Recent progress of advanced energy storage materials for flexible and wearable supercapacitor: From design and development to applications

Journal of Energy Storage 27 (2020) 101035

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Recent progress of advanced energy storage materials for flexible and wearable supercapacitor: From design and development to applications ⁎

T ⁎

Chandu V.V. Muralee Gopia, , Rajangam Vinodha, Sangaraju Sambasivamb, Ihab M Obaidatb, , ⁎ Hee-Je Kima, a b

School of Electrical and Computer Engineering, Pusan National University, Busandaehak-ro 63 beon-gil, Geumjeong-gu, Busan 46241, Republic of Korea Department of Physics, United Arab Emirates University, Al Ain, UAE

A R T I C LE I N FO

A B S T R A C T

Keywords: Flexible substrates Energy storage materials Supercapacitor Energy density

Wearable electronic devices, such as electrical sensors, flexible displays, and health monitors have gained considerable attention and experienced rapid progress. Recently, the progress of flexible and wearable supercapacitors (SCs) have received considerable attention due to their ease of fabrication, low cost, flexible integration into textiles, long cycle life, fast charging/discharging, high efficiency, and capability to bridge the energy/power gap between conventional capacitors and batteries/fuel cells. In this context, the recent progress and achievements of flexible and wearable supercapacitors are presented, especially, the rational design and synthesis of novel nanostructured electrode materials on various flexible-based substrates, such as, carbon cloth, graphene coated fabric, silver coated fabric, nickel coated fabric, copper/nickel coated polyester fabric (CNF), etc. are summarized. The latest representative techniques and active materials of recently developed wearable supercapacitors with superior performance are summarized. Lastly, the current challenges, and future research directions and perspective in optimizing and developing the energy storage performance and function of flexible and wearable supercapacitors for their practical applications are addressed.

1. Introduction Recently, numerous efforts have been dedicated to design and development of ecological, sustainable and renewable energy storage systems with high electrochemical performances to address the rapid depletion of fossil fuels, increasingly worsening environmental pollution and global warming and urgent needs of environmental friendly alternative devices in modern electronic industry [1,2]. In recent years, extensive research has been focused to investigate and development of flexible energy storage systems, with the primary goal of applying flexible electronics to devices such as flexible displays, portable electronics, wearable devices, electronic sensors, health monitors, power backup, mobile phone, laptops, and etc [3-6]. The design and fabrication of electrochemical energy storage systems with high flexibility, high energy and power densities dominate the majority of current rechargeable energy storage markets. Conventional Li-ion based batteries (LiB) (<500 W h Kg−1) are not well suit for portable/wearable electronics due to the problem of heavy, bulky and have low performance. Also, the heat generated from the commercially available LiB's can affect the human skin, which limits their use in wearable systems [7,8]. For these reasons, supercapacitors considered as a promising ⁎

alternative to conventional LiB's [9,10]. In particular, supercapacitors are gained considerable attention owing to their rapid charge-storage capability (i.e., low discharge time: supercapacitor: 1–10 s vs. LiB: 10–60 min) and enhanced cyclic stability (supercapacitor > 30,000 h vs. battery >500 h) [11,12]. Unfortunately, the supercapacitors generate the lower energy density than the conventional batteries, which restricted the usage of supercapacitors in electronic applications. Low energy density mainly comes from low voltage window and specific capacitance (E = 1/2 CV2). If the above short slabs were resolved, supercapacitors would be commercialized and widely used. Although some of the recent literatures have claimed to enhance the energy density of supercapacitors to 100–150 W h Kg−1, which is almost comparable to those of LiB's (120–170 W h kg−1) [13-15]. Thus, it is highly desirable to design and develop a novel anode and cathode materials to fabricate the flexible supercapacitors with high energy density. The energy storage performance of supercapacitors is mainly depend on various factors, such as the electrochemical behaviors of the electrode materials, the choice of electrolyte, and the potential window of the electrodes. Tremendous research efforts have been directed in the design and development of advanced electrode materials for flexible

Corresponding authors. E-mail addresses: [email protected] (C.V.V. Muralee Gopi), [email protected] (I.M. Obaidat), [email protected] (H.-J. Kim).

https://doi.org/10.1016/j.est.2019.101035 Received 28 September 2019; Received in revised form 20 October 2019; Accepted 23 October 2019 2352-152X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. An overview of the latest progress in the field of flexible/wearable supercapacitors.

(CNTs), graphene, carbon fiber, graphitic carbon and their composites have gained much attention as flexible electrodes due to their large surface area, excellent conductivity, high stability and good mechanical behaviors. However, the EDLC materials suffer from low capacitance, which is a huge obstacle for enhancing the energy density of supercapacitors. To overcome this issue, surface modification is a key finding to increasing the energy density. In detail, exfoliation, surface activation, N-doping or S-doping and composite of carbon materials with the iron complexes can effectively enhance the capacitance of EDLC-based materials [35,36]. Besides, a light weight and flexible current collector is also a primary component in the design of wearable supercapacitors. Therefore, lot of efforts have been focused on the fabrication of flexible electrodes for supercapacitors applications. So far, several flexible electrodes, such as flax fiber textile, carbon foam, carbon nanofibers, Ni-cotton fabrics, carbon cloth, graphite foam, Ti foil and functionalized graphene sheets and etc. for flexible SCs because of their light weight, high flexibility and large accessible surface area. In this section we have efficiently presented recent breakthroughs of design and development of various anode materials for flexible supercapacitor applications. Recently, Zhang et al. [37] reported a hierarchical flexible electrode material by directly growing CNT on carbonized natural flax fabric (Fig. 2a), which have promising applications in flexible energy storage devices owing to their mechanical flexibility, large accessible surface area, and stable high-rate performance. Fig. 2b depicts hair-like structure, denoting successful growth of CNT. Here, the CNT loading was controlled by the growth time of 10, 15, and 20 min over carbonized fabric (CF) were used, and accordingly electrodes were indicated as CFCNT-1, CF-CNT-2, CF-CNT-3. Fig. 2c shows the cyclic voltammetry (CV) plots of electrodes at a scan rate of 5 mV s−1 in 6 M KOH electrolyte. All the CV plots retain a nearly rectangular shape, representing good capacitive behavior. Besides, galvanostatic charge-discharge (GCD) profiles of electrodes exhibit typical symmetrical linear charge-discharge behavior (Fig. 2d). Compared with the capacitance of pure CF electrode, the CF loaded with CNT exhibited the enhanced specific capacitance, by enhancing the specific surface area of the electrodes. Moreover, the cycling stability of CF-CNT-2 exhibited the very stable capacitance with a high retention of 96% over 5000 cycles (Fig. 2e). Also, Zhang et al. [38] synthesized a hierarchical three dimensional (3D) carbon foam (CF) containing a significant amount of multiscale pores (MSP) (including macro-, meso‑, and micropores). Fig. 3a shows schematic of the CF-MSP. Initially, Silica-embedded carbon foam (CFSiO2) was synthesized by mixing silica spheres (about 200 nm in

supercapacitors with appropriate structural properties to facilitate effective electron transport and ionic diffusion [16-25]. Herein, we provide a comprehensive view of the recent progress and advances made in flexible and wearable supercapacitors by categorizing various flexible an ode and cathode electrode materials. To facilitate further research and development, some future research trends and directions are finally discussed. This review is aimed at delivering readers with a comprehensive insight into the fundamental understanding, new electrode materials and novel device designs of flexible and wearable supercapacitors. Accordingly, as depicted in Fig. 1, this review briefly describes the latest scientific progress in flexible and wearable supercapacitors. 2. Progress of electrode materials for flexible and wearable supercapacitors Cost-effective, environmental friendly and flexible electrode materials with high stability, outstanding electrochemical property and excellent mechanical performance are essential as a vital characterization of flexible supercapacitors. A supercapacitor mainly composed of current collectors, two active electrode materials, electrolyte and a separator. Based on the charge storage mechanism, generally supercapacitors are classified into electric double‐layer capacitors (EDLCs), pseudocapacitors, and hybrid supercapacitors [26]. EDLCs store the charge electrostatically at the electrode-electrolyte interface, while pseudocapacitors store charge through electrochemical Faradaic redox reactions. EDLCs mainly employ carbonaceous materials (including activated carbon, carbon nanotubes (CNTs), graphene and graphitic carbon) [27-30], while that for pseudocapacitors are transition metal oxides (RuO2, MnO2, Fe2O3 and etc.) and conducting polymers (e.g. polypyrrole, polyaniline, poly(3,4-ethylenedioxythiophene) and etc.) [31]. As per the working mechanism, EDLCs deliver high power density and outstanding stability but a low capacitance, whereas pseudocapacitors possess the opposite behavior. Interestingly, the hybrid supercapacitor device composed of combination of both EDLC and pseudocapacitor electrode materials, which results an extended operating potential window, higher energy densities and high capacitances [3234]. 2.1. Flexible substrates to fabricate anode based electrode materials for supercapacitors Carbon based materials, such as activated carbon, carbon nanotubes 2

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Fig. 2. (a) Schematic of the fabricated CF-CNT electrode. (b) SEM image of the CF-CNT-2 sample. (c) CV plots (d) GCD profiles of CF-CNT hybrids at a constant scan rate of 5 mV s−1 and current density of 1 A g−1. (e) Cycling stability of CF-CNT-2 electrode over 5000 cycles. Reproduced from ref. [37] with the permission of the John Wiley & Sons Ltd.

After reducing the Ni(Ac)2 with carbon species and the Ni nanoparticles acted as catalysts for the growth of CNTs. Using the 70:20:10 wt ratio of PAN/PVP/Ni(Ac)2 precursor solution, Y. Qiu et al. achieved a dense CNTs vertically aligned on the CNFs (Fig. 4b and inset). The TEM (Fig. 4c) and HR-TEM (inset of Fig. 4c) revealed that the formed VCNTs are multiwalled with a diameter of about 30–60 nm and a wall thickness of 10–20 nm. Fig. 4d shows the CV curves of the symmetric twoelectrode VCNTs/CNF based EDL capacitor in a 1 M NaOH electrolyte at various scan rates. All the CV curves exhibit a nearly rectangular shape, revealing a nearly ideal capacitive behavior. Using C = A/(2 mνΔV), the specific capacitance of VACNTs/CNFs is calculated to be 213.6 F/g at a scan rate of 10 mV/s, and 128.0 F/g at a higher scan rate of 1000 mV/s. Furthermore, VACNTs/CNFs-based EDL capacitors are fabricated using EMIMBF4 ionic liquid electrolyte. Fig. 4e depicts the GCD plots of the VACNTs/CNFs-based EDL capacitors in EMIMBF4 ionic liquid electrolyte at various current densities. Based on the equation of C = 2I(Δt/ΔV), the VACNTs/CNFs-based EDL capacitor delivered a the specific capacitances of 146.8, 132.1, 94.8, 76.9, and 63.0 F/g at 0.5, 1, 2, 5, and 10 A/g, respectively. Further, the EDL capacitor delivered a high specific energy of 70.7 Wh/kg at a current density of 0.5 A/g (Fig. 4f). An optical image of a fully fabricated EDL capacitor is shown in inset of Fig. 4f. After charging, the flexible EDL capacitor can light up a blue LED indicator (3.2 V). Also, Y. Li et al. [40] also successfully used CNFs as flexible current collectors for supercapacitor applications. Y. Li et al. synthesized the NSCPCNFs by electrospinning a metal–organic framework ZIF-67 and a thiourea (TU) incorporated polyacrylonitrile (PAN) precursor with subsequent carbonization at 700 °C, 800 °C and 900 °C for 2 h with a heating rate of 5 °C min−1 in a N2 atmosphere, respectively (Fig. 5a). The corresponding samples are named as NSCPCNF-700, NSCPCNF-800 and NSCPCNF-900. The PVA-H2SO4 gel is used for the fabrication of FSC and the structure of the FSC is shown in Fig. 5b. As shown Fig. 5c,

diameter) with chitosan solution, followed by gelation with glutaraldehyde, freeze-drying, and carbonization. Then, a piece of CF-SiO2 was soaked in 2 M sodium hydroxide (NaOH) aqueous solution to dissolve the embedded SiO2 spheres. This sample is indicated as CF. Finally, CF was then soaked in 1.0 M KOH aqueous solution, followed by drying and further annealing at 800 °C in nitrogen atmosphere to generate the CF-MSP. CF-MSP exhibits the porous structure and higher surface area than the CF. CV and GCD profiles of the 3-electrode system revealed the quasi-rectangular curves and isosceles triangular shaped profiles, revealing that the CF-MSP has nearly ideal capacitive performance and efficient ion transfer (Fig. 3b,c). Moreover, a symmetric quasi-solid state supercapacitor device was fabricated via assembly of two identical pieces of CF-MSP electrodes filled with lithium hydroxidepolyvinyl alcohol (LiOH-PVA) gel electrolyte. The device has excellent EDL capacitive behavior (Fig. 3d,e), denoting the infiltration of gel electrolyte and ion diffusion in the electrodes are efficient. Also, the symmetric supercapacitor showed an outstanding power density of 250 kW kg−1 at an energy density of 2.8 Wh kg−1. The findings pave the way for enhancing rate capability of supercapacitors and improving their capacitances at ultrahigh current densities. Recently, carbon textiles such as CNT yarns and carbon cloth have been greatly used as flexible and binder-free electrodes to growth a various electroactive materials and achieve high capacitance behavior. Further, vertically grown carbon nanotubes (VCNTs) exhibited a high surface to volume ratio, high electronic and ionic conductivity and favorable packing density together with suitable pore volume, which are results an impressive capacitance. Very recently, Y. Qiu et al. reported the efficient and facile synthesis of VCNTs over carbon nanofibers (CNFs) for high energy flexible supercapacitor applications [39] (Fig. 4a). Initially, various weight ratios (85:10:5; 75:20:5; 70:20:10) of PAN (polyacrylonitrile)/PVP (polyvinylpyrrolidone)/Ni(Ac)2 (Ac = acetate) nanofibers are prepared by electro-spinning method. 3

