Supporting Information for Dynamic and Galvanic Stability of Stretchable Supercapacitors By Xin Li, Taoli Gu and Bingqing Wei* Department of Mechanical Engineering, University of Delaware, Newark, DE 19716 (USA) *Corresponding author: weib@udel.edu Telephone: 1-302-831.5-6438 FAX: 1-302-831.5-3619 Materials and Methods. The SWNT macrofilm electrodes were prepared by chemical vapor deposition, and attached on 1-mm-thick rectangular pre-strained PDMS (Dow Corning 184) to form buckled structure (Supporting information Fig. S1a) as the stretchable electrodes. Elastomeric separator was prepared by electrospinning polyurethane to form a nonwoven porous membrane (Supporting information Fig. S1b). The as-prepared elastomeric electrospun polyurethane separator is electronically insulating and mechanically robust under reversible stretching/releasing. The electrospinning process is cost-effective and the thickness of the separator can be easily controlled by adjusting the electrospinning time. In our study, we fabricated the separator with a thickness ~10 µm. The scanning electron microscopy image of the as-prepared polyurethane separator was obtained on a JEOL JSM-7400F microscope. The image of the buckled SWNT film was captured on an Olympus BX60 microscope and Nikon DS-Fi1 camera. 1
Detail of Electrochemical Measurement. The cyclic voltammograms (CVs), electrochemical impedance spectroscopy (EIS), galvanostatic charge-discharge and self-discharge measurements were performed to characterize electrochemical behavior of the fully stretchable supercapacitors under dynamic stretching/releasing as well as dynamic bending, using EG&G PARSTAT 2273 and Arbin BT4+ test systems. The dynamic testing was conducted on a programmable custom-made stretchable stage. The organic electrolyte was 1.0 M tetraethylammonium tetrafluoroborate (TEABF 4, battery grade, Alfa Aesar) in propylene carbonate (PC, Alfa Aesar). The stretchable supercapacitors were assembled in a glove box filled with pure argon. ZView software was used to obtain the Randle's equivalent circuit parameters. Copper foils were used as the current collectors. 2
Optical and SEM photos. Figure S1a shows the optical image of the buckled SWNT film, demonstrating a uniform periodic wavy pattern, which is attributed to the strong bonding between the SWNT macrofilms and the PDMS substrates. Figure S1b shows the SEM image of the electrospun PU elastomeric separator with nonwoven porous structure. The fiber diameters range from 0.8 µm to 1.2 µm. The interconnected pores create excellent channels for the mass transport of mobile ions. a b Figure S1. a) Optical image of the buckled SWNT film b) SEM image of the electrospun elastomeric separator. 3
The specific capacitance of the stretchable supercapacitor under fixed strains (0% strain and 31.5% strain) at various scan rates from 50 mv s -1 to 2000 mv s -1 is summarized in Figure S2. It shows that the specific capacitances of the cell at 31.5% strain outperform that at 0% strain by various ratios, which could be attributed to the different ion diffusion at different scan rates. The trend that the specific capacitance at 31.5% strain is higher than that of originally un-strained cell remains the same. Specific capacitance (F g -1 ) 60 50 40 30 20 10 0 50 mv s -1 0 500 1000 1500 2000 Scan rate (mv s -1 ) 0% strain 31.5% strain Figure S2. Summary of the specific capacitance versus scan rates (from 50 mv s -1 to 2000 mv s - 1 ) at 0 % strain and 31.5 % strain. 4
Randle's Equivalent circuit and AC impedance fitting results. R S R CT W O C DL Table S1 AC impedance Equivalent circuit fitting parameters in ZView R S (Ω) R CT (Ω) W or (Ω) C DL (µf) 0 % strain 11.6 11.38 19.34 9.55 31.5 % strain 11.9 3.38 7.74 10.22 DSR 1.11 % s -1 11.78 4.89 33.07 9.67 DSR 2.22 % s -1 11.85 3.97 30.65 9.81 DSR 4.46 % s -1 11.79 3.50 27.26 9.90 5
Wavy SDC caused by DSR mode between 0% and 31.5% strain Figure S3. Self-discharge curve (SDC) of the supercapacitor under in-situ DSR mode between 0% strain and 31.5% strain, with strain rate of 2.22 % s -1 (red curve) and 4.46% s -1 (black curve). Compared to the strain rate of 4.46% s -1, the slower self-discharge of supercapacitor under the strain rate of 2.22 % s -1, could be attributed to the higher Warburg ion diffusion resistance W or, as presented in Table S1. 6
1.5 1.0 Fully Bent Flat Dynamic bending (2.1% s -1 ) from Flat to Fully Bent Current (ma) 0.5 0.0-0.5-1.0-1.5-1.5-1.0-0.5 0.0 0.5 1.0 1.5 Potential (V) 7 mm Fully Bent 19 mm Flat Figure S4. The CV curves of the stretchable supercapacitor at the flat state (blue curve), fully bent state (black curve) and dynamic bending (from flat to fully bent state) at the strain rate of 2.1% s -1. The schematic shows that at the flat state, the end-to-end distance of the cell is 19 mm, whereas the end-to-end distance reduces to 7 mm at its fully bent state. The scan rate is set at 50 mv s -1. 7
Movie clip 1: Schematic animation of the dynamic stretching/releasing of the stretchable supercapacitor Movie clip 2:The stretchable supercapacitor is under in situ dynamic stretching/releasing under strain rate of 4.46 % s -1 Movie clip 3: The galvanostatic charge-discharge curve at current density of 10 A g -1, when the supercapacitor is under in situ dynamic stretching/releasing under strain rate of 4.46 % s -1. 8