Ultra-thin coaxial fiber supercapacitors achieving high energy and

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Ultra-thin coaxial fiber supercapacitors achieving high energy and power densities Caiwei Shen, Yingxi Xie, Mohan Sanghadasa, Yong Tang, Longsheng Lu, and Liwei Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12047 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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ACS Applied Materials & Interfaces

Ultra-thin coaxial fiber supercapacitors achieving high energy and power densities Caiwei Shena,1, Yingxi Xiea,b,1,*, Mohan Sanghadasac, Yong Tangb ,Longsheng Lua,b and Liwei Lina a

Department of Mechanical Engineering, University of California at Berkeley, Berkeley, CA, USA

b

School of Mechanical & Automotive Engineering, South China University of Technology, Guangzhou,

Guangdong, China c

Aviation and Missile Research, Development, and Engineering Center, US Army, Redstone Arsenal, AL,

USA * Corresponding author: [email protected] 1

Both authors contributed equally to the paper

Abstract Fiber-based supercapacitors have attracted significant interests because of their potential applications in wearable electronics. Although much progress has been made in recent years, the energy and power densities, mechanical strength, and flexibility of such devices are still in needs of improvement for practical applications. Here we demonstrate an ultra-thin micro coaxial fiber supercapacitor (µCFSC) with high energy and power densities (2.7 mWh/cm3 and 13 W/cm3), as well as excellent mechanical properties. The prototype with the smallest reported overall diameter (~13 µm) is fabricated by successive coating of functional layers onto a single micro carbon fiber via a scalable process. Combining the simulation results via the electrochemical model, we attribute the high performance to the well-controlled thin coatings that make full use of the electrode materials and minimize the ion transport path between electrodes. Moreover, the µCFSC features high bending flexibility and large tensile strength (more than 1 GPa), which make it promising as a building block for various flexible energy storage applications.

ACS Paragon Plus Environment


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Keywords: Micro supercapacitors, Carbon fiber, Flexible energy storage, Wearable electronics, Multifunctional electronics 1 Introduction The research in wearable electronics has attracted strong interests in recent years with the inspirations of matured commercial products such as smart wristbands and watches. Fiber-based electronics represent the next level of ministrations and further advancements from the current two-dimensional (2D) wearable systems to one-dimensional (1D) structures 1. Clearly, both 1D power generation sources 2–4 and energy storage devices 5,6 will be indispensable components in systems based on 1D structures. For example, flexible fiber supercapacitors

6–9

and batteries

10,11

have been developed

for the possible integrations with electronics such as the “smart textiles”

12

. Traditionally,

supercapacitors have two parallel planar electrodes sandwiching an electrolyte layer in between. Such supercapacitors can be made flexible by using thin layers of solid electrolyte and electrodes or flexible conductive materials such as carbon nanotubes

13

and graphene 14. However, these 2D structures won’t

allow water or air to pass through and their mechanical properties and fabrication processes are not compatible with textiles. On the other hand, fiber-based supercapacitors

6

are flexible with higher

degrees of freedom when compared with 2D films and they are potentially compatible with textiles in terms of similar geometric dimensions, mechanical properties and fabrication processes. Previously, three configurations of fiber supercapacitors have been reported based on: (1) parallel electrodes

8,9,15–18

, (2) twisted electrodes

19–25

, and (3) coaxial electrodes

26–36

, respectively. For

supercapacitors using parallel electrodes, a flexible substrate is used to assemble the electrodes and a layer of electrolyte is applied afterwards. For supercapacitors using either twisted or coaxial electrodes with electrolyte in between, they could be directly woven into textiles. Since the coaxial fiber supercapacitors generally have larger interface areas, shorter ion transport path, and better stability upon mechanical bending 6 (Figure S1), they have been the focuses of several prior works. However, factors and choices in sizes, materials, and fabrication processes in the state-of-art coaxial fiber supercapacitors


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have limited their performances in energy density, power density, mechanical strength, and manufacturing cost. Specifically, the layer thicknesses and fiber diameter have great influences on the electrochemical performances and mechanical properties. Electrodes and electrolyte layers with small thicknesses may result in higher power density by shortening the ion transport pathes

37

but small size

fibers may also cause poor structural rigidity. As such, several reported coaxial fiber supercapacitors have diameters of 100 microns to several millimeters

26,28,29,38

, which are much larger than those of

wearable fibers of 10-50 µm in diameter such as cotton, silk, and polyester. For metal-wire based coaxial supercapacitors 26,28, heavy mass lowers their energy density based on the total weight of the system. For carbon nanotubes (CNTs) based coaxial supercapacitors

