Supporting Information. Ultra-thin coaxial fiber supercapacitors achieving high energy and power densities

Transcription

1 Supporting Information Ultra-thin coaxial fiber supercapacitors achieving high energy and power densities Caiwei Shen a,1, Yingxi Xie a,b,1, *, Mohan Sanghadasa c, Yong Tang b, Longsheng Lu a,b and Liwei Lin a 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: yingxixie@berkeley.edu 1 Both authors contributed equally to the paper

2 1 Fabrication and characterization details Synthesis and Fabrication of the inner electrode layer: One single CF (TORAYCA T700S) with 6.7±0.2 μm diameter and 4-6 cm long was used as the inner core material. The CF was immersed in acetone overnight and activated by 65 wt% nitric acid in 200 C for 2 hours before further processes. The MnO2 nanoflakes and PPy nanolayer were deposited on the CF via the electrochemical deposition process. A solution of 20 mm Mn(NO 3 ) 2 and 100 mm NaNO 3 is used for the MnO2 nanoflakes coating, 0.1 M LiClO4 and 5wt% pyrrole monomer is used for the PPy nanolayer coating respectively. 1 The twoelectrode configuration was used in the deposition process with a platinum wire as the counter electrode, and the CF as the working electrode. A constant current of 0.5 μa/cm is used for 5 minutes to electrochemically deposit MnO 2 nanoflakes, and a 2 μa/cm current is used for 2 minutes to electrochemically deposit the PPy layer. Then the CF was washed by deionized water (DI water) and dried between each process. Fabrication of the electrolyte layer: the gel electrolyte was prepared by adding 1 g of Phosphoric acid (H 3 PO 4 ), and 1 g of polyvinyl alcohol (PVA, 180,000mw) powder into 11 g of deionized water. The mixture was heated to 100 C with stirring until the solution became clear. To coat the electrolyte onto the MnO2/PPy-coated CF, the CF was dipped into the gel electrolyte and pulled up vertically at the speed of 0.2 mm/s. After the electrolyte was solidified, the dip coating process was repeated 3 times. Fabrication of the outer electrode layer: multi-walled carbon nanotubes were dispersion in Dimethylformamide (7-8wt%, carboxylic acid functionalized) and used as outer electrode. To coat the outer electrode onto the electrolyte-coated CF, the CF was dipped into the carbon nanotubes dispersion and pulled up 3 to 4 times before drying in ambient temperature. Characterization: The optical images were obtained on a Mitutoyo MF-U microscope. The SEM images were obtained using a LEO 1540 Scanning Electron Microscope. The cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge discharge measurements were performed using a Gamry Ref 600 electrochemical station. To make the electric connection, two pieces of low-melt fusible bismuth based alloy (#117 Alloy, Rotometals, Inc.) thin films were placed on a hot plate and melted. Then the μcfsc electrodes were soaked into the liquid metal. After the metal was cooled down and solidified, the two pieces of metal thin films were connected to the electrochemical station.

3 2 Supporting figures a Electrolyte b A possible ion c Active layer Fiber core transport path Ion transport path _ Substrate Figure S1. Cross-section schematics of three configurations of fiber-based supercapacitors. (a) Parallel; (b) Twisted; (c) Coaxial.For the same thicknesses of the electrodes and the electrolyte layer, the coaxial structure requires minimum amount of electrolyte and provides the shortest ions transport path as compared with electrodes that are in parallel or twisted.

4 Figure S2. A possible roll-to-roll process design for continuous production of μcfscs.

5 R i T i T e T o Scale down R o Scale up T = T i + T e + T o Figure S3. Schematics showing the layer thickness definition and the scaling of CFSCs. The volumes of different layers, and the volume ratios are explained as follows: For a fixed length L, the volume of: carbon fiber core inner electrode V core = πlr i 2 (1) V i = πl[(r i + T i ) 2 R i 2 ] (2) electrolyte outer electrode V e = πl[(r i + T i + T e ) 2 (R i + T i ) 2 ] (3) V o = πl[(r i + T i + T e + T o ) 2 (R i + T i + T e ) 2 ] (4) When we fix R i, and vary T CL (Figure 2d and 2e), we fix the volume ratios of two electrodes and electrodes to electrolyte (except when T e 0) as: V i : V o = constant, and (V i + V o ): V e = constant When we scale R i and T CL together (Figure 2f), we fix the volume ratios of all layers as: V i : V o = constant, (V i + V o ): V e = constant, and (V i + V o + V e ): V core = constant

