Design Strategies for Flexible Supercapacitors: Comparison
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Please note this is a comparison between Version 2 by Jason Zhu and Version 1 by Inho Nam.

The spread of wearable and flexible electronics devices has been accelerating for a wide range of applications. Development of an appropriate flexible power source to operate these flexible devices is a key challenge. Supercapacitors are attractive for powering portable lightweight consumer devices due to their long cycle stability, fast charge-discharge cycle, outstanding power density, wide operating temperatures and safety. Much effort has been devoted to ensure high mechanical and electrochemical stability upon bending, folding or stretching and to develop flexible electrodes, substrates and overall geometrically-flexible structures. Supercapacitors have attracted considerable attention and shown many applications on various scales.

  • supercapacitors
  • flexible electronics
  • wearable devices

1. Paper and Other Paper-Thin Substrates

Paper is a promising substrate for constructing flexible energy storage devices due to its large surface area and mechanical strength. It serves as excellent support for loading active materials and electrolytes and has features that can improve the life cycle with high power density and energy density relative to conventional rigid electrodes when physical stress is applied [15][1]. The two main electrode designs in paper SCs are the “sandwich” and the “in-plane” devices [16][2]. Yuan et al., fabricated sandwich-type supercapacitors using a carbon nanoparticle (CNP)/MnO2 nano-rod hybrid design and H3PO4/polyvinyl alcohol (PVA) electrolyte [12][3]. The supercapacitors fabricated on paper-thin carbon fabric were lightweight, flexible and twisted without compromising the structurally integrated devices. The cyclic voltammetry (CV) results showed only subtle changes in electrochemical performance at various bending angles and retention of 97.3% of its initial capacity even after 10,000 charge-discharge cycles [12][3].
While almost every paper type SC uses conventional sandwich electrodes, this design cannot compete with new configurations based on in-plane interdigitated electrodes that feature higher power and energy densities [16][2]. Nam et al., fabricated transparent and ultra-bendable supercapacitors with in-plane interdigitated electrodes using a masking method [17][4]. They deposited Au and active materials (MnO2) on polyethylene terephthalate (PET) and assembled transparent PVA/H3PO4 gel polymer electrolyte at the last stage. This paper-thin supercapacitor displayed superior capacitance stability under in-plane bending and compressive conditions [17][4]. The capacitance increased by ~1.2 times when a flat supercapacitor was curved with the internal highest bending rate covering the transparent supercapacitor. Then, the compression bending of the electrolyte caused pressure in the direction perpendicular to the PET substrate in the PVA/H3PO4 electrolyte membrane, resulting in a closer interaction between the electrode and electrolyte membrane [18,19,20,21][5][6][7][8].
The interdigitated electrode pattern can be made using masking or by direct printing. The example above used the masking approach. Printing enables mass production of a thin digital design pattern since the inks are amenable to producing ultra-thin patterns on pre-engineered substrates. The printing process begins by dispersing inorganic nanoparticles (NPs) or organic dye in a proper solvent to develop an ink of appropriate viscosity. Printing technology is particularly well suited for the manufacture of flexible, low-cost, portable products because of the generality and broad applicability of substrates and inks.
The layer-by-layer (LbL) assembly is an easy way to accurately control the number of active materials loaded on diverse substrates based on interdependent interactions between species, regardless of the size and shape of the substrate [23,24][9][10]. Ko et al., fabricated bendable paper-like SCs using LbL assembly based on hybrid asymmetric structural composition [25][11]. Au-soaked paper was used as substrate with MnO cathode, Fe3O4 anode, PVA/Na2SO4 polymer gel electrolyte and a separator. The contact resistance between adjacent NPs was minimized to increase the areal capacitance and rate performance by directly bridging all the interfaces of either metal, metal oxide nanoparticles, or both, through a small TREN ligand (tris(2-aminoethyl)amine). The assembled SC showed a specific power density of 128.9 kW/kg, specific energy density of 121.5 W h/kg and energy density value of 267.3 μW h/cm2. The device exhibited an area power value of 15.1 mW/cm2 and areal capacitance of 1.35 mF/cm2 at a high NP loading amount of >4.09 mg/cm2, and about 90% of the initial capacitance was retained after 5000 cycles. The SC also exhibited outstanding mechanical stability under various stress conditions, which is crucial for practical implementation. There was no significant change in the CV shape during bending or wrapping, indicating a solid and stable connection between the paper substrate, NP and electrolyte [25][11].

