Elsevier

Electrochimica Acta

Volume 247, 1 September 2017, Pages 400-409
Electrochimica Acta

Unveiling Polyindole: Freestanding As-electrospun Polyindole Nanofibers and Polyindole/Carbon Nanotubes Composites as Enhanced Electrodes for Flexible All-solid-state Supercapacitors

https://doi.org/10.1016/j.electacta.2017.07.038Get rights and content

Highlights

  • Polyindole nanofibers were fabricated and employed for the first time in supercapacitors.

  • Remarkable specific capacitance of Polyindole nanofibers as high as 238 F g−1 (1.0 A g−1) is reported.

  • A single-step supercapacitor assembly with a high energy density of 17.14 W h kg−1 at a power density of 426 W kg−1 is demonstrated.

Abstract

Polyindole(Pind) is one of the conducting polymers (CPs) which previously was less studied but of recent is gaining attention for energy storage applications. In all the few previous reports, when Pind was employed as electrode active material in supercapacitors, the capacitance was reported low with reasonable values only being obtained as a composite with other materials. The reasons underlying the poor performance of Pind and Pind nanocomposites are thought to be: 1) inactive morphology and limited surface area, 2) poor conductivity, and 3) poor electrode fabrication techniques. To address the trio, we employed the traditional, easy and scalable electrospinning technique to fabricate high surface area electroactive Pind nanofibers. Further, a little percentage (10 wt.%) of carbon nanotubes (CNTs) were added to enhance the conductivity of Pind and to study the effect of our fabrication route on the nanocomposites. Significant capacitance improvements of up to 238 F g−1 and 476 F g−1 at 1.0 A g−1 for Pind and Pind/CNT freestanding electrospun electrodes, respectively were achieved. Moreover, we report the significant performance of the all-solid-state symmetric, flexible and binder-free supercapacitor fabricated by a one-step and scalable method of as-electrospun Pind/CNT nanofibers on the stainless steel fabric current collector. The supercapacitor showed a high energy density of 17.14 W h kg−1 at a power density of 426 W kg−1 and capacitance retention of 95% after 2000 cycles. We strongly believe that we have set a stage for Pind to compete in a healthy race with other CPs as a next generation electrode material for supercapacitors.

Graphical abstract

Polyindole and polyindole/Carbon nanotubes nanofibers were fabricated via electrospinning as flexible electrodes. They showed an improved capacitance of up to 238 F g−1 and 476 F g−1 at 1.0 A g−1 for Pind and Pind/CNT, respectively.

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Introduction

All organic polymers were previously known to be super electrical insulators until 1977 when Hideki Shirakawa and the group discovered that polyacetylene when doped with chlorine, bromine, or iodine vapor, reveals a remarkable electronic conductivity[1]. This incredible discovery earned them a Nobel Prize in Chemistry in 2000. With this breakthrough, from then, the science community became increasingly interested in this field, and this led to the discovery of various other conducting polymers (CPs). Among which include polyaniline (Pani)[2], polypyrrole (Ppy)[3], polyindole (Pind)[4], polythiophene[5], with their corresponding derivatives, and many others. The CPs have mainly found use in energy[6], biomedicine[7], sensors[8], [9] and corrosion protection applications[10]. In the energy applications, CPs are commonly employed in supercapacitors (SCs) as electrode active materials so as to improve supercapacitor efficiency by increasing their energy storage and reducing self-discharging. They are capable of doing this by undergoing a redox reaction thereby storing charge in the bulk of the material[11].

The development of SCs as energy storage devices of recent is popular. Because it is one of the answers to the energy crisis and growing concerns with regards to pollution caused by over-exploitation of fossil fuels[12], [13]. The SCs have exceptional electric properties (higher power delivered per unit mass than Li-ion batteries and higher energy stored per unit mass than dielectric capacitors) with fast charge–discharge, longer cyclic stability, facile dynamics of charge propagation, easy operational mechanism, and low fabrication costs[12], [14], [15], [16]. Charge storage and the active constituent materials form a basis for classification of the family of which supercapacitors are part. They can be the electrical double-layer capacitors(EDLCs) based on high surface area carbonaceous materials[17], [18], [19] and the pseudo-capacitors(PCs) or redox SCs based on transition metal oxides or CPs[20], [21]. Among various CPs for PCs, of recent, Pind is starting to receive attention as one of the prominent candidates for use as enhanced electrode material in SCs. It has properties of both poly(para-phenylene) and Ppy self-possessed such as good thermal stability[22], high redox activity[23], and also slow degradation rate properties[24], in comparison to the competitors Pani and Ppy. Pind has recently been applied in supercapacitors, but, reasonable specific capacitance being obtained only with the help of other materials such as the carbon-based materials[25], [26], [27] transition metal oxides[28] or a combination of both[29], [30], usually inform of in-situ as-polymerized nanospheres and nanowires composites. So far the as-polymerized pure Pind specific capacitance reported is in the range of 21.8 to 49 F g−1 [31], [32]. The reported performance is insufficient and this might be due to a number of factors such as; (1) the inactive morphology and the limited surface area of the as-polymerized active materials, (2) the poor conductivity of Pind which is dictated by the polymerization conditions and doping, and (3) the poor electrode fabrication techniques.

