Carbon nanotubes as electrode for supercapacitors

Research paper in the pro­ceed­ings of 2nd Inter­na­tion­al Con­fer­ence on Mate­ri­als for Ener­gy (EnMat II), Karl­sruhe, Ger­many, 12–16 May, 2013.

One-dimen­sion­al car­bon nanos­truc­tures have been known and fab­ri­cat­ed for more than a hun­dred years and were orig­i­nal­ly rWe describe a fast and cost-effec­tive process for the growth of car­bon nanofibers (CNFs) at a tem­per­a­ture com­pat­i­ble with Both sil­i­con wafers and ther­mal­ly oxi­dized sil­i­con wafers are diced into 14×14 mm2 pieces to fit the cir­cu­lar active area with 11 mm diam­e­ter used in voltam­me­try. 50 nm of tung­sten is sput­tered on both sides of the chips for edge cov­er­age to have bet­ter elec­tri­cal con­tact of back side and grown side. A cat­a­lyst lay­er con­sist­ing of alu­minum (5 nm) and iron (2 nm) is deposit­ed using elec­tron beam evap­o­ra­tion. The CNTs are grown by chem­i­cal vapor depo­si­tion at 700 °C using acety­lene and hydro­gen gasses as car­bon source and car­ri­er. First, the cat­a­lyst is pre­treat­ed at 500 °C in the envi­ron­ment of con­tin­u­ous hydro­gen flow at around 8 mbar pres­sure. Then acety­lene is intro­duced and the tem­per­a­ture is raised to 700 °C with­in a few sec­onds. Sam­ple (1) con­sists of: Si, W, Al, Fe; sam­ple (2) con­sists of: Si, SiO2, W, Al, Fe. Mea­sure­ments were car­ried out by a three elec­trode sys­tem with Ag/​AgCl as ref­er­ence elec­trode, Pt as counter elec­trode and 1M KOH as elec­trolyte. The capac­i­tance was cal­cu­lat­ed from the voltam­mo­gram (Fig­ure 1). The voltam­me­try was car­ried out with 5 cycles per sam­ple. Sam­ple (1) yields a capac­i­tance of 0.0475 F and (2) a apac­i­tance of 0.04 F for the active geo­met­ri­cal sur­face at sweep rate 20 mV/​s (Table 1). Cal­cu­lat­ed capac­i­tances are from the voltam­mo­gram val­ues, where the capac­i­tance is the absolute val­ue between ‑0.1 — 0.1 V divid­ed by 2 and divid­ed by the sweep rate. C = Δ|I| /​ s, where Δ|I| is the dif­fer­ence in cur­rent, s is the sweep rate (dE/​dt) and C is the capac­i­tance. An esti­ma­tion of CNT weight using SEM pic­tures yields approx­i­mate­ly 0,3 mg. The mea­sured weight from a scale is in the range 0.8–1.4 mg which gives a spe­cif­ic capac­i­tance of 13P1: 46.9 ± 12.7 F/​g and 14P1: 39.3 ± 10.7 F/​g for the two sam­ples respec­tive­ly. Future improve­ments of these CNT elec­trodes will be to pro­duce longer nan­otubes and a more dense struc­ture. Both these para­me­ters will increase the sur­face area and by that yield a high­er capac­i­tance for the elec­trode. By con-trolling the ver­ti­cal align­ment of the CNTs in com­bi­na­tion with pro­duc­tion meth­ods con­tain­ing cheap mate­ri­als and by using indus­tri­al fab­ri­ca­tion tech­niques the ener­gy den­si­ty can be improved. This makes ver­ti­cal­ly aligned CNT a very promis­ing mate­r­i­al as elec­trode mate­r­i­al for supercapacitors.

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