2. Textile Configuration

Carbon fiber is an up-and-coming candidate for flexible substrates due to its high mechanical strength and electrical conductivity. The textile’s 3D network can provide fast electron and ion conduction paths and high loading quantity of active materials. The textile scaffolding can be fabricated from carbon nanotubes (CNT), graphene fiber, metal, et cetera [26,27,28,29,30,31][12][13][14][15][16][17]. Dong et al., chose activated carbon fiber cloth (ACFC) as the body material to design ACFC/CNT and ACFC/MnO2/CNT composites [31][17]. The ACFC/MnO2/CNT textile electrode has a long operating life and excellent flexibility as fiber and textile electrodes. The manufactured textile electrode showed an areal capacitance of 2542 mF/cm2, power density of 16,287 μW/cm2 and energy density of 56.9 μWh/cm2. Textile and fiber electrodes provide excellent cycling performance and structural flexibility. 
Cheng et al., produced textile electrodes woven from CNT/graphene fibers (GF) which have high electrical conductivity and surface area by pre-intercalating Fe3O4 nanoparticles [32][18].
The CNT/GFs retain outstanding flexibility of the GFs by bending them into loops or springs without structural breaks. The folding state of the textile supercapacitor exhibited a CV curve similar to that when in the flat state. The capacitance decreased initially during cycle tests but leveled out at a stable value after 1000 cycles (0.4 mF/cm2); the box-shaped CV curve also slowly contracted and stabilized over 200 folding cycles [32][18].
Cakici et al., reported highly flexible carbon-based textile well covered with MnO2 structures [33][19]. Owing to its 1D construction, carbon textiles are the most commonly used current collectors in energy storage applications. MnO2 can be grown directly on the surface of carbon textile collectors with a horizontal 1D structure and thus can produce a supercapacitor electrode without using a conductive additive or a binder. The charge and discharge capacitance remained at the initial value of 461 F/g after 5000 cycles, and the capacitance maintenance ratio of the carbon textile-MnO2 hybrid device was 99.7%. Furthermore, the Coulombic efficiency was maintained at 99.3%, indicating the stability of the device. The fabricated composite electrode has indicated a specific capacitance of 463 F/g at 1 A/g in 1.0 M Na2SO4 electrolyte and good cycling stability by maintaining excellent capacitance at high C-rate [33][19].

3. Wire Configuration

Unlike the traditional paper type flexible SCs, the recently introduced wire type SCs are smaller in size, lighter in weight, more flexible and can be transformed into almost any shape, knotted and even woven into textiles. Structural type is crucial for the device assembly and operational stability in wire type SCs. Three structural designs have been developed to date: parallel [34][20], twisted [35][21] and coaxial [36][22] configurations. Li et al., fabricated wire-shaped supercapacitor electrodes built through growing CuCo2O4 nanostructures onto Ni wires [37][23]. In general, it is hard to control the morphology of the grown nanostructure. To deal with these issues, they induced facile capillary action to assemble the single-walled CNTs (SWCNTs) and graphene oxide (GO) directly on the nickel wire. The adsorption of CNTs on parallel Ni surfaces is facilitated by the shape of the nanowires, which significantly enhances conductivity and promotes electrolyte penetration. These symmetrical all-solid wire-shaped SCs exhibit outstanding EDLC performance in addition to ultrahigh flexibility and mechanical properties.
The two fiber electrodes in a parallel structure are physically separated from each other and can become mechanically unstable. Therefore, Ren et al., reported a flexible and wearable EDLC wire by twisting two aligned multi-walled carbon nanotube (MWCNT)/ordered electrodes [35][21]. The ordered mesoporous carbon (OMC) particles are tightly bundled by aligned MWCNTs, allowing more effective use of the high surface area of OMC components. The CV curve of the EDLC wire was well maintained at a scan rate of 1 mV/s during the 1000 cycle bending process [35][21]. Parallel and twisted fibers placed in the center of the devices exhibit the shortest distance, while the fibers located on the outside of the devices show the maximum distance. The longer the distance between the cathode and anode, the longer the ion diffusion path, which results in higher internal resistance and lower power density [38,39][24][25].
Owing to the relatively low ion mobility of gel electrolytes compared to liquid electrolytes, it is necessary to optimize the distance between the fiber electrodes to improve the overall performance of the flexible device. On the other hand, the coaxial type demonstrates a more homogeneous distance between electrodes and shows efficient charge/ion transfer [40][26]. In addition to improving the charge/ion transfer, the coaxial type is regarded as a more mechanically stable configuration [41,42][27][28]. This design also allows the flexibility to merge two different devices into one device to perform the original function.
Yu et al., fabricated freestanding CuO@AuPd@MnO2 SCs using coaxial nano-whiskers (NWs) [36][22]. Thin AuPd was deposited onto the CuO NWs to act as a current collector and the electrodeposited MnO2 acted as the anode. It can be easily fabricated by placing the outer tubular electrode (both pre-coated with gel electrolyte) over the two electrodes partitioned by a separator which has ionic porosity. Built from these NWs, this supercapacitor showed outstanding bendability and flexibility, high energy density, high power and excellent cyclic stability. In addition, the box and symmetrically shaped CV curves show the ideal pseudocapacitive property of MnO2 and excellent reversible oxidation-reduction reaction. This device maintained 93.4% of its initial capacitance after bending 100 times at 180 degrees, showing considerable bendability [36][22]. However, it is hard to accurately assemble a multi-layered core-sheath design into long fibers with a small diameter. Consequently, it is necessary to develop a simple process for producing new configurations that can keep a constant distance between the electrodes [43,44][29][30]. Nam et al., called this difficulty an “energy lag effect” [45][31]. When two types of electrodes are formed between the planar and cylindrical electrode, the electric field is different. For example, the electric field in a normal charging plane is presumed to be homogeneous in the 1D direction. In contrast, the electrodes of a wire-shaped energy storage device have a cylindrical structure, and in reality, an electric field that attenuates in 2D is generated. To avoid the effects of this energy lag effect, Nam et al., proposed a dual planar-helix structure for the electrodes, which has an entire wire type but the capacity is analytically equivalent to that of a normal 2D planar SC. The electro-capacity and the ohmic resistance of the planar and double-helix designs were investigated using CV and galvanostatic charging-discharging (GCD). The capacitance density of a dual planar-helix supercapacitor (1.9 F/cm3 at 10 mV/s) was found to be three times higher than that of a double helix type (0.66 F/cm3 at 10 mV/s) made using the same materials. Furthermore, the CV curves showed stable electrochemical performance under twisting deformation [45][31].
Guo et al., developed a wire-type supercapacitor with a parallel double helix structure (PDHS) for stable operation even under deformed states [47][32]. They wrapped two symmetrical titanium @MnO2 (Ti@MnO2) fiber electrodes around flexible nylon fiber and separated them by a spatial gap filled with LiCl/PVA gel as the electrolyte. A commercially available metal Ti fiber with high conductivity was used as current collector and MnO2 was electrochemically deposited as active material for the cathode and anode. 

