Fiber Supercapacitors Utilizing Pen Ink for Flexible/Wearable Energy Storage

A novel type of flexible fiber/wearable supercapacitor that is composed of two fiber electrodes – a helical spacer wire and an electrolyte – is demonstrated. In the carbon-based fiber supercapacitor (FSC), which has high capacitance performance, commercial pen ink is directly utilized as the electrochemical material. FSCs have potential benefits in the pursuit of low-cost, large-scale, and efficient flexible/wearable energy storage systems. Flexible/stretchable devices for energy harvesting and storage, such as flexible solar cells,1 flexible chemical batteries,2 and flexible supercapacitors,3 are being extensively studied to meet aesthetic demands and the needs of specific environments. The pursuit of thinner, lighter, highly elastic, and efficient flexible electronics has not ceased. Common fibrous and textile materials can be used to build suitable electronic device units or integrated modules that show incomparable advantages in the development of efficient and beautiful flexible portable/wearable electronics.4 Existing fiber-shaped energy conversion devices include fiber solar cells (light is converted to electricity),5 fiber nanogenerators (mechanical energy is converted to electrical energy),6 and fiber thermoelectric generators (heat is converted to electrical energy).7 Once the energy is captured from the environment, energy storage becomes necessary. Excess energy is stored for later use, not only to avoid wasting energy, but also to maintain stable system operations. Therefore, the development of lightweight, high power/energy density, and flexible energy storage devices for flexible/wearable electronics is highly significant. Lithium-ion batteries and supercapacitors (or electrochemical capacitors) are at the forefront of current storage technologies.8 Compared with lithium-ion batteries, supercapacitors possess fast charging/discharging capacity, ultrahigh power delivery, exceptional cycle life and efficiency, and a wide range of operating temperatures and safety,9 which confirm their potential in hybrid electric vehicles, metro trains, as power and memory back-up for various mobile electronic devices,10 and in realizing energy integration after being combined with energy conversion devices. Electrochemical double-layer capacitors store energy in the double layer by means of adsorption of oppositely charged ions. Electrode materials generally use active materials such as activated carbon,11 onion-like carbon,12 carbide-derived carbon,13 carbon nanotubes,14 and graphene15 because of the high specific surface area of these materials, which makes them suitable for reversible ion adsorption or charge storage. Carbon materials are abundant and electrochemically stable. However, relatively high internal series resistance, limited surface area accessible to the electrolyte ions, and pore morphology requirements restrict the capacitance performance of carbon-based supercapacitors. Several carbon materials (e.g., carbon nanotubes, carbon onions) exhibit good electrochemical capacitance but involve complex preparation,12, 14 which make them costly and inapplicable to large-scale manufacturing. In the work reported here, we designed a novel and efficient flexible fiber supercapacitor (FSC) that consists of two fiber electrodes, a helical spacer wire, and an electrolyte. Despite a few studies on FSCs,16 their device structure is not ideal because direct contact of electrodes inevitably occurs, causing serious leakage and low chemical capacitance. In this research, the use of a spacer wire enables the efficient separation of the two fiber electrodes and a scalable preparation. Commercial pen ink is employed as the active material for the first time, allowing us to obtain good electrochemical capacitance performance, with an areal capacitance of 11.9–19.5 mF cm−2, an energy density of 1.76 × 10−6–2.70 × 10−6 Wh cm−2, and a power density of up to 9.07 mW cm−2. Common pen ink shows fine dispersion and strong adhesion. A simple dip-coating method can be used to form a uniform film on fibrous substrates. Compared to numerous existing carbon materials, pen ink possesses advantages such as low cost, high stability, and well-established industrial production. This ink carbon material remains robust after 15000 electrochemical cycles. Furthermore, the FSC does not show any apparent degradation in the bending test, which indicates good flexibility. Low-cost, high-performance FSCs provide an alternative strategy toward efficient flexible storage devices and wearable energy management. Figure 1a shows the structural diagram of an FSC that includes two fiber electrodes, a spacer wire, and an electrolyte. The fiber electrode is composed of a conductive fibrous substrate and electrochemically active materials. The spacer wire is evenly twisted onto the surface of one fiber electrode with a specific pitch. Two fiber electrodes are then placed closely parallel and packaged into a flexible plastic tube filled with electrolyte to obtain the FSC. A spacer is required in traditional flat supercapacitors to effectively prevent short circuits caused by the direct contact of two electrodes. Common spacers also