Full Length ArticleUnderstanding and controlling the rest potential of carbon nanotube-based supercapacitors for energy density enhancement
Graphical abstract
Introduction
It is important to devise and explore new ways to improve the performance of energy storage devices to meet the ever-increasing energy demands of society. Supercapacitors are promising energy storage systems with high power density, fast charge-discharge dynamics, and long cycle lifetime. These characteristics make supercapacitors suitable for broad range of applications, such as grid power buffers, harvested energy storage devices, energy recovery devices, hybrid electrical vehicles, uninterruptible power supply sources, and memory backup devices [1], [2]. Although supercapacitors exhibit higher energy density than conventional capacitors, the energy density of a typical supercapacitor is lower than that of other energy storage devices, such as batteries [3], [4]. Therefore, it is highly desirable to improve the energy density characteristic of supercapacitors while maintaining their advantageous power performance.
Researchers have developed various strategies to enhance the energy density of supercapacitors by incorporating redox materials and/or using asymmetric electrode configurations [5], [6], [7], [8]. For example, metal oxides and conductive polymers can be used as pseudo-capacitive materials to significantly improve capacitance. Recently, it has been demonstrated that incorporation of redox molecules such as hydroquinone and decamethylferrocene into electrolytes can improve the capacitance and/or the cell voltage of capacitors [9], [10]. Combining supercapacitor-type and battery-type electrodes can also improve the energy density of supercapacitors. Since all of the above-mentioned supercapacitors involve redox chemical reactions, enhancement of their energy density could be accompanied by a certain reduction in their power performance.
One interesting and promising approach would be to improve the energy density of EDLCs without the involvement of redox chemical reactions. This is because EDLCs are generally faster than pseudo-capacitors, because the operation of EDLCs is based on facile physical adsorption and desorption of ions rather than on redox chemical reactions. We posit that properly adjusting the rest potential (E0) of EDLCs should increase their cell voltage and thus the energy density. The rest potential E0 is defined as the potential at which the positive and negative electrodes are placed relative to the reference electrode when the cell voltage is 0 V. The value of E0 significantly affects the characteristics of EDLCs [11], [12]. When positive and negative electrodes contain equal amounts of active material, the potentials of two electrodes will symmetrically deviate from E0 during charging. If E0 is located away from the center of the electrolyte stability window, one of the electrodes will reach the stability window limit before the other electrode will approach its limit on the other side [13]. Therefore, positioning E0 at the center of the stability window will make full use of the stability window and the operating voltage of the system will attain a maximal value [14], [15].
Unfortunately, in most of the electrolyte/electrode systems, E0 is not properly positioned for maximizing the energy density. A few commercial organic electrolytes such as tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile have E0 at the center of their stability windows. However, if E0 could be controlled, it might be possible to utilize previously overlooked electrolytes for developing high-voltage EDLCs. A few methods, such as changing ions in an electrolyte, mass balancing, and charge injection, have been developed to adjust the value of E0 [15], [16], [17], [18]. However, these methods adjust E0 only temporarily or have some unwanted issues related to fabrication.
Here, we introduce a novel method for adjusting E0 by surface charge modification of electrodes for energy density enhancement of EDLCs. Treating the surface of single-walled carbon nanotubes (SWNTs) in two different ways, i.e., by acid treatment or PEI coating, changes the charge and the Fermi energy of the SWNTs in the opposite directions. This suggests a method for enforcing the value of E0 towards the center of the stability window of the electrolyte, which would increase the cell voltage and energy density. Moreover, surface modification such as PEI coating is durable and thus PEI-SWNT EDLCs exhibit excellent cycle stability. We demonstrate the improvement of the energy density based on the E0 shift, for two different electrolytes. We believe that this demonstration opens up the opportunity to explore various other electrolytes for energy density improvement of EDLCs.