4. Origami-Shaped SCs

The origami-based approach represents another alternative that allows deformability compared to conventional methods using freely deformable materials and mechanically designed structures. Origami is the ancient art of folding 2D sheets of paper with advanced folding along predefined creases and can be used to fabricate flexible and compact 3D structures [48][33]. Nam et al., fabricated an origami-based, all-solid-state, bendable supercapacitor system, assembling the analog of a series circuit [49][34]. This supercapacitor consisted of periodically isolated electrodes (IEs) and sectionalized electrolytes. These are important elements of a single system for the densely packed series circuit analogs. The sectionalized electrolytes and IEs were produced by easily designing with a graphite rod onto a paper substrate. The sectionalized ion transferring paper (SITP) substrate exhibits stable folding characteristics that are natural for ordinary paper. The characteristics of SITP allow origami construction.

5. Biomimetic Configuration

Nature is a collection of technologies that have long been stabilized, optimized and made efficient and sustainable through the process of evolution. The collaboration between material scientists and biologists is important, namely, in the development of biomimetics, which refers to the imitation and application of systems and mechanisms as well as the structure and functions of organisms adapted to the environment through evolution. It seeks to develop stable and energy-efficient technologies by utilizing the structural features of living things such as multifunctionality, adaptability, resilience and self-organization capabilities. Examples of biomimetics range from materials to robotics [50[35][36],51], human organs and tissue development [52,53,54][37][38][39] and power supplies [46,55][40][41].

5.1. Actin-Myosin Induced Omni-Directional Stretchable System

The sarcomeres in muscle tissue are composed of myosin molecules and actin filaments. Myosin molecules act as wheels and actin filaments function as tracks and they undergo permanent and reversible stretching in living bodies. Nam et al., imitated the structure of sarcomere to investigate complete and independent stretchable all-solid-state SCs [55][41]. In this system, graphene-CNT layer and PVA/H3PO4 were used as electrodes and gel electrolyte, respectively. CNT adapts as a roll of myosin and graphene acts like a floated track (actin filaments). CNT turns into a stretching motion when it receives external stress because of its high elasticity, and by connecting graphene and CNT with van der Waals interaction, the interfacial stress and slip stress are diminished at various deformed states. The graphene/CNT-layered structures demonstrate highly stable electrochemical performance under twisting and biaxial and uniaxial transformation.

