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 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.

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.