Flexible Fiber-shaped Supercapacitors Research Articles

Flexible Fiber Shaped Self-Powered System Based on Conductive PANI for Signal Sensing and Energy Harvesting.

As science and technology advance, people are increasingly inclined to use sustainable and portable wearable electronic devices. The traditional supporting power source, batteries, suffers from issues of flexibility and lifespan, severely constraining the development of wearable devices. Alternatively, the self-powered system, serving as a power source, can effectively collect energy from the surrounding environment, achieving maintenance-free operation and high adaptability, which has attracted widespread research. The coaxial fiber-structured self-powering system proposed in this study is based on a supercapacitor (SC) and a triboelectric nanogenerator (TENG). The carbon fiber (CF) has polyaniline (PANI) and rGO connected to it, and a friction layer of silicone rubber is wrapped around the outside. The conductivity of the fiber was increased by multiple PANI graftings, and a coaxial fiber-type TENG with a 2 mm diameter was created. Following weaving, the TENG displays a high power density of 576 mW m-2 and an open-circuit voltage of 160 V and a short-circuit current of 9 μA. In addition, the flexible fiber-shaped supercapacitor uses NiAl-LDHs@CF as the negative electrode and AC@CF as the positive electrode, showing a specific capacitance of up to 281.4 mF cm-2. Furthermore, the SC and TENG are assembled into a coaxial self-power supply system, which has excellent performance and shows extensive potential applications in the field of wearable device power supply.

Simple construction of molybdate anions-intercalated NiCo-based layered double hydroxides nanosheet arrays on carbon fibers for enhanced flexible fiber-shaped supercapacitors

As the energy supply devices for miniaturized, wearable, portable and intellectualized electronic products, flexible fiber-shaped supercapacitors (FSCs) have attracted great attention by virtue of their peculiar superiorities such as ultra-fast charge/discharge ability, ultra-high output power, ideal service life, outstanding mechanical flexibility, and superior compatibility with the human body. The performance enhancement of FSCs is highly dependent on the high-performance active electrode materials. In this work, a series of molybdate anions-intercalated NiCo based layered double hydroxides (NiCoMo-LDHs) have been synthesized on flexible carbon fiber (CF) by a simple electrodeposition strategy by regulating the Mo content and the growth time. It turns out that the intercalation of molybdate anions can significantly widen the interlayer spacing and reduce the crystallinity of the synthesized LDH materials. The morphological differences between the LDHs with different Ni/Co/Mo ratios are analyzed, and the morphological evolution process of the materials with the specific Ni/Co/Mo ratio of 1:3:0.2 are systematically investigated. The well-developed NiCoMo-LDH nanosheets electrode with appropriate Ni/Co/Mo ratio (NCM2–500) delivers the best electrochemical performances among all the synthesized LDHs. In particular, the synthesized NCM2–500 fiber electrode exhibits excellent specific capacitance, superior electrical conductivity and favourable cycling stability. Additionally, the assembled parallel type NCM2–500//AC FSC device using NCM2–500 and activated carbon (AC) as the cathode and anode with the assistance of PVA/KOH gel electrolyte demonstrates a brilliant specific energy of 36 Wh cm−1 at 845 W cm−1 (nearly 17 Wh kg−1 at about 400 W kg−1) and a high cycle lifespan of maintaining 103 % of the pristine capacity after 5000 cycles. This FSC device is an ideal candidate for next-generation applications due to its remarkable electrochemical properties.

CuO@NiCo-LDH core-shell structure for flexible fiber-shaped supercapacitor electrode material

Metal fiber electrodes are of great interest for their excellent electrical conductivity and flexibility, however the limited surface area is unfavorable for the electrochemical performance. Herein, CuO nanowires in-situ growth on one dimensional (1D) Cu wire were obtained by wet chemistry and annealing method, and NiCo-LDH nanoplates were further growth on CuO nanowires by hydrothermal method to fabricate CuO@NiCo-LDH. The combination of CuO nanowires and NiCo-LDH nanoplates creates a three-dimensional (3D) core-shell structure, which improves the specific surface area and the electrochemical properties. In addition, there is a synergistic effect between the highly reactive of NiCo-LDH and the well-conducting of CuO/Cu. The composite presents a high specific capacitance of 1220.4 mF cm−2 at 1 mA cm−2. Moreover, the CuO@NiCo-LDH//activated carbon paper (ACP) asymmetric supercapacitor exhibits an excellent energy density of 20.1 μWh cm−2 at the power density of 556.3 μW cm−2, outstanding cycling stability with specific capacitance retention of 87.1 % after 6000 cycles and good bending performance. This paper provides a novel idea for the reasonable construction of electrode material structure.

