can reach up to 21.6 mWh/cm3, outperforming the performance of the thin-film batteries. Additionally, Bao et al. manufactured the planar graphene-based linear tandem MSCs (LTMSCs) on diverse substrates (e.g. A4 paper and nylon membrane) with symmetric and asymmetric structures via mask-assisted spray coating method [30]. The parallel linear mask was used with both the width of single electrode and interspace of 0.1 cm. A symmetric cell was operated in the voltage of 0-0.8 V in the PVA/H2SO4 electrolyte and displayed the areal capacitance of 4.9 mF/cm2 at the scan rate of 2 mV/s. The graphene-based LTMSs with ten cells could extend the potential up to 8 V, demonstrating the excellent uniformity in performance of the devices. 29

Besides, the AMSCs with MnO2 nanosheets/PH1000 (PEDOT:PSS) as positive electrode and graphene as negative electrode in PVA/LiCl gel electrolyte extended the voltage to 1.8 V. Three printed AMSCs connected serially can readily drive a LED bulb. Stamp-assisted printing was recently developed to fabricate flexible on-chip MSCs, which is a simple route to imprint diverse patterns on the substrates with the assistance of stamps [115, 123, 124]. This method eliminates the stringent conditions and the needs of expensive equipment to some extent. For example, Gao et al. used this technique to transfer the insulating ink pattern to the conductive glass, followed by the electroplating of Ni film, removing the marker ink, attaching Kapton, peeling off the Kapton, electrodepositing MnO2 and drop-casting gel electrolyte to get the flexible nanotextured Ni/MnO2 based on-chip MSCs (Fig. 7c) [124]. The width of the electrode fingers and interspace distance of the interdigital pattern were 1.4 mm and 300 µm, respectively. Such resolution was comparable to that of screen printing technique [68]. As-fabricated devices displayed a high areal capacitance of 4.15 mF/cm2 at the scan rate of 0.02 V/s. Stamp-assisted printing is also beneficial for preparing the series and parallel devices without any external metal wires. The series and parallel MSCs can obviously extend the output voltage and current, respectively. In addition to the insulating ink, the conductive ink can also be directly imprinted on the flexible substrates as electrodes via the stamp-assisted printing method. For instance, Nicolosi et al. demonstrated a strategy to achieve scalable production of flexible all-MXene based MSCs through design of the pad and cylindrical stamps 30

[115]. In the work, 12 interdigital MXene-based MSCs were generated at one time using the pad stamps. At the same time, the cylindrical stamps with a handle can be applied to prepare the planar MSCs, which greatly reduced the production period.

2.6 Other fabrication methods Besides the above commonly used techniques, some other methods were also developed to prepare some flexible on-chip MSCs [125-132]. For example, Chen et al. fabricated the flexible planar ZnCo2O4-based MSCs with the channel width of 150 µm through mechanical scribing process [133]. The mechanical scribing system consisted of a two-dimensional platform with high-precision guide rails and a needle vertically over the platform controlled by a computer program. With the movement of the platform along X-Y axes, the needle would scribe the designed patterns on the electrode material film. Therefore, this method is convenient for large-scale fabrication of flexible on-chip MSCs. The prepared MSCs delivered the maximum energy density of 0.065 µWh/cm2, the power density of 0.092 mW/cm2 and the capacitance retention of 92% after 10000 cycles, respectively. Lei et al. applied a three-dimensional translation stage system to scribe the interdigital nanoporous gold film and successfully achieved the separation between the fingers of 100 µm [134]. After electrodeposition of MnO2 and PPy as electrodes, the devices showed the capacitance of 1.27 mF/cm2, the maximum energy density of 45.3 mWh/cm3 and the high power density of 440.4 W/cm3, respectively. Besides, Li et al. simply employed an oily mark pen to make a serpentine pattern to partially cover the conductive Ti foil for the following electrodeposition process [135]. It is a 31

straightforward method for electrode designs without any mask or special instruments. The interspace between the two fingers is 0.5 mm, which may be further narrowed with a thinner mark pen. The flexible planar MSCs exhibited the capacitance of 26.6 mF/cm2 or 53.2 mF/cm3 at the current density of 0.1 mA/cm2 and high energy density of 4.7 mWh/cm3 at the power density of 80 mW/cm3. In a recent work, Alshareef et al. found that the ink patterns written by mark pen can be used in both lift-off and etching processes to fabricate flexible on-chip MSCs [136]. The marked area was used to protect the underlying ITO layer from etching. After removing the ink by ethanol, the PEDOT was electrodeposited on the conductive interdigital pattern. A maximum energy density of 12 mWh/cm3 was achieved for the flexible planar MSCs.