27

, low mechanical tensile strength (0.01-0.1

GPa) is a major concern. On the other hand, highly graphitized carbon fibers have high tensile strength (4-8 GPa) but they have small electric double-layer capacitance (EDLC) due to low surface areas. Finally, coaxial supercapacitors with electrolyte

29

or electrode

27,38

fabricated by laboratory-based

wrapping processes won’t be compatible with the large-scale, low-cost fabrication processes in the textile industries. Here we demonstrate high-performance (both high energy and power densities), flexible, and mechanically strong micro coaxial fiber supercapacitors (µCFSCs) via a potentially scalable fabrication process to achieve an overall diameter of ~13 µm - the smallest fiber supercapacitor device in the literature. The small size and proper design of each active layer not only guarantee high flexibility, but also benefit both energy and power densities, as further supported by simulation of a general electrochemical model of 1D CFSCs. As such, this micro coaxial fiber supercapacitor could be a promising building block as flexible energy storage systems for applications in wearable electronics.

2 Results and discussion 2.1 Structure and material design of µCFSCs



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The structure of our µCFSC is illustrated in Figure 1a. We start with a low-cost, commercially available micro carbon fiber

39

with a measured diameter of 6.7±0.2 µm as the core material due to its

high electrical conductivity (0.7 x 105 S/m), light weight (1.8 g/cm3), and large mechanical strength (4.9 GPa). While the fiber core serves as both mechanical support and current collector, a thin layer of MnO2/Polypyrrole (PPy) composite is electrodeposited as the inner electrode coating in which MnO2 provides ultrahigh capacitance

40–43

and PPy protects and stabilizes the MnO2 44,45 (Figure 1b). The 650

nm-thick composite layer improves the capacitance of a bare carbon fiber by 1000 times without increasing much volume (Supporting information part 3). A layer of polyvinyl alcohol (PVA)/H3PO4 (1.0±0.5 µm thick) and a layer of CNTs (Figure 1c, 1.5±0.5 µm thick) are dip-coated as the solid-state electrolyte and the outer electrode coating, respectively. Such processes result in a thin and pinhole-free layer of electrolyte with a very short ion transport path, and a porous and conductive network of CNTs with high capacitance. The final µCFSC device has a diameter of about 13 µm, even thinner than the cotton fibers in wearable fabrics (Figure 1d). Such a small size guarantees a high bending flexibility of the device, and no mechanical failure is observed even at a small bending radius of 150 µm (Figure 1e). Moreover, a single µCFSC is also strong enough to lift a US quarter coin of 5.67 g, which corresponds to a tensile strength of at least 0.42 GPa (Figure 1f). The high strength and the scalable fabrication process make µCFSCs potentially compatible with a continuous roll-to-roll process for batch production, and a possible future process is proposed in Figure S2. Threads made of µCFSCs in parallel may be produced and woven into various forms of macro-scale flexible supercapacitors. 2.2 Equivalent circuit model of µCFSCs The potential applications may require µCFSCs of at least centimeters long, which is 1000 times of their diameter. As such, it is reasonable to treat the µCFSC as 1D device that is described by a transmission line circuit model, as illustrated in Figure 2a. To a first-order approximation, the local EDLC and pseudo capacitance are lumped into a single capacitor at the electrode-electrolyte interfaces as dCi and dCo for the inner and outer electrode, respectively. The local electrical and ionic resistances are represented as dRi, dRo, dRe for the inner electrode, outer electrode, and electrolyte (including


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electrolyte in porous electrodes), respectively. A set of differential equations can be derived to solve the circuit model for voltage and current under various charge-discharge conditions (see Supporting information part 4). As a result, parameter dependency of the energy and power densities of µCFSCs can be analyzed.

Figure 1. (a) Schematic of the µCFSC composed of a carbon fiber as the core, a layer of MnO2/PPy as the inner electrode coating, a layer of PVA/H3PO4 as the solid-state electrolyte, and a layer of CNTs as the outer electrode coating. SEM images of the surface of (b) the MnO2/PPy layer, and (c) the CNTs layer; (d) SEM image of a µCFSC put on a wearable cotton fabric. (e) SEM image of a µCFSC with a knot in the middle. (f) Photo of a US quarter coin (5.67g) lifted by a µCFSC.



