Highly-Efficient Dendritic Cable Electrodes for Flexible

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Highly-Efficient Dendritic Cable Electrodes for Flexible Supercapacitive Fabric Yuxin Yang, Nannan Zhang, Baofeng Zhang, Yu Xin Zhang, Changyuan Tao, Jie Wang, and Xing Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11263 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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ACS Applied Materials & Interfaces

Highly-Efficient Dendritic Cable Electrodes for Flexible Supercapacitive Fabric Yuxin Yang,†,§,# Nannan Zhang,†,# Baofeng Zhang,⊥ Yuxin Zhang, § Changyuan Tao, § Jie Wang,*,‡,⊥ Xing Fan*,†,§ †

The State Key Laboratory of Mechanical Transmissions, Chongqing University, Chongqing

400044, P. R. China. ‡

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National

Center for Nanoscience and Technology (NCNST), Beijing 100083, P. R. China §

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, P.

R. China. ⊥ Electronic

Materials Research Laboratory, Key laboratory of the Ministry of Education &

International Center of Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, P. R. China #

Y. Yang and N. Zhang contributed equally to this work.

KEYWORDS: supercapacitor; flexible; electrodeposition; dendritic cable electrode; fabric;

1 Environment ACS Paragon Plus


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ABSTRACT: In search for cloth-like wearable energy-storage devices with both high energy density and high power density, metal fibers surrounded by micro metal dendrites, as current collectors, are either rooted inside a thick layer of carbon particles or wrapped with flower-like nano NiO in a similar manner to the root or stem system of natural plants, to form dendritic cable-like negative or positive electrodes. These dendritic cable electrodes could be further combined or woven into flexible solid-type super-capacitive garland or fabric, together with cotton wires. Benefitting from the ultra large interface of the metal dendrites current collector, it can be charged up to 1.8V, and give an energy density of 0.1408 mWh.cm-2 and a power density of 3.01 mW.cm-2, which is capable of directly starting a small electric car with a short and flexible piece of supercapacitor.

1. INTRODICTION The demand for wearable power source has given an enormous impetus to flexible energy harvesting and storage devices.1-8 For the flexible energy storage devices like super-capacitor or lithium ion battery, metal oxide or carbon materials were sandwiched layer-by-layer between flat metal or carbon foil, or wrapped over conductive wires.9-16 Emerging wire-type devices have attracted extensive attention in recent years. They could be enrolled, twisted or woven into various shapes, to facilitate the dressing.17-20 A key issue obstructing the way to the commercial application is their relatively low energy density and power density. The most common strategy towards high energy density is to increase the thickness and density of the active layer.21-24 Unfortunately, it often leads to inefficient charge transfer and poor mechanical cohesion stability on the electrode interface, thus limits the power density of the flexible device. Many efforts have been devoted to the active oxides layer, and have assembled various oxide structures, like the hierarchical structure.25,26 To further improve the charge transfer between the electrolyte/oxide


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interface and the current collector, a few type of porous current collectors with high specific area have been applied, for which more active materials could be patterned within a smaller thickness but a larger area.27-30 Nevertheless, many nano pores are too smaller for oxide units, and ion channels twist inside the nano porous structure are inefficient. Thus, it is important but challenging to design novel flexible current collector with both more micro warehouses for nano oxides and more efficient ion channels, in search for highly-capacitive flexible energy storage devices.31,32 Since the very ancient time, the plants have been growing on the earth in a flexible, sustainable and space-saving manner,33-35 and have constructed an efficient life system. On one hand, solar energy harvested from leaves and nutrients from the soil was transferred efficiently through a dendritic networks of stem and roots to sustain the life-related photosynthetic reaction. On the other hand, the dendritic root system penetrated into the soil can also reduce the soil erosion. The dendritic architecture of the root and stem system plays a very critical role in the nutrients collection and transfer, which is similar to the charge transfer towards the current collector inside supercapacitors or batteries. 36,37 Herein, highly-capacitive supercapacitor fabric was fabricated by weaving two types of flexible dendritic cable electrodes together with cotton wires. These two types of dendritic cable electrodes were both assembled based on micro metal dendrite arrays around metal fibers. One is fabricated by rooting the metal dendrites inside a thick layer of carbon particles. The other is fabricated by wrapping the metal dendrites with flower-like nano NiO. They can be combined together to assemble solid-type flexible super-capacitors in the form of cable or fabric. With an energy density of 0.1408 mWh.cm-2 and power density of 3.01 mW.cm-2, a short piece of supercapacitor could directly start up a small electric car.