3. Hybrid ion capacitors Due to the relatively low energy density of the conventional flexible on-chip MSCs, hybrid ion capacitors combining with both the battery-type anode and capacitor-type cathode were investigated in the last few years. In 2018, Bao et al. reported the flexible all-solid-state planar lithium-ion micro-capacitors (LIMCs) for the first time [22]. With conductive electrochemically exploited graphene (EG) as metal-free current collectors, the interdigital electrodes with Li4Ti5O12 (LTO) nanospheres anode and activated graphene (AG) cathode were constructed on nylon membrane by mask-assisted deposition (Fig. 8a). The ion gel LiTFSI-P14TFSI-PVDF-HFP with good ionic conductivity and thermal stability up to 350°C was used as the electrolyte and coated on the interdigital electrodes to get the flexible LIMCs. Fig. 8b showed the 32

mechanism of the flexible LIMCs. In the charging process, Li+ intercalated into the crystal lattice of the LTO nanospheres and TFSI- quickly adsorbed on the surface of AG. During the discharge process, the Li+ de-intercalated from the LTO and TFSIdesorbed from the AG. To fabricate hybrid ion-capacitor, balancing the charge between the anode and cathode is very important for the electrochemical performance. Devices with different thickness ratios (LTO:AG) in the range from 1:2 to 1:6 were measured in the potential of 0-3 V and the corresponding volumetric capacitances were displayed in Fig. 8c. It was observed that the maximum volumetric capacitance was 28.4 mF/cm3 at the current density of 0.1 mA/cm2 with the thickness ratio of 1:3. The fabricated LIMCs also exhibited excellent cycling stability and flexibility. More importantly, the ionic conductivity of the electrolyte enhanced with the improved temperature owing to the decreased viscosity. The energy and power densities of the LIMCs at different temperatures were compared with the AG//AG MSCs and some other commercially available energy storage devices as shown in Fig. 8d. The prepared LIMCs delivered the outstanding energy density of 25.5 mWh/cm3 at 0°C (0.01 mA/cm2), 53.5 mWh/cm3 at room temperature (0.02 mA/cm2), 50 mWh/cm3 at 50°C (0.05 mA/cm2) and 54.6 mWh/cm3 at 80°C (0.05 mA/cm2), respectively, which were much higher than those of AG//AG MSCs, commercial supercapacitors, Li thin-film battery and Al electrolytic capacitors [79, 81, 137]. The results illustrated that the planar LIMCs with good electrochemical performance and excellent flexibility can be the highly-competitive candidates for the on-chip power sources. In addition to Li-ion hybrid MSCs, the zinc-ion hybrid MSCs were also fabricated 33

recently due to the advantages of Zn metal, such as abundant reserves, high stability and easy preparation [138, 139]. For instance, Qu et al. reported an aqueous Zn-ion MSC with Zn anode and CNT cathode in gelatin-based ZnSO4 electrolyte [139]. In the preparation process, the interdigital CNT film on the PI tape was obtained by laser cutting. The scanning electron microscopy (SEM) images of pristine and laser-engraved CNT film were shown in Fig. 8e and 8f. Afterwards, Zn2+ was reduced to Zn and electroplated on one side of the interdigitated CNT micro-electrode in a three-electrode system. Fig. 8g exhibited the SEM image of the Zn nanosheets, which nearly vertically grown on the CNT electrode. The fabricated flexible device displayed a high areal capacitance of 83.2 mF/cm2 at the current density of 1 mA/cm2, much higher than the CNT-based symmetric MSC for the reason of the synergistic effect of both energy storage mechanisms. The high energy and power densities of the Zn-ion MSC were achieved to be 29.6 µWh/cm2 and 8 mW/cm2, respectively. After 6000 cycles, 87.4% of the initial capacitacne retained as shown in Fig. 8h. Notably, the in-situ electroplating process can replenish the losing Zn content causing by the irreversible consumption in the repeated charging and discharging measurements. As a result, a higher capacitance of 76 mF/cm2 was offered after the re-plating Zn anode, manifesting that a longer lifetime can be realized of the devices. The photographs of both devices at different bending states were exhibited in Fig. 8i. The Zn-ion MSCs in series and parallel constructions were also prepared for practical application, revealing that the fabricated hybrid ion MSCs can meet with the requirements of the flexible and portable electronics. 34