Constructing a Hierarchical Ternary Hybrid of PEDOT:PSS/rGO/MoS2 as an Efficient Electrode for a Flexible Fiber-Shaped Supercapacitor

Improving the specific capacitance and energy density of a fiber-shaped supercapacitor (FSSC) is critical to its applications as an energy storage device for advanced smart wearable electronics. In this paper, a heterogeneous poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/reduced graphene oxide/molybdenum disulfide (PEDOT:PSS/rGO/MoS2) fiber was prepared to achieve a high-performance and durable electrode for the FSSC. As indicated, pseudocapacitive MoS2 was in situ grown on a highly conductive acid-treated PEDOT:PSS/rGO assembly with a hierarchical structure. This structural design emphasizes the wrinkled morphology and high conductivity of the PEDOT:PSS/rGO backbone to maximize the MoS2 deposition and accelerate its electron transfer, which fully utilizes the pseudocapacitance of MoS2 and provides additional capacitance contribution to the obtained device. Attributed to the synergy, the prepared fiber electrode in the FSSC exhibits high volumetric/areal specific capacitance (325.8 F cm–3/405.3 mF cm–2 at 1 A cm–3 or 1.2 mA cm–2) and excellent rate performance (82%, 1–10 A cm–3). The corresponding device shows an ultrahigh volumetric energy density of 6.9 mW h cm–3 at a high power density of 173.6 mW cm–3 and an areal energy density of 8.5 μW h cm–2 at 215.9 μW cm–2, together with excellent cycle stability and mechanical flexibility, outperforming most of the previously reported FSSCs. Accordingly, the proposed strategy provides a great opportunity to develop a high-performance FSSC for further wearable electronics.

Construction of flexible fiber-shaped boron-doped diamond film and its supercapacitor application

Flexible fiber-shaped supercapacitors (FSSCs) are promising candidates as electrode materials for the development of deformable electronic devices. Although tremendous efforts have been focused on the preparation of flexible electrode materials, traditional FSSCs materials face problems of inferior stability and complicated processes. Boron-doped diamond (BDD) holds promise as a FSSC electrode, owing to its well-established preparation process, strong acid and alkali corrosion resistance, environmentally and skin-friendly characteristics. Here, we reported a novel strategy for the construction of BDD-based FSSCs by growing a BDD film on a flexible tantalum (Ta) fiber substrate through hot filament chemical vapor deposition technique. Results showed that the BDD fiber film featured 10 folds improvement in specific capacitance than a planar BDD electrode. A symmetric supercapacitor device was assembled using the BDD fiber electrode and achieved an energy density of 25.6 mJ cm−2 at a power density of 0.6 mW cm−2, and a desirable stability with higher capacitance retention of 93.5% after 20,000 cycles. Furthermore, the symmetric BDD fiber device exhibited satisfactory bendability with high specific capacitance retention under various bending deformations. The findings in this research work hold promise for the fabrication of high performance flexible FSSCs.

Joule heating-induced faradaic electrode-decorated graphene fibers for flexible fiber-shaped hybrid supercapacitor with high volumetric energy density

The rational design of flexible, portable, and wearable fiber-based electrodes via selective hybridization of materials has motivated researchers to focus on the advancement of miniaturized energy-storage systems with excellent efficacy. Here, we introduce a novel Joule-heating approach for manufacturing hybridized fibers consisting of bark-like NiO as a faradaic shell decorated over electrically treated graphene fibers (EGFs@NiO) as a capacitive core for high-performance hybrid supercapacitors. Each Joule-heating process was conducted for a very short period (∼100 s) to achieve highly restored graphitic structures with a reduced internal resistance of GFs to expedite the electrochemical kinetics and successful hybridization of EGFs with NiO for promoting redox reactions. Moreover, an all-solid-state hybrid supercapacitor was constructed using EGFs@NiO as a positive electrode and chemically reduced GF (CGF) as a negative electrode; the supercapacitor exhibited a volumetric capacitance of 838.3 F cm−3 (volumetric capacity: 139.7 mAh cm−3), wide operating window of 1.5 V, and volumetric energy density of 34.49 mWh cm−3 at a power density of 374.97 mW cm−3. The energy efficiency of the process was evaluated based on the ratio of the volumetric capacitance of the device to the process time, and the proposed supercapacitor outperformed the recently reported asymmetric/hybrid supercapacitors based on the conventional manufacturing process. Thus, this work contributes to the development of fast, scalable, and cost-effective techniques for fabricating wearable electrodes for real-time applications.

