Elsevier

Applied Surface Science

Volume 433, 1 March 2018, Pages 765-771
Applied Surface Science

Full Length Article
Understanding and controlling the rest potential of carbon nanotube-based supercapacitors for energy density enhancement

https://doi.org/10.1016/j.apsusc.2017.10.044Get rights and content

Highlights

  • Surface charge of SWNTs strongly affects the rest potential of SWNT-EDLCs.

  • A shift in the rest potential is attributed to a change in the Fermi level of SWNTs.

  • Adjusting the rest potential can increase the voltage and energy density of EDLCs.

  • PEI coating of SWNTs improves the cell voltage.

  • The proposed method can be extended to EDLCs with various electrolytes.

Abstract

We present a novel method for enhancing the energy density of an electrical double layer capacitor (EDLC). Surface modification of single-walled carbon nanotube (SWNT) electrodes significantly affects the rest potential (E0) of EDLCs; acid treatment and polyethyleneimine (PEI) coating of SWNTs shift E0 toward more positive and more negative values, respectively. Adjusting E0 towards the center of the electrolyte stability window can increase the cell voltage and hence the energy density. PEI coating on SWNTs increases the cell voltage from 0.8 V to 1.7 V in tetrabutylammonium perchlorate (TBAP)/tetrahydrofuran (THF) electrolyte, and from 2.5 V to 3.1 V in tetraethylammonium tetrafluoroborate (TEABF4)/3-cyanopropionic acid methyl ester (CPAME), respectively. Moreover, PEI-SWNT EDLCs exhibit excellent cycling stability (92% of capacitance retention over 10000 cycles). We attribute the shift in E0 to a change in the Fermi level of SWNTs owing to the surface charge modification. Injection of electrical charge into PEI-SWNTs consistently yielded similar trends and thus validated our hypothesis. Our results may help to push various electrolytes that have been overlooked so far to new frontiers for obtaining high energy-density supercapacitors.

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)

  • M. Monthioux et al.

    Sensitivity of single-wall carbon nanotubes to chemical processing: an electron microscopy investigation

    Carbon

    (2001)
  • Y. Ryu et al.

    High electrical conductivity and n-type thermopower from double-/single-wall carbon nanotubes by manipulating charge interactions between nanotubes and organic/inorganic nanomaterials

    Carbon

    (2011)
  • D. Lee et al.

    Low-cost transparent, and flexible single-walled carbon nanotube nanocomposite based ion-sensitive field-effect transistors for pH/glucose sensing

    Biosens. Bioelectron.

    (2010)
  • B.E. Conway

    Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications

    (1999)
  • M. Winter et al.

    What are batteries fuel cells, and supercapacitors?

    Chem. Soc. Rev.

    (2004)
  • J.R. Miller et al.

    Electrochemical capacitors for energy management

    Science

    (2008)
  • Cited by (13)

    • A review of carbon materials for supercapacitors

      2022, Materials and Design
      Citation Excerpt :

      In addition, acid treatment and polyethyleneimine coating of CNTs can significantly change the rest potential of CNTs-based supercapacitors. By adjusting the rest potential, the working voltage of the supercapacitors can be increased, thereby enhancing the energy density [98]. Metal oxides have high theoretical capacity, but the high resistance and low surface area make the actual capacity much lower than the theoretical value.

    • Self-assembled reduced graphene oxide films with different thicknesses as high performance supercapacitor electrodes

      2020, Journal of Energy Storage
      Citation Excerpt :

      According to the reports in the past decade, it is well known that the physical and chemical properties of electrode materials are one of the crucial factors in determining the energy storage performance of the supercapacitor [4]. Carbon materials (such as activated carbon, carbon fiber and carbon nanotubes) [5–7], conducting polymers (such as polyaniline, polypyrrole, and polythiophene) [8–10] and transition metal oxides (such as RuO2, MnO2, and NiO) [11–14] are investigated to use as the active material for the supercapacitor electrodes. Among them, carbon based materials are the dominant electrode material because of their high electronic conductivity, low cost, long-term cycling life, and suitable to various electrolyte systems, which are also the main commercial electrode material with a large electrochemical double layer capacitance [15].

    • Synergistic effects of water and carbon dioxide on the reversible thermoelectric behaviors of polyethyleneimine/single-walled carbon nanotubes composites

      2020, Synthetic Metals
      Citation Excerpt :

      The electron-rich polymer PEI has been often used as n-type dopant for SWCNTs [20]. The highest occupied molecular orbital (HOMO) of PEI is higher than the Fermi level of SWCNTs, and electrons move from PEI to the conduction band of SWCNTs, making the SWCNTs n-type [40]. In different gas environment, it can be found O2 has little impact on the change of Seebeck coefficient, while H2O or H2O/CO2 has great influence on them.

    View all citing articles on Scopus
    1

    These authors contributed equally to this work.

    View full text