Fig. 8 (a) Schematic diagram of the fabrication of LTO//AG-LIMCs by mask-assisted filtration. (b) Mechanism illustration of LTO//AG-LIMCs. (c) Volumetric capacitance vs. various thickness ratio of anode and cathode at the current density of 0.1 mA/cm2. (d) A Ragone plot of the all-solid-state planar LTO//AG-LIMCs at different temperatures and other commercially available energy storage devices. Reproduced with permission [22]. (e) The SEM images of the (e) pristine CNT paper, (f) microelectrode and (g) anode after plating. The inset shows the magnified image of the anode material. (h) Cycling performance of the ZmSC at the current density of 5 mA/cm2. (i) Photographs of SmSC under the bent and twisted conditions as well as ZmSC in the bent state. Reproduced with permission [139].

4. Multifunctional flexible on-chip MSCs Multi-functionality is an important demand for the wearable and portable energy storage devices. With the development of electrode materials, electrolyte and flexible 35

substrates, the intrinsically multifunctional flexible planar MSCs were gradually investigated, such as stretchable, self-healable, electrochromic and thermoreversible self-protection

properties.

Herein,

the

recent

progress

of

the

essentially

multifunctional flexible on-chip MSCs are introduced in detail. 4.1 Stretchable MSCs Stretchable on-chip MSCs with the features of barely mechanical or electrochemical performance attenuation under various tensile strain conditions have gained tremendous interest in the past few years. Generally, there are four types of stretchable MSCs, including porous textiles, honeycomb-like, bridge-island and wavy/bucked structures [32, 61, 140]. For example, Li et al. designed the skin-mountable interdigital graphite/MnO2-based MSCs by pencil drawing followed with deposition processes on the medical adhesive tape [141]. The prepared flexible on-chip MSCs not only exhibited superior flexibility at different bending states but also presented stable stretching performance under tensile strain from 100% to 130%, revealing the potential practicability of the devices. Komvopoulos et al. developed a honeycomb-like stretchable MSC arrays on PDMS substrates [142]. The honeycomb structure can bear a large tensile deformation without affecting the MSCs and the interconnections. The device possesses excellent electrochemical stability even at 150% stretching, 180° bending and 60° twisting conditions. In addition, the stretchable MSC arrays have a wide application in wearable electronics.

36

Fig. 9 (a) Schematic illustration of the structure and fabrication process of a biaxially stretchable planar supercapacitor. (b) Surface SEM images of MWCNT, Mn/Mo mixed oxide, and Mn/Mo@MWCNT electrodes, respectively. (c) The scheme of the organic gel electrolyte. (d) CV curves of the Mn/Mo@MWCNT based supercapacitor at various scan rates of 10-300 mV/s. (e) GCD curves measured at the current densities of 0.3-0.8 mA/cm2. (f) The areal capacitance calculated based on GCD curves at different current densities. (g) GCD curves tested under a biaxial strain of 10-50% and the corresponding normalized capacitance values at a current density of 0.8 mA/cm2. (h) The capacitance retention as a function of stretching/releasing cycle number under different biaxial strains. (i) GCD curve dynamically recorded during finger bending/spreading motion while the device is attached to an index finger. Reproduced with permission [72].