require a porous structure for ion transport, so they usually have porous polymer membranes.17 In this research, however, the well-designed helical spacer wire also prevents the FSC from short circuiting during bending and plays a key role in the efficient transport of ions and the easy preparation of large-size FSCs. Architecture of the FSC and morphology of the electrode. a) Schematic diagram of the FSC. b) SEM image of the plastic fiber electrode coated with pen ink film. c) SEM image of ink nanoparticles at high magnification, with a particle size of around 20 nm. d) Photograph of a flexible FSC packaged using plastic tube (Φ inner = 0.9 mm, Φ outer = 1.8 mm). The excellent flexibility of the FSC was demonstrated by wrapping the FSC around a 7.5 mm-diameter glass rod. Commercial pen ink was deposited on the surface of the fiber electrode by means of a simple dip-coating process and acted as the active material. Figure 1b shows a scanning electron microscopy (SEM) image of a plastic fiber electrode with ink coating, which reveals that the pen ink forms a relatively uniform film on the fibrous substrate. Viewing the SEM image at high magnification (Figure 1c) reveals that the ink film is composed of nanoparticles about 20 nm in size that bond together to form a porous morphology. Our previous Raman investigation verified that the active component in pen ink is mainly graphite carbon nanoparticles.18 A specific surface area of 27.4 m2 g−1 of ink nanoparticles was obtained by the Brunauer–Emmett–Teller (BET) method, implying a potentially substantial surface area for the electrochemical adsorption of electrolyte ions. Figure 1d shows a photograph of a prepared FSC wrapped around a glass rod, which displays the excellent flexibility and bending resistance of the FSC. The electrochemical characterization of the FSC was performed on an Autolab workstation (Metrohm, Switzerland). The electrochemical performance was then tested by cyclic voltammetry (CV) and galvanostatic charge/discharge in 1 M Na2SO4. As the ink film thickness increased, the electrochemical performance of FSCs increased correspondingly (Figure S1 in the Supporting Information). An ink film approximately 21 μm thick was deposited onto a 150 μm nickel wire substrate and used as the fiber electrode. In consideration of the flexibility and mechanical strength of the ink film, we did not further increase the film thickness. Figure 2a shows the cyclic voltammograms recorded by a two-electrode system with a voltage window from 0 to 1.0 V. All of the CV curves are close to rectangular at the different scan rates of 0.1, 0.5, 1.0, and 10.0 V s−1. This result illustrates the good electrochemical performance of the FSC. The nickel wire substrate contributed minimally to overall capacitance; high capacitance was mainly contributed by the ink coating (Figure S2 in the Supporting Information). An areal capacitance value of 9.5 mF cm−2 was calculated from CV at a scan rate of 1.0 V s−1, which is significantly higher than the reported areal capacitance (1.7 mF cm−2) of a carbon-onion-based micro-supercapacitor at the same scan rate.12 Figure 2b shows the relationship between the discharge current and the scan rates. The discharge current shows a linear dependence on scan rates ranging from 0 to 1.0 V s−1, indicating the high power output capability of the FSC. When the scan rates exceed 1.0 V s−1, the current values deviate from the linear region, probably owing to the limited ion diffusion in the electrode and the conductivity of the electrode materials.12 The stability of a supercapacitor is an important parameter in evaluating its potential for practical applications. Figure 2c shows the change of capacitance as the number of cycles increases, obtained from a CV test at a scan rate of 1.0 V s−1. The capacitance slightly increases as the number of cycles increases to 15000, which confirms its good electrochemical stability. Electrochemical characterization of the FSC. For (a–g), the FSC length is 6.4 cm, the electrode diameter is 191 μm (including the 150 μm nickel wire substrate and the 21 μm-thick ink film), and the electrolyte is an aqueous solution of 1 M Na2SO4. a) CV curves of the FSC at different scan rates ranging from 0.1 to 10.0 V s−1, where the rectangular curves of a dual battery can be observed. b) The relationship between the discharge current and the scan rates. The red straight line shows the linear range extending to 1.0 V s−1, indicating the high power output capability of the FSC. c) Capacitance value versus the number of cycles during long-term cycling (scan rate: 1.0 V s−1). For 15000 cycles, the capacitance is basically unchanged and even rises slightly at a late stage, demonstrating good electrochemical stability. Inset: Cyclic voltammograms (1.0 V s−1) of a number of selected cycles (1st, 5000th, 10000th, 15000th). d) Galvanostatic charge/discharge curves for the FSC at high current density (13 mA cm−2). The symmetric triangular curves and relatively low IR (current times resistance) drop indicate excellent capacitive behavior. e) Impedance spectroscopy of the FSC shows an equivalent series resistance (ESR) of about 1.4 Ω and that a low ESR favors the reduction of IR drop during