Fig. 1. Schematics of supercapacitors

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1 Electrically tuned supercapacitors Tazima S. Chowdhury and Haim Grebel Department of Electrical and Computer Engineering and Electronic Imaging center New Jersey Institute of Technology, Newark, NJ 712, USA. Fast charging and discharging of large amounts of electrical energy make supercapacitors ideal for shortterm energy storage. 15 In its simplest form, the supercapacitor is an electrolytic capacitor made of an anode and a cathode immersed in an electrolyte. As for an ordinary capacitor, minimizing the charge separation distance and increasing the electrode area increase capacitance. In supercapacitors, charge separation is of nanometer scale at each electrode s interface (the Helmholtz double layer); making the electrodes porous increases their effective surface area. 6 8 A separating layer between the anode and the cathode electrodes is used to minimize unintentional electrical discharge (Fig. 1). Here we show how to increase the capacitance of supercapacitors by more than 45% when modifying the otherwise passive separator layer into an active diodelike structure. Active control of supercapacitors may increase their efficiency during charge and discharge cycles. Controlling ion flow in electrochemical cells is the first step toward a new type of iontransistors. Fig. 1. Schematics of supercapacitors ionic charges electronic charges porous electrode current collector separator Specifically, the capacitance of a typical parallel plate capacitor is given by: C= A/d. Here is the effective permittivity of the cell's electrolyte, A is the area of each electrode and d is the effective charge separation length. Most efforts to date were devoted to optimizing the anode, cathode and the electrolyte. We instead modified the passive separator layer 911 and made it electrically active An ordinary dielectric separator layer presents pure resistance to the ion flow while the electronic pn junction in Fig. 2a adds internal polarity to the cell. In electronic terms, this capacitorwithincapacitor affects the effective dielectric constant of the cell: if its polarity opposes the internal cell s polarity, then charges would be drawn to the plates of the outer capacitor and the overall cell s capacitance would increase. The electronic diodelike structure partially screens the ions flowing through it, as well. 14 Vext Current (ma) anode cathode 2 pn gate membrane Voltage (V) Fig. 2. Diodelike midcell gate structure: blue electronic charges; red ionic charges; green/blue layers ptype/ntype layers. Electronic, currentvoltage (IV) curve for a dry gate membrane. We used functionalized single wall carbon nanotubes (SWCNT) for the structured pn gate element because they tend not to oxidize easily The junction(s) were deposited on a polyamide film with.5micron nanopores. The electrolyte was 1 M NaCl (see experiments in the SI section). The cell's capacitance was determined from discharge experiments. 2 Two other methods (2 and 3electrode set ups) produced similar results and are described in the SI section. The bare separator membrane reduced the cell capacitance by ca 3% with respect to cells without it. This is because it provided pure resistance to the flowing ions in the cell. Placing the SWCNT pn junction(s) on the bare membrane restored the cell capacitance to its initial value. Biasing the gate membrane by a mere 3 mv further increased the cell capacitance by more than 15% (Fig. 3). Here, data points were taken with a delay of two minutes. The delay between the positive (red dots) and negative (blue dots) bias branches was 14 hours to let the cell restabilized by diffusion fluctuations at Vg= V. If one takes the data continually, then the CapacitanceVg curve saturates at the maximal capacitance value, suggesting a lasting polarizing effect. 2 1

2 Capacitance (mf) Fig. 3. Discharge experiments: we started with the positive bias branch. The cell capacitance starting value was 137 mf; its end value was 16 mf. After 14 hours of rest, and without altering the cell, measurements were resumed along the negative bias branch: the initial cell capacitance was 142 mf, ending with a cell capacitance value of 152 mf. 15. M. J. O Connell, P. Boul, L. M. Ericson, C. Huffman, Y. Wang, E. Haroz, C. Kuper, J. Tour, K. D. A. and R. E. Smalley, Chem. Phys. Lett., 342 (21) M. Shim, A. Javey, N. W. S. Kam and H. Dai, J. Am. Chem. Soc., 123, (21) 17. A. L. M. Reddy, M. M. Shaijumon, S. R. Gowda, and P. M. Ajayan, J. Phys. Chem., 114, (21) M. Kaempgen, C. K. Chan, J. Ma, Y. Cui, and G. Gruner, Nano Letts., 9, (29) C. Wan, L. Yuan, and H. Shen, Int. J. Electrochem. Sci., 9 (214) Meryl D. Stoller and Rodney S. Ruoff, Energy Environ. Sci., 21, 3, References 1. Patrice Simon and Yury Gogotsi, Nature Materials, 7 (28) John R. Miller and Patrice Simon, Science, 321 (28) ZhongShuai Wu, Guangmin Zhou, LiChang Yin, Wencai Ren, Feng Li and HuiMing Cheng, Nano Energy, 1 (212) R. Kötz, and M. Carlen, Electrochimica Acta, 45 (2) Alberto Varzi, Corina Täubert, Margret Wohlfahrt Mehrens, Martin Kreis, and Walter Schütz, Journal of Power Sources, 196 (211) E. Frackowiak, and F. Beguin, Carbon, 4 (1) (22) 7. M. Mastragostino and C. Arbizzani, Journal of Power Sources, 97 (21) I. H. Kim and K. B. Kim, Electrochemical and Solid State Letters, 4, (21) A62A S. Sreevatsa and H. Grebel, ECS Transactions, 19 (29) R.R. Nair, H.A. Wu, P.N. Jayaram, I.V. Grigorieva, A.K. Geim, Science 335 (212) Y.M. Shulga, S.A. Baskakov, V.A. Smirnov, N.Y. Shulga, K.G. Belay, G.L. Gutsev, Journal of Power Sources 245 (214) Amrita Banerjee and Haim Grebel, Electrochem. Commun. (21) doi:1.116/ j.elecom Joel Grebel, Amrita Banerjee and Haim Grebel, Electrochimica Acta, 95 (213) H. Grebel and A. Patel, Chem. Phys. Letts. 64 (215)