In another work, Ha et al. fabricated stretchable MSC arrays through making the relative stiff islands of active devices on both sides of a soft thin film, which were electrically connected with embedded liquid metal of Galinstan [143]. In the process of uniaxially stretching to 40%, the concentrated strain was inflicted on the soft thin film according to the finite element method (FEM) analysis. Moreover, the 37

normalized capacitance nearly remained unchanged under the strain of 40%. They also reported a biaxially stretchable MSC arrays via implanting the active devices based on the PET substrate within the soft elastomer of Ecoflex [144]. The mechanical and electrochemical properties of the island MSC arrays also remained invariable under biaxial strain of 50%, demonstrating the promising application of the stretchable bridge-island MSC arrays in bio-implantable and portable electronics. In addition to the static testing conditions, the mechanical and electrochemical properties of the stretchable planar MSCs should also be stable in the dynamic states. Ha

et

al.

demonstrated

the

buckled

MWCNTs/Mn-Mo

mixed

oxide

(Mn/Mo@MWCNT) based flexible on-chip MSCs in organic electrolyte [72]. Fig. 9a showed the schematic illustration of the interdigitated MSC on a semi-transparent silicon rubber film. Due to the higher electrical conductivity compared with pure Mn oxide, the Mn/Mo mixed oxide film was selected as the electrode materials. The size of the Mn/Mo mixed oxide in the composite film is smaller than that electrodeposited without MWCNT film as displayed in Fig. 9b, which can effectively increase the specific surface area. The organic gel electrolyte composed of adiponitrile, succinonitrile,

lithium

bis(trifluoromethanesulfonyl)imide,

and

poly-(methyl

methacrylate) (ADN/SN/LiTFSI/PMMA) was applied, which is stable for at least a month in ambient air (Fig. 9c). The electrochemical performance of the fabricated stretchable planar supercapacitors was measured in the potential of 0-2 V (Fig. 9d and 9e). The maximum areal capacitance was obtained to be 7.5 mF/cm2 at the current density of 0.3 mA/cm2 (Fig. 9f). The GCD curves at different biaxial strain from 0% 38

to 50% showed no significant change in Fig. 9g, indicating that the steady normalized capacitance of the devices. Furthermore, ∼ 90% of the capacitance retention remained after 1000 stretching and releasing cycles under different biaxial strain (Fig. 9h). The photographs of the device before and after biaxially stretching for 50% were shown in the inset of Fig. 9h. Fig. 9i presented the GCD curves of the device, which was dynamically evaluated during the index finger bending and spreading motion. No obvious variations were found, manifesting the outstanding mechanical and electrochemical stability of the skin-like stretchable MSCs.

4.2 Self-healable MSCs Self-healable function has been a research hotspot in electronics for the reason that the mechanical damage is usually the biggest problem that prevented the electronic devices from working properly. It is also important for flexible on-chip MSCs to possess the self-healable function for practical applications. In the past several years, there are many reports on developing self-healable supercapacitors with sandwich or wire-shaped structures [145, 146]. However, the relevant self-healable on-chip MSCs are rare reported. In 2018, Gao et al. developed a highly self-healable 3D MSC with MXene-rGO composite aerogel wrapped with a self-healing carboxylated polyurethane (PU) shell (Fig. 10a) [23]. The PU encapsulating material realized the reconnection through the rich interfacial hydrogen bonds once it was destroyed. The photographs of the 3D MXene-rGO electrode covered with PU at the pristine, wound and after self-healing states were displayed in Fig. 10b. The electrochemical properties of the devices in original state and after different cutting-healing cycles 39

were measured (Fig. 10c-e). Based on the CV curves, 81.7% of the initial capacitance value retained after the fifth self-healing process. The slightly reduced capacitance resulted from the deviation of reconnection between the separated parts. Besides, the increased equivalent series resistance also demonstrated a little mismatch of the electrodes. After five healing process, the 3D MSC still can drive a photodetector, confirming the good restoration of the device. Self-healable 3D MSCs may be good candidate for next generation of multifunctional electronics with long span life.

Fig. 10 (a) Schematic diagrams of the manufacturing process of MXene−rGO self-healing MSCs. (b) Photographs of the 3D self-healable MSC at original, after cutting and after self-healing states, respectively. (c-e) The CV, GCD and Nyquist curves of the initial and after several self-healing cycles of MSC. Reproduced with permission [23], Copyright 2018.