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The maximum possible energy density of the µCFSC is estimated based on the series capacitance of the total inner electrode capacitance (Ci) and the total outer electrode capacitance (Co), which are determined by the volumetric capacitance and the volume fraction of each electrode. For a carbon fiber core with a fixed base radius, thicker coating layers provide larger capacitance per overall volume due to higher ratios of active energy storage materials. However, thicker coating materials result in lower mechanical strength per cross-section area since the coating materials are weaker than the carbon fiber core. In Figure 2b, we estimate the overall volumetric capacitance and mechanical strength as functions of the total thickness of the coating layers (Figure S3), while the radius of the carbon fiber core is fixed to be 3.35 µm. For the size of choice in this work, the total thickness of three coating layers is ~3.15 µm, which corresponds to 73 % of the total volume for high capacitance. Meanwhile, a high overall mechanical strength of 1.3 GPa is guaranteed, which is 27% of that of a bare carbon fiber. The maximum power density and rate dependency of the device, on the other hand, are greatly affected by the electrical resistance of inner electrode (Ri) and outer electrode (Ro), and ionic resistance of electrolyte (Re), which are determined by the geometry of each layer and the electrical or ionic conductivity of each material. The electrical conductivity of carbon or metal-based electrodes is typically in the ranges of 104-108 S/m, while the ionic conductivity of solid electrolyte is generally lower than 10-1 S/m (about 10-2 S/m for PVA/H3PO4 used here)

46,47

. Therefore, Re is the limiting resistance

even in CFSC where the geometry is in favor of ion transports. Intuitively, thinner electrode and electrolyte coatings result in higher power performance because of shorter ion transport paths between two electrodes. We use the circuit model to investigate the energy density and power density of CFSCs with different total thicknesses of coating layers (TCL, Figure S3) and a fixed carbon fiber core radius of 3.35 µm, and plot the results (known as Ragone plots) in Figure 2c. It can be seen that thinner total coating layers only slightly improve the power density, but drastically lower the energy density. In addition, if the thickness of the electrolyte layer (Te) is approaching zero, the energy density does go up due to the higher volume fraction of the electrode materials, while the improvements in power density is negligible. These calculations suggest that our size choice represents a reasonable volume ratio between


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the carbon fiber core and coating layers to balance the mechanical strength, energy density, and power density.

By keeping volume ratio of the CFSCs, their performances with scaled overall radius of the

device (Figure S3) are plotted in Figure 2d. It is found that the maximum energy density remains about the same and the power density increases with smaller fiber sizes and saturates around the size of 13 µm - our device. Intuitively, the power performance is limited by the ionic resistance, Re, for CFSCs with large diameters.

For devices with very small diameter, the contribution of the electrical resistances, Ri

and Ro, could become comparable to that of Re such that further reduction of Re won’t make large improvements of the power density.

Figure 2. (a) A transmission line circuit model of 1D CFSC, in which the differential capacitors at the electrode-electrolyte interface represent local capacitance effects, and differential resistors are local electrical and ionic resistances in the radial and axial directions. (b) Calculations of the volume



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capacitance and mechanical strength of the CFSC as functions of the total thickness of coating layers (including inner electrode layer, electrolyte, and outer electrode layer), when the radius of the fiber core is fixed at 3.35 µm. (c) Energy density versus power density (Ragone plots) calculated by the model of CFSCs with different thicknesses of coating layers. (d) Ragone plots showing the simulation results of CFSCs with scaled sizes, in which the volume ratio of each layer with respect to the fiber core is fixed. 2.3 Fabrication of µCFSC A bare carbon fiber under SEM is shown in Figure 3a. The fiber surface is smooth at first, and becomes rough after successive electrodeposition of MnO2 and PPy (Figure 3b and 3c, Figure S4). The composition of the coating layers is further confirmed by X-ray photoelectron spectroscopy (XPS, Figure S5) and Energy Dispersive X-ray Spectroscopy (EDS, Figure S6). The thickness of the inner electrode coating can be controlled by the current and time during the deposition process, and here we use a 650±150 nm-thick layer to achieve a capacitance of 58 µF/cm, which corresponds to a volumetric capacitance of 386 F/cm3 for the inner electrode coating only, or 115 F/cm3 including the carbon fiber core. A 1.0±0.5 µm-thick layer of PVA/H3PO4 is dip-coated as the electrolyte layer, which looks transparent under optical microscope (Figure S4) and smooth under SEM (Figure 3d). The thickness can be controlled by adjusting the viscosity of the electrolyte solution and the pulling speed of the dip-coating process (Supporting information part 5). A 1.5±0.5 µm-thick layer of CNTs with a volumetric capacitance of about 95 F/cm3 (See details in Supporting information part 3) is then coated by dipping the fiber into a CNTs dispersion for 5 times (Figure 3e, Figure S4). For better performance, the final µCFSC device is dipped into 85 wt% H3PO4 to soak the nanoporous CNTs layer with sufficient electrolyte. Figure 3f shows the SEM image of the cross-section of the µCFSC, in which the coaxial structure of the carbon fiber core, the inner electrode coating, the electrolyte, and the outer electrode coating are clearly seen and distinguishable.


