Application of Carbon Nanomaterials to an Active Electrode Material in Electric Double Layer Capacitors

Abstract

Electric double layer capacitors (EDLCs), also called ultracapacitor or supercapacitor, perform charge and discharge by the electric double layer which arises in the interface of an electrode and electrolyte. Therefore, EDLCs have advantages that provide higher specific power and perform longer cycle life compared with secondary batteries. This brings a number of industrial applications including back-up power supply in electronic circuits, uninterruptible power source, power supply for rapid heating of the welding drum in laser printer and copy instrument. In this study, we used carbon nanoballoon (CNB) [1, 2], carbon nanocoil (CNC), and onion-like carbon (OLC) instead of commercially-used activated carbon (AC) as an electrode material of EDLCs. The specific surface area of CNB is 16 times lower than that of AC, but higher than those of CNC and OLC. Electrochemical impedance spectroscopy of the EDLCs revealed that the CNB and CNC electrodes had a much lower internal resistance than the AC electrode, which indicates a low capacitance maintenance factor at the high current density. When the potential window was 1 V both for the aqueous and the organic electrolytes, the specific capacitance of the EDLC evaluated in the aqueous electrolyte was larger than that in the organic electrolyte. CNCs were prepared using automatic chemical vapor deposition (CVD) system with a consecutive substrate transfer apparatus. A mixed solution of Fe2O3 and SnO2 particles, which are a catalyst for CNCs, was dropped on a graphite substrate, and the substrate was calcined at 350°C for 10 min. The substrates were then placed in the center of a CVD furnace and annealed at 780°C in a N2 atmosphere with a N2 gas flow rate of 1400 sccm for 5 min. CNCs were then synthesized by adding C2H2 gas with a flow rate of 350 sccm. AcB was prepared using a twin-torch arc discharge apparatus, in which arc discharge occurred between graphite electrodes in N2 atmosphere. AcB is mainly composed of cocoon-shaped carbon nanoparticles with a lot of amorphous ingredients. CNB was obtained by heating AcB in a Tammann oven in Ar atmosphere at 2600ºC for 2 h. CNB consists of a hollow particle and has a high electrical conductivity. The prepared AC, CNC, AcB, CNB, and OLC were observed using scanning electron microscopy and transmission electron microscopy. Laser Raman spectroscopy (excitation wavelength: 532 nm) was used to evaluate the crystallinity of the carbon nanomaterials. We used a two-electrode cell with two coin-type electrodes for the electrochemical measurement of EDLCs. Coin-type electrodes were prepared by the following procedure. First, 10 wt. % of polytetrafluoroethylene (PTFE) dispersion liquid was dropped onto 90 wt. % of CNC, CNB, or OLC. In the case of AC, 10 wt. % of PTFE was dropped onto 80 wt.% of AC and 10 wt.% of KB, which is used as an conductive agent. Then each of them was mixed for 15 min by an automatic mortar. The mixed material was put into the jig (inner diameter: 15 mm) and was pressed by 14 MPa for 10 min at room temperature. An electrochemical measurement system (Hokuto Denko Corp., Tokyo, Japan, HZ-5000) was used for the electrochemical measurement. Cyclic voltammetry (CV), galvanostatic charge/discharge test, and electrochemical impedance spectroscopy (EIS) were used. As a result, the specific surface areas of the carbon nanomaterials were in the order: AC, CNB, CNC, and OLC. The EDLC using AC showed the highest internal resistance. Although the EDLC using AC showed the highest specific capacitance at a low current density, it rapidly decreased as the current density increased, indicating a low capacitance maintenance factor. When the current density was larger than 1.0 A g−1, the specific capacitance of the EDLCs using CNB and CNC became equivalent to that using AC. By the evaluation of Ragone plots, the EDLCs using CNC and CNC had a slightly larger energy density than that using AC at a high power density.