Abstract
We investigated the effect of the diameter size of single-walled carbon nanotubes (SWCNTs), on their high-temperature energy storage behavior in an electric double layer capacitor (EDLC) using the ionic liquid triethyl(2-methoxyethyl) phosphonium bis(trifluoromethylsulfonyl)imide (P222(2O1)-TFSI). We used four SWCNT samples with diameter sizes ranging from 0.8 to 5 nm, and evaluated their electrochemical charge storage behavior through galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). We found that for the SWCNTs with small average diameter of 1 nm, the value of the electrode capacitance measured at a current density of 5 mA g−1 increased from 15.8 at room temperature to 27.5 F g−1 at 150 °C, and the value measured at a current density of 80 mA g−1 increased from 14.0 at room temperature to 22.1 F g−1 at 150 °C. The larger diameter samples on the other hand did not show any significant change in their capacitance with temperature. We calculated the size of the interstitial tube spaces from the Raman spectra of the samples, and used density functional theory (DFT) calculations to estimate the sizes of the cation and anion of the electrolyte. The obtained results suggest that the temperature-induced changes in the electrolyte properties improved the ion accessibility into the otherwise constrained space inside the small diameter SWCNTs, while the spaces inside the larger SWCNTs already provided easily accessible storage sites hence good performance at room temperature, making the increase in temperature of little to no effect on the charge storage performance in such SWCNTs.
Highlights
The use of electric double layer capacitors (EDLCs) is being increasingly expanded to many applications, including some which take place under elevated temperatures, like electric and hybrid vehicles, electric aircra, aerospace electronics, and oil drilling rigs
The energy storage is largely dependent on the structural properties of the electrode material. This raises the logical question of the effect of the structural properties of the electrode on the energy storage performance of the EDLCs at elevated temperatures, and the design factors to be considered in such systems, especially when the device is expected to operate under both elevated temperatures and high charging rates
To the best of the authors' knowledge, this factor has remained largely unaddressed, apart from a study that compared the behaviour of carbon nanotubes to that of nanoonions used as electrodes for EDLCs operated at elevated temperatures.[6]
Summary
The use of electric double layer capacitors (EDLCs) is being increasingly expanded to many applications, including some which take place under elevated temperatures, like electric and hybrid vehicles, electric aircra , aerospace electronics, and oil drilling rigs. Increasing the temperature resulted in an improvement in the performance at high current density (Fig. 4), following similar trends as those seen in the GCD measurements.
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