discharging. f) Relationship between the area-specific capacitance and the discharge current. The area-specific capacitance remains at 12 mF cm−2 under a high discharge current of 17 mA cm−2, a value that is significantly higher than those reported in the literature for carbon-based micro-supercapacitors (0.4–2 mF cm−2). g) Ragone plot for the FSC showing that the performance is comparable to that reported previously in the literature. h) Capacitance versus length shows a linear relationship between the capacitance and the device length. The electrochemical properties of FSCs were investigated further by means of galvanostatic charge/discharge. Figure 2d reveals the galvanostatic charge/discharge curves for the FSC at a high current density of 13 mA cm−2 (the charge/discharge curves under different current densities are presented in Figure S3 in the Supporting Information). The curves in Figures 2d and S3 present a typical symmetrical triangular shape, further verifying that the capacitance of the prepared FSC originates from the electric double layer at the ink film–electrolyte interface. Test of the FSC under high current density also reveal that the voltage maintains a linear dependence on time even with ultrafast charge/discharge rates. Figure S4 (Supporting Information) derives the IR drop at different discharge currents, which indicates the small IR drop remaining in the FSC at a high discharge current. The results above fully elucidate that our FSC bears a relatively small internal series resistance. Electrochemical impedance spectroscopy (EIS) in Figure 2e also proves the low series resistance of FSC, which is always beneficial for reducing ohmic losses during discharge. These results confirm the excellent capacitive performance of the pen ink carbon-based FSC. A conventional flat supercapacitor consists of two planar electrodes and a planar spacer, and has a two-dimensional planar structure. In comparison, the FSC proposed in this study has a one-dimensional (1D) linear structure. Hence, the effect of length on capacitance was studied further. As shown in Figure 2h, varying the FSC length from 3 to 17 cm causes a linear increase in capacitance values. Thus, the desired energy output can be conveniently achieved by means of a simple adjustment in size of the prepared FSCs. The capacitance per unit length of the FSC is a critical parameter in its evaluation because FSC is 1D. The length-specific capacitance derived from Figure 2h is about 0.504 mF cm−1, which is significantly higher than that of previous FSCs.16 Metal wire substrates possess high density, which is not favorable for fabricating lightweight, highly flexible, and wearable FSCs. Hence, to achieve these ideal characteristics of FSCs, the metal wire substrate was replaced by Au-coated plastic fiber and carbon fiber in FSC preparation. The corresponding CV investigations were conducted using a two-electrode system in 1 M H2SO4. The results are presented in Figure 3a. The CV curves with a nearly rectangular shape also demonstrate good chemical capacitive behavior. The calculated area-specific capacitance of the carbon fiber-based FSC is about 26.4 mF cm−2, approximately 4.8 times that of the Au-coated plastic fiber-based FSC (5.5 mF cm−2). A likely reason for this difference is that the carbon fiber bundle is composed of multiple monofilaments, which have a considerably higher total surface area than the plastic fiber (Figure S6, Supporting Information). In addition to the active ink material, the carbon fiber substrate also makes a small contribution to the electrochemical capacitance (Figure S7, Supporting Information). High specific surface area, mechanical strength, chemical stability, and a price advantage make carbon fiber an ideal material for producing low-cost, high-performance FSCs. Characterization of the flexible FSC. a) Cyclic voltammograms (0.1 V s−1) of FSCs based on Au-coated plastic fiber (black curve) and carbon fiber (red curve) substrates. Electrolyte: 1 M H2SO4. The FSCs are 5.5 cm long. b) Cyclic voltammograms of a quasi-solid FSC with plastic fiber substrate at different bending states of 0°, 180°, and 360°. Scan rate: 0.25 V s−1. Electrolyte: PVA/H2SO4 gel electrolyte. Device length: 5.5 cm. A slight drop in capacitance at high curvature is shown (e.g., bending states of 180° and 360°). c) Schematic illustration of the different bending states in (b). Liquid electrolyte brings the risk of electrolyte leakage for capacitors. Thus, gel or polymer electrolytes are being explored, instead of liquid electrolytes, to prepare safe, all-solid-state supercapacitors.22 To realize FSCs that can be truly woven, poly(vinyl alcohol) (PVA)/H2SO4 gel electrolyte was used to fabricate a highly flexible quasi-solid-state FSC. Figure 3b shows the CV curves at different bending states (the different bending states are illustrated schematically in Figure 3c) for a gel electrolyte–based FSC with Au-coated plastic fiber substrate. As shown in Figure 3b, the FSC capacitance dropped only slightly after the high curvature, which may result from the partial damage of the ink film in highly bent states. We also attempted to perform a more systematic investigation of the bending tolerance (Figure S8, Supporting