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Fig. 3. (a) Schematic illustration of the synthesis of CF-MSP. (b) CV and (c) GCD profiles of CF-MSP electrodes at various scan rates and current densities in 3electrode system. (d) CV and (e) GCD curves of a symmetric quasi-solid state supercapacitor device at different scan rates and current densities. Reproduced from ref. [38] with the permission of the American Chemical Society.

performance flexible supercapacitors. In recent years, many flexible/ stretchable polymer based supercapacitor devices have been demonstrated [41]. Recently, Chen et al. developed a facile and scalable route to prepare flexible and binder-free carbon nanofibers yarn@polypyrrole@reduced graphene oxide (CNY@PPy@rGO) coreshell fiber electrode for high performance symmetric fiber-shaped all-solid-state supercapacitors [42]. Recently, other wearable fabric such as Ni-coated cotton fabric is used by Y. Yang et al. and successfully fabricated the composite fabric electrode via a direct alternating filtration of MWCNT and RGO on woven Ni-cotton current collector [43]. The fabrication process of the composite fabric electrode is shown in Fig. 6a. Typically, pre-cleaned cotton fabric was first coated with a conductive Ni layer by a polymerassisted metal deposition (PAMD) route. Next, MWCNT and RGO hybrids were coated on the surface of as-prepared Ni-cotton by vacuum filtration, in which the Ni-cotton was served as the filtering membrane. One alternating filtration cycle resulted in the preparation of one MWCNT/RGO bi-layer on cotton, and fabricated the around ten bilayers by repeating the alternating filtration for ten times. As a result,

the nanofiber structure was obtained and the nanofibers were connected with each other randomly to form a conductive network, which is beneficial for charge transport. High magnified SEM shown in Fig. 5d exhibits a lot of pores with rough surface. Moreover, the TEM and HRTEM images of NSCPCNF-800 delivered a porous nanofibers structure and regular carbon polyhedral structure (Fig. 5e and f). The Inset of Fig. 5f depicts the SAED pattern, which exhibit a amorphous carbon structure of NSCPCNF-800. Fig. 5g shows the structure and charge process of the FSC (NSCPCNF-800//NSCPCNF-800). The CV and GCD curves of FSC are shown in Fig. 5h and i. CV and GCD plots of FSC show asymmetrical shapes, denoting that a faradaic reaction occurs during the electrochemical process, which should be caused by the N, S dual doping. Interestingly, a specific capacitance of 65 F g−1 is obtained at a high current density of 10 A g−1, which reaches 63% of value at 0.5 A g−1 (103 F g−1), demonstrating its excellent rate performance. Further, energy density of 14.3 W h kg−1 at a power density of 250 W kg−1 can be achieved, and a value of 9 W h kg−1 can be maintained even at a high power density of 5000 W kg−1 (Fig. 5j). These results conclude that the NSCPCNF could be a promising electrode material for high4

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Fig. 4. (a) Schematic of the VACNTs/CNFs electrode structure. (b,c) SEM and TEM image of VACNTs/CNFs electrode. (d) CV of VACNTs/CNFs based EDL capacitor at various scan rates. (e) GCD and (f) Ragone plots of VACNTs/CNFs-based EDL capacitors in EMIMBF4 ionic liquid electrolyte. Inset shows the optical image of VACNTs/CNFs-based flexible EDL capacitor and a lighted LED indicator. Reproduced from ref. [39] with the permission of the American Chemical Society.

different current densities (Fig. 7c). The as-developed SSC exhibited a highest gravimetric capacitance of 4.76 F g−1 at 0.4 A g−1 and still maintained 61% of the highest gravimetric capacitance (2.95 F g−1) when current density increased by 20 times to 8 A g−1 (Fig. 7d). Further, the 3D-GCA SSC delivered a maximum energy density of 0.43 W h kg−1 at a power density of 76.36 W kg−1 and maximum power density of 4079.9 W kg−1 at 0.26 W h kg−1 (Fig. 7e). In series combined two fully charged SSCs can power a red light emitting diode for 3 min, digital times for 5 min and a small electric fan for several seconds (inset of Fig. 7e). Furthermore, the properties of the carbon-materials can be modified by the technique of doping. Among the doping, the nitrogen doping with nitrogen atom is highly active as a redox center, which could achieve large pseudocapacitance, making N-doped graphene (NG) an electroactive material for supercapacitors. Recently, X. Wang et al. fabricated the microwave-assisted synthesis of N-doped-graphene-assembled monoliths (M-NGM) for high-performance supercapacitors with using current collectors [45]. The schematic illustration of M-NGM is shown in Fig. 8a. Firstly, a facile solution synthesis of N-doped graphene is prepared and then the self-assembly of NG to form NGM using filtration and freeze-drying. After, microwave treatment of NGM to give M-NGM with a unique nanosac-in-sheet structure. The photographic images of the as-prepared free standing M-NGM is shown in Fig. 8b, 8c. The SEM images clearly show that NG sheets in M-NGM are wrinkled (Fig. 8d) and many bulging sac-like structures with a diameter less than 100 nm on the surface of each NG sheet (Fig. 8e). Without using current collectors or conductive additives, a symmetric solid-state supercapacitor was fabricated using two symmetrical pieces of M-NGM electrodes, which revealing the potential application of M-NGM in supercapacitors. Here, PVA-H2SO4 is used as the electrolyte to fabricate the supercapacitor. All the CV curves at various scan rates show nearly rectangular shape, indicating the ideal supercapacitor performance (Fig. 8f). As shown in Fig. 8g, all the GCD plots show a low IR drop and

the mass loading of MWCNT/RGO composites enhanced from 0.4 to 23.7 mg cm−2 (Fig. 6b), and the film exhibited a final thickness of 400 µm (Fig. 6c). The flexible MWCNT/RGO fabric electrodes were used to prepare waterproof and wearable all-solid-state symmetric supercapacitors with help of PVA/LiCl polymer gel electrolyte (Fig. 6d). As depicted in Fig. 6e,6f, all the CV and GCD curves of the supercapacitor delivered a nearly rectangular sand standard triangular shapes at various scan rates and current densities. The capacitance of the fabricated flexible supercapacitor exhibited a 2.7 F cm−2 at 20 mA cm−2 when using ten-bilayered MWCNT/RGO hybrid fabric electrode. Furthermore, the flexible supercapacitor exhibited a improvement in the capacitance of 3.2 F cm−2 over 10,000 cycles (Fig. 6g). Therefore, with the record-breaking device performance, the ease of fabrication, and the cost-effective of the materials needed, such a fabric-type solid state supercapacitors are very promising for use as wearable energy storage devices. Cheng Zhu et al. demonstrated the fabrication strategy for 3Dprinted graphene composite aerogels (3D-GCAs) with designed architecture for micro-supercapacitor applications [44]. The fabrication process scheme of 3D-GCAs is demonstrated in Fig. 7a. Briefly, the composite inks are prepared by mixing of both GO and graphene nanoplatelets (GNP) and silica fillers, as well as a catalyst (R-F solution with sodium carbonate) to form a homogeneous, highly viscous and thixotropic ink. After, the ink is loaded into a syringe barrel and extruded through a micronozzle to pattern 3D structures. Finally, the printed structures can be processed into aerogels using gelation, supercritical drying and carbonization, followed by etching of silica with hydrofluoric acid. A quasi-solid-state supercapacitor (SSC) was prepared by assembly of a thin separator and two identical 3D-GCAs (inset of Fig. 7d). The fabricated supercapacitor device is termed as 3D-GCA SSC. Significantly all CV plots remained quasi-rectangular shape at various scan rates (Fig. 7b). Moreover, chronopotentiometry curves exhibited a liner and symmetric shape with negligible IR drops at 5

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Fig. 5. Schematic illustration of N, S dual-doped flexible hierarchically porous carbon polyhedra embedded carbon nanofibers (NSCPCNFs) preparation (a) and flexible supercapacitor (FSC) structure (b). SEM (c,d) and TEM (e,f) image of NSCPCNF-800. Schematic illustration of the structure and charge process of FSC (g). Electrochemical performance of NSCPCNF-800//NSCPCNF-800: CV plots of at various scan rates (h), GCD curves at various current densities (i) and Ragone plot of FSC (j). Reproduced from ref. [40] with the permission of the Royal Society of Chemistry.

cm−2 even at 17 mA cm−2. Fig. 9g depicts the volumetric energy and power densities of the as-prepared symmetric SC. The maximum energy density of the fabricated SC is 9.2 mW h cm−3 at a power density of 12 mWcm−3. These results concluded that the facial material synthesis and superior performance make the Fe2O3@ACC composite promising for practical applications in high performance energy storage systems. Moreover, recently, H. Liang et al. fabricated the PH3 plasma activated Fe2O3 (Fe2O3-P) on carbon cloth [47]. The plasma activation strategy could efficiently tune the surface area, conductivity and vacancy (defect) concentration, which are significantly boost the performance of Fe2O3 anodes. Initially FeOOH nanorods were synthesized by hydrothermal method on carbon cloth substrate and then the Fe2O3 nanorods were obtained by the calcination of FeOOH precursor at 450 °C in air at 3 h. The obtained porous Fe2O3 nanorods were then subjected to PH3 plasma to get PH3 activated Fe2O3 (Fe2O3-P) nanorods. As shown in SEM image of Fig. 10a, Fe2O3 delivered the morphology of nanorods, which are aligned on the skeleton of carbon cloth. As depicted in Fig. 10b, the nanorod morphology was well retained after the plasma treatment, while the carbon cloth turns black (bottom of Fig. 10b). TEM image further indicated the porous nanorod morphology (Fig. 10c). The electrochemical performances of the Fe2O3 and Fe2O3-P electrodes were evaluated using a three-electrode configuration in 1 M Na2SO4 electrolyte solution. The Fe2O3-P electrode delivered a higher current density than that of pristine Fe2O3 electrode, demonstrating a great improvement in pseudocapacitive performance due to plasma activation (Fig. 10d). GCD results are shown in Fig. 10e, indicating that the larger covered area can be observed for Fe2O3-P, suggesting higher capacitance. The Fe2O3-P electrode delivered a much higher areal capacitance of 340 mF cm−2 (369 F g−1) at 1 mA cm−2, compared to 66 mF cm−2 (86 F g−1) of Fe2O3 (Fig. 10f). Further, Fe2O3-P electrode also

the symmetrical triangular shapes at various current densities. As-fabricated supercapacitor was exhibited a specific capacitance and area capacitance of 364 F g−1 (at 0.5 A g−1) and 894 mF cm−2, respectively. Further, the M-NGM supercapacitor delivered a maximum energy density of 12.2 W h kg−1 and a high power density of 16.2 KW kg−1 (Fig. 8h). Therefore, the M-NGM could be a promising free-standing electroactive material for supercapacitor applications. Due to the excellent electronic conductivity and large surface area, the activated carbon cloth gained the great attention as a most appreciate matrices for loading the electroactive materials and its flexibility is desirable for wearable electronic devices. Recently, J. Li et al. prepared the porous Fe2O3 nanospheres anchored on activated carbon cloth (Fe2O3@ACC), which acted as excellent electroactive material for supercapacitors [46]. The fabricated Fe2O3@ACC electrode and the structure of symmetric are depicted in Fig. 9a. A facile hydrothermal technique was used to fabricate porous Fe2O3 nanospheres on the activated carbon cloth, which can be directly used as an electrode for supercapacitor applications. The SEM image in Fig. 9b clearly shows that the particles are roughly spherical structure and compactly stacked on the carbon fibers. The HR-TEM image is depicted in Fig. 9c, which clearly exhibited the lattice fringes of the (110) plane could be found with interplanar spacing of 0.37 nm. Interestingly, the electrochemical properties of the Fe2O3@ACC electrode are measured in the positive and negative potential windows (Fig. 9d). The CV results of the electrode exhibited the ideal rectangular shape under the positive and negative potential windows at 10 mV s−1. Also, the assembled symmetric SC in 3 M LiNO3 aqueous electrolyte delivered an ultrahigh operating voltage window from 0 to 1.8 V (Fig. 9e). Fig. 9f depicts the GCD plots of symmetric SC various current densities, which exhibited area specific capacitance of 1565 mF cm−2 at 1 mF cm−2 and maintained a 750 mF 6