4.3 Electrochromic MSCs Stimuli-responsive devices as indicators can respond to the external irritations, such as voltage, light, heat, pH and magnetic field, etc. They play increasingly significant roles in the modern electronics. Electrochromic function as a kind of stimuli-response is achieved through the variable colors of electrodes under different potentials. The commonly involved electrochromic materials include polyaniline, polypyrrole, polythiophene, WO3, NiO and so on [147-149]. Flexible on-chip MSCs can also 40

possess the electrochromic functions by choosing proper electrode materials. For example, Feng et al. demonstrated a stimulus-responsive MSC (SR-MSC) with a reversible electrochromic window on flexible PET substrate [20]. Fig. 11a showed the preparation process of the SR-MSC including patterning the 2D exfoliated graphene and 1D V2O5 nanoribbons (EG/V2O5) hybrid film, removing the residual area and covering the PVA/LiCl gel electrolyte with methyl viologen (MV2+). The fabricated flexible device displayed reversible electrochromic phenomenon in the voltage of 0-1 V due to the conversion between the viologen (MV2+, colorless) and its radical ion derivative (MV+, purple), as shown in Fig. 11b. The PET-supported SR-MSC also exhibited good flexibility under different bending angles (Fig. 11c). The in situ UV-vis spectro-electrochemical technique was employed to investigate the electrochromic effect by detecting the absorbance changes at 550 nm (Fig. 11d). In the charging process, the absorbance increased and eventually attained the highest amount at 550 nm when the voltage reached up to 1 V. As a consequence, the colorless MV2+ was gradually transformed into purple MV+. While during the discharging process, the color of the EG/V2O5 MSC and corresponding absorbance reduced step by step. The changed color provided the direct visualization of the energy storage state without extra equipment. In addition, ultrathin electrochromic MSCs as wearable tattoo energy devices with negligible performance degradation under various movements and deformations were proposed by Cheng et al [150]. In their work, the self-assembled vertical gold nanowires (v-AuNWs), PANI and PVA/H3PO4 served as current collector, 41

electrochromic material and electrolyte were used to prepare the flexible planar MSCs. In the potential range from -0.8 V to 0.8 V, the PANI changed the color at various states. The fabricated devices exhibited superior electrochemical performance, flexibility and cycling stability. Arbitrary shapes can be fabricated by using different masks, making it possible to fabricate devices used in different conditions and to be used in the future human-device interaction experience.

Fig. 11 (a) Schematic fabrication of SR-MSCs. (b) The electrochromic mechanism of viologen. (c) CV curves of the EG/V2O5 based MSC under different bending angles at the scan rate of 10 mV/s. The inset shows the photograph of the device in bending state. (d) The UV–vis spectra of EG/V2O5 based MSC at different voltages from 0 to 1.0 V during one charge–discharge cycle. Reproduced with permission [20].

4.4 Thermoreversible self-protection MSCs Despite of the rapid development of the flexible on-chip MSCs in the past years, the heat produced during the charging/discharging cycles is still a serious problem to date. It is a detriment for the electronics to work at high temperature for a long time. Besides, a great deal of heat as well as the increased pressure inside may possibly lead 42

to severe explosion or combustion. Therefore, solving this issue is of great importance for safe usage of energy storage devices. Using thermoresponsive hydrogel electrolytes with sol-gel transformation in the cooling-heating process is an effective approach to suppress the generation of heat [151-154]. For example, Feng et al. reported

a

flexible

thermoswitchable

poly(N-isopropylacrylamide)-g-methylcellulose

MSC and

(TS-MSC) lithium

with chloride

(PNIPAAm/MC/LiCl) as electrolyte on PI substrate [155]. The PNIPAAm/MC/LiCl electrolyte changed from the sol to gel state in the temperature range of 30-80°C. The ionic conductivity of the thermosensitive electrolyte greatly reduced from 10 to 3.4x10-2 mS/cm with increased temperature. As-fabricated TS-MSC exhibited the areal capacitance of 1.83 mF/cm2 at 30°C and 0 mF/cm2 at 80°C, respectively. Recently, Xu et al. constructed an all 3D-printing TS-MSC with a electrolyte consisting of H2SO4-dissolved poly(ethylene oxide)-g-methylcellulose (MC-g-PEO H2SO4), which can be used in a lower temperature [24]. Fig. 12a showed the mechanism illustration of the reversible thermoresponsive electrolyte. In the cooling state, the conductive ions shuttled freely in the solution. When the temperature was elevated, the polymer molecular chains were intertwined together to limit the movement of ions, leading the device to a halt-like state. The reaction is reversible according to the temperature. The CNT was used as electrode material and was printed on various substrates, such as slice, paper and plastic tape, etc., making it possible to roll up the device (Fig. 12b-e). Fig. 12f displayed the GCD curves of the TS-MSC

