Elsevier

Applied Surface Science

Volume 439, 1 May 2018, Pages 833-838
Applied Surface Science

Full Length Article
Covalent organic framework-derived microporous carbon nanoparticles coated with conducting polypyrrole as an electrochemical capacitor

https://doi.org/10.1016/j.apsusc.2018.01.103Get rights and content

Highlights

  • COF nanoparticles are prepared via a condensation reaction.

  • COF-derived microporous carbon (MPC) nanoparticles are synthesized.

  • Polypyrrole (Ppy) is electrochemically polymerized.

  • The specific capacitance with Ppy coating is enhanced up to 2.55 F cm−2.

  • The enhancement is due to the synergetic effect of pseudocapacitance and reduced resistance.

Abstract

We report a high-performance electrochemical capacitor based on covalent organic framework (COF)-derived microporous carbon (MPC) nanoparticles and electrochemically polymerized polypyrrole (Ppy) as a pseudocapacitive material. The COF, Schiff-based network-1 (SNW-1) nanoparticles are prepared via a condensation reaction between melamine and terephthalaldehyde, and the resultant MPC film is prepared via a screen-printing method. The MPC film exhibits a bimodal porous structure with micropores and macropores, resulting in both a large surface area and good electrolyte infiltration. Ppy is synthesized potentio-statically (0.8 V vs. Ag/AgCl) by varying the reaction time, and successful synthesis of Ppy is confirmed via Raman spectroscopy. The specific capacitance with the Ppy coating is enhanced by up to 2.55 F cm−2 due to the synergetic effect of pseudocapacitance and reduced resistance.

Introduction

Although electrochemical capacitors are a relatively new energy storage system compared to lithium batteries, they have attracted significant interest from many research groups due to their high power density, high durability, and lower environmental impact [1]. Electrochemical capacitors can be classified into two categories according to their energy storage mechanism: electric double layer capacitor (EDLC) and pseudocapacitor. The former is known for its rapid charge–discharge rate and long lifetime, while the latter is relatively similar to secondary batteries and known for its high capacitance [2], [3], [4], [5].

Although many types of compounds have been used as pseudocapacitor active materials, carbon-based materials have usually been applied in EDLCs due to their high surface area and high capacitance [6]. Therefore, various kinds of nanostructured carbons such as one-dimensional (1D) carbons [7], [8], graphene [9], [10], and microporous carbons [11], [12] have been prepared by various synthetic methods. Recently, metal organic frameworks (MOFs) have also been used as carbon sources due to their organic linkers and high surface area [13], [14]. Furthermore, covalent organic frameworks (COFs), which are similar to MOFs except for the absence of metal ions, have also appeared as sources of microporous carbon particles [15], [16].

For pseudocapacitors, there must be faradaic current between the active materials and electrolyte, resulting in a redox reaction under applied potential. Among pseudocapacitive materials, conducting polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) [17], [18], polypyrrole (Ppy) [19], [20], and polyaniline (PANI) [21], [22] have been extensively applied as active materials due to their high conductivity, capacitance, and ease of synthesis. In particular, Ppy has been widely adopted as the active material of electrochemical capacitors in various applications due to its high conductivity, reactivity, and capacitance [23], [24], [25].

Herein, we first report the combination of conducting polymer Ppy and microporous carbon (MPC) derived from a COF, Schiff-based network-1 (SNW-1), without any additives or binders. After the synthesis of the SNW-1 nanoparticles, the nanoparticle dispersion was directly printed onto carbon sheets, followed by thermal annealing under Ar conditions. The porous structure was characterized by a field emission scanning electron microscope (FE-SEM) and the pore size distribution was calculated by the Horvath-Kawazoe (HK) method. To enhance the electrical conductivity and capacitance, Ppy was electrochemically polymerized onto the bimodal carbon film with varying reaction times. The electrochemical properties of the as-prepared electrodes were characterized by cyclic voltammetry and constant current charging-discharging method.

Section snippets

Materials

Melamine (99%), terephthalaldehyde (99%), pyrrole (reagent grade, 98%), ethylene glycol (EG, anhydrous, 99.8%), p-toluenesulfonic acid monohydrate (ACS reagent, ≥98.5%), and sodium p-toluenesulfonate (95%) were purchased from Sigma–Aldrich. Dimethyl sulfoxide (DMSO, anhydrous, 99.7%) was obtained from Fisher BioReagents. Isopropanol (IPA, 99%), acetone (99.5%), tetrahydrofuran (THF, 99%), and dichloromethane (99.5%) were provided by Daejung Chemical Co., Korea. All the chemicals were used

Synthesis and characterization

The SNW-1 nanoparticles are synthesized from two monomers; namely, melamine and terephthalaldehyde (Fig. 1a). The synthesis is confirmed by using FT-IR spectroscopy (Fig. 1b). The melamine exhibits some distinctive absorption bands at 3417 and 3468 cm−1, which are assigned to the H-N-H stretching vibration, and at 1650 cm−1, which is attributed to the H-N-H deformation [27]. The other monomer, terephthalaldehyde, shows absorption bands at 1683 and 2870 cm−1, which are assigned to the Cdouble bondO and Csingle bondH

Conclusion

An electrochemical capacitor based on a bimodal porous carbon film was fabricated using COF-derived carbon nanoparticles and post-treatment with Ppy. The electrochemical capacitor with the bare MPC nanoparticles prepared via a screen-printing method showed a specific capacitance of 244.7 mF cm−2 when using a 1 M KOH aqueous electrolyte. The high surface area was attributed to the presence of intra-micropores from the MPC and the good electrolyte infiltration was possibly due to the presence of

Acknowledgements

This work was supported by the National Research Foundation (NRF) grant funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A4A1014569, NRF-2017R1D1A1B06028030, NRF-2016R1A2B4014256) and the Center for Advanced Meta-Materials (CAMM) (NRF-2014M3A6B3063716).

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