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aa

b

c

d

e

f

1 2 3 4

Figure 3. SEM images of: (a) a bare carbon fiber, (b) a carbon fiber coated with MnO2, (c) a carbon fiber coated with MnO2/PPy as the inner electrode, (d) after the electrolyte coating, (e) after the CNTs coating layer as the outer electrode, and (f) the cross-section of the µCFSC showing clear boundaries between the four layers. 1- carbon fiber core; 2- MnO2/PPy; 3- electrolyte; and 4- CNTs layer. Scale bars are 2µm in all figures. 2.4 Electrochemical characterizations To make electrical connections to the two electrodes of a single µCFSC, one end of the carbon fiber is fixed by a metal clip during the whole fabrication and testing process (Figure S7). The other end with outer electrode layer is connected by the soldering process to a conductive wire. Cyclic voltammetry (CV) tests are conducted to characterize the electrochemical performance of the device. Figure 4a shows the CV curves of a single µCFSC at scan rates from 20 mV/s to 1000 mV/s, which indicate excellent capacitive behavior of the prototype since the shapes of the curves are nearly rectangular and the currents are almost proportional to the scan rates (Figure S8). The capacitance of the device is calculated as the integration of the charge divided by the voltage range (See equations in Supporting information part 6). The volumetric capacitance as a function of scan rate is plotted in Figure 4b, together with the



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results simulated by using the 1D transmission line circuit model. The µCFSC shows a capacitance of 25.8 µF/cm, or 19.4 F/cm3, or 2.7 mWh/cm3 at the scan rate of 20 mV/s, which is a very high value since the total volume of the two electrodes, the electrolyte, and even the mechanical support (carbon fiber core) are all included. The predictions from the circuit model are not far from the experimental results, and the deviation is probably due to the uncertainties in the dimension measurements and the simplified electrochemical impedances 48 used in the circuit model. The energy and power densities of our µCFSC are plotted in Figure 4c, including experimental results from other reported CFSCs, and simulation results using our circuit model and reported parameters (See details in Supporting information part 7). From both experimental and simulation results, our µCFSCs show higher energy and power densities than other reported CFSCs

26–35,38,49

. The

high values are mainly due to the high capacitance of the active materials and the compact design with short paths for ion transports between electrodes. Specifically, our design has positive and negative electrodes occupying over 52 % of the total volume using thin layers of MnO2/PPy and CNTs with measured high capacitances of 386 F/cm3 and 95 F/cm3, respectively. Other CFSCs have larger sizes but lower performances, such as the ones using CuO/AuPd/MnO2 nanowires (410 µm in diameter, 37 F/cm3 and 31 vol% for the electrodes)

26

, multiwalled carbon nanotubes/carbon fibers (800 µm in diameter,

14.1 F/cm3 and 27 vol% for the electrodes) 29, or carbon nanotubes (43 µm in diameter, ~32 F/cm3 and 25 vol% for the electrodes) 27, For other fiber-based supercapacitors using twisted electrodes and parallel electrodes, researchers often ignore the sizes of separator, electrolyte, and mechanical support in the performance calculations to achieve capacitances between 2.5 to 300 F/cm3, and energy density up to 6.3 mWh/cm3 in the literature

6,17

. As a result, the comparison of those work with coaxial fiber

supercapacitors, which have separator, electrolyte, and mechanical support all included in a single 1D device, is difficult.


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Figure 4. (a) Cyclic voltammetry (CV) curves of a µCFSC prototype under different scan rates from 20 to 1000 mV/s. (b) Volume capacitances calculated from experimental results and the circuit model versus scan rate. (c) Ragone plots of results from our experiments, simulations from our circuit model, and experimental results from prior works. (d) Nyquist plot of the impedance of a single µCFSC with the frequency range from 0.1 to 100k Hz. (e) Capacitance retention results of the µCFSC during the 20,000 charge/discharge cycling tests. (f) CV curves of a straight µCFSC and a µCFSC that is bent with a radius of about 100 µm in the middle. Other electrochemical measurements, such as impedance spectroscopy and cycling tests of a single µCFSC are recorded. Figure 4d shows the Nyquist plots before and after 20,000 charge/discharge cycles with the frequency range from 0.1 to 100k Hz. The impedance doesn’t change much even after long-term cycling. For the initial impedance, the crossover frequency at a phase angle of -45° is 160 Hz, which corresponds to a relaxation time of only 6.3 ms. The same result can also be obtained through