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2. EXPERIMENTAL SECTION 2.1. Fabrication of dendritic Ni cable. Water was obtained from a water-purification system (HMC-WS10, Korea). All reagents were of analytical grade. Ni dendritic were directly deposited on metal fibers through a template-free electro-deposition process at a suitable range of current density using self-made all-round reactors (Figure S1). The central axis (Ni wire) diameters of the as-grown electrodes were set to about 150µm. The metal zinc sheet anode was rolled into a cylinder. The nickel wire is used as the cathode, which is vertically erected at the center of the reactor. The electrolyte was filled up between the cathode and the anode. The electrolyte was composed of NiSO4 (0.97M), NiCl2 (0.31M), and H3BO3 (0.6M). Both the growth voltage and growth time were experimentally optimized. 2.2. Assembly of NiO electrode and activated carbon electrode. As is demonstrated in Figure 1 and Figure S2, the NiO nano-sheets were grown on dendrites via the alcohol thermal method. The alcohol thermal reaction was conducted for 20 h at 80 oC in a solution of Ni(NO)3 (1M, 20mL), urea (4M, 20mL), ethanol (water/ethanol, V/V=2:7) and PEG-2000 (10g.L-1). Before the alcohol thermal reaction, a seed layer of Ni(OH)2 was deposited by immersing the metal dendritic cable into a seed-layer solution, and then dried at room temperature. The seed-layer solution (water/ethanol=1:1) contained Ni(NO)3 (1M, 5mL), urea (4M, 5mL) and ethanol (10 mL). After the alcohol thermal reaction, the electrode was sintered at 300oC under a nitrogen atmosphere for 3h. The negative electrode is then prepared by direct coating a layer of activated carbon (AC) on the nickel dendritic using activated carbon (AC) suspension, which includes activated carbon (Kuraray, YP-50F), polyvinylidene fluoride (PVDF) and 1-Methyl-2pyrrolidinone. The ratio of activated carbon and PVDF is 9:1. Then, the activated carbon electrode was dried at 80oC for 3h.


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2.3. Electrochemical characterization. Electrochemical measurements were carried out on a CHI660D electrochemical workstation (Shanghai Chenhua, China), using a standard threeelectrode system. For the electrochemical test of the super-capacitive electrode, a Pt foil (2 cm×2 cm) was employed as the counter electrode, whereas a saturated calomel electrode was employed as the reference electrode. The distance between the working electrode and the counter electrode was 1 cm, whereas the electrolyte was 1M KOH aqueous solution. Electrochemical impedance spectroscopy (EIS) measurement was performed at 0V, in a frequency range from 100 kHz to 0.10 Hz, with voltage amplitude of 5 mV. 2.4. Asymmetric supercapacitor devices. An asymmetric supercapacitor was made by using NiO/Ni-dendrites and AC/Ni-dendrites electrodes as the positive electrode and negative electrode. A piece of glassy fiber filter paper was used as the separator and 1M KOH aqueous solution was used as the electrolyte. All the electrochemical measurement were performed using CHI660D (Shanghai Chenhua, China). 2.5. Structure characterization. SEM (S4800, Hitachi, Japan) and TEM (JEM-100CX, JEOL, Japan) were employed to identify the morphology and structure of the samples. The actual specific area was measure by linear voltammetry method and also calibrated using BET method (3H-2000PS1, Beishide Instrument Technology co. Ltd., China). For the linear voltammetry measurement, the current was regarded as being in direct proportion to the surface area, when the current density was sufficiently small. It is more suitable for tiny metal dendrites, which is too small for the adsorption-desorption isotherm method. The surface area of the dendritic cable electrode was calculated by comparing the current with that of an ideal cylinder electrode (See Supporting note 3 for more details).



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3. RESULTS AND DISCUSSION

Figure 1. (a) The device fabrication process for the asymmetric flexible supercapacitor based on the dendritic cable electrodes. SEM images for (b) the dendritic metal substrate. (c) The carbon coated negative electrode. (d) The NiO grown on the dendrites of the positive electrode. (e) Fabricating a flexible textile-type or fiber-like supercapacitor from the dendritic cable electrode.