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Fig. 6. Schematic illustration of the fabrication of composite fabric samples using alternating filtration. (b) Mass loading of carbon nanomaterials with respect to filtration cycles. (c) Cross-sectional SEM image of the 10-bilayer composite fabric electrode. (d) Fabricated water-proof all-solid-state fabric type supercapacitor. (e) CV and (f) GCD plots of supercapacitor at various testing parameters. (g) Cycling stability of as-fabricated device measured at 20 mA cm−2. Reproduced from ref. [43] with the permission of the John Wiley & Sons Ltd.

lamellar hybrid comprising vanadium nitride nanodots intercalated carbon nanosheets (VNNDs/CNSs) for high-performance flexible supercapacitors [48]. Fig. 11a schematically shown the synthesis procedure for the 2D VNNDs/CNSs. The intercalated polyaniline are insitu converted into 2D CNSs via confining carbonization, while the sandwiched V2O5 layers are converted into 0D VNNDs with the diameter of 3–5 nm forming 0D-in-2D pillared lamellar structure. SEM image show that V2O5.1•6H2O nanosheets have a lateral size of 0.5–2 µm and thickness of 4–10 nm (Fig. 11b). The SEM image in Fig. 11c exhibited

possesses a superior cycling stability with 83% capacitance retention over 5000 cycles at 20 mA cm−2 (Fig. 10g). Therefore, the new plasma activation is a useful technique for improving the performance of flexible supercapacitor. Nanostructured transition metal nitrides such as vanadium nitride (VN) have gained much attention as promising electrode materials for SCs due to their unique properties of intrinsic metallic conductivity, large pseudocapacitance, and high density. Recently, Q. Li et al. reported that a spatially confined strategy to develop 0D-in-2D pillared

Fig. 7. (a) Schematic diagram of fabrication process of three-dimensional hierarchical graphene aerogels. (b) CV and (c) CD profiles of 3D-GCA collected in 3 M KOH SSC at various scan rates and current densities. (d) Gravimetric capacitance and capacitive retention values. Inset shows the schematic diagram of the 3D-GCA SSC. (e) Ragone plot of the 3D-GCA SSC. Reproduced from ref. [44] with the permission of the American Chemical Society. 7

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Fig. 8. (a) Schematic illustration of the M-NGM electrode. (b,c) Photographic images of free-standing and bent M-NGM. (d,e) SEM images of M-NGM at different magnifications. (f) CV and (g) GCD profiles of M-NGM supercapacitors with PVP-H2SO4 electrolyte at various scan rates and current densities. (h) Ragone plot of MNGM supercapacitor. Reproduced from ref. [45] with the permission of the American Chemical Society.

the VN shells, respectively (Fig. 12c). The electrochemical measurements of the as-developed electrode were carried out using three electrode setup in 3 M KOH aqueous electrolyte solution. As depicted in Fig. 12d, all the CV profiles at different scan rates delivered an approximately rectangular shape without severe polarization, demonstrating excellent reversibility and high rate performance for TiN@VN NWAs. Further, the symmetric shapes of the GCD plots at different current densities also denote the excellent reversibility (Fig. 12e). Due to the excellent core-shell heterostructures, the TiN@VN NWAs delivered a specific capacitance of 1255.2 mF cm−2 at 1 mA cm−2 and even maintains at specific capacitance of 854.6 mF cm−2 at high current density of 10 mA cm−2, demonstrating high rate performance TiN@VN NWAs. These findings provided the convenient and effective strategy for the design of new electroactive materials on flexible CNTF for next generation wearable SCs. Many of the reports demonstrated that the energy storage performance of SCs is mainly based on the active materials. However, for practical applications, the weight/volume of the device including the package and non-active material substrates are also key parameters to achieve high-performance SCs. Indeed, the non-active material substrate can be heavy in weight and large in volume, which can be more than 10 times than that of the electroactive material. However, it is a big challenge to fabricate high-energy storage devices with a low content of non-active materials, yet the device performance can be drastically deteriorated by reducing or eliminating the use of these nonactive but conductive/stable substrates. With above consideration, C. Guan et al. developed a novel hierarchical iron oxide nanoparticles decorated on 3D ultrathin graphite foam-carbon nanotube forest substrate (GF-CNT@Fe2O3) [50]. The schematic of the growth procedure of GF-CNT@Fe2O3 is shown in Fig. 13a. In detail, Co-Ni catalyst is adopted to synthesize CNTs forest on the graphite foam. Further, the GF-CNT

the PANI/V2O5 hybrid nanosheets have a similar morphology to V2O5.1•6H2O. The SEM image in Fig. 11d revealed that the VNNDs/ CNSs have nanosheet morphology similar to the PANI/V2O5 nanosheets. The magnified SEM image of the VNNDs/CNSs in Fig. 11e exhibited that a large number of nanodots are well anchored on the 2D nanosheets, these nanodots closely resembles the pillars which are sandwiched between the interlayer of nanosheets forming 0D-in-2D pillared lamellar structure. The flexible all solid-state symmetrical device composed of the VNNDs/CNSs electrodes and PVA-KOH gel electrolyte, shown in Fig. 11f. After full charging, three SCs connected in series can lighten a red LED with a turn-on voltage of 2 V. All the CV curves at various scan rates exhibited the quasi-rectangular shape with distortion, indicating high reversibility and rate capability (Fig. 11g). The device delivered an excellent cycling stability with 91% capacity retention over 10,000 cycles (Fig. 11h). Further, the flexible symmetrical SC delivered a high volumetric energy density of 30.9 WhL−1 and high power density of 64,500 WL−1. These results provide insights into design and development of high-performance electrode materials for flexible SCs. Recently, Q. Li et al. fabricated the hierarchical vanadium nanosheets with the titanium nitride nanowire arrays (TiN@VN NWAs) on a flexible carbon nanotube fiber (CNTF), which can be successfully used as a novel binder-free anode material for SCs [49]. TiN NWAs are initially coated on the CNTF using hydrothermal route followed by nitrogenization treatment in ammonia/argon. After, VN NSs are grown on the TiN NWAs by hydrothermal method followed by nitrogenization. As depicted in Fig. 12a, VN NSs are homogeneously anchored on the TiN NWAs surface to form a 3D hierarchical core-shell structure. The TEM studies also confirmed the TiN@VN NWAs core-shell heterostructures (Fig. 12b), and the HR-TEM image exhibited the lattice fringes with spacing of 0.245 and 0.341 nm, indexed to the (200) and (220) plane of 8

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Fig. 9. (a) Schematic illustration of the Fe2O3@ACC sample and the structure of symmetric supercapacitor. (b). SEM and (c) HR-TEM images of the Fe2O3 nanospheres. (d) CV profiles of Fe2O3@ACC electrode in both positive and negative potential regions. (e) CV and (f) GCD profiles of Fe2O3@ACC symmetric supercapacitor with 3 M LiNO3 electrolyte at various scan rates and current densities. (h) Ragone plot of Fe2O3@ACC symmetric supercapacitor. Reproduced from ref. [46] with the permission of the Elsevier.

2.2. Flexible substrates to fabricate cathode based electrode materials for supercapacitors

substrate coated with nano-crystalline Fe2O3 using atomic layer deposition. SEM image in Fig. 13b clearly depict the CNTs have been covered over on the 3D graphite foam surface. Enlarge image in Fig. 13c clearly reveal the CNTs were uniformly coated with many small Fe2O3 nanoparticles over graphite foam. TEM image shown in Fig. 13d show that CNT is uniformly covered with tiny Fe2O3 nanoparticles. These nanoparticles facilitate the access of electrolyte to large surface area and thus achieving high energy storage performance. The electrochemical performance of the GF-CNT@Fe2O3 is investigated with ALD cycles of 200, 400 and 600, respectively (denoted as GF-CNT@ 200Fe2O3, GF-CNT@400Fe2O3 and GF-CNT@600Fe2O3). The CV plots in Fig. 13e revealed that the GF-CNT exhibited a rather rectangular shape, typically for EDLC, while the CV plot of GF-CNT@400Fe2O3 delivered apparent redox peaks (pseudocapacitive behavior). The area of the GF-CNT@400Fe2O3 is much higher than the GF-CNT, revealing the capacitance is enhanced after the Fe2O3 coating. The charge-discharge plots shown in Fig. 13f also show the capacitance increased after the Fe2O3 coating. At 20 mA cm2, GF-CNT@400Fe2O3 delivered an areal capacitance of 470.5 mF cm−2, which is 4 times than that of GFCNT (93.8 mF cm−2). Interestingly, the GF-CNT@400Fe2O3 delivered an excellent cycling stability of 111.2% of the initial capacitance even after 50,000 cycles at 20 mA cm−2 (Fig. 13g). The above findings reveal that the GF-CNT@Fe2O3 is rather stable supercapacitor anode with high energy storage performance. Therefore, in this section we discussed about various flexible substrates to fabricate the high-performance anode materials for supercapacitors. Table 1 lists recent advancements in flexible anode materials and/or symmetric devices with their corresponding electrochemical performance.

Tremendous efforts have been made in the development of metalbased materials (metal oxides/nitrides/sulfides/carbides). Compared to carbon-based materials, metal-based materials, such as, pseudocapacitive materials (MnO2 and RuO2) and battery-type materials (NiO, Co3O4, NiCo2O4, Ni(OH)2, etc.) delivered excellent energy storage behavior owing to their surface and diffusion-controlled electrochemical redox reactions with electrolyte ions. Porous metals such as nickel foam, nano-porous, gold/silver and stainless steel mesh have been usually used for fabricating pseudocapacitive materials, but their rigidness leads to devices less flexible and also lower energy densities. To address this issue, carbon cloth, silver-sputtered textile cloth, carbon nanotube fibers, copper/nickel coated polyester fabric, Soft Pt foil, Cu/ PET fiber, and stainless steel (SS) wire mesh are attractive materials for fabricating flexible electrodes due to their high flexibility and strength. Among the various substrates, the carbon cloth is very suitable as the substrate material of flexible electrode material due to its good electrical conductivity, chemical stability, good mechanical strength and flexibility. Moreover, pure carbon cloth has no pseudocapacitive properties. Hence, it is very wise and feasible to fabricate flexible electrode materials by loading transition metal compounds with pseudocapacitive/batteries properties on carbon cloth. W. Zeng et al. fabricated the unique core-shell ZnO@C@NiCo2O4 nanorod sheet arrays (NRSAs) on a carbon cloth substrate using facile hydrothermal and electrodepositon route [51]. Benefiting from the introduction of carbon obtained from metal-organic frameworks (MOFs) and the core-shell structure, the composite electrode achieved the higher energy storage performance (2650 F g−1 at 5 A g−1). 9

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Fig. 10. SEM images and digital photographs of (a) Fe2O3 and (b) Fe2O3-P nanorods. (c) TEM image of Fe2O3-P nanorods. (d) CV and (e) CD plots of Fe2O3 and Fe2O3P nanorods electrodes at constant scan rate and current density. (f) Capacitance values of electrodes. (g) Cycling behavior at 20 mA cm−2. Reproduced from ref. [47] with the permission of the Elsevier.

