Carbon Dioxide Activated SiC-CDC: Attractive Material for Supercapacitor Electrodes

Abstract

Nanostructured carbide-derived carbons (CDC) were synthesized from SiC powders (SiC-CDC) via gas phase chlorination at temperature 1000 ºC [1]. Thereafter the CDCs were additionally activated by CO2 treatment method, resulting in nearly two-fold increase in specific surface area. The results of X-ray diffraction, high-resolution transmission electron microscopy (Figs. 1a and b) and Raman spectroscopy showed that the synthesized CDC materials are mainly amorphous, however containing small graphitic crystallites. The low-temperature N2 sorption experiments (Fig. 1c) were performed and the specific micropore surface areas from 1100 m2 g-1 up to 2270 m2 g-1 were obtained, depending on the extent of CO2 activation. The energy and power density characteristics of the supercapacitors based on 1 M (C2H5)3CH3NBF4 solution in acetonitrile and SiC-CDC as an electrode material were investigated using the cyclic voltammetry, electrochemical impedance spectroscopy, galvanostatic charge/discharge and constant power discharge methods. The cycling efficiency, i.e., the so–called round trip efficiency (RTE) has been calculated as a ratio of charge released and accumulated during discharging and charging of the supercapacitors. The calculated RTE values for all CO2 activated systems remained within the range from 98 to 99 %, showing that the CO2 activated SiC-CDC powders are promising materials for various energy storage applications. The specific capacitance values (C m), calculated from the Nyquist plots at ac frequency ƒ = 1 mHz depend on the SiC-CDC material used and C m values are very high for CO2 treated systems if compared with untreated SiC-CDC. C m values obtained from Nyquist plots are somewhat higher, but nevertheless in a good agreement with the values obtained using CV and CC methods. C m values for additionally CO2 treated SiC-CDC are comparable with data obtained for other CDC materials (TiC-CDC, VC-CDC, WC-CDC) studied. The maximum C m values (125 - 130 F g-1) have been calculated for mainly microporous SiC-CDC 1000 ºC activated with CO2 at 950 ºC for 8 h, however having well developed mesopores inside 2 – 4 nm region (Fig. 1c). The Ragone plots (Fig. 1d) calculated to the total material weight of two electrodes for the supercapacitors based on different SiC-CDC electrodes have been obtained from the constant power tests within the cell potential range from 3.0 V to 1.5 V. The mass fraction of active carbon material per electrode is 0.9, taken into account the PTFE binder and Al current collector. Very good performance has been established for SiC-CDC 1000 ºC, additionally activated at 900 °C during 8 h. The activation time 16 h at T = 950 ºC seems to be too long and therefore we have established different activation times for SiC-CDC additionally CO2 activated supercapacitors if various activation temperatures have been used. This conclusion is in a good agreement with N2 sorption data as well as our data published before [2,3]. Thus, differently from TiC-CDC the additional CO2 activation step has enormous influence on power densities of supercapacitor cells completed.