According to our design concept (Figure 1a), metal wire or metal coated polymer wire surrounded by micro metal dendrites formed a dendritic cable. Either the positive electrode or the negative electrode was fabricated by assembling nano functional units on the dendritic metal cable, in a similar way to the stem full of flowers or the root in the soil. A complete device can


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then be assembled by combining the stem-like positive electrode and the root-like negative electrode together. Herein, the hierarchical dendritic metal cable served as a type of promising current collector for supercapacitor or batteries. Similarly, for natural plants with similar dendritic structures, nutrition and energy can be efficiently harvested from the atmosphere and the soil, and then be efficiently distributed over the plants for life related reactions. Meanwhile, the structure is very flexible, which could survive the wind, or even be bundled into corolla. Scanning electron microscopy (SEM) images are presented for the dendritic metal substrate (Figure 1b), the carbon coated dendritic negative electrode (Figure 1c) and the NiO grown on the dendrites of positive electrode (Figure 1d), respectively. The micro metal dendrites grown around a thin wire are composed of tiny branches of several tens of micrometer in diameter. The dendritic cable electrode with a diameter of approximately 1-2 millimeter looks like a long and thin green bristle grass, giving a large specific area of 3.79 m2/g (Figure S3). For the negative electrode, the dendritic metal substrates is rooted in a soil of commonly-used highlysupercapacitive activated carbon (AC) particles with a size of about 600nm (Figure S4), which are too big for entering nano pores of traditional porous electrode. Comparing with traditional flat electrode or porous metal electrode, the micro dendrites confers upon the electrode not only the high specific area for efficient current collection, but also enough rooms, even deep inside the bulky volume, to accommodate more widely-used capacitive materials,27,36 such as AC particles. Besides, the tiny dendrites can act as a claw to hold the carbon particles when electrode bends, which is similar to the role of plant roots in preventing soil erosion.37 For the positive electrode, pieces of NiO nano petals with a thickness of 20nm and a width of 200nm (Figure 1d and Figure S4) were assembled on the dendritic metal substrates, like flowers or leaves on the stem. Upon bending, the gaps in-between dendrites allows a large relative shift, which would



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release the stress on the capacitive materials covered the metal dendrites. It is noteworthy that, whether decorated with nano flower or rooted in AC soil, the as-fabricated flexible electrode still have the shape of cables. Thus, different shapes of flexible super-capacitor, can be assembled by twisting, weaving or closely lying side-by-side together (Figure 1e).

Figure 2. (a) Calculation of the apparent area for the Ni dendritic cable. (b) Apparent diameter (D) and actual area (S) of Ni dendritic cable at different growth voltages, Length=1.5 cm. (c-e) SEM images of Ni dendritic cables at different growth potentials: 10V, 25V, and 35 V, respectively. (f) Apparent diameter (D) and actual area (S) of Ni dendritic cable at different growth time, Length=1.5 cm.

The dendritic cable electrodes possess very high specific area, which is derived from the hierarchical micro dendrites. As is indicated in Figure 2a, the apparent area of the dendritic cable electrode could be calculated via the following equation. S= π × d × h

(1)

It is found that, the actual specific surface area even reaches up to 10 times of the apparent area (Figure 2b). When the deposition voltage is 30V, the surface area is 5.9cm2, which is 80 times


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that of the bare nickel wire. Besides, it is also closely related with the voltage during the electrodeposition process. As the voltage increases from several volts to several tens of volts, the area of the dendritic cable electrode first increase and then decrease, forming a peak point at about 30V. The SEM images for dendritic cable electrodes grown at different voltage are also presented in Figure 2. At a low voltage, only a layer of metal grains would pile around the fiber, which is same as the common electro-plating process (Figure 2c). Well-grown metal dendrites only occur in a certain range of voltage (Figure 2d). At a high voltage, dendrites begin to crosslink with each other and form a previously reported porous structure (Figure 2e). The metal dendrite growth process are also similar to the branch growth process of a plant. During which, new small branches grow from elder branches, smaller and newer branches will then grow from these small branches (Figure S1). The formation of metal dendrites is a result of the self-organization during the electro-field-driven ion migration in the solution, which could be described and mathematically simulated via the diffusion-limited aggregation model.38-41 During the electro-deposition, each step of the ion migration could be regarded as the vector sum of two parts. One is the electric-field-driven migration towards the electrode along the electric field line. The other one is the random migration caused by micro thermal disturbances inside the solution, of which the direction is completely random. Without the random migration, there will be only a dense layer of metal. Furthermore, the relative speed of the electric-field-driven migration to the random migration would vary with the voltage, leading to the structure change of the dendrite. Meanwhile, both the radius and the specific area of the dendritic cable electrode would increase with the growth time (Figure 2f and Figure S5). The structure of the dendritic cable electrode could be finely adjusted by varying factors, such as the voltage, the growth time, the



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content of the electrolyte, the pH value and so on, which exhibits a great potential for further optimization.