Fig. 15c shows that the NiCo2O4 nanowires surface was becomes rougher and larger after the deposition of NiMoO4, in which the NiCo2O4 nanowires are decorated with many tiny NiMoO4 nanosheets. Such composite core-shell NiCo2O4@NiMoO4 nanowires flexible electrode achieved the flowing properties: (1) the synergic effect of NiCo2O4 and NiMoO4 can further enhance the electrochemical performance; (2) the binder-free architecture can offers a rapid electron transfer path, effectively improving the rate performance; (3) the coreshell nanostructure is more stable, providing an outstanding cycling stability at high current density. The electrochemical behavior of NiCo2O4@NiMoO4 electrode was examined in a three-electrode setup using 3 M KOH electrolyte. Fig. 15d depicts the comparative GCD plots of the three electrodes at 2 mA cm−2. The results indicates that the discharge time of NiCo2O4@NiMoO4 electrode is longest, demonstrating its best specific capacitance. Further, CV test was also conducted at various scan rates (Fig. 15e). CV results of NiCo2O4@NiMoO4 electrode exhibit the well-defined redox peaks in each CV plot are indexed to the reversible faradaic redox reactions of Ni/Co-O, Ni/Co-OOH with OH−. The electrons and electrolyte ions transport in the electrode is schematically illustrated in Fig. 15f. NiCo2O4 has higher electronic conductivity, so electrons can effectively transfer from the carbon cloth to the NiMoO4 nanosheets through the backbone of the NiCo2O4 nanowires. Recently, H. Liang et al. also used the flexible carbon paper substrate to fabricate the metal phosphides via PH3 plasma for supercapacitor applications [55]. The high reactivity of plasma supports rapid and low temperature conversion of hydroxides into monometallic, bimetallic, or even more complex nanostructured phosphides. Initially, metal hydroxide nanoplates precursors were grown on carbon paper using hydrothermal method and then synthesized the hydroxide nanoplates into phosphide porous nanoplates using PH3 plasma, as shown in Fig. 16a-c. SEM image in Fig. 16d depicts the NiCoP exhibits a hierarchical structure with nanoplates lying aslant or perpendicular to the substrate. Further, the HRTEM image in Fig. 16e exhibited the

Besides, Zhou and co-workers facially developed promising uniformly stacked lamella micro-/nanostructured Co9S8 materials on carbon cloth using two-step hydrothermal method [52]. Initially, CoCO3 particles composed of nanosheets are grown on carbon cloth and then vulcanized with sodium sulfide, as illustrated in Fig. 14a. As shown in Fig. 14b, uniformly stacked lamella structure of Co9S8 particles are grown over carbon cloth surface, such special micro/nanostructures are more suitable as electrode materials for supercapacitors. It is evident from the schematic demonstration of crystal structure (Fig. 14c), it is clearly seen that how the CoCO3 crystal converts into the Co9S8 crystal, which is due to the CO32− ions in the CoCO3 crystal are gradually replaced by S2− ions as time goes on. Fig. 14d depicts the GCD profiles of Co9S8 at various current densities, which delivers the highly linear and nearly symmetric nature of the Co9S8 indicate that an outstanding reversibility and a rapid I-V response are obtained (Fig. 14d). As result, the flexible Co9S8 electrode achieved excellent electrochemical properties including a high specific capacitance (1475.4 F g−1 at 1 A g−1), a good rate capacity (80.2% retention at 20 A g−1). Further, at various bending angles, the Co9S8 electrode achieved the similar specific capacitance (Fig. 14e), which demonstrates the Co9S8 on carbon cloth can be served as a flexible electrode material for supercapacitors. Recently, X. Zheng et al. developed the new interface structure by embedding a TiO2 layer between ZnO and Ni(OH)2 to support drive electrons towards the Ni(OH)2/electrolyte reaction interface [53]. The stair-like bands is favourable for electron transport, which results in a lower equilibrium potential and a higher capacitance. Besides, L. Huang et al. prepared the hierarchical core-shell NiCo2O4@NiMoO4 nanowires were grown on carbon cloth via a two-step hydrothermal reaction, which can served as a flexible binder-free electrode for supercapacitors [54]. The schematic preparation process of NiCo2O4@NiMoO4 structures on carbon cloth is depicted in Fig. 15a. SEM image shown in Fig. 15b exhibited the aligned NiCo2O4 nanowire morphology with diameter of 200 nm, which were grown on carbon fiber substrate. 10

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Fig. 11. (a) Schematic illustration of the preparation of the VNNDs/CNS. (b) SEM images of (b) V2O5•1•6H2O, (c) PANI intercalated V2O5 nanosheets, and (d) VNNDs/CNSs. (e) High-resolution SEM image of VNNDs/CNSs. (f) Schematic and digital photos of the flexible solid-state SC showing the outstanding flexibility boding well for portable applications such as power a red LED. (g) CV plots of the supercapacitor at various scan rates. (h) Cycling stability at 10 A g−1 and inset depicts the CV plots of solid-state SC before and after cycling test. (i) Ragone plot of solid-state SC composed of VNNDs/CNSs. Reproduced from ref. [48] with the permission of the Elsevier.

NiMoO4•xH2O core-shell heterostructures on carbon fabric using facile hydrothermal route with CoMoO4 nanowires (NWs) as the core and NiMoO4 nanosheets (NSs) as the shell [57]. This core-shell structure could provide rapid charge transport, abundant active sites, and good strain accommodation. As a result, the CoMoO4@NiMoO4•xH2O sample achieved high energy storage performance with a high specific capacitance of 1582 F g−1, excellent cycling life with the capacitance retention of 97.1% after 3000 cycles and good rate capability. Cotton fabrics (CFs) gained considerable attention as they can be easily converted into conductive cotton fabrics (CCFs) through hightemperature carbonization for energy storage applications. However, their poor conductivity and low hydrophilicity constitute big obstacles for supercapacitors. Hence, the conductivity and hydrophilicity of CCF is enhanced by combine with transition metal oxides/hydroxides, supporting both EDLC and pseudocapacitance behaviors for effective improvement of capacitance. Very recently, T. Xia et al. fabricated the growth of an unique nanostructured Ni(OH)2 layer on CCF using facile high-temperature carbonization process and subsequent electrochemical deposition (ED) treatment [58]. In detail, a schematic illustration of the fabrication process of the Ni(OH)2 layer on CCF is depicted in Fig. 17a. The Ni(OH)2 layer on CCF exhibited the equilibrium contact angle of 0°, as depicted in Fig. 17b. The Ni(OH)2@CCF exhibited a pores and petals morphology (Fig. 17c). The electrochemical performance of the as-assembled Ni(OH)2@CCF//Ni(OH)2@CCF SC devices was examined in the symmetrical two-electrode configuration.

lattice spacing of 0.22 nm, which is well agree with D-spacing of the (111) planes of NiCoP. The electrochemical performance of NiCoP nanoplates was carried out using three-electrode setup using in 1 M KOH. Fig. 16f depicts the CV profiles of NiCoP at different scan rates. All the CV profiles exhibit a pair of redox peaks associating with the faradaic redox reactions. Further, the potential plateaus in the non-linear GCD curves are associated to the typical faradaic reactions (Fig. 16g), well agree with the CV profiles. Interestingly, the NiCoP flexible electrode achieved a high specific capacity of 194 mAhg−1 at 1 A g−1 and it was retained up to 194 mAhg−1 at 10 A g−1 with a capacity retention of 87%. Moreover, after 5000 cycles, the NiCoP electrode exhibited the capacity retention of 81% with a good Faradaic efficiency, revealing an excellent electrochemical stability of material. Hence, the plasma-assisted method can be generally applied to develop different nanostructured metal phosphides for various applications. S. Liu et al. reported the growth of CuCo2S4 tubular nanostructures on flexible carbon fiber for use as positive electrode materials for supercapacitors [56]. The CV plots of CuCo2S4 at various scan rates exhibited a pair of welldefined redox peaks corresponding to the Faradaic reactions of Cu2+/ Cu+ and Co4+/Co3+/Co2+ related to OH− in the electrolyte. The GCD plots delivered obvious voltage plateaus, denoting the typical battery behavior of battery-type materials. Interestingly, the CuCo2S4 electrode achieved the specific capacity of 875 C g−1 at 1 A g−1 and it was retain 323 C g−1 when the current density increased to 15 A g−1. Recently, J. Wang and co-workers demonstrated the growth of CoMoO4@ 11

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Fig. 12. (a) SEM, (b) TEM and (c) HR-TEM image of TiN@VN NWAs/CNTF. (d) CV and (e) GCD profiles of TiN@VN NWAs/CNTF electrode at various scan rates and current densities. Reproduced from ref. [49] with the permission of the John Wiley & Sons Ltd.

revealing the pseudocapacitance behaviors of Ni(OH)2. The assembled symmetric supercapacitor achieved a high specific capacitance of 131.43 F g−1 at 0.25 A g−1 with a high energy density (35.78 Wh kg−1 at a power density of 0.35 kW kg−1, and it can reach 18.28 Wh kg−1 at

The CV profiles exhibited the combination of both pseudocapacitance and EDLC behaviors, and there is no distortion even at high scan rates (Fig. 17d). Further, GCD plots in Fig. 17e exhibited distinct nonlinearities, which were quite different from those for EDLC capacitors,

Fig. 13. (a) Growth process of GF-CNT@Fe2O3 staring form graphite foam. (b) SEM, (c) TEM and (d) HR-TEM images of GF-CNT@Fe2O3. (e) CV and (f) CD profiles of GF-CNT@400Fe2O3 and GF-CNT. (g) Cycling stability of GF-CNT@400Fe2O3 over 50,000 cycles at 20 mA cm−2. Reproduced from ref. [50] with the permission of the American Chemical Society. 12

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[43] [44] [45] [46] [47] [48] [49] [50] 2675 mF cm−2 at 20 mA cm−2 63.6 F g−1 at 10 A g−1 364 F g−1 at 0.5 A g−1 2775 mF cm−2 at 1 mA cm−2 369 F g−1 at 1 mA cm−2 573.1 F g−1 at 0.5 A g−1 1255.2 mF cm−2 at 1 mA cm−2 659.5 mF cm−2 at 5 mA cm−2 PVA-LiCl 3 M KOH PVA-H3PO4 3 M LiNO3 1 M Na2SO4 PVA-KOH 3 M KOH 2 M KOH MWCNT/RGO 3D-printed graphene aerogel lattice microwave-assisted N-doped-graphene monoliths (M-NGM) Porous Fe2O3 nanospheres@ACC Fe2O3-P VNNDs/CNSs TiN NWAs@VN NSs CNT@Fe2O3

2-electrode 2-electrode 2-electrode 2-electrode 3-electrode 2-electrode 3-electrode 3-electrode

2-electrode

Carbon nanofibers yarns (CNY) Ni-cotton fabrics self-supporting self-supporting Carbon cloth Carbon cloth Carbon paper carbon nanotube fibers Graphite foam

PVA/H3PO4

a high power density of 14.00 kW kg−1) (Fig. 17f). Moreover, three symmetric supercapacitors were assembled in series and power a green LED and even light up eight green LEDs simultaneously, suggesting a great potential for real-world applications (Fig. 17g). Carbon nanotube fibers (CNTFs) are considered as promising candidates for electrodes owing to their lightweight nature, outstanding flexibility, high electrical conductivity and chemical inertness. As the electrode materials, the conductivity and strength of CNTFs should be satisfied with the necessity of supercapacitor. X. Wang and co-workers synthesized the manganese–nickel–cobalt sulfide (MNCS) multi-tripod nanotube arrays (NTAs) on a CNTFs surface using cost-effective hydrothermal process and anion exchange [59]. Fig. 18a depicted the schematic of the fabrication process of MNCS multi-tripod NTAs on CNTFs. Initially, trimetallic carbonate hydroxide precursors were generated by a simple hydrothermal process (Step 1) by dissolving of Mn2+, Ni2+, Co2+ and hydrolysis products of urea (OH− and CO32−). The Mn-Ni-Co precursors exhibit nanowire morphology and are vertically aligned on the CNTFs surface. Then, the anion-exchange reaction produces in the conversion of Mn-Ni-Co nanowire arrays (NWAs) precursor into MNCS multi-tripod nanotube arrays (NTAs). SEM image in Fig. 18b exhibit that the top ends of the MNCS array touched, which forming overlap joints of multi-tripod nanoarray structure. Also, a large number of tiny particles are distributed on the surface of all arrays. EDX mapping results evidently identify that the Mn, Ni, co and S elements are uniformly distributed in the MNCS NTA (Fig. 18c). The electrochemical performance of the as-fabricated Ni-Co NWA (NCO NWA), Mn-Ni-Co NWA (MNCO NWA) precursors and MNCS NTAs are examined using three-electrode configuration in 1 M KOH aqueous electrolyte. Fig. 18d depicts the comparative GCD plots of the NCO NWA, MNCO NWA precursors and MNCS NTAs at constant current density of 2 mA cm−2. As expected, the MNCS NTA sample delivered a much longer discharge time than the NCO and MNCO samples at the same current density, denoting that the MNCS NTA electrode has a higher specific capacitance. Interestingly, the MNCS NTA electrode exhibited a high volumetric capacity of 2554.5 F cm−3 (7.025 F cm−2) at 1 mA cm−2, which is about 2.5 and 5.4 times that of the MNCO NWAs (1010 F cm−3) and NCO NWAs (477.3 F cm−3). CV plots of MNCS NTS electrode are depicted in Fig. 18e, which exhibited a pair of redox peaks in the 0 to 0.4 V potential range, implying typical pseudocapacitive behavior. Also, the CV plots maintained a similar CV shape without any distortion even at high scan rate, indicating that the MNCS NTA electrodes are favorable for rapid charge and discharge processes. Further, the MNSC NTS electrode maintained the capacitance retention of 94.8% to the initial capacitance over 5000 cycles at 10 mA cm−2 (Fig. 18f). Besides, G. Nagaraju et al. utilized the cost-effective conductive textile substrate (CTs) for supercapacitor applications [60]. In detail, hierarchical 3D porous nanonetworks of nickel–cobalt layered double hydroxide (Ni–Co LDH) nanosheets (NSs) are deposited on flexible CTs by a simple two-electrode system based electrochemical deposition (ED) method. Under an external cathodic voltage of −1.2 V for 15 min, the Ni–Co LDH NSs were grown on conductive fibers with good adhesion. As a pseudocapacitor electrode, the flexible Ni–Co LDH NSs electrode provides rapid charge transfer, abundant active sites and good structural stability, which lead to superior electrochemical performance. Interestingly, the Ni–Co LDH NSs electrode exhibited the specific capacitance of 2105 F g−1 at 2 A g−1 and an outstanding capacitance retention of 89.5% was achieved over 2000 cycles in 1 KOH electrolyte at 10 A g−1, indicating a good electrochemical stability. Commercial carbon textile cloth (CTC) has outstanding mechanical flexibility, strength, and conductivity, thus has considered for realization of wearable electronic devices. However, the electrochemical stability of grown nanostructures on their ligaments is unsatisfying and the CTC is very expensive. To address this issue, J. Zhu et al. reported a significant advance toward wearable textile supercapacitors by developing synthesizing nanotube-built multitripod arrays of FeCo2S4–NiCo2S4 composites on a silver-sputtered textile cloth (SSTC)