5.2. Endoskeleton Structure Energy Storage System

Regardless of how stretchable and foldable electrodes are developed, fabrication of fully flexible electrical devices is not possible as long as an external hard passive cover exists. The passive cover, electrodes and electrolytes are loaded from the outside to the inside in conventional systems, which have a structure similar to exoskeleton systems like insects. The hard-cover provides stability against physical attacks but limits flexibility. To circumvent this intrinsic problem, Nam et al., proposed an oppositely ordered structure, in other words, endoskeleton structure. They used graphene-CNT layer electrodes for stretchable electrodes and PVA/H3PO4 as electrolyte because PVA has enough tensile yield strength (23 MPa) and stretchability for use as an external layer [60][42]. Polypropylene sheets were used as a porous and internal scaffold, which played the roles of a skeleton and ion transferring substrate. The pores are arranged hexagonally to minimize strain and stress while deformed, resulting in serpentine networks. These endoskeleton structured SCs showed great capacitance stability under folded and stretched states. The specific capacitance assessments according to scan rates of 0.1, 0.3 and 0.5 mV/s were 144, 95, and 73 F/g under 15% stretched state and the achieved Coulombic efficiency was about 90% regardless of the deformation states. Furthermore, the specific capacitance values were maintained at over 97% and 90% after 50 cycles of bending and stretching, showing steady electrochemical and mechanical performance. 

6. Micro-Supercapacitors

With an industrial focus on miniaturized autonomous electrical devices, reducing the thickness and size enough to be carried has commanded attention, along with flexibility to easily integrate them into circuits of micro-devices. All-solid-state micro-supercapacitors (MSCs) are especially encouraging to meet the aforementioned purposes [61,62][43][44]. However, ions of the electrolyte in conventional electronic devices with piled structures hardly penetrate deep inside the active electrode materials, leading to low C-rates accompanied by comparatively low power and energy densities [63,64,65][45][46][47]. Hence, in-plane MSCs using interdigitated structures were developed to provide high power density because of their short ion diffusion length and densely packed electrodes [66][48]. Diverse patterning approaches have been established to prepare interdigitated electrodes for MSCs including photolithography with laser scribing [67[49][50][51],68,69], etching [70,71,72][52][53][54] and printing [73,74,75][55][56][57].
Liu et al., developed carbon-type flexible all-solid-state MSCs using a mask-free plasma etching method. CNT electrode and PVA/H3PO4 gel polymer electrolyte were used. They compared interdigitated SCs and conventional sandwich SCs using the same electrodes with the former exhibiting higher capacitance [68][50]. The power and energy densities could be handled easily by changing the dimension of interdigitated electrodes per unit area. The MSC with 12 electrodes demonstrated a capacitance of 2.02 F/cm3 (scan rate = 10 mV/s).
The studies mentioned above featured electrodes using carbon based-materials, which form an electrical double layer (EDL) at the surface. They are commonly used as active materials for SCs because of their good electric conductivity and large surface areas. Surface area is an important indicator for the performance of SCs, as ions are stored only at the surface by adsorption and desorption. Metal oxides are also outstanding candidates as they store ions over redox reactions on the surface. However, most pseudocapacitive materials do not have a large specific area nor satisfactory electrical conductivity, both of which are necessary. In this regard, Lee et al., increased the surface area using laser processing and fabricated flexible MSCs [76][58]. Laser-induced sintering of the metal oxide precursors allows fabricating considerably porous electrodes, forming incomplete crystal growth and agglomeration. They used a polyimide (PI) film as the flexible substrate, silver conductor, PVA as gel electrolyte and MnO2 and Fe2O3 as active materials. They maximized the operating voltage and achieved high volumetric energy density by using an asymmetrical configuration of hetero-pseudocapacitive metal oxides, namely, MnO2 and Fe2O3. By controlling the laser scan rate, the porosity of silver and metal oxides could be managed. As the scan rate increased, the size of pores decreased and simultaneously the number of pores per unit area increased. The measured CV and GCD of both electrodes showed high stack capacitance values of 160.5 F/cm3 (Fe2O3) and 136 F/cm3 (MnO2). Furthermore, the CV curve was continuously maintained under different bending angles (0°, 45°, 90°, and 120°), showing good deformability of the device [76][58].
Industrial-scale applications demand rapid production of large-area interdigitated electrodes at a low cost. In this regard, the gravure printing method is a promising option, as it provides high-speed, roll-to-roll deposition of materials at high resolution (<30 µm) [77,78][59][60]. Zhang et al., fabricated interdigitated MSCs by gravure printing on a PI substrate with Ag electrodes, graphene active material and PVA-H2SO4 solid electrolyte [73][55]. The gravure-printed MSCs exhibited a high capacitance value of 6.65 mF/cm2, high power density (0.35 mWh/cm3 at 300 mW/cm3) and high energy density (1.41 mWh/cm3 at 25 mW/cm3).

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