Figure 3. (a) Nyquist plots of the positive electrode and the negative electrode. Electrochemical impedance spectroscopy analysis was at a bias voltage of 0V. (b) Charge-discharge curves of the NiO positive electrode. (c) Charge-discharge curves of the AC negative electrode. (d) The capacitances of the NiO positive electrode and the AC negative electrode at different charge-discharge current. (e) Cyclic voltammetry curves of the NiO positive electrode and the AC negative electrode. (f) The cycle stability of the dendritic cable negative electrode. SEM images of the AC layer before and after 10000 times of cycle are also inserted. (g) The capacitance of the dendritic cable electrodes at different growth voltage. (h) The capacitance of electrodes with different length. (i) The electrode capacitance at different bending angles.


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Highly-efficient super-capacitive positive and negative electrodes were respectively assembled based on the dendritic cable electrodes (Figure 3a and Figure S6). For the positive electrode, NiO nano pedals are grown on the metal dendrites. With a length of 1.4 cm and a diameter of 1.3mm, its capacitance reaches up to 0.88F at a current load of 5mA, giving a coulomb efficiency of 96.5% (Figure 3b). For the negative electrode, the metal dendrites are buried in a soil of activated carbon particles. With a length of 1.4cm and a diameter of 2mm, its capacitance reaches up to 1.0F at a current load of 5mA, giving a coulomb efficiency of 98.5% (Figure 3c). The charge collection of the metal dendritic is very efficient. From a discharge current range of 5mA to 30mA (Figure 3d), the capacitance of the positive electrode only decrease by less than 4.2%, and the capacitance of the negative electrode only decrease by 19.1%, showing a good charging-discharging performance at a large current. Besides, the ions transfer inside the closely packed AC layer would be less efficient than that in the nano structured NiO layer, which leads to a relatively faster drop of the capacitance at a large current. The chemical stability of both the dendritic positive electrode and the dendritic negative electrode were investigated via the cycle voltammetry analysis (Figure 3e). On the negative side, no side-reactions on the carbon-coated dendritic electrode were observed in a voltage range of 0V to -1.4V, taking advantage of the high alkalinity of the electrolyte. After 10000 times of charge-discharge cycle, the capacitance of the negative electrode would even slightly increase (Figure 3f). The branches of micro dendrites would firmly grab the activated carbon particles and reduce the drop of carbon particles from the metal substrate. The increase of the capacitance could be attributed to the separation of unnecessary carbon particles physically adhered to the surface of the carbon layer, which previously blocked the passage of the electrolyte. As is observed in Figure 3f and confirmed via N2 adsorption-desorption isotherm measurement, after



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10000 times of charge-discharge cycle, large crystals on the surface of the AC layer were replaced by more smaller particles. The carbon layer would be more porous with the specific area increasing from 318.93m2.g-1 to 484.93m2.g-1 (Table S1), which would provide more channels for the electrolyte to permeate in or out. On the positive side, there is also no evident side-reactions on the NiO-coated dendritic electrode in a voltage range of -0.2V to 0.6V (Figure 3e). For one thing, the Ni oxide on the electrode would obstruct the contact of the electrolyte with the metal substrate. For another thing, the oxidation product of metal Ni at alkaline condition is insoluble capacitive hydroxide, which will cover the metal surface and block the further corrosion of the metal electrode, without reducing the capacitance of the electrode. As a matter of fact, after 4000 times of chargedischarge cycle with a charging voltage up to 0.4V, the capacitance of the positive electrode still retain more than 80% of its capacitance (Figure S7), of which the stability is almost the same with that of the NiO–based super-capacitive electrode assembled on inert carbon electrode.42,43 Besides, both electrodes based on the dendritic metal arrays can reach a higher capacitance than that of the previously reported porous current collector (Figure 3g), which is derived from the well-aligned dendrite structure, and is closely related with the growth voltage and time (Figure 2 and Figure S8a). As the voltage increased, the electrode capacitance first increased and then decreased. At a voltage of 30V, the areal specific capacitance of the NiO/Ni electrode on dendritic cable substrate could be nearly twice as that on porous Ni foam substrate at the same NiO growth and capacitance testing condition (Figure 3g and Figure S9). As the growth time increases, the height of dendrites increase, along with monotonously-increased electrode capacitance (Figure S8b). For the porouos metal foam, many nano pores in traditional porous Ni foam will be blocked by NiO nanosheets or AC particles with a size of several hundreds of