–

[42] 92.57 F g−1 at 0.1 A g−1

– 0.43 12.2 9.2 mWh cm−3 – 30.9 WhL−1

[37] [38] [39] [40] g−1 at 0.1 A g−1 F g−1 at 1 A g−1 F g−1 at 0.5 A g−1 g−1 at 0.5 A g−1 191 F 374.7 146.8 103 F – 2.8 70.7 14.3 3-electrode 2-electrode 2-electrode 2-electrode Flax fiber textile Carbon foam Carbon nanofibers Carbon nanofibers

CF-CNT CF-MSP Carbon nanotubes N, S dual-doped porous carbon polyhedra embedded carbon nanofibers (NSCPCNFs) CNY@PPy@rGO// CNY@PPy@rGO

6 M KOH LiOH-PVA EMIMBF4 PVA-H2SO4

Reference Specific capacitance/areal capacitance at current density Energy density (W h Kg−1) Electrolyte Measurement configuration Current collector Electrode materials

Table 1 Summary on the electrochemical performance of various flexible and wearable anode materials for supercapacitors. The mass energy density is measured based on the total weight of the symmetric device.

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Fig. 14. (a) Schematic illustration of fabrication process of Co9S8 on carbon cloth. (b)SEM image of Co9S8 on carbon cloth. (c) Schematic structure of rhombohedral CoCO3 and cube Co9S8. GCD plots of Co9S8 on carbon cloth at (d) various current densities and (e) at various bending angles at 20 A g−1. Reproduced from ref. [52] with the permission of the Royal Society of Chemistry.

CFS-CNS structures exhibited the high surface area and also support the fast charge transfer. In three-electrode setup, the CFS–CNS structures on CNF achieved the high specific capacity of 183.4 mA h g−1 at 1 A g−1, with an excellent rate capability of 172.4 mA h g−1 at 8 A g−1 and exceptional cycling stability with 99.2% retention over 3000 cycles in aqueous 3 M KOH electrolyte. Moreover, the symmetric supercapacitor was fabricated using the hierarchical CFS-CNS structures with KOH electrolyte. The assembled fabric based CFS–CNS//CFS–CNS symmetric SC delivered a maximum energy density of 80.2 W h kg−1 at 1000 W kg−1 and achieved an outstanding cycling lifespan with 97.02% retention over 3000 cycles. Also, the symmetric SC showed the excellent flexibility to sustain different deformations including bending and twisting. Generally, textile fibers could be weaved into woven and nonwoven fibrous texture. The non-woven textile fibers possess a highsurface area compared to the woven textiles due to the intertwined and disorderly arranged fibrous network. Thus, the non-woven conductive textile substrate (NWCTs) containing of metallic Cu thin film on polyethylenterephthalate fibers can be used as a flexible and highly conductive electrode due to the low resistivity, three-dimensional (3D) fibrous framework, good porosity, and cost effectiveness. Such high conductive behavior and porous fibrous-structure framework current collector support a fast electron transport to improve charge storage properties and probably enable the use for wearable energy storage devices. Cha and co-workers facially prepared the ultrathin β-Ni(OH)2 nanosheets (NSs) on non-woven conductive textile substrate (NWCTs) using simple electrochemical deposition method [63]. The binder-free hierarchical β-Ni(OH)2 NSs on NWCTs exhibited an excellent energy storage performance (specific capacitance of 2185.6 F g−1 at 5 A g−1 and capacitance retention of 95% over 1000 cycles) in 1 M KOH electrolyte for pseudocapacitor applications. Moreover, Nagaraju et al. demonstrated a facile and cost-effective growth of Co3O4 nanoplate arrays (NPAs) on nickel (Ni) coated polyethylenterephthalate (PET) fibers woven conductive fabric substrate using the two electrode system based electrochemical deposition (ED) method [64]. Then, the Co3O4 NPAs on flexible substrates were directly used as a binder-free electrode, achieving a specific capacitance of 145.6 F g-1 at 1 A g−1 with

[61]. This novel hierarchical architecture is highly porous and also facilitates rapid charge transfer for electrochemical reactions. The silver coating not only supports the homogeneous nucleation and induces growth of long precursor nanowires, but also further improves the composites electrical conductivity and enhances the interfacial bonding force between the grown architectures and SSTC. A piece of pristine textile cloth was obtained from a lab coat (Fig. 19a) followed by silver sputtering and sulfuration (Fig. 19b). Fig. 19c schematically demonstrates the fabrication process of ternary metal mixed sulfides on a SSTC. The SEM image of FeCo2S4–NiCo2S4 exhibited the multitripod arrays grown on SSTC (Fig. 19d). SEM-energy-dispersive X-ray spectroscopy (EDS) spectrum (Fig. 17e) depicts elements’ characteristic peaks, evidencing the presence of the Ni, Fe, Co, and S signals, and no impurities can be identified (Fig. 19e). The solid-state symmetric supercapacitor was assembled with FeCo2S4–NiCo2S4 electrodes and an electrolyte poly(vinyl alcohol) (PVA)/KOH film as separator. Fig. 19f showed the CV profiles of the supercapacitor at various scan rates within a potential window of 0 to 1 V. All the CV plots achieved the similar CV shape without any shape distortion, indicating the efficient ionic and electronic transports in the electrode materials. Further, the symmetric supercapacitor exhibited the non-linear charge-discharge profile, which is due to the contribution of pseudocapacitance from the sulfides composite (Fig. 19g). Importantly, solid-state supercapacitor achieved a high energy density of 46 W h kg−1 (at 1070 W kg−1) and high energy specific power of 4723 W kg−1 (at 17.1 W h kg−1) (Fig. 17h). Therefore, silver sputtering is a promising and simple physical process than can completed in few minutes and appropriate to almost all kinds of fiber cloth, thus silver sputtering on textile cloths provides a general, cost-effective, green, and scalable route to wearable energy-storage devices. Very recently, Gopi and co-workers successfully utilized the copper/ nickel coated polyester fabric (CNF) as a flexible substrate to deposit the electroactive materials for supercapacitor applications [62]. Typically, microsphere-like CoFe2Se4–CoNiSe2 (CFS–CNS) nanostructures are prepared via a facile hydrothermal route and facially integrated on flexible and conductive CNF substrate for SCs. The novel hierarchical 14

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Fig. 15. (a) Schematic illustration of the preparation process of NiCo2O4@NiMoO4 on carbon cloth. SEM images of (b) NiCo2O4 nanowires and (c) hierarchical coreshell NiCo2O4@NiMoO4 nanowires on the carbon cloth. (d) Comparative charge-discharge plots of as-prepared electrodes at 2 mA cm−2. (e) CV profiles of NiCo2O4@ NiMoO4 electrode at various scan rates. (f) Schematic illustration showing the merits of NiCo2O4@NiMoO4 on carbon cloth during electrochemical measurements. Reproduced from ref. [54] with the permission of the Nature publishing groups.

89.15% capacitance retention over 1000 cycles at 3 A g−1 in 1 M KOH electrolyte solution. As is well known, the transparent conductive films (TCFs) based on metallic nanowire (NW) interconnected networks deliver high conductivity and good transparency, which are considered as a promising current collector with high efficiency for electron transport. However, it is a big challenge to effectively attach active materials to metallic NW surfaces without damaging their structures and consequently enhance the electrochemical performance of TFS electrodes. To address this issue, very recently, J. Liu et al. prepared the flexible and transparent electrodes based on one-dimensional Ag nanowire@NiCo/NiCo(OH)2 core–shell nanostructures with a tunable composition using electrodepositon method for supercapacitor applications [65]. Here, the highly conductive Ag nanowire (NW) network considered as a current collector, and NiCo/NiCo(OH)2 nanostructures were grown on the Ag NWs surface. The fabrication process of the core-shell Ag NW@NiCo/ NiCo(OH)2 nanostructures is demonstrated in Fig. 20a. At first, a PVA solution was deposited over clean PET substrate using a spin-coating route. Then, the Ag NW dispersion in ethanol was dropped on the PVA film surface to produce Ag NW percolation networks by another spin-

coating process. Finally, the Ag NW@NiCo/NiCo(OH)2 core-shell structures was fabricated using the electrodepositon method. To examine the synergistic effects of Ni and Co elements, various concentration ratios of Ni to Co (1:0, 0.75:0.25, 0.5:0.5, 0.25:0.75 and 0:1) were applied during the electrodepositon. Fig. 20b depicts the SEM image of the Ag NW film, denoting that the diameter of the Ag NWs ranges from 80 to 150 nm. It can be clearly seen that the deformations of the NWs caused by the mechanical press, which are marked by red dashed circles. As shown in Fig. 20c, the Ag NWs surfaces were uniformly covered by the active material and the average diameter of the NWs increased to 500 nm at Ni : Co = 0.5 : 0.5. To examine the synergistic effects of Ni and Co elements, the electrochemical performances of the samples with various ratios of Ni to Co were elaborately measured in 1 M KOH solution. Fig. 20d depicts the CV profiles of five samples at a constant scan rate of 5 mV s−1. All the samples exhibited the distinct faradaic redox peaks around 0.1 V and 0.3 V, denoting that the electrochemical behavior of the samples is mainly occurred by the reversible valence state changes between Ni2+/Ni3+and Co2+/Co3+. Ni–Co ratio of 0.5:0.5 electrode exhibited the high peak intensity, revealing the largest electrochemical activity (Fig. 20e). At 0.1 mA cm−2, 15

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Fig. 16. (a-c) Schematic illustration of the fabrication process of metal phosphide nanostructures. (d) SEM and (e) HRTEM image of NiCoP nanoplates on carbon paper substrate. (f) CV and (g) GCD plots of NiCoP electrode at various scan rates and current densities. (h) Cycling stability at 10 A g−1. Reproduced from ref. [55] with the permission of the Elsevier.

energy density (E = 0.5CV2), the energy density could be expanded by enhancing either the specific capacitance (C) or operating potential window (V). To address this key issue and make the SCs for commercial applications, fabrication of battery-supercapacitor hybrid (BSHs) or hybrid supercapacitor (HSC) device constructed with battery-type/ pseudocapacitive positive and carbon-based negative materials have recently gained enormous attention with advantages of both high energy and power densities. This efficient strategy obvious enhances the potential window and capacitance, and hence improves the energy density by assembly of intrinsic charge-storage capabilities of cathode electrode (pseudocapacitive/battery-type materials) and porous anode electrode (carbon-based materials) for BSHs and HSCs. Very recently, Sekhar and co-workers fabricated the flexible asymmetric supercapacitor (ASC) by assembly of nickel-cobalt layered double hydroxide nanosheets on Ag NWs-fenced carbon cloth (NC LDH NSs@Ag@CC) as a positive electrode and activated carbon coated carbon cloth (CC) as a negative electrode, as depicted in Fig. 21a [66]. The electrochemical behavior of the ASC was tested with a potential of 0–1.6 V. Fig. 21b depicts the CV profiles of the ASC conducted at various scan rates. In distinct to the redox peaks, the ASC exhibited the quasi-rectangular shapes, which are due to the presence of both WDLC and battery-type materials. The symmetrical nature of GCD curves illustrated the well balancing mass between the both positive and negative electrodes (Fig. 21c). The fabricated ASC achieved the areal capacitance values of 230.2, 218.5, 213.8, 209.6, 206, 202.7, 198.4 and 198.1 mF cm−2 at 1, 2, 3, 5, 7, 10, 15 and 20 mA cm−2, respectively. More importantly, the ASC achieved a maximum energy density of 78.8

the GCD plot Ni–Co = 0.5:0.5 electrode exhibited the longest discharge time than the other samples, illustrating the superior electrochemical performance (22.2 mF cm−2). It is worth noting that Ni : Co = 0.75:0.25, 0.5:0.5 and 0.25:0.75 electrodes exhibit areal capacitance retentions of 88.9%, 91.9% and 97.6% after 5000 cycles, which are markedly larger than the 53.2% of 1:0 and the 70.7% of 0 : 1 samples (Fig. 20f). These findings gave appropriate evidence to confirm that the prepared Ag NW@NiCo/NiCo(OH)2 electrode achieved outstanding electrochemical properties and it showed potential for application in flexible and transparent energy storage devices. Therefore, in this section we discussed about various flexible substrates to fabricate the high-performance cathode materials for supercapacitors. Table 2 lists recent advancements in flexible cathode materials and/or symmetric devices with their corresponding electrochemical performance. 3. Next generation flexible and wearable hybrid supercapacitor (HSC) or asymmetric supercapacitor (ASC) or batterysupercapacitor hybrid (BSH) Nowadays, supercapacitors (SCs) are gained special consideration as an excellent energy storage device for multifarious applications owing to their rapid charge-discharge rates, high power density and outstanding cycling performance. SCs could be more attractive candidates with higher energy density, higher power density and ultra-cycling performance for next-generation high-performance energy storage devices. However, SCs delivers lower energy density than rechargeable batteries, which limit their commercialization. According to equation of 16

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Fig. 17. (a) Schematic illustration of the fabrication of Ni(OH)2@CCF. (b) Photo for the water contact angle (CA) measurement of Ni(OH)2@CCF. (c) SEM image of Ni (OH)2@CCF at electrodepositon period of 200 s. (d) CV plot and (e) GCD profiles of Ni(OH)2@CCF//Ni(OH)2@CCF at different testing. (f) Ragone plot of symmetric supercapacitor. (g) Green LEDs powered by the Ni(OH)2@CCF//Ni(OH)2@CCF SCs in series. Reproduced from ref. [58] with the permission of the American Chemical Society.

