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

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 CO 2 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 N 2 sorption experiments (Fig. 1c) were performed and the specific micropore surface areas from 1100 m 2 g -1 up to 2270 m 2 g -1 were obtained, depending on the extent of CO 2 activation. The energy and power density characteristics of the supercapacitors based on 1 M (C 2 H 5 ) 3 CH 3 NBF 4 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 CO 2 activated systems remained within the range from 98 to 99 %, showing that the CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 activated supercapacitors if various activation temperatures have been used. This conclusion is in a good agreement with N 2 sorption data as well as our data published before [2,3]. Thus, differently from TiC-CDC the additional CO 2 activation step has enormous influence on power densities of supercapacitor cells completed. References 1. E. Tee, I. Tallo, H. Kurig, T. Thomberg, A. Jänes, E. Lust, Electrochim. Acta , 161 , 364 (2015). 2. I. Tallo, T. Thomberg, K. Kontturi, A. Jänes, E. Lust, Carbon , 49 , 4427 (2011). 3. I. Tallo, T. Thomberg, H. Kurig, A. Jänes, K. Kontturi, E. Lust, J. Solid State Electrochem ., 17 , 19 (2013). Acknowledgements The present study was supported by the Estonian Center of Excellence in Science project 3.2.0101.11-0030, Estonian Energy Technology Program project 3.2.0501.10-0015, Material Technology Program project 3.2.1101.12-0019, Project of European Structure Funds 3.2.0601.11-0001, Estonian target research project IUT20–13 and personal research grant PUT55 of the Estonian Ministry of Education and Research, and projects 3.2.0302.10-0169, 3.2.0302.10-0165. Authors would like to thank Prof. Kyösti Kontturi from Aalto University (Finland) for HRTEM measurements. Figure 1