No abstract

No abstract

No abstract

Cyclic voltammetry, constant current charge/discharge, electrochemical impedance spectroscopy and constant power discharging methods have been applied to establish the electrochemical characteristics of electric double–layer capacitor (EDLC) consisting of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and micro/mesoporous carbon electrodes prepared from silicon carbide derived carbon (SiC-CDC) that have been additionally activated with carbon dioxide (CO2). The electrochemical characteristics for EDLCs (region of ideal polarizability, characteristic time constant, specific series and parallel capacitances) are significantly dependent on the CO2 activation extent of the SiC-CDC materials. The calculated coulombic efficiency values for CO2 activated systems vary within the range from 97 to 99.8% (calculated using integrated charge density values). The energy efficiencies from 82 to 86.5% show that the CO2 activated SiC-CDC powders are much more suitable for various power and energy storage applications compared to untreated SiC-CDC. From impedance spectroscopy the highest capacitance value of 170 F g−1 at 3.6 V has been established for SiC-CDC with the largest activation burn–off extent of carbon from SiC-CDC. The Ragone plots for the carbon materials synthesized show noticeable influence of the CO2 activation extent on the stored specific energy and specific power values delivered.

Different mico-mesoporous silicon carbide-derived carbons (SiC-CDC) were synthesized via gas phase chlorination at 1100 • C and thereafter activated at 900 • C and 1000 • C with H 2 O steam using Ar and CO 2 as the carrier gases. The physical characterization data show that these materials are mainly amorphous, the structure does not change remarkably during the activation process and the surface chemistry of the differently activated and treated materials remains the same and there are no functional groups at the SiC-CDC surface. N 2 , Ar and CO 2 sorption measurements indicate an increase in the specific surface area and pore size distribution with increasing the activation temperature, whereas the influence of the carrier gas during synthesis is minimal. Although the specific surface areas and pore size distributions differed, the electrochemical parameters in 1 M (C 2 H 5 ) 3 CH 3 NBF 4 acetonitrile solution for all SiC-CDC materials were similar -specific gravimetric capacitances 130 ± 18 F g −1 and volumetric capacitance 67 ± 14 F cm Activated carbon materials are widely studied for several energy storage applications. In electrical-double layer capacitors (EDLC) different carbon materials are used for preparing the hierarchically micromesoporous electrodes.1-6 The use of activated carbon electrodes in EDLC-s is taking advantages of its micro-mesoporous structure to reduce the electrode's electrical resistance and improve energy storage performance.1 Organic and waste materials are quite often being used to prepare cost-effective carbon materials, but carbide-derived carbons (CDC) have an unique nanoporous structure with a narrow pore size distribution, a possibility to fine-tune the pore size and volume and also they have high electric conductivity.1,6-9 These properties make the CDC materials especially promising for energy storage/power generation applications. 2,4,8 To improve the carbon materials specific surface area and average pore size they are often activated by gas phase or liquid phase activation. Since gas phase activation enables cleaner production it is more favorable than liquid phase chemical activation.9,10 Carbon dioxide and H 2 O steam are the most widespread activating agents due to the cost-efficiency and endothermic nature of their reactions which allows better process control. 1Previously we have shown that gas phase activation with CO 2 of silicon carbide-derived carbon (SiC-CDC) resulted in doubling of the Brunauer-Emmett-Teller (BET) specific surface area and noticeable increase in pore size.12 Due to that the electrochemical parameters where significantly enhanced.12,13 Román et al. have reported that while CO 2 produces narrow micropores on the carbons and widens them as activation time is increased, steam yields pores of all the sizes from the early stages of the process and results in wider pore size distribution and more obvious development of macropores in the carbon materials.9,11 Steam activation has been widely used before for different activated carbons, but it is not m...