Surface Modification of Commercial Cotton Yarn as Electrode for Construction of Flexible Fiber-Shaped Supercapacitor

In this study, we report on the rational design and facile preparation of a cotton-reduced graphene oxide-silver nanoparticle (cotton-RGO-AgNP) hybrid fiber as an electrode for the building of a flexible fiber-shaped supercapacitor (FSSC). It was adequately characterized and found to possess a well-defined core−shell structure with cotton yarn as a core and a porous RGO-AgNP coating as a shell. Thanks to the unique morphological features and low electrical resistance (only 2.3 Ω·cm−1), it displayed attractive supercapacitive properties. When evaluated in a three-electrode setup, this FSSC electrode delivered the highest linear and volumetric specific capacitance of up to ca. 12.09 mF·cm−1 and ca. 9.67 F·cm−3 with a satisfactory rate capability as well as a decent cycling stability. On the other hand, an individual parallel symmetric FSSC cell constructed by this composite fiber fulfilled the largest linear and volumetric specific capacitance of ca. 1.67 mF·cm−1 and ca. 0.67 F·cm−3 and offered the maximum energy density, as high as ca. 93.1 μWh·cm−3, which outperformed a great number of graphene- and textile yarn-based FSSCs. Impressively, bending deformation brought about quite a limited effect on its electrochemical behaviors and almost no capacitance degradation took place during the consecutive charge/discharge test for over 10,000 cycles. Consequently, these remarkable performances suggest that the currently developed cotton-RGO-AgNP fiber has considerable application potential in flexible, portable and wearable electronics.

Flexible fiber-shaped supercapacitors based on graphene/polyaniline hybrid fibers with high energy density and capacitance

Fiber-shaped supercapacitors (FSCs) are promising energy storage devices for portable and wearable electronics due to their miniaturized size, flexibility, and knittability. Despite the significant progress in this area, it is still a challenge to develop large capacitance and high energy density FSCs for practical applications. In this work, a hybrid fiber composed of reduced graphene oxide and polyaniline nanoparticles (r-PANI-GOF) is synthesized via in situ synthesis of polyaniline nanoparticles both on the surface and inside of graphene fibers. The areal specific capacitance of a single r-PANI-GOF electrode is as large as 1755 mF cm−2 in the three-electrode system. The r-PANI-GOF hybrid fibers were also used as electrodes for making an all-solid-state FSCs. This whole device has a specific areal capacitance of up to 481 mF cm−2 and a high areal energy density of 42.76 μWh cm−2. The hybrid fiber electrodes with a high capacitance, and excellent flexibility may become new candidates for the development of fiber-shaped high-performance energy storage devices.

Activated carbon fiber yarns with birnessite-type MnO2 and oxygen-functional groups for high-performance flexible asymmetric supercapacitors

Carbon fiber yarn (CFY) is deemed to be an ideal fiber flexible substrate because of its excellent mechanical flexibility, conductivity, and low-cost. In this study, positive electrode was prepared by hydrothermal process to form a birnessite-type MnO2 interconnected 3D nanostructure loading on the carbon fiber yarn (MnO2@CFY). Using an acid-etching process, the oxygen-containing functional groups successfully introduce to the surface of carbon fiber yarn (OCFY) as the negative electrode. The asymmetric all-solid-state flexible fiber-shaped supercapacitor (AFFSC) based on as-prepared MnO2@CFY and OCFY delivers an excellent volumetric energy density of 11.05 mWh cm−3 at a volumetric power density of 100.32 mW cm−3, maintaining 95.33% capacitance retention after 5000 bending cycles.