6 Figure S4. Optical photos of (a) the carbon fiber core; (b) after the electrodeposition of polypyrrole; (c) after the dip-coating of electrolyte; (d) after the dip-coating of outer electrode. All scale bars are 20μm. Figure S5. X-ray photoelectron spectroscopy (XPS) of the inner electrode materials composed of bare carbon fiber (CF), carbon fiber deposited with MnO 2 (CF-MnO 2 ), and carbon fiber deposited with MnO 2 and PPy (CF-MnO 2 -PPy). The CF sample only shows peaks of carbon (C) and oxygen (O), while the CF- MnO 2 sample shows Mn 2p 1/2 and Mn 2p 3/2 peaks, and the CF-MnO 2 -PPy sample has nitrogen (N) on the surface.

7 Figure S6. Energy Dispersive X-ray Spectroscopy (EDS) of the inner electrodes composed of bare carbon fiber (CF), carbon fiber deposited with MnO 2 (CF-MnO 2 ), and carbon fiber deposited with MnO 2 and PPy (CF-MnO 2 -PPy). The CF sample is mainly composed of Carbon, while the CF-MnO 2 sample is composed of Carbon, Oxygen, and Manganese, and the CF-MnO 2 -PPy sample has additional Nitrogen and Chlorine, because of ClO - group doped in PPy. Coaxial fiber 50μm Figure S7. (Left) Optical photo the μcfsc as compared with a human hair. (Right) Optical microscope photo of a twisted μcfsc.

8 Average Current ( A/cm) Scan rate (V/s) Figure S8. The current as the function of scan rate for a μcfsc prototype. Figure S9. (a) Evolution of the imaginary part of capacitance vs. frequency for the µcfsc, in which the peak at ~160 Hz corresponds to a relaxation time of 6.3 ms. (b) Fitting result of the impedance spectrum of the µcfsc using an equivalent circuit model. The circuit is composed of a series resistance (R s =9.3 kω), a capacitance (C=11.3 μf), a charge transfer resistance (R ct =0.13 kω), a Warburg impedance (Z w =0.32 μs s 1/2 ), and a constant phase element (CPE=0.63 μs s ). Relatively small charge transfer resistance and diffusion impedance are due to the shortened distance between two electrodes of the device.

9 3 Inner electrode and outer electrode tests and performance calculation 3.1 Inner electrode tests We test the electrochemical performance of the inner electrode (including carbon fiber core and active layers) by using a three-electrode system with the fiber as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl reference electrode. The capacitances per length and per volume of different fiber electrodes are summarized in Table S1, and typical CV curves are plotted in Figure S6. Table S1. Capacitance summary of different fiber electrodes Fiber electrode Capacitance per length (μf/cm) a Average diameter (μm) Capacitance of Active Layers (F/cm 3 ) b Capacitance of Total Electrode (F/cm 3 ) c Bare carbon fiber (CF) ±0.2 N/A CF treated in HNO ±0.2 N/A 0.85 Treated CF + E-dep MnO2 (0.5 μa/cm, 5 min) Treated CF + E-dep MnO2 (0.5 μa/cm, 5 min) + E-dep Ppy (2 μa/cm, 0.5 min) Treated CF + E-dep MnO2 (0.5 μa/cm, 5 min) + E-dep Ppy (2 μa/cm, 2 min) Treated CF + E-dep MnO2 (0.5 μa/cm, 5 min) + E-dep Ppy (2 μa/cm, 5 min) 2 7.8± ± ± ± a Calculated from CV curve at 100mV/s scan rate; 20% error from length measurement b Capacitance divided by the volume of active layers c Capacitance divided by the total volume of carbon fiber and active layers

10 Current (ma/cm 2 ) Current ( A/cm) Bare CF x 100 Treated CF x 100 CF/MnO2/PPy Potential VS. Ag/AgCl (V) Figure S10. Typical CV curves of electrodes of bare CF, HNO 3 treated CF, and CF coated with MnO 2 (0.5 μa/cm, 5 min) and PPy (2 μa/cm, 2 min) tested in a three-electrode system. The currents of bare CF and treated CF are enlarged by 100 times to compare with the CF/MnO2/PPy electrode, and the scan rate is 100 mv/s for all curves. 3.2 Outer electrode test and calculation The volumetric capacitance of the outer electrode (dip-coated CNT) is estimated by using a two-electrode symmetric supercapacitor cell. A thin PVA/H 3 PO 4 solid electrolyte film of ~10 μm is first prepared by drying of the electrolyte solution in a Petri dish. Then the film is dipped into the CNT dispersion, so that both sides of the film are coated with CNT (7±4 μm-thick in a typical coating). The CNT-coated electrolyte film is dried, dipped into 85 wt% H 3 PO 4 for a few seconds, and cut into squares of ~1 cm 2. Silver paint and metal wires are used to connect the CNT electrodes on two sides of the film. The capacitance of such a symmetric cell is tested, and the volumetric capacitance of such a CNT layer is calculated to be ~95 F/cm Voltage (V) Figure S11. CV curve of a two-electrode symmetric supercapacitor cell using CNT electrodes and PVA/H 3 PO 4 solid electrolyte film at a scan rate of 100 mv/s.