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nanometers. Therefore, not only many area detectable by N2 adsorption-desorption isotherm method is actually unreachable, but also ion channels for the electrolyte is inefficient. The dendritic cable possess a larger usable area. In another word, well-designed dendrite structure on the cable can provide more micro rooms, which are capable of accommodating more active units in the size range of several hundred nanometers, and remain enough spaces for building up an efficient fractal channel structure for ions transfer. Meanwhile, charges from the tips of dendrites could be collected efficiently, and converge into the end of a longer cable, thanks to the high conductivity of the metal. The capacitance of the dendritic cable electrode increases linearly with the length of the cable (Figure 3h). By finely controlling the growth conditions, like voltage, time and additives, the dendritic structure could be further optimized to give better performance. In addition, taking advantages of the micro dendrites, the flexibility of the dendritic cable electrode are also very good. Its capacitance does not decrease even at a bending angle of 180o (Figure 3i), and retains over 80% of its original value after repeatedly bending for more than 45 times (Figure S10). For one thing, different from that in the metal foam, gaps in-between dendrites could also allow a large relative shift and release the stress of the capacitive layer at a bending state. For another thing, the metal branch of the micro dendrites is about tens of micrometers in diameter. After they aligned in an array, the external mechanic pressure could be well shared, which enable the cable electrode to endure quite an amount of stress. A new supporting movie was added as supporting movie S1, which demonstrated the performance of a dendritic cable electrode under external stress of a blade.



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Figure 4. (a) The cyclic voltammetry curve for a complete supercapacitor. (b) The capacitance of a capacitor at different current density. The apparent outer cylinder area is employed when calculating the areal specific capacitance, as is also discussed in Figure 2a. (c) The electrode potential of positive electrode and negative electrode at different capacitor voltage for wire type, dendritic cable type and flat type electrodes. (d) Stability of the supercapacitors based on dendritic cable electrodes. (e) A comparison among different capacitors assembled on wire type, dendritic cable type and flat type electrodes, respectively. (f) The relationship between the capacitance and the projected area for supercapacitor based on different types of electrode.

Complete flexible super-capacitors were then assembled by combining the positive cable electrode and the negative cable electrode together. After device structure optimization, the supercapacitor could be charged up to 1.8V, and give a high specific capacitance of 313mF.cm-2 @ 3.2 mA.cm-2 (Figure 4a,b and Figure S11), which is calculated using the apparent area around the fiber-shaped supercapacitor device, as also defined in Figure S12. Even at a large current of 16.3 mA.cm-2, its capacitor still retain above 211mF.cm-2. Thus, a high energy density of 0.1408 mWh.cm-2, and high power density of 3.01 mW.cm-2 is achieved.


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In order to find the origin of the high capacitor voltage, the electrode potential on the positive electrode or the negative electrode at different capacitor voltage was investigated (Figure 4c). The capacitor voltage is determined by the potential difference between the positive electrode and the negative electrode. Comparing with the supercapacitor composed of the flat electrodes or the fiber electrodes, the supercapacitor composed of dendritic cable electrodes hold a lower electrode potential for both positive electrode and the negative electrode. At a high capacitor voltage of 1.8V, the electrode potential on the positive dendritic cable electrode is only 0.55V, which is much lower than 1.2V of that on flat electrode. Meanwhile, the electrode potential on the negative dendritic cable electrode is -1.25V, which is also much lower than -0.6V of that on flat electrode. Under such a circumstance, both the oxygen evolution reaction and the metal oxidation on the positive would be largely inhibited, while the hydrogen evolution reaction on the negative does not emerge evidently owing to the highly alkaline electrolyte and the overpotential on the negative. In another word, the electrode potential distribution inside the supercapacitor based on dendritic cable electrode is more reasonable in such a highly alkaline electrolyte. In fact, after 5000 times of charge-discharge cycle, the capacitance of the capacitor would only decrease by less than 5% (Figure 4d). There is no obvious difference for the shape of the cycle voltammetry curve before and after the charge-discharge cycle test. As is indicated in Figure 4e, the capacitor composed of dendritic cable electrode occupied a much smaller space than that of the traditional flat electrode, while gave the highest capacitance among the three types of electrode. It can be attributed to the high specific area of the dendritic current collector. Furthermore, in order to achieve a higher capacitance for the full capacitor, the capacitance of the positive electrode should be matched well with that of the negative electrode. As is compared in Figure 4f, when the dendritic cable positive electrode is paired with traditional