form

25°C

to

70°C,

indicating 43

the

downward

trend

of

the

charging/discharging time with the elevated temperature. Notably, 100% shutdown of the performance was achieved at 80°C. The cyclic measurement in the alternative cooling-heating process was also tested, exhibiting a good reversibility of the electrochemical properties (Fig. 12g). The impedance related to the migration of ions increased with the improved temperature, which revealed that the sol-gel transition of the polymer effectively hindered the transport of the conductive ions (Fig. 12h).

Fig. 12 (a) Schematic illustration of reversible self-protection for the thermoresponsive electrolyte. (b) Photographs of MSCs printed on slide and (c) on paper (d) being rolled up as well as (e) printed on plastic tape. (f) GCD curves of the MSC measured at different temperatures. (g) Thermoreversible specific capacitance behavior of the TS-MSCs upon heating−cooling cycles. (h) Temperature-dependent impedance spectra of TS-MSCs upon heating from 25 to 80°C. Reproduced with permission [24].

5. Integration and applications With the advantages of tiny size, light weight as well as the planar configuration, it is possible to integrate the flexible on-chip MSCs with other functional electronic devices to get on-chip integrated systems. Up to now, there are mainly two types of integrated systems have been developed. One is the all-in-one self-driven system with integrated energy harvester powered MSCs and functional electronic devices (sensors, 44

detectors, etc.) [10,14,19,21,25,42,68,74,87,97,133,156-161]. The most commonly used energy harvesters include solar panel, fuel cell, piezoelectric generator (PEG) and TENG, etc. The other type of integrated systems is MSCs-driven sensing systems. In this section, the integrated systems are highlighted.

Fig. 13 (a-c) Integrated system with tandem AMSC bridging solar cell and gas sensor. Reproduced with permission [68]. (d-f) Self-powered system with integrated on-chip PFCs and MSCs for powering red LED. Reproduced with permission [25]. (g-i) An integrated energy harvesting system on PI. Reproduced with permission [74]. (j-k) Self-charging bracelet constructed with TENG, power management module, double-sided MSCs and portable electronics. Reproduced with permission [157] (l-o) A wearable wristband with integrated multiplexed sweat sensor arrays and MSCs. Reproduced with permission [159].

Yan et al. developed a flexible paper-based integrated system consisting of a gas sensor, tandem MnO2/PPy based asymmetric MSCs and commercial available Si-based solar cell (Fig. 13a) [68]. The asymmetric MSCs acted as bridge between the solar cell and the gas sensor. After being charged by the solar cell for 78 s, the voltage of the asymmetric MSCs reached up to 2.82 V. The corresponding charging curve of 45

the asymmetric MSCs by the solar cell and discharging curve at 0.1 mA/cm2 were shown in Fig. 13b. Simultaneously, the PANI-based sensor can be driven properly by the charged tandem asymmetric MSCs at 1 V. Fig. 13c depicted the response and recovery curves of the PANI-based gas sensor through alternately inputting the gaseous NH3 and HCl, manifesting the feasibility of the designed sustainable self-driven system. Fan et al. fabricated a monolithically integrated wearable wristband including the commercial Si-based solar cells array, supercapacitors with MnO2/rGO/PEDOT:PSS electrode materials and SnO2 gas sensor through inkjet printing [160]. In the work, the output voltage of the solar cells can reach 2.8 V when the self-powered system was exposed to the table lamp. The SnO2 gas sensor exhibited good sensitivity in ethanol and acetone, which are significant for monitoring drunk-driving and diabetes, respectively. Besides, Ha et al.