Information) and pressure tolerance (Figure S9, Supporting Information) of the flexible FSC, which showed that the FSC has acceptable resistances to bending and compression. In any case, there is still room for further improvement to enhance the mechanical performance of FSCs for practical applications. In the future, we hope to prepare conducting polymers (e.g., polyaniline, polypyrrole) with high electrochemical activity to use instead of the ink carbon film. Polymer film can relieve stress during bending, and thus create more flexible FSCs. Compared with previously reported FSCs,16 the FSCs proposed in this study show better potential for practical applications. First, a spacer design is not provided in the previous FSCs, which inevitably leads to direct contact between two electrodes and causes a short circuit, especially when preparing large-size devices or during the bending process. Contact between fiber electrodes also occurs when no spacer wire exists, which causes a short circuit of the capacitor (Figure S10, Supporting Information). Second, the FSCs can be easily scaled-up using a simple dip-coating process. Previous FSCs were only 0.5 mm long, whereas those in this study can reach tens of centimeters, or even a few meters, in length. Third, the FSC prepared in this work have an obviously better capacitance performance. Meanwhile, energy density and power density are two to three orders of magnitude higher, and are thus closer in performance to flat-structured supercapacitors and micro-supercapacitors.12, 22 Compared with conventional flat supercapacitors, the FSCs in this work are more flexible and lightweight, and are thus more suited to specific applications such as the preparation of wearable or highly flexible electronics for energy storage. Moreover, FSCs are expected to be combined with fiber energy conversion devices to obtain composite wearable energy systems used in scavenging and storing energy. This report also introduces the direct use of low-cost commercial pen ink as active material to fabricate FSCs that have excellent capacitive performance and high electrochemical stability superior to other carbon materials.23 Given their low-cost materials, efficient and simple preparation process, safety, light weight, high electrochemical performance, and environmental friendliness, FSCs lead the way to developing large-scale, flexible/wearable energy storage devices. In summary, commercial pen ink was adopted for the first time as an active material for supercapacitors. By means of a simple and facile process, flexible ink-based fiber electrodes were produced on common fibrous substrates to achieve highly flexible FSCs. Besides excellent flexibility and bending resistance, FSCs exhibit good capacitance, power density, and energy density that are comparable to those of traditional flat supercapacitors. FSCs based on pen ink also demonstrate a stable cycling life of over 15000 cycles. This work promotes a viable strategy for large-scale preparation of low-cost, lightweight, highly efficient, and flexible/wearable energy storage devices. Preparation of electrodes and device fabrication: The pen ink (from Hero, Shanghai Ink Factory in China) and plastic fiber were used in this work as purchased. Nickel wires (Alfa Aesar, 99.7% purity, diameter = 0.150 mm) were ultrasonically cleaned in acetone for 20 min. Carbon fibers (Toray, M40JB) were ultrasonically washed in a mixed solution of acetone/ethanol/deionized water (v:v:v = 1:1:1) for 20 min. Au film was deposited onto plastic fiber by magnetron sputtering (JCP 200). The conductivity of the Au-coated plastic fiber was measured to be 10 Ω cm−1. The dip-coating method was adopted to prepare all the required fibrous electrodes. The procedure was as follows: nickel wire, carbon fiber, and Au-coated plastic fiber were separately dipped into ink solution, taken out, and placed on a hot stage for drying. This dip-coating procedure was repeated until the desired film thickness was achieved, giving the final fibrous ink electrodes. An insulated enameled wire (diameter = 0.080 mm) was evenly wound onto one ink electrode at about 200 μm pitch to act as the helical spacer wire. The resulting electrode was placed close to and parallel to another ink electrode. Two active ink electrodes were fixed with glue and then inserted into a flexible plastic tube filled with electrolyte to prepare a FSC. The gel electrolyte was prepared by dissolving 10 g of PVA in a mixed solution of 10 mL sulfuric acid and 100 mL deionized water under vigorous stirring at 80 °C to form a transparent gel. Characterization and electrochemical measurement: The surface morphology of the electrode was characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800). The specific surface area of ink nanoparticles was determined by the BET method using a Micromeritics ASAP 2010 by adsorption of at A Autolab was used to electrochemical CV and galvanostatic charge/discharge was performed using with a 10 and a range of 1 Supporting is from the or from the and contributed to this The current work is by the and of We at of for during of to are as are but not or are as by the The is not for the or of any by the than be to the corresponding for the