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flat-type negative electrode, the space it occupied would be almost ten times larger than that when two dendritic cable electrodes paired together. Smaller space occupation is of special importance for wearable applications, which makes the dendritic cable a superior current collector for wearable energy-storage devices. The all-dendritic-cable-electrode supercapacitor provide a very promising solution. Finally, a complete all-solid cable-like supercapacitor was fabricated using the transparent gel electrolyte. Its capacitor voltage reaches up to 1.8V, while the specific capacitance reaches up to 207.5 mF.cm-2 @ 3.2 mA.cm-2 (Figure 5a and Figure S13). A photo for the solid-state gel electrolyte is demonstrated in Figure 5b. The electrolyte is solid, flexible and transparent, which would make the supercapacitor easier to encapsulation. With such a type of solid-type electrolyte, the dendritic cable electrode can even be woven with cotton wires into a supercapacitive fabric (Figure 5c and Figure S14). This type of energy storage fabric could be derived into tent or cloth, which is promising for many outdoor applications. As demonstrated in Figure 5d and Figure S15, two cable-like supercapacitor with a length of 3cm were connected in series and was bended into a garland shape. After fully charged, it can directly start up a small electric car (see Supporting movie S2), By scaling up into a tent in the future, it may be possibly applied in real vehicles, like caravan.


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Figure 5. (a) Galvanostatic charge-discharge curves of a flexible supercapacitor using gel electrolyte (current density =6.5mA.cm-2). Insert is a photo of the transparent solid gel electrolyte. (b) A photo of a cable like flexible supercapacitor. (c) A capacitive fabric using gel electrolyte and its electrochemical performance. (d) A demonstration for driving a small electric car using two series-connected cable-like supercapacitor bended into a garland shape.

4. CONCLUSION In summary, solid-type supercapacitor fabric was fabricated by weaving dendritic cable positive and negative electrodes together with cotton wires. These two types of electrodes were both assembled on micro metal dendrites arrays around metal fibers, as an imitation to the 17 Environment ACS Paragon Plus


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efficient nutrients collection and storage system in natural plants. The cable negative electrode was assembled by rooting the metal dendrites inside a thick layer of carbon particles. The positive electrode was assembled by wrapping the metal dendrites with flower-like nano NiO. Flexible super-capacitor in the form of cable or fabric were then assembled. Comparing with the strategy of assembling a thick layer of dendritic capacitive oxide on a smooth current collector, our strategy of assembling a thin layer of capacitive oxide on a dendritic cable current collector would further shorten the average charge transfer distance while reserve a high usable area. With good mechanical and charge-discharge stability, it could be charged up to 1.8V, outputting an energy density of 0.1408 mWh.cm-2 and power density of 3.01 mW.cm-2. After fully charged, two supercapacitor connected in series can directly starting a small electric car. The idea of the micro dendritic electrode structure provided an inspiring strategy to promote the performance of nano materials in final devices, which is suitable for future mass production and could be extensively applied to other types of energy harvesting and storage devices.

ASSOCIATED CONTENT Supporting Information Supporting Information Available: Setups for the growth of metal dendrite; Measurement of the surface area of the dendrite; More SEM and TEM images for the activated carbon particles and the nano NiO; Influence of preparation conditions on the metal dendrite; More electrochemical analysis and mechanic bending test results of electrodes and devices; Calculation of the apparent area; The preparation process of the asymmetric supercapacitor; A demonstration movie for the supercapacitor to start up a small electric car. These materials are available free of charge via the Internet at http://pubs.acs.org. 18 Environment ACS Paragon Plus

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AUTHOR INFORMATION Corresponding Authors *Email: [email protected]; *Email: [email protected]; Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21676033), the State Key Laboratory of Mechanical Transmissions (SKLMT-ZZKT-2017Z01) and the Fundamental Research Funds for the Central Universities (106112016CDJZR225514).

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Abstract Graphic


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Highly-Efficient Dendritic Cable Electrodes for Flexible

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