Design and fabrication of high performance flexible supercapacitor with polypyrrole@carbon fiber yarn electrode and redox active dopants

The flexible fiber-shaped electrodes that polypyrrole (PPy) and hydroquinone (HQ) derivatives coated on the surface of carbon fiber yarn (CFY) were synthesized by a simple one-step electrosynthesis. Besides, the effects of the size and structure of the HQ derivatives on the electrochemical performance of the doped PPy@CFY electrodes were analyzed. The study suggested that sodium lignosulfonate doped PPy@CFY (LS-PPy@CFY) exhibited a high specific capacitance of 245.89 F cm−3 at 0.26 A cm−3. The all-solid-state flexible fiber-shaped supercapacitor (AFFSC) based on two parallel LS-PPy@CFY electrodes showed a high specific capacitance of 50.08 F cm−3 and an excellent energy density of 4.45 mW h cm−3. Moreover, the AFFSC achieved prominent cyclic stability, maintaining 92.7% capacitance retention after 25,000 cycles, as well as excellent flexibility and mechanical stability which kept 89% capacitance retention after 5000 bending cycles. In addition, the AFFSC also exhibited good voltage and current scalability and potentiality in its practical application.

Boosting storage properties of reduced graphene oxide fiber modified with MOFs-derived porous carbon through a wet-spinning fiber strategy

Supercapacitors that are light weight and flexible, while occupying a low volume and demonstrating good mechanical properties are in demand for portable energy storage devices. Graphene composite fibers are supposed to be ideal electrodes for flexible fiber-shaped supercapacitors. Integration of MOFs-derived porous carbon into graphene fibers provides desirable electrochemical and mechanical properties. Herein, a general strategy is shown for the preparation of MOFs-derived porous carbon/reduced graphene oxide fibers. Close-packed and aligned graphene sheets along with porous MOFs-derived porous carbon can achieve outstanding mechanical properties through synergistic effects. Consequently, a large specific capacitance of 56.05 F cm−3, a good tensile property of 86.5 MPa and a high retention of 96.6% after 10 000 cycles can be achieved with the composite fibers. Moreover, a further deposition of polyaniline (PANI) and manganese dioxide (MnO2) by in situ growth on the fabricated composite fibers provide an improvement in specific capacitance with value of 74.21 F cm−3 and 65.08 F cm−3, respectively. The above results demonstrate the promising application of composite fibers as a flexible and stable electrode and substrate for energy storage devices.

Hierarchically interconnected conducting polymer hybrid fiber with high specific capacitance for flexible fiber-shaped supercapacitor

Polypyrrole (PPy) based fiber-shaped supercapacitor (FSSC) is attracting increasing attention as energy storage devices of flexible and wearable electronic products in recent years. However, its application is limited by the low specific capacitance and poor rate capability, mainly suffering from inferior conductivity and inhibited ions channels of fiber electrode. Here, a high-performance flexible FSSC is constructed based on hierarchically interconnected porous poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/PPy composite fiber prepared by in situ chemical polymerization of pyrrole on a novel highly-conductive PEDOT:PSS hydrogel support. The good electronic and ionic conductor properties, hierarchically porous structure and π-π interaction of conducting polymer hybrid fiber ensure fast electrons transfer/ions diffusion and the efficient utilization of whole electrode. Benefiting from the well-designed hybrid architecture and synergistic effect, the PEDOT:PSS/PPy hybrid fiber in FSSC exhibits superb volumetric/areal/length specific capacitance (393.8 F cm−3, 770.6 mF cm−2 and 18.9 mF cm−1 at 2.0 A cm−3) and excellent rate performance (52.8% capacitance retention even at ultrahigh current density of 50 A cm−3). The FSSC device displays record-high energy density (8.3 mWh cm−3 or 16.2 μWh cm−2 at such a high power density of 389.1 mW cm−3 or 761.5 μW cm−2) compared to other previously-reported PPy based FSSCs, as well as long-term cycle stability and exceptional flexibility. The outstanding electrochemical and flexural performances of our PPy based FSSC are competent for portable and wearable electronics.