11 4 1D transmission line model for CFSCs Figure S12. Equivalent circuit of 1D transmission line model for CFSCs Parameters and definitions: dr i = dx (5) 2 σ i πr i dx (6) dr o = σ o π(r 2 o R 2 i ) dr e = R o R i σ e π(r i + R o ) dx dc = C dx (8) σ i, σ o, σ e are conductivity constants (S m) C is capacitance per length (F/m) (7) C is capacitance per volume (F/m 3 ) R i, R o are radii (m) Equations: A 1 2 V i x 2 + A 2 2 V o x 2 = 0 (9)

12 (V i V o ) = A t 3 2 V i x 2 + A 4 V i t ( 2 x 2 ) (10) A 1 = σ i πr i 2 (11) A 2 = σ o π(r o 2 R i 2 ) (12) A 3 = σ 2 i πr i C 2 πr = σ i (13) i C Boundary conditions: A 4 = (R o R i ) σ i πr i 2 σ e π(r i + R o ) = σ i (R o R i ) R i 2 σ e (R i + R o ) (14) V i (X, t) t V o (0, t) = 0 (15) V i (0, t) = 0 x (16) V o (X, t) = 0 x (17) = δ (scan rate) (18)

13 5 Dip-coating theory Figure S13. Schematics of dip-coating process In the dip-coating process, the electrolyte near the fiber must move at the same velocity as the fiber due to the liquid viscosity. The viscosity of the liquid, the gravity force and the capillary force all affect the dipcoating process. Researchers have proposed the relationship between the coating layer thickness and other parameters as 2,3 : h = 1.34rCa Ca 2 3 (19) Where h is the coating layer thickness, r is the radii of the fiber, and Ca is the capillary number, which is computed by Ca = ηv γ (20) where η and γ are the electrolyte viscosity and surface tension, respectively, and V is the pulling velocity.

14 6 Equations to calculate capacitance, energy density, power density For Cyclic Voltametry Scan rate Capacitance Energy C = Idt ΔV r = dv dt = IdV ΔV r (21) (22) Power For Galvanostatic charge/discharge E = VIdt = 1 VIdV (23) r P = E Δt = E r ΔV (24) Capacitance Energy C = IΔt ΔV (25) Power E = I Vdt (26) P = E t (27) Volumetric C(or E, P) = C(or E, P) Total volume of the device (28)

15 7 Details of data used in Figure 4c The simulation data are obtained by using our equivalent circuit model and key parameters (e.g. core radius, coating layer thicknesses, and volumetric capacitances of electrode materials) reported in our work, ref. 26, and ref. 29. Other plotted experimental data are all volumetric values. They are either directly reported in the papers (such as ref. 26, 31, 34*), or calculated from other metrics as long as the sizes of the device are provided. For example, a few papers report energy/capacitance per length (ref , 32, 33) or per area (ref. 35) and we divided the values by πr 2 or r/2 (r is the radius of the whole device) to convert it to volumetric values. * A correction is made to the data from ref. 34 ( which reports a power density of W/cm 3 (corresponding to a charging time of s), but should be mw/cm 3 (corresponding to a charging time of s) according to their test results shown in Figure 3 and equation (5). Reference (1) Tao, J.; Liu, N.; Ma, W.; Ding, L.; Li, L.; Su, J.; Gao, Y. Solid-State High Performance Flexible Supercapacitors Based on Polypyrrole-MnO2-Carbon Fiber Hybrid Structure. Sci. Rep. 2013, 3, 1 7. (2) Quéré, D.; Qu, D. Fluid Coating on a Fiber. Annu. Rev. Fluid Mech. 1999, 31 (1), (3) White, D. A.; Tallmadge, J. A. A Gravity Corrected Theory for Cylinder Withdrawal. AIChE J. 1967, 13 (4),