μWh cm−2 and the power density of 12.1 mW cm−2 with the high potential window of 1.6 V. Further, the ASC delivered a good cycling stability with a capacitance retention of 88.1% after 2000 cycles (Fig. 21d). Also, the serially connected 2 ASC devices power a commercial red LED for 160 s, as shown in the inset of Fig. 21e. In view of comfort for wearable electronics, cotton-textile radiation-proof clothes commonly used for pregnant woman cloth (PWC) are selected as the flexible substrate to construct wearable energy storage devices, which have the properties of flexible, green, renewable, breathable and excellent conductivity. Hence, Liang et al. successfully fabricated the PWC supported all-solid-state flexible supercapacitor (Co-Ni LDH/PWC// FeOOH/PWC) and achieved a high energy density of 65.76 W h kg−1 and power density of 1426.15 W kg−1 [67]. Recently, N. Yu et al. successfully fabricated the flexible ASC by assembly of carbon cloth supported Co3O4 nanosheets and MnO@C nanosheets as the positive and negative electrodes, respectively [68]. The ASC device achieved a high operating potential of 1.7 V, a high specific capacitance of 166 F g−1, an outstanding 59.6 W h kg−1, an excellent rate capability and superior cycling stability. Further, the fabricated ASC displayed excellent flexibility and mechanical stability even under severe bending states. Zhao et al. rationally developed the Fe2O3 nanoneedle arrays

(Fe2O3 NNAs) and NiCo2O4/Ni(OH)2 hybrid nanosheet arrays (NiCo2O4/Ni(OH)2 HNAs) on SiC nanowire (SiC NW), which were effectively used as the negative electrode and positive electrode, respectively [69]. The schematic illustration of the fabrication process of the SiC NWs @Fe2O3 NNAs negative electrode and SiC NWs@ NiCo2O4@Ni(OH)2 HNAs positive electrode on carbon cloth is shown in Fig. 21f. In negative electrode, initially, FeOOH precursor is grown homogeneously on the entire surface of the SiC NWs directly deposited on carbon cloth using hydrothermal method. Next, using the annealing process, the FeOOH precursors are converted into a Fe2O3 NNAs. In case of positive electrode, a facile two-step electrodepositon and calcination process are used to prepare the NiCo2O4@Ni(OH)2 hybrid NSAs on SiC NWs. The ASC device is constructed by SiC NWs@Fe2O3 NNAs and SiC NWs@NiCo2O4/Ni(OH)2 HNAs electrode materials, as schematically illustrated in Fig. 21g. Fig. 21h shows the CV plots of the ASC at various scan rates between 0 and 1.75 V. It was clearly seen from CV plots that the each CV plot exhibited a pair of symmetrical redox peaks, which are attributed to the reversible redox reactions associated with the electrolyte ions insertion/extraction. The voltage plateaus and nearly symmetric charge/discharge features in all GCD plots manifests the pseudocapacitive behavior and superior coulombic efficiency 17

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Fig. 18. (a) Schematic illustration of the fabrication process of MNCS multi-tripod NTAs on CNTFs. (b) SEM image and (c) EDX Mapping of MNCS NTAs on CNTFs. (d) Comparative GCD plots of different electrodes at 2 mA cm−2. (e) CV plots of MNCS NTAs electrode at various scan rates. (f) Cycling stability of MNCS NTAs electrode at 10 mA cm−2. Reproduced from ref. [59] with the permission of the Royal Society of Chemistry.

[72]. The asymmetric flexible fiber-shaped supercapacitors (FSSCs) achieved high specific capacitance (22.94 F g−1 at 0.5 A g−1)***, excellent energy density (7.66 Wh kg−1), cyclability, and flexibility in PVA/KOH gel electrolyte, due to their improved operating potential window, high conductivity, and sufficiently synergistic effects of Ni–Co DHs, pen ink, and nickel coated CF. Very recently, S. Liu et al. prepared the flexible quasi-solid-state ASC using F-Co2MnO4-x/carbon fiber (CF) as the positive electrode, Fe2O3/CF as the negative electrode and PVA/ KOH as the gel electrolyte [73]. The as-fabricated ASC device achieved a high energy density of 64.4 W h kg−1 at a power density of 800 W kg−1. P. Yang et al. also fabricated the high-performance flexible solid-state ASC with a α-MnO2 nanowires/carbon fabric and amorphous Fe2O3 nanotubes/carbon fabric [74]. The solid-sate ASC achieved a volumetric capacitance of 1.5 F/cm3 at 2 mA/cm−2 and high energy density of 0.55 mWh cm−3. Carbon nanotube fibers (CNTFs) are considered as the promising current collector for high-performance fiber-shaped asymmetric supercapacitors (FASCs) owing to their high electrical conductivity, light weight, excellent flexibility, large specific surface area and outstanding mechanical properties. Hence, to enhance the electrochemical performance of the fibrous electrodes, researchers have dedicated potential efforts to depositing nanostructured active materials on them. Recently, J. Guo et al. successfully fabricated a FASC device using Zn-Ni-Co ternary oxides (ZNCO) nanowire arrays (NWAs) on CNTF as the positive electrode and the vanadium nitride nanosheets (NSs) on CNTF as

(Fig. 21i). Moreover, from the GCD plots, the as-fabricated ASC device achieved specific capacitance as high as 242 F g−1 at 4 A g−1 and it maintains a 106 F g−1 even at 30 A g−1, revealing a high rate capability. More importantly, the ASC device exhibited the maximum energy density of 103 W h kg−1 at a power density of 3.5 kW kg−1. These results provide a fresh route for developing next-generation high-energy storage and conversion systems. Many researchers are paid more attention to fabricate the flexible supercapacitors using the carbon cloth or carbon fiber as a substrate. For instance, Q. Liu et al. assembled the flexible solid-state ASC by the Co9S8@NiCo2O4 electrode with activate carbon (AC) electrode and polyvinyl alcohol (PVA)/KOH as gel electrolyte [70]; and achieved a high specific capacitance of 250 F g−1 at 1 A g−1, and delivered a high energy density of 86 W h kg−1 at 792 W kg−1. Recently, J. Balamurugan et al. fabricated the free‐standing NiMo-S and Ni-Fe-S nanosheets (NSs) on carbon cloth for high‐performance flexible ASC by a facile hydrothermal method followed by sulfurization [71]. As-developed flexible ASC (Ni-Mo-S NS//Ni-Fe-S NS) achieved a high volumetric capacity of ≈1.9 mAh cm−3, outstanding energy density of ≈82.13 Wh kg−1 at 0.561 kW kg−1, exceptional power density (≈13.103 kW kg−1 at 61.51 Wh kg−1) and an excellent cycling stability of (≈95.86% of initial capacity over 10,000 cycles). L. Gao et al. successfully fabricated the asymmetric flexible solid-state supercapacitor using Ni–Co double hydroxides (DHs)/ nickel/pen ink/carbon fiber and pen ink coated on highly conductive nickel/carbon fiber as positive and negative electrodes, respectively 18

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Fig. 19. (a) A photograph of a lab coat. (b) Digital photos pristine textile cloth cut from the lab coat, after silver sputtering, and resulted product after sulfuration. (c) Schematic illustration of fabrication process of FeCo2S4–NiCo2S4 on SSTC. (d) SEM image of FeCo2S4–NiCo2S4 on SSTC. (e) SEM-EDS spectrum and corresponding atomic percentages of different elements. (f) CV plots and (g) CD profiles of an all-solid-state symmetric supercapacitor at various scan rates and current densities. (h) Ragone plot of all-solid-state symmetric supercapacitor. Reproduced from ref. [61] with the permission of the John Wiley & Sons Ltd.

of LiCl–PVA gel electrolyte. The schematic illustration of the preparation process of the stretchable FASC is depicted in Fig. 22e. The schematic structure and cross-sectional structure of the stretchable FASC is shown in Fig. 22f and 22g. Fig. 22h showed the wrapping of the modified CNT fibers around a pre-stretched elastic fiber. As depicted in Fig. 22i, the CV profiles of the stretchable FASC device are investigated at various scan rates in the potential window of 0–1.8 V. All the CV plots exhibit quasi-rectangular shapes without any redox peaks, denoting the outstanding reversibility and desirable capacitive behaviors of the stretchable FASC device. Further, GCD plots delivered the nearly linear and symmetric nature in the potential window of 0–1.8 V, as shown in Fig. 22j. The FASC device achieved a high capacitance of 278.6 mF cm−2 at 0.6 mA cm−2, and maintained a capacitance of 205 mF cm−2 even at high current density of 6 mA cm−2, indicating an outstanding rate capability. Further, the stretchable FASC exhibited a high energy density of 125.37 μWh cm−2 at the power density of 540 μW cm−2 (Fig. 22k). Moreover, the stretchable FASC device showed negligible changes in the GCD plots at a 2 mA cm−2 with increasing strains from 0% to 100%, demonstrating the great flexibility of FASCs (Fig. 2l). However, the carbon-based textiles/fibers/papers suffer from high hydrophobicity and low conductivity, and they are not appropriate for clothing substrates. Also, the carbon textile/paper-based electrodes are expensive and the complicated processes are required to attain

the negative electrode [75]. The all-solid-sate FASC device was prepared in a twisting configuration, which is schematically shown in Fig. 22a. First, the VN NSs precursors were grown on CNTFs using solvothermal method and followed by annealing to achieve VN NSs. Simultaneously, ZNCO NWAs precursor was also synthesized using hydrothermal process and annealed in air to obtain the ZNCO NWAs. Then, the FASC was assembled using VN/CNTF and ZNCO/CNTF electrodes were twisted together closely and coated by a thin layer of KOH/PVA gel electrolyte. Fig. 22b depicts the CV plots of the FASC at various scan rates and showed the rectangle-like shapes even at high scan rates, suggesting the ideal capacitive behavior and excellent reversibility. Further, FASC achieved the triangle-like and almost symmetric GCD plots at different current densities, consistent with the CV results (Fig. 22c). Moreover, the as-assembled FASC achieved a high specific capacitance of 50 F cm−3 and an extraordinary energy density of 17.78 mWh cm−3 at the power density of 80 mW cm−3 (Fig. 22d). Besides, Q. Zhang et al. developed a simple and low-cost route to fabricate highly capacitive hierarchically structured MnO2@PEDOT:PSS@ oxidized carbon nanotube fibers (MnO2@PEDOT:PSS@OCNTF) positive electrode and flower-like MoS2nanosheets@CNTF (MoS2@CNTF) negative electrode for flexible and stretchable FASCs [76]. An all-solidstate stretchable FASC was fabricated that attached the MnO2@PEDOT:PSS@OCNTF and MoS2@CNTF after coating them with a thin layer

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Fig. 20. (a) Schematic illustration of the fabrication process of the Ag NW@NiCo/NiCo(OH)2 electrode. SEM images of (b) Ag NW and (c) Ni : Co = 0.5 : 0.5. (d) CV plots of various electrodes at 5 mV s−1. (e) GCD profiles of different electrodes at 0.1 mA cm−2. (f) Capacity retentions for 5000 cycles of five samples. Reproduced from ref. [65] with the permission of the Royal Society of Chemistry.