3 Electrically tuned supercapacitors Tazima S. Chowdhury and Haim Grebel Department of Electrical and Computer Engineering and Electronic Imaging center New Jersey Institute of Technology, Newark, NJ 712, USA. Supplementary Information Experiments and Methods Aqueous solutions were prepared using deionized water. Single wall carbon nanotubes were obtained from Cheaptubes with a purity better than 9%. The CNTs were refluxed in dilute nitric acid to remove metal catalyst particles, which are typically used in the growth of CNT. The nitric acid was later washed away and replaced by DI water. The nanotubes were functionalized with polymers: ptype and ntype tubes were obtained by wrapping the tubes with PVP and PEI, respectively. 15 The ptype and ntype nanotubes were suspended in DI water using a horn probe sonicator for 8 hours. Each layer was drop casted on a hydrophilic nanofiltration TS8 filter using vacuum. Copper leads were attached to areas far removed from the electrolyte using silver epoxy. The thickness of each film was estimated as a few microns. Junctions with an insulator were made by spinning a thin layer of PMMA on the ptype film before the deposition of the ntype film. Metallic capacitors were constructed by first sputtering of Au/Pd alloy on one side of a bare membrane and joint two such pieces together. The resistance of the metallic film was 3 K /cm. We did not attempt to increase the surface area of either the anode, or the cathode and so the working and counter electrodes were made of graphite rods with a diameter of 5 mm. Ag/AgCl was used as a reference electrode and 1 M of sodium chloride solution served as an electrolyte. Metrohm PGSTAT potentiostat/galvanostat was used to acquire the cyclic voltammetry curves with a voltage sweep between 1 mv to 1 mv at various scan rates. Electrochemical Impedance spectroscopy tests were performed using the FRA32 module of the same system. The frequency ranged from 1 mhz to 1 khz with a perturbation amplitude of.24 V. The system also enables discharge measurement. Results 3electrode system: Nyquist plot and Bode plots shown in Fig. S1a,b, respectively, exhibit the cell's characteristics at the high and low frequency regions. Above 1 Hz, the cell's impedance becomes more resistive.

4 Imaginary Z'',(Ω) Real Z',(Ω) Fig. S1. Nyquist plot and Bode plot of a cell with a pn gate electrode in 1 M NaCl (experiment and simulation). Phase Angle, (θ) Frequency, logf (Hz) Cyclic Voltammetry (CV) for several cell s construction are shown in Fig. S2a at Vg=. In Fig. S2b we demonstrate that the capacitance value were maintained after 8 cycles. Current, (ma) Potential,(V) Fig. S2. Cyclic voltammetry of electrochemical cell with three types of gate electrodes on hydrophilic (TS8) membrane. The scan rate was 1 mv/s. Overall capacitance as a function of number of cycle. The upper blue curve was obtained for a pn gate membrane; the middle pink curve belongs to a pin junction(s), namely, sucsession of ptype, insulator (a thin PMMA film) and ntype films; the lower green curve was obtained for an allmetallic capacitor. Capacitance (mf) Number of Cycles In Fig. S3. we show the tuning of the capacitance by the gate voltage bias.

5 Capacitance (mf) Fig. S3. Control over the cell's capacitance by varying the biasing on the pn gate electrode. The pside faced the working electrode and the scan was starting from zero for either the positive or the negative branches. Note the asymmetry in the curve; it is attributed to the diodelike gate structure. 2electrode system CV data for diodelike gate membrane is shown in Fig. S4a. The cell s capacitance is shown in Fig. S4b. Current (ma) Voltage (V) Fig. S4. 2electrode CV at scan rate of 1 mv/s at Vg=. Cell s capacitance vs gate voltage. There was a delay of 1 hr between the data collection for the, first negative and, then positive branches. Another look at the discharge data is provided in Fig. S5. We started with the negative bias branch. We drained the electrolyte (keeping it for a reuse) checked the electrodes and put back the same electrolyte. The action of drainage and filling the electrolyte discharged the gate membrane as can be seen from the starting capacitance values. Capacitance (mf) 47

6 Capatitance (mf) Fig. S5. Cell capacitance as a function of the gate voltage, Vg. The asymmetry between the negative and positive branches is attributed to the diodelike electronic bias. The Vg= and Vg= differ by 1 mv.