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evolution of the imaginary part of capacitance as a function of frequency (Figure S9a). An equivalent circuit model is used to fit the Nyquist polt (Figure S9b). The impedance at high frequency and the fitted result both show a series resistance of ~9 kΩ for a single µCFSC device. In fact, for one dimensional fiber-shaped supercapacitors, the impedance should be inversely proportional to the cross-section area. Considering that the diameter of our device is only ~13µm (cross-section area = 1.3×10-6 cm2), the normalized series resistance should be ~0.01 Ω · cm2. For the potential applications, many µCFSCs will be connected in parallel to reduce the overall impedance. Figure 4e shows the capacitance retention of the µCFSC during 20,000 charge/discharge cycles. The device shows an increase of capacitance in the first 10,000 cycles, which is probably due to the gradual infiltration of electrolyte into the nanoporous structures of both electrodes, as similar tendency is observed in other works

8,20,50,51

. Over 85% of the

initial capacitance is retained after 20,000 cycles, demonstrating an excellent cycle stability of the device. In addition to the high electrochemical performance, the ultra-thin µCFSC shows intrinsically high mechanical flexibility. All 1D fibers may be bent in any direction with any angle, while the bending radius is limited by the size and flexibility of the fiber. Our µCFSC can be bent at a radius of less than 50 µm without breaking, as shown in the optical photo in Figure S5. We can also tie the µCFSC into a knot with a radius of ~100 µm, as shown in the SEM image in Figure 1e. We tested the performance of a µCFSC with a bending radius of ~100 µm in the middle and compared the results with that of a straight µCFSC in Figure 4f. The two curves are very close to each other, and the bent device works as good as the straight one. The slight deviation of the performance may be caused by the errors during the coating of CNT layer, which shows a performance error of ±20% for different samples. Such a small bending radius means that the device is very difficult to break through bending, and this value can hardly be achieved by fibers with large diameters in the range of a few hundred microns. The small size and high flexibility make our µCFSC promising in realizing energy storage systems to be integrated with clothes, as clothes are usually woven by fibers with dimeters of tens of microns for comfortableness and softness.


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For practical applications, threads composed of many µCFSCs in parallel have to be used to increase the overall current. Here we twist 7 µCFSCs (Figure 5a) and test the CV curve (Figure 5b) and Galvanostatic charge–discharge curve (Figure 5c) of the thread. The thread shows a capacitance of more than 5 times of a single µCFSC due to variations in each µCFSC. We also test three µCFSCs connected in series to achieve a higher voltage compared with a single one. Figure 5d shows the schematic of how three µCFSCs are connected, and Figure 5e shows the photo of the real setup, in which the metal connections are much bigger than the µCFSCs. Figures 5f and 5g compare the CV and galvanostatic curves of a single µCFSC and three in series at the same test conditions, respectively. The operation voltage is tripled and the capacitance becomes one third when three fibers are in series. a

b

c

f

g

300μm d

e

A single μCFSC

Metal contact

1cm

Figure 5. (a) SEM image of a thread made of seven twisted µCFSCs. (b) CV curves of a single µCFSC and seven µCFSCs in parallel at the scan rate of 1 V/s. (c) Galvanostatic charge–discharge curves of a single µCFSC and seven µCFSCs in parallel at the current density of 1 µA/cm. (d) Schematic of three µCFSCs connected in series. (e) Photo of three µCFSCs connected in series. (f) CV curves of a single µCFSC and three serial ones at the scan rate of 1 V/s. (g) Galvanostatic charge–discharge curves of a single µCFSC and three serial ones at the current density of 1 µA/cm.



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3 Conclusions In conclusion, we propose and demonstrate an ultra-thin micro coaxial fiber supercapacitor with the smallest reported overall diameter (~13 µm) achieving high energy density (up to 2.7 mWh/cm3) and power density (up to 13 W/cm3). We also develop a simplified circuit model which agrees with our experiments. It reveals that scaling down the diameter of CFSCs by using thinner layers of active materials while fixing the volume ratios of each layer will improve the performances, especially the energy density, power density and flexibility. With the use of scalable fabrication process, we believe such µCFSCs are promising building blocks for future flexible and wearable power systems. Supporting Information Fabrication and characterization details and images. Mathmatical models of CFSCs.

Acknowledgements This work was supported in part by the BSAC, Berkeley Sensor and Actuator Center, the National Science Foundation Industry/University Cooperative Research Center, and the US Army Aviation and Missile Research, Development and Engineering Center.

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Ultra-thin coaxial fiber supercapacitors achieving high energy and

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