integrated stretchable

self-powered devices containing twelve parallel-linked MSCs based on (Ppy)/CNT electrode materials, tandem Si-based solar cells and a fragmentized graphene foam-based strain sensor on a piece of Ecoflex substrate [161]. The MSCs array can be charged by the solar cells and then wirelessly power the strain sensor. In order to confirm the wearable properties, the integrated system was attached on the wrist and successfully measured the arterial pulse and external stress variation under sunlight. The change in strain was reflected by the resistance in this process. Therefore, the self-powered systems of solar cell and MSCs displayed the promising application in wearable electronics. Ideally, the Si-based solar cell in these works can be replaced with a flexible one to realize the fully flexible systems on a single substrate. 46

In addition to solar cell, fuel cell can also be applied in the integrated system. Mai et al. constructed a flexible self-charging energy chip with four photocatalytic fuel cells (PFCs) in series and two tandem asymmetric MSCs (Fig. 13d) [25]. The PFCs with TiO2 photoanode and Ag counter electrode with urea as fuel under UV exposure were used to output stable electric energy. Meanwhile, the asymmetric MSCs with NiCoP@NiOOH positive electrode and zeolite imidazolide framework derived carbon (ZIF-C) negative electrode exhibited excellent electrochemical performance, flexibility and cycling stability. In the integrated system, the energy conversion and storage were realized at the same time. Fig. 13e showed the charging and discharging curves of the asymmetric MSCs powered by two PFCs. It was observed that the asymmetric MSCs were charged to 1.2 V after 288 s and then discharged for 703 s at the current density of 0.1 mA/cm2. The cycling test of the integrated devices was performed and shown in Fig. 13f, indicating that the self-charging system can continuously supply energy for the wearable and portable electronics. Converting the mechanical energy into electricity is a crucial and promising way to supply power for the electron devices. Integrated systems driven by piezoelectric generator-charged MSCs were also developed. For example, Zhu et al. fabricated an integrated system on PI/Au substrate containing a PEG, a diode bridge rectifier circuit, two tandem multilayered graphene-based MSC (MG-MSC) line-filters, MG-PANI MSCs array and pressure/gas sensors (Fig. 13g) [74]. According to piezoelectric effect, the alternating current (AC) signal was produced via pressing the PEG with a frequency of 110 HZ, which can be transformed into direct current (DC) output by the 47

MG-MSC line-filter and stored in the MG-PANI MSCs eventually. After charging, the voltage of the MG-PANI MSCs can reach 1.5 V. Fig. 13h presented the regular current response of a pressure sensor. Meanwhile, the PANI gas sensor responded properly with the NO2 and NH3 input (Fig. 13i), signifying that the sensors were successfully powered by the MG-PANI MSCs array. Besides, transferrable PPy nanowires-based MSCs were also fabricated by Zhi’s group, as mentioned above [27]. In the work, the PEG and PPy nanowires-based MSCs were integrated together and successfully lit up the LED signal lamp. This work can effectively promote the development of low-cost, transferrable as well as high-performance MSCs. The mechanical energy from human movement with the feature of inexhaustibility can be harvested and converted into electricity. Zhang et al. presented a self-charging smart bracelet (SCSB) composing of a flexible freestanding TENG, power management module (PMM), stretchable double-sided MSCs based on CNT-PDMS elastomer and portable electronics (Fig. 13j) [157]. During human normal motions, the mechanical energy can be collected by the TENG and simultaneously stored in the MSCs through the PMM. To further confirm the practicality, the SCSB on the wrist was employed to power a pedometer directly. Fig. 13k showed the V-t curve of the SCSB at different operating modes, such as charging mode, working mode and sustainable mode. In the beginning, the linear increased curve demonstrated a charging process. After the voltage reached around 3 V, the MSCs can be used to drive the pedometer, which led to a linear drop on the V-t curve. The TENG and pedometer can operate at the same time and achieve a sustainable mode eventually. 48