Recent advances in flexible fiber-shaped supercapacitors

With the continuous development of today's flexible electronic products, fiber-shaped supercapacitors (fiber-shaped supercapacitors, FSCs) have attracted continuous attention. That’s due to their advantages such as light weight, controllable volume, good bending and tensile properties, and weavable. Fiber-shaped supercapacitors, with their unique one-dimensional fiber structure, can be combined with various other electrical or power generation devices into multifunctional integrated fiber-shaped electronic devices, which have huge application prospects in the field of wearable electronic textiles. This article describes the latest developments in fiber-shaped supercapacitor devices. Firstly, different fiber substrates are introduced and their advantages and disadvantages are analyzed as well. It also summarizes the electrode materials such as carbon materials, metal oxides and sulfides, conductive polymers, and hybrid nanocomposites of fiber-shaped supercapacitors. By analyzing the differences and characteristics of different electrode materials, it is shown that different electrode materials are suitable for different uses in fiber-shaped supercapacitors. Then we also summarize the application of fiber-shaped supercapacitors in cooperation with other devices to form integrated devices, including integration with general power devices, sensors, other photoelectric conversion devices and other power generation devices into hybrid devices and applied to practice. Finally, by summarizing the recent development results of fiber-shaped supercapacitors and the current challenges in the field, some current bottlenecks and problems of fiber-shaped supercapacitors are proposed, and some suggestions and ideas for the future development direction are put forward.

Hierarchical core-shell fibers of graphene fiber/radially-aligned molybdenum disulfide nanosheet arrays for highly efficient energy storage

Flexible fiber-shaped supercapacitors have broad application prospects in wearable and portable electronics and smart textiles, however, often suffered from their relatively low capacitance and energy density. In this paper, a unique core-shell fiber composed of graphene fiber and radially-aligned molybdenum disulfide nanosheet arrays (GF/MoS2) has been rationally designed and prepared. The morphology and structure of well-designed core-shell fibers were studied by Raman, X-ray diffraction (XRD), scanning electron microscope (SEM) and energy-dispersive x-ray spectroscopic (EDX) elemental mapping. All-solid-state, fiber-shaped symmetric supercapacitors were constructed by packaging two parallel GF/MoS2 core-shell fibers with PVA/H2SO4 electrolyte. Owing to the synergistic effects between graphene and MoS2, the fiber-shaped supercapacitor exhibits an enhanced areal capacitance up to 189.73 mF cm−2 and a high energy density of 14.665 μW h cm−2. The fibrous supercapacitors also exhibit good stability and flexibility. This work provides a strategy to design core-shell fibrous electrodes for high performance fiber-shaped supercapacitors, which also holds great potential for future flexible electronic devices.

CoNi2S4 Nanoplate Arrays Derived from Hydroxide Precursors for Flexible Fiber-Shaped Supercapacitors.

A high-quality porous CoNi2S4 nanoplates array was in situ synthesized on carbon fibers (CFs) by a hydrothermal method via a CoNi-layered double hydroxide (LDH) precursor transformation process. The CoNi2S4@CFs electrode exhibits largely enhanced supercapacitor performance with a specific capacitance of 1724 F/g at 1 A/g, in comparison with that of the CoNi-LDH (1302 F/g) precursor. Furthermore, the CoNi2S4@CF electrode shows an extremely high rate capability with capacity retention of 79% under a charge density of 60 A/g, whereas the retention rate of CoNi-LDH@CFs is only ∼34%. The abundant pore structure, improved electrical conductivity, and lower internal resistances of CoNi2S4@CFs (1.0 Ω) compared to those of CoNi-LDH@CFs (9.5 Ω) are responsible for the enhancement of energy storage performance. By using the CoNi2S4 nanoplate array as the positive electrode, an all-solid-state asymmetric fiber-shaped supercapacitor was further obtained, which exhibits outstanding flexible, foldable, and wearable capability. In view of the component tunability for LDH materials, the hydroxide precursor transformation method with merits of mild conditions and easy operation can be extended to the synthesis of a variety of metal sulfides for broad applications in electronic devices.

