Carbon nanotubes as electrode for supercapacitors

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.