environment and one of the fastest-growing waste streams in the world. Meanwhile, these wastes are precious and contain significant amounts of valuable metals. Among the different e-waste sources, electrical cable wires are largely composed of metallic fibers such as copper and aluminium, which can be utilized for various electronic and energy-related applications. Hence, recycling of these e-waste cable wires for fabrication of fiber-type supercapacitors applications not only decreases the fabrication cost of SCs but also reduces the depletion of fossil fuel usage, which provides new taught for real-life applications along with eco-friendliness. Inspired from recycling approach of e-waste, G. Nagaraju et al. utilized the Cu fibers from waste cable wires as a lowcost current collector and grown the core-shell-like nanostructure materials over them for high-performance fiber-based hybrid supercapacitor (FHSC) [79]. Nagaraju and co-workers fabricated the solidstate FHSC using forest-like nickel oxide nanosheet grafted carbon nanotube coupled copper oxide nanowire arrays (NiO NSs@CNTs@CuO NWAs/Cu fibers) as positive electrode and activated carbon coated carbon fibers as a negative electrode with a gel electrolyte [79]. Fig. 23a depicts the schematic of the fabrication process of FHSC using positive and negative electrodes. CV plots of FHSC measured at different scan rates with a potential window of 0–1.55 V (Fig. 23b). CV plots exhibited the outstanding capacitive behavior rather than the redox peaks, which is due to the inclusion of EDLC material. Further, the GCD plots of FHSC exhibited a symmetric charge-discharge times, denoting the excellent rate capability of the device (Fig. 23c). More importantly, the FHSC achieved a maximum energy density of 26.32 W h kg−1 with a power density of 219.03 W kg−1 (Fig. 23d). Moreover, two serially connected FHSCs were easily attached to human shirt to operate the LED and multifunction electronic display (Fig. 23e,f). On the other hand, L. Gao et al. successfully assembled the flexible fiber supercapacitor (FFSC) using low-cost, wasted nickel fiber (NF), cellular

these substrates. Hence, it is crucial to pursue suitable alternatives with low-cost and eco-friendliness for carbon-based textiles. Papers and textiles have been employed as ideal substrates owing to their cost-effectiveness, flexibility and highly porous structures, which can absorb electroactive materials. However, papers and textiles should be coated with electrically conductive materials due to insulating behavior. Conductive materials, such as carbon nanotubes (CNTs) or silver nanowires, have been mainly incorporated into the paper or textile substrates to improve their electric conductivity. Recently, Y. Co et al. introduced the metallic paper-based supercapacitor (MP-SC) electrodes using ligand-mediated layer-by-layer assembly, which can directly bridge all the interfaces of metal and/or metal oxide nanoparticles (NPs) through small molecules [77]. As a result, Y. Co et al. fabricated the hybrid metallic paper-based asymmetric supercapacitors (MP-ASCs) composed of alternating MnO/Au NP (positive electrode) and alternating Fe3O4/Au NP electrode (negative electrode) [77]. The as-fabricated MP-ASC achieved a maximum specific capacitance of 222 F g−1 at 1 mA cm−2 and outstanding energy density of 121.5 W h kg−1. On the other hand, metal wires, such as Ni wire, copper wire and stainless steel have been examined as current collectors for flexible and wearable supercapacitors due to their acceptable conductivity. However, the large diameter of the wires reduces the flexibility of wearable electronics. Recently, Wang and co-workers assembled the flexible ASC consisting NiCo2O4/ultrafine nickel wire (positive electrode) with Fe3O4/ultrafine nickel wire (negative electrode) [78]. As-fabricated flexible ASC exhibited an energy density of 32.6 Wh kg−1 at a power density of 846 W kg−1. Recently, electronic waste (e-waste) such as expired household appliances (electrical cable wires, fans, televisions, heaters, etc.) and information technology electronic equipment's (computers, mobile phones, circuit boards, etc.) is becoming major concern for 20

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[51] [52] [53] [54] [55] [56] [57] [58] [59]

[60] [61] [62] [63] [64]

[65]

2650 F g−1 at 5 A g−1 1475.4 F g−1 at 1 A g−1 1981 F g−1 at 2 A g−1 1325.9 F g−1 at 1 A g−1 194 mA hg−1 at 1 A g−1 875 C g−1 at 1 A g−1 1582 F g−1 at 1 A g−1 131.43 F g−1 at 0.25 A g−1 7.025 F cm−2 at 1 mA cm−2

2105 F g−1 at 2 A g−1 – 577.4 F g−1 at 1 A g−1 2185.6 F g−1 at 5 A g−1 145.6 F g−1 at 1 A g−1

22.2 mF cm−2 at 0.1 mA cm−2

– – – – – – – 35.78 –

– 46 80.2 – –

–

Reference Specific capacitance/areal capacitance/specific capacity at current density Energy density (W h Kg−1)

1 M KOH 3-electrode

copper foam (CF), and graphene sheets (GSs) as well as layered double hydroxides (LDHs) [80]. The schematic of the assembled FFSC is shown in Fig. 23g, which consisting of NiCo LDH@ GSs@CF-NF (as the positive electrode), activated carbon (AC) (as the negative electrode), and PVA/KOH as gel electrolyte. CV profiles at various scan rates exhibited the battery-type capacitor and double-layered capacitor features, demonstrating an efficient ion transportation (Fig. 23h). Further, the GCD plots in Fig. 23i delivered nearly linear and symmetric behavior, indicating the outstanding electrochemical reversibility. From the GCD plots, the areal capacitance of the FHSC was found to be 350.9 mF cm−2 at a current density of 1 mA cm−2. Most importantly, the FFSC achieved excellent energy density of 109.6 μWh cm−2 at a power density of 749.5 μW cm−2 and maintained 64.2 μWh cm−2 even at a power density of 14,997.6 μW cm−2 (Fig. 23j). At various bending angles from 0 to 180 deg, the FFSC exhibited capacitance retention of decay of only 5% (Fig. 23k). Furthermore, the FFSC was connected to various devices, weaved into the cotton shirt, and even flexed to fit our body shape. As shown in Fig. 23l and 23m, the curved FFSC was twined around a finger successfully powered a digital watch and also integrated with a headphone. Further, the FFSC connected to lab cloth to power electronics under bending deformation (Fig. 23n). As depicted in Fig. 23o, a motor fan also powered with the serially connected FFSCs. This design principle creates new opportunities for emerging a fibershaped energy-storage device to attain high-energy output for applications in an extensive range of fields. Some other low-cost and flexible current collectors are also used for the fabrication of the supercapacitors [81-84]. Gopi and co-workers successfully fabricated the flexible hybrid supercapacitor (FHSC) by assembling the hierarchical Ni(OH)2 nanoneedle arrays with NiO–NiCo2O4 nanosheet arrays (Ni(OH)2 NNAs@NiO–NiCo2O4 NSAs) on CuNi fabric as the positive electrode and graphene-ink (G-ink) on Cu–Ni fabric as the negative electrode with KOH as the electrolyte solution [81]. As-prepared FHSC device (Ni(OH)2 NNAs@NiO–NiCo2O4 NSAs// graphene-ink) exhibited an outstanding specific capacitance of 273.1 F g−1, a superhigh energy density of 97.1 W h kg−1 and an excellent cycling stability with 94.7% retention after 5000 cycles. Recently, X. Wang et al. developed the sodium‐ion hybrid supercapacitors (Na‐HSCs) by containing the flexible Al-plastic film supported graphene shells over T‐Nb2O5 nanowires (Gr‐Nb2O5) on as an anode and an activated carbon as a cathode delivered high energy density of 112.9 Wh kg−1 at a power density of 80.1 W kg−1 [82]. S. Lei et al. fabricated the flexible all-solid-state asymmetrical supercapacitor by sandwiching the Dacron cloth supported Cu(OH)2 nanobelt arrays (positive electrode) between two carbon nanofibers (CNF) matrices (negative electrodes), using KOH-PVA gel as the electrolyte and as the separator. As a result, the fabricated textile-based flexible device (Cu(OH)2/Cu/Dacron// CNF/Dacron) delivered a high areal capacitance of 195.8 mF cm−2 at 1 mA cm−2 and an energy density of 3.6 × 10−2 mWh cm−2 at a power density of 0.6 mW cm−2 [83]. G. Nagaraju et al. assembled the battery supercapacitor hybrid (BSH) in a sealed configuration with core-shelllike nickel cobalt layered double hydroxide nanosheets adhered to nickel cobalt layered double hydroxide nanoflake arrays on nickel fabric (NC LDH NFAs@NSs/Ni fabric) as a battery-type positive electrode and activated carbon coated fabric (AC@CF) as a negative electrode with a slice of filter paper as a separator in 1 M KOH solution [84]. More importantly, a fabric-based BSH achieved a stable operational potential window of 1.6 V, a large areal capacitance of 1147.23 mF cm−2 at 3 mA cm−2, and a high energy density of 0.392 mWh cm−2 at a power density of 2.353 mW cm−2. A summary of the electrochemical performance of flexible and wearable hybrid supercapacitor (HSC) or asymmetric supercapacitor (ASC) or battery-supercapacitor hybrid (BSH) designs based on different positive and negative electrodes with their corresponding electrolytes is shown in Table 3. With the excellent energy storage capabilities (rapid charge-discharge rates, high power density, acceptable energy density and outstanding cycling performance), the flexible supercapacitor can be used for some practical

NiCo/NiCo(OH)2

1 M KOH PVA-KOH 3 M KOH 1 M KOH 1 M KOH 3-electrode 2-electrode 2-electrode symmetric 3-electrode 3-electrode

conductive textile silver-sputtered textile cloth copper/nickel coated polyester fabric non-woven conductive textile substrate (NWCTs) Nickel (Ni) coated polyethylenterephthalate (PET) fibers woven conductive fabric substrate Ag nanowire

3M 6M 6M 3M 1M 1M 2M 6M 1M

KOH KOH KOH KOH KOH KOH KOH KOH KOH 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 3-electrode 2-electrode 3-electrode Carbon cloth Carbon cloth Carbon cloth Carbon cloth fiber Carbon paper Carbon fiber Carbon fabric carbonized cotton fabric carbon nanotube fibers

ZnO@C@NiCo2O4 arrays Co9S8 3D ZnO@TiO2@Ni(OH)2 NiCo2O4@NiMoO4 NiCoP CuCo2S4 CoMoO4@NiMoO4•xH2O Ni(OH)2 MNCS multi-tripod nanotube arrays (NTAs) Ni–Co LDH NSs FeCo2S4–NiCo2S4 microsphere-like CoFe2Se4eCoNiSe2 β-Ni(OH)2 NSs Co3O4

Electrolyte Measurement configuration Current collector Electrode materials

Table 2 Summary on the electrochemical performance of various flexible and wearable cathode materials for supercapacitors. The mass energy density is measured based on the total weight of the symmetric device.

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Fig. 21. (a) Schematic demonstration of the fabricated asymmetric supercapacitor. (b) CV and (c) GCD plots of the ASC measured at various scan rates and current densities. (d) Ragone plot and (e) cycling life of the ASC device. Reproduced from ref. [66] with the permission of the Elsevier. (f) Schematic of the fabrication process of SiC NWs @Fe2O3 NNAs negative electrode and SiC NWs@NiCo2O4@Ni(OH)2 HNAs positive electrode on carbon cloth. (g) Schematic of the assembled ASC device. (h) CV plots and (i) GCD profiles of ASC device at various testing's. (j) Ragone plot of ASC device. Reproduced from ref. [69] with the permission of the John Wiley & Sons Ltd.

Fig. 22. (a) Schematic illustration of the fabrication process of fiber-shaped asymmetric supercapacitors (FASCs). (b) CV profiles and (c) GCD plots of FASC at various scan rates and current densities. (d) Ragone plot of FASC. Reproduced from ref. [75] with the permission of the Elsevier. (e) Schematic of the fabrication of the stretchable FASC. (f) Structure of the FASC. (g) Cross-sectional structure of the stretchable FASC. (h) Wrapping of the modified CNT fibers around a pre-stretched elastic fiber. (i) CV, (j) GCD curves and (k) Ragone plot of FASC. (l) GCD plots of the as-prepared FASCs with increasing strain from 0% to 100%. Reproduced from ref. [76] with the permission of the Elsevier.

applications, such as, piezoelectric supercapacitors, photo-supercapacitors, shape memory supercapacitors, microbial supercapacitors, electrochromic supercapacitors, self-healing supercapacitors,

integrated supercapacitor-sensor device, and etc. [85-91].

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Fig. 23. (a) Schematic illustration of the fabrication proves of PVA-KOH electrolyte coated fiber‐based hybrid supercapacitor. (b) CV plots, (c) GCD curves and (d) Ragone plot of FHSC. (e,f) Two serially connected FHSC powering LED and electronic display. Reproduced from ref. [79] with the permission of the John Wiley & Sons Ltd. (g) Schematic illustration of the assembled flexible fiber-based solid asymmetrical supercapacitor (SASC) device (NiCo LDH@GSs@CF-NF//AC). (h) CV plots, (i) GCD curves and (j) Ragone plot of the SASC. (k) Capacitance retention of the device at different bending angles. Practical application of the supercapacitor packaged with different electronic devices. The SASC (l) powered a digital watch, (m) integrated with a headphone, (n) attached to lab-cloth to power electronics and (o) connected to motor fan. Reproduced from ref. [80] with the permission of the American Chemical Society.