Section snippets
Preparation of electrodes and electrolytes
The electrodes in this study were fabricated using three different types of SWNTs: raw SWNTs (0.7–1.4 nm in diameter, Sigma-Aldrich), acid-treated SWNTs (COOH-SWNTs), and PEI-coated SWNTs (PEI-SWNTs). SWNT solutions were prepared as follows. For the solution of raw SWNTs, as-received SWNTs were dispersed in propylene carbonate (anhydrous, 99.7%) by tip sonication (Sonics & Materials, VC 750) for 1 h (1 g L−1). COOH-SWNTs were prepared by slightly modifying the procedure described elsewhere [19].
Characterization of composition, surface charge, and morphology of electrode materials
The three types of electrode materials, COOH-SWNTs, raw SWNTs, and PEI-SWNTs, have different element compositions and hence different surface charges. While carbon is the dominant element in all of the three materials, COOH-SWNTs and PEI-SWNTs contain more oxygen and nitrogen than raw SWNTs, respectively, as revealed by XPS (Table 1). The results suggest the presence of COOH groups and PEI polymers on the SWNT surface after the treatments. Both XPS and TGA indicate that the weight percentage of
Conclusion
We demonstrated a novel strategy for controlling E0 of SWNT-based EDLCs, which allows to increase their energy density. Simple surface modifications of SWNTs, such as acid treatment and PEI attachment, significantly shifted E0. Especially, PEI coating of SWNTs increased the cell voltage from 0.8 V to 1.7 V in TBAP/THF and from 2.5 V to 3.1 V in TEABF4/CPAME, respectively. Moreover, the PEI-SWNT EDLC exhibited excellent cyclability (capacitance retention over 10000 GCD cycles was 92%). We attributed
Acknowledgement
This work was financially supported by a National Research Foundation of Korea (NRF) grant that was funded by the Korean government (NRF-2017R1A2B2006209).
References (32)
- et al.
Templated mesoporous carbons for supercapacitor application
Electrochim. Acta
(2005) - et al.
Comparison of commercial supercapacitors and high-power lithium-ion batteries for power-assist applications in hybrid electric vehicles: i. Initial characterization
J. Power Sources
(2002) - et al.
A new type of high energy asymmetric capacitor with nanoporous carbon electrodes in aqueous electrolyte
J. Power Sources
(2010) - et al.
Long-term cycling of carbon-based supercapacitors in aqueous media
Electrochim. Acta
(2009) - et al.
A symmetric carbon/carbon supercapacitor operating at 1.6 V by using a neutral aqueous solution
Electrochem. Commun.
(2010) - et al.
Adjustment of electrodes potential window in an asymmetric carbon/MnO2 supercapacitor
J. Power Sources
(2011) - et al.
Adjusting electrode initial potential to obtain high-performance asymmetric supercapacitor based on porous vanadium pentoxide nanotubes and activated carbon nanorods
J. Power Sources
(2015) - et al.
Purification of carbon nanotubes grown by thermal CVD
Phys. E
(2007) - et al.
Electrochemical detection of nanomolar dopamine in the presence of neurophysiological concentration of ascorbic acid and uric acid using charge-coated carbon nanotubes via facile and green preparation
Talanta
(2016) - et al.
Modified carbon nanotubes: an effective way to selective attachment of gold nanoparticles
Carbon
(2003)
Sensitivity of single-wall carbon nanotubes to chemical processing: an electron microscopy investigation
Carbon
High electrical conductivity and n-type thermopower from double-/single-wall carbon nanotubes by manipulating charge interactions between nanotubes and organic/inorganic nanomaterials
Carbon
Low-cost transparent, and flexible single-walled carbon nanotube nanocomposite based ion-sensitive field-effect transistors for pH/glucose sensing
Biosens. Bioelectron.
Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications
What are batteries fuel cells, and supercapacitors?
Chem. Soc. Rev.
Electrochemical capacitors for energy management
Science
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These authors contributed equally to this work.