To deal away with the issues mentioned above, for the best results when adopting Pind in SCs, it is, therefore, paramount to improve Pind electroactive properties by increasing its surface area and conductivity. And, ensuring that the high surface area electrode materials have active morphology and geometry. Active morphology and high surface area may be easy to obtain. However, the electrode fabrication techniques are known to tamper with the two. Therefore, the maintenance of these properties throughout the electrode fabrication process is of great importance. Electrospinning is an ideal, the best, easy and scalable technique to fabricate high surface area nanofibers with diameters ranging from micrometers to tens of nanometers using electrostatic force[33]. Zhijiang and coworkers[34] previously reported electrospun Pind nanofiber membranes (electrospun with acetonitrile) with significantly high specific surface areas, better crystallinity, and electronic conductivity, and a more effective doping/de-doping properties than Pind polymer powder hence promising in potential electronic applications. Also recently Gergin et al. [35] employed Polyacrylonitrile(PAN) as a carrier in dimethylformamide (DMF) to electrospin Pind, and the resultant Pind nanofibers showed significantly high specific surface areas and a more efficient electronic conductivity. With the mentioned properties, Pind was tested as a fiber polymer electrode for lithium ion secondary batteries[34] and sensors[35]. To the best our knowledge, only this literature exists concerning the electrospun Pind nanofibers and applications thereof. It is worth noting that Pind is extremely hard to electrospin into nanofibers. The complexity is due to the presence of the benzene and pyrrole rings in the polymer backbone which makes its polymer chains extremely rigid and insoluble in many organic solvents. Therefore, this prevents Pind-based electrospinning solutions from achieving sufficient chain entanglements to achieve the minimum solution viscosity required for successful electrospinning. To deal away with these problems, electrospinnable polymers such as Polyethylene oxide (PEO), Polyvinyl alcohol (PVA) and others related, can be thought of as helpers to be added to Pind to form electrospinnable polymer blends. However, these polymers are necessary evils, as they diminish the conductivity of Pind. Therefore, minimum weight percentage should be used as much as possible.

In this study, we were able to electrospin Pind into high surface area electroactive nanofibers with the help of PEO (10 wt. %) as the carrier polymer to obtain freestanding flexible electrodes. The as-electrospun nanofibers without binders were studied to understand their efficiency when used as electrode active materials in supercapacitors. Also, a small percentage of CNTs (10 wt.%) was introduced into the electrospinning solution to improve the conductivity of the nanofiber membrane for better performance. Finally, a flexible symmetric all-solid-state supercapacitor based on high surface area and electroactive Pind/CNT binder-free nanofiber electrodes on stainless steel fabric current collector was assembled and tested. It should be noted that our application of Pind and Pind/CNT electrospun nanofibers in supercapacitors together with the electrospinning route are hereby studied and reported for the first time. Moreover, our aim to solve the three major problems resulting in the previously reported poor capacitance properties of Pind and Pind nanocomposites; such as (1) poor electro-activeness/morphology, (2) poor conductivity, and (3) inefficient fabrication techniques have also been solved. The results showed significant improved performance.

Section snippets

Materials

Indole monomer (99% purity), ammonium persulphate (APS), Sulphuric acid (H2SO4) and p-toluenesulfonic acid (p-TSA) were acquired from Sinopharm Chemical Reagent Co. Ltd, Shanghai. PEO (Mw = 400,000 g/mol) and Multi- walled CNTs were purchased from Sigma-Aldrich. Chloroform ( > 99.5%), the solubilizing media was purchased from Shanghai Lingfeng Chemical Reagent Co. Ltd. Flexible stainless steel fabric was purchased from Shenzhen Hui Zhengxin Material Co., Ltd (China). Double deionized water was used

Morphology and polymer characterization results

Fig. 1a is FTIR spectra of the monomer indole, Pind polymerized nanoparticles and the electrospun nanofibers. The comparison of the nature of the vibrational modes appearing in the polymer and monomer can assist in the explanation of polymerization mechanism involved. The strong peaks observed at 3431 cm−1 (polymer) and 3402 cm−1 (monomer) are characteristic of N-H bond absorption. In the monomer, this peak is narrower whereas, in the polymer, the peak is wide and is shifted as per the spectrum

Conclusions

In summary, we have shown that the previous insufficient capacitance values of Pind and Pind-based nanocomposites were significantly contributed by the poor fabrication techniques of the electrode materials. The previously reported in-situ slurry-based fabrication methods cared less at generating and preserving active morphology of Pind or Pind based nanocomposites, of which both factors are paramount in obtaining electrode active materials with high capacitance values. In this work, Pind and

Acknowledgements

The authors thank Ifra Marriam (Donghua University) for the video and device bending tests design. This work was funded by the “Chenguang Program” supported by Shanghai Education Development Foundation, Shanghai Municipal Education Commission (15CG32), Fundamental Research Funds for the Central Universities (2232015D3-20), Science and Technology Commission of Shanghai Municipality (16JC1400700), and National Natural Science Foundation of China (51673088).

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