4. Conclusions and future challenges

and implantable medical devices. This review presented the recent progress in the different flexible current collectors for the growth of various anode and cathode materials, and novel cell designs of flexible supercapacitors. To date, numerous researchers are focused on the development of various new electroactive materials on flexible substrates

Flexible supercapacitors are gaining considerable attention as promising energy storage systems with various technological applications, such as wearable/portable electronics, smart clothing, electronic skins

Table 3 Summary on the electrochemical performance of various flexible and wearable hybrid supercapacitor (HSC) or asymmetric supercapacitor (ASC) or battery-supercapacitor hybrid (BSH). The mass and areal energy densities are measured based on the total weight and area of the HSC or ASC or BSH. Electrode materials

Current collector

Electrolyte

Energy density

Specific capacitance/volumetric capacitance at current density

Reference

NC LDH NSs@Ag//AC Co-Ni LDH/PWC// FeOOH/PWC

Carbon cloth Pregnant women cloth (PWC, Cotton/Ni + Cu+Ni) Carbon cloth Carbon Cloth

1 M KOH PVA-KOH

78.8 μWh cm−2 65.76 W h kg−1

230.2 mF cm−2 at 1 mA cm−2 –

[66] [67]

3 M KOH 2 M KOH

59.6 W h kg−1 103 W h kg−1

166 F g−1 at 2 A g−1 242 F g−1 at 4 A g−1

[68] [69]

Carbon Carbon Carbon Carbon Carbon Carbon

PVA/KOH PVA/KOH PVA/KOH PVA/KOH PVA/LiCl PVA/KOH

86 W h kg−1 82.13 W h kg−1 7.66 W h kg−1 64.4 W h kg−1 0.55 mWh cm−3 17.78 m Wh cm−3

250 F g−1 at 1 A g−1 103 mA h g−1 at 2 mA cm−2 22.94 F g−1 at 0.5 A g−1 180 F g−1 at 1 A g−1 1.5 F cm−3 at 2 mA cm−2 50 F cm−3 at 0.1 A cm−3

[70] [71] [72] [73] [74] [75]

Carbon nanotube fibers Commercial paper/Au Nickel wire Cu fiber Copper foam-nickel fiber copper/nickel coated polyester fabric

LiCl-PVA 1 M Na2SO4 PVA/KOH PVA/KOH PVA/KOH 3 M KOH

125.37 μW h cm−2 121.5 W h kg−1 32.6 W h kg−1 26.32 W h kg−1 109.6 μW h cm−2 97.1 W h kg−1

278.6 mF cm−2 at 0.6 mA cm−2 222 F g−1 at 0.65 A g−1 81.6 F g−1 at 1 A g−1 93.42 F g−1 at 0.7 mA 350.9 mF cm−2 at 1 mA cm−2 273.1 F g−1 at 1 A g−1

[76] [77] [78] [79] [80] [81]

Al-plastic film Dacron cloth

1 M NaClO4 PVA/KOH

– 195.8 mF cm−2 at 1 mA cm−2

[82] [83]

Polyester fabric/Ni

1 M KOH

112.9 W h kg−1 3.6 × 10−2 mWh cm−2 0.392 mWh cm−2

1147.23 mF cm−2 at 3 mA cm−2

[84]

Co3O4 nanosheets//MnO nanosheets SiC NWs@NiCo2O4@Ni(OH)2 HNAs//SiC NWs@Fe2O3 NNAs Co9S8@NiCo2O4//AC Ni-Mo-S NS//Ni-Fe-S NS Ni−Co DHs/pen ink/nickel//pen ink/nickel F-Co2MnO4-x/CF//Fe2O3/CF MnO2 NWs//Fe2O3 NTs Zinc-Nickel-Cobalt ternary oxides// vanadium nitride MnO2@PEDOT:PSS@OCNTF// MoS2@CNTF MnO/Au NP//Fe3O4/Au NP NiCo2O4//Fe3O4 NiO NSs@CNTs@CuO NWAs//AC NiCo LDH@ GSs//AC (Ni(OH)2 NNAs@NiO– NiCo2O4 NSAs// Graphene-ink Gr-Nb2O5//AC Cu(OH)2 nanobelt arrays//carbon nanofiber matrices core−shell-like NC LDH NFAs@NSs/Ni fabric//AC@CF

cloth cloth fiber fiber fiber nanotube fibers

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and cell designs (symmetric and asymmetric/hybrid) for flexible supercapacitors. In addition, the latest research discoveries of flexible supported new electroactive materials with controlled architectures hold great promise towards exceptional enhancement in the electrochemical performance in terms of high energy, power densities and cycling stability. The performance of flexible supercapacitor is generally expressed in terms gravimetric and volumetric scales that foster novel electrode design progress, such as flexible substrate-supported and freestanding designs. It is crucial to develop the flexible, lightweight and effectively utilize the entire electrode materials to obtain high energy storage performance, thus avoiding the extra weight of current collectors and binders or additives, it is delicate and difficult to handle. Conversely, a great number of flexible substrates (carbon cloth, metal foils/wires, textiles, carbon nanotube fibers, PETs and etc.) have been examined to attain high-performance of flexible supercapacitors. Due to their high electrical conductivity, high surface area, light weight and high flexibility, carbon nanotube fibers- and carbon-based current collectors are gained considerable attention for flexible supercapacitor application. However, the electrochemical stability of nanostructures on their ligaments is not satisfactory and the price of carbon-based current collectors is relatively expensive. Hence, conductive textilebased current collectors are facially developed using metal deposition over textiles. In addition, extensive efforts have been devoted towards exploring new anode and cathode electroactive materials that enhance overall flexible SC performance. Several new anode and cathode materials, such as carbon foam-multiscale pores (CF-MSP), N, S dual-doped porous carbon polyhedra embedded carbon nanofibers (NSCPCNFs), porous Fe2O3 nanospheres@ACC, VNNDs/CNSs, TiN NWAs@VN NSs, NiCo2O4@NiMoO4, MNCS multi-tripod NTAs, FeCo2S4–NiCo2S4, microsphere-like CoFe2Se4eCoNiSe2, etc. have been found to be promising electroactive materials for flexible supercapacitor application because of their good conductivity and high specific capacitance. More importantly, the high-performance anode and cathode materials are assembled to develop the flexible battery-supercapacitor hybrid (BSH) or hybrid supercapacitor (HSC) or asymmetric supercapacitor (ASC) with improved energy density and can be successfully applied in various potential applications. This is a significant step towards attaining smart SCs. Furthermore, based on our foregoing discussions, we think that the following issues should be paid much attention in future researches on high-energy density flexible supercapacitor applications. First, in practical applications, the energy density of flexible supercapacitor is based on the weight/volume of the whole device including the package and nonactive material substrates. Indeed, the nonactive material substrate is heavy in mass and large in volume compared to electroactive material weight. Thus, it is crucial to reduce the weight (and volume) fraction of the electrochemically inactive components in flexible supercapacitor to achieve high energy and power densities. Second, the performance of flexible supercapacitor mainly depends on the electroactive material. Hence, the selection of electrochemically active materials is crucial to obtain high electrochemical performance. Third, integration of various anode or cathode materials achieved the higher energy storage performance including high energy density and enlargement of voltage window. Therefore, more efforts should be taken to elucidate new synthesis methods to synthesize composite electrode materials that meet industrial requirements. Fourth, electrolytes can be optimized in flexible supercapacitor. More focus on enhancing the device performance in terms of capacitance, energy density, stability and operating cell voltage by choosing new redox-active electrolytes is required. Electrolytes with higher ion conductivity and larger operation voltage are worth developing. Fifth, the sealing of the flexible supercapacitor is crucial to achieve stable performance. In this context, the research should also focus on the sealing of the supercapacitor. Sixth, the flexibility and mechanical strength of flexible

supercapacitor need to be thoroughly investigated. Generally, many researchers are tested on the electrochemical properties of fabricated flexible supercapacitor at various bending angles, but it is not clear how many cycles of flexible supercapacitor can tolerate. Seventh, cycling stability is also a crucial parameter for real time applications. Most of the studies tested the cycling stability around 3000 to 10,000 cycles only. Hence, the researchers should develop the highly stable flexible supercapacitor to sustain more cycles. Eight, another challenge is the high production cost of the flexible supercapacitor for the practical implementation. Hence, future studies should focus on should focus on fabricating highly flexible and energetic supercapacitor by choosing low-cost raw materials. Ninth, the integration of flexible supercapacitor with various electronic gadgets is gained considerable attention for real time applications. Thus, new flexible, bendable, foldable and stretchable device designs need to be explored. Tenth, the integration of flexible supercapacitor with other functional electronic devices (such as lithium-ion batteries, solar cells and sensors) in a single component is of great interest. Researches on these integrated and flexible devices are in the dominant. Following suggested efforts on optimisation, smart supercapacitors would play a crucial role in the development of low-weight, flexible and wearable capacitive devices in the near future. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements This work was supported by BK 21 PLUS, Creative Human Resource Development Program for IT Convergence, Pusan National University, Busan, South Korea. Also, this work was supported by UAEU Program for Advanced Research (UPAR) under Grant no. 31S312. References [1] D. Cai, D. Wang, B. Liu, Y. Wang, Y. Liu, L. Wang, H. Li, H. Huang, Q. Li, T. Wang, ACS Appl. Mater. Interfaces 5 (2013) 12905–12910. [2] G.K. Veerasubramani, A. Chandrasekhar, S.M.S. P, Y.S. Mok, S.J. Kim, J. Mater. Chem. A 5 (2017) 11100–11113. [3] C. Choi, H.J. Sim, G.M. Spinks, X. Lepro, R.H. Baughman, S.J. Kim, Adv. Energy Mater. 6 (2016) 1502119. [4] J.Y. Tang, P. Yuan, C.L. Cai, Y.B. Fu, X.H. Ma, Adv. Energy Mater. 6 (2016) 1600813. [5] A. Vlad, N. Singh, C. Galande, P.M. Ajayan, Adv. Energy Mater. 5 (2015) 1402115. [6] L. Ma, Y. Zhao, X. Ji, J. Zeng, Q. Yang, Y. Guo, Z. Huang, X. Li, J. Yu, C. Zhi, Adv. Energy Mater. 9 (2019) 1900509. [7] A. Nazari, S. Farhad, Appl. Therm. Eng. 125 (2017) 1501–1517. [8] W.T. Navaraj, C. García Núñez, D. Shakthivel, V. Vinciguerra, F. Labeau, D.H. Gregory, R. Dahiya, Front. Neurosci. 20 (2017) 1–20. [9] D.P. Dubal, N.R. Chodankar, D.H. Kim, P. Gomez-Romero, Chem. Soc. Rev. 47 (2018) 2065–2129. [10] B. Song, L. Li, Z. Lin, Z.K. Wu, K.S. Moon, K. S, C.P. Wong, Nano Energy 16 (2015) 470–478. [11] C. Liu, X. Yan, F. Hu, G. Gao, G. Wu, X. Yang, Adv. Mater. 30 (2018) 1705713. [12] W. Xu, Z. Jiang, Q. Yang, W. Huo, M.S. Javed, Y. Li, L. Huang, X. Gu, C. Hu, Nano Energy 43 (2018) 168–176. [13] Y. Wang, X. Yang, A.G. Pandolfo, J. Ding, D. Li, Adv. Energy Mater. 6 (2016) 1600185. [14] J.X. Feng, S.H. Ye, A.L. Wang, X.F. Lu, Y.X. Tong, G.R. Li, Adv. Funct. Mater. 24 (2014) 7093–7101. [15] M. Yu, D. Lin, H. Feng, Y. Zeng, Y. Tong, X. Lu, Angew. Chem. Int. Ed. 56 (2017) 540154459. [16] H. Li, Z. Tang, Z. Liu, C. Zhi, Joule 3 (2019) 613–619. [17] A. Afif, S. Rahman, A.T. Azad, J. Zaini, M.A. Islan, A.K. Azad, J. Energy Storage 25 (2019) 100852. [18] F. Liu, L. Zeng, Y. Chen, R. Zhang, R. Yang, J. Pang, L. Ding, H. Liu, W. Zhou, Nano. Energy 61 (2019) 18–26. [19] S. Dai, W. Xu, Y. Xin, M. Wang, X. Gun, D. Guo, C. Hu, Nano. Energy 19 (2016) 363–372. [20] Y. Huang, F. Cui, J. Bao, Y. Zhao, J. Lian, T. Liu, H. Li, J. Mater. Chem. A 7 (2019) 20778–20789. [21] Q. Xue, J. Sun, Y. Huang, M. Zhu, Z. Pei, H. Li, Y. Wang, N. Li, H. Zhang, C. Zhi,

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