Tungsten oxynitride nanowires as negative electrode for fiber-shaped supercapacitor

Holey tungsten oxynitride nanowires are vertically aligned on carbon nanotube fibers and applied as negative electrode of high performance supercapacitor. Due to the rich nitrogen doping and holey structure, this negative electrode displays an excellent electrical conductivity, a high volumetric capacitance of 265 F cm−3 at 0.08 A cm−3 and a good capacitance retention of 94.15% after 20000 cycles at 1.0 A cm−3. When it is applied in an asymmetric flexible fiber-shaped supercapacitor matched with a manganese-based positive electrode, the device achieves a remarkable volumetric capacitance of 52 F cm−3 at 0.5 A cm−3 and an extraordinary volumetric energy density of 23.4 mW h cm−3 at 0.45 W cm−3 with a maximum working voltage of 1.8 V. Therefore, this simple fabrication of stable negative materials opens a new pathway for application of high energy density asymmetric fiber-shaped supercapacitor in new generation flexible energy storage devices.

Flexible fiber-shaped supercapacitors with high energy density based on self-twisted graphene fibers

In this research, self-twisted graphene fibers are successfully prepared by a simple method. The assembled self-twisted graphene fiber exhibits excellent mechanical properties, the elongation is up to 26.3% and the tensile intensity reaches up to 244.8 MPa, which is higher than that of the previously reported graphene fibers. Based on the remarkable mechanical properties of self-twisted graphene fibers, the flexible fiber-shaped supercapacitors are fabricated. The fiber-shaped supercapacitor demonstrates outstanding flexibility, even at the knotted state the capacitance is very stable. Furthermore, the fiber-shaped supercapacitor exhibits excellent supercapacitive performance with high energy density (25.5 mWh cm−3) and long-term cycling stability, retaining 91% of the capacitance over 10,000 cycles. The outstanding performances of flexible fiber-shaped supercapacitors indicate the self-twisted graphene fibers have promising applications in portable and flexible energy storage devices.

MXene Hybrid Fibers: Highly Conductive Ti3C2Tx MXene Hybrid Fibers for Flexible and Elastic Fiber‐Shaped Supercapacitors (Small 8/2019)

MXene Hybrid Fibers: Highly Conductive Ti₃C₂T<i>_x</i> MXene Hybrid Fibers for Flexible and Elastic Fiber‐Shaped Supercapacitors (Small 8/2019)

NiO-bridged MnCo-hydroxides for flexible high-performance fiber-shaped energy storage device

Flexible fiber-shaped supercapacitors (FFSSs) hold promising prospect to meet the increasingly high requirements of the wearable electronics. However, today it remains a great challenge to construct advanced supercapacitor with high areal capacity and favorable rate capability to achieve superior energy density in facile route is a great challenge. Herein, we directly used the low-cost nickel wire as the fiber substrate to in-situ grow compacted NiO buffer layer capable of strongly grafting the outer MnCo-Layered double hydroxide (MnCo-LDH) with high electrochemical reversibility. Compared to MnCo-LDH directly growing on nickel fiber in the absence of NiO, the NiO@MnCo-LDH exhibited 210% enhancement in areal capacity (165.6 mC cm−2/368.1 mF cm−2 at 0.5 mA cm−2) and ultrahigh rate capability (85% retention at 20 mA cm−2), as synthesized NiO buffer not only served as “nano glue” to strongly immobilize the active materials on the metal substrate but also positively supplied extra capacitance. Thusly, the assembled hybrid/asymmetric fiber device presented a high energy density of 0.0198 mWh cm−2 at a power density of 0.38 mW cm−2 to drive a digital watch, demonstrating its promising potential application in electronic devices. This rational design sheds light on the synthesis of nickel fiber-based supercapacitor with high energy delivery.

Facile Synthesis of Na-Doped MnO2 Nanosheets on Carbon Nanotube Fibers for Ultrahigh-Energy-Density All-Solid-State Wearable Asymmetric Supercapacitors.

Flexible fiber-shaped supercapacitors hold promising potential in the area of portable and wearable electronics. Unfortunately, their general application is hindered by the restricted energy densities due to low operating voltage and small specific surface area. Herein, an all-solid-state fiber-shaped asymmetric supercapacitor (FASC) possessing ultrahigh energy density is reported, in which the positive electrode was designed as Na-doped MnO2 nanosheets on carbon nanotube fibers (CNTFs) and the negative electrode as MoS2 nanosheet-coated CNTFs. Owing to the excellent properties of the designed electrodes, our FASCs exhibit a large operating potential window (0-2.2 V), a remarkable specific capacitance (265.4 mF/cm2), as well as an ultrahigh energy density (178.4 μWh/cm2). Moreover, the devices are of outstanding mechanical flexibility.