Alshareef et al. prepared a self-charging power band through integrating a TENG and MXenes-based MSCs into the silicone rubber [97]. This configuration can persistently converted the mechanical energy derived from the contact with human skin into electricity and stored in the MSCs. Consecutively clapping with a frequency of 5 Hz for 30 min can charge the MSC to 0.6 V. Furthermore, the charging rate can be enhanced with the increased frequency. In addition to clapping, other motions (hand shaking or walking) may also make the TENG operate properly. As a wearable and portable application, the fabricated band can be used to power a watch as well as the thermos-humidity indoor and outdoor, illustrating that the integrated system can be promising to power different electronics. Besides, Wang’s group also fabricated a flexible self-charging unit consisting of a LIG-TENG and LIG-MSCs. These two parts were electrically isolated by the sandwiched PI substrate. The MSCs array can be charged to 3 V in 117 min by the LIG-TENG, which successfully powered the LED and hygrothermograph. Interestingly, the self-charging system can be embedded into the insole, manifesting its potential wearable applications [19]. Flexible integrated MSC-driven sensing system represents an important research direction that can effectively monitor various signals (toxic gas, physiological health, temperature, light and pressure, etc) [14, 21, 87, 133, 158]. For instance, Shen et al. designed a self-driven gas sensing system with the PPy-based concentric circles structured MSC array as power source on the same flexible substrate [10]. The as-fabricated MWCNT/PANI gas sensor showed good selectivity to ethanol as well as high detection capability of less than 1 ppm at room temperature. Importantly, 49

personalized monitoring drunken driving system was successfully realized through the integrated devices. Guo et al. prepared a wearable wristband of AC-based MSCs through screen printing method, which facilitated the design of on-chip integration systems [106]. The AC nanospheres with high specific surface area of 2534 m2/g were derived from glucose and the flexible wristband can be used as power unit to drive the electronic watch or LED. In another work, Shen et al. developed a wearable self-powered sweat monitoring system for personal health application [159]. In the system, the NiCo2O4-based MSCs served as power source was used to drive the sensor arrays including glucose sensor, [Na+] sensor and [K+] sensor (Fig. 13l). The integrated devices can be worn on the body to supervise the perspiration in real time. During exercise, the response currents of glucose, [Na+] sensor and [K+] sensors presented upward trends, demonstrating that the corresponding concentrations increased in the sweat (Fig. 13m-n). The smart integrated system can be the promising monitoring candidate for personal health in the future.

6. Conclusions and outlook In conclusion, the recent progress of flexible on-chip MSCs was systematically summarized in this paper. We first briefly summed up the often involved soft substrates, electrode materials and electrolytes in the flexible on-chip MSCs. Following this part, various typical methods to manufacture the flexible on-chip MSCs were classified, including photolithography, plasma etching, laser lithography, microfluidic etching and printing. At the same time, the advantages and disadvantages 50

of these techniques are also briefly generalized. Multi-functionality is an important demand for the wearable and portable energy storage devices. Accordingly, multifunctional on-chip MSCs were also highlighted in this paper. With the successful fabrication of flexible on-chip MSCs, highly compacted integrated systems were recently developed, which were also highlighted in the paper in detail. Despite the fast research progresses on the flexible on-chip MSCs, there are still some problems/issues need to be solved. For example, optimizing the parameters (gap, width, length and the numbers of fingers) of the planar interdigital electrodes is a challenge for improving the electrochemical performance of the device. To further promote the development of the flexible on-chip MSCs for wearable electronics, the future research emphasis should focused on as follows: (1) Although the electrodes were designed with different patterns including hair brush, concentric circle and parallel column, the fractal structures with edging effect should also be considered to improve the electrochemical performance. (2) Printing is a simple and efficient method to prepare flexible on-chip MSCs. The preparation of printable ink is the key step to fulfill printing purpose. Unfortunately, currently, it is still a very issue that needs further investigations. (3) In addition to the micromachining techniques mentioned above, it is still highly desirable to develop other simple, low-cost, environmentally friendly and universal method to fabricate flexible on-chip MSCs with miniaturization and high precision. (4) Self-healing is an important function to restore the electrical properties and structural integrity of the device. As a result, developing the self-healable materials 51

has strategic significance for the long-lived energy storage devices. Meanwhile, other multi-functionality of the flexible on-chip MSCs should be explored for the wearable electronics. (5) Although several integrated systems have been developed, the performance and integration is still far from the request of practical applications. Design highly compacted systems with extremely small size, thickness, and multifunctionalities, is still a big challenge for researchers in this field. Besides, the biocompatible and implantable devices should be further fabricated for future wearable and portable electronics applications.

Acknowledgements This work was supported by National Natural Science Foundation of China (51672308, 51972025 and 61888102).

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: