Flexible Supercapacitor Stackable with Solar Cells Based on PEDOT-Carbon Nanotube Nanocomposite Electrodes Using Ionic Liquid Gel Electrolytes for Solar Electricity Storage
Authors: Obeidat Amr M , Rastogi Alok C 
Published: February 2020

Electrically conducting polymers (ECP) showing pseudo-capacitive property are actively researched as supercapacitor electrodes for energy storage. Nanocomposites with structured carbons contribute additional charge storage through electric double layer component enhancing the charge storage ability.  The nanocomposites with CNTs hold additional promise to compensate for weak mechanical property of ECPs useful for flexible supercapacitor devices and enhance its electronic conductivity. Conventionally, the supercapacitor performance is evaluated using anion dopants in aqueous medium having voltage limitation and in organic electrolytes which are toxic. Ionic liquid gel electrolytes are promising but not widely studied for supercapacitor applications. Besides enabling stable high voltage operation, ionic liquid gel electrolyte makes supercapacitor assembly simple which can be configured as flat stackable layered device. These attributes are exploited in integration with the solar cells for the storage of solar electricity.

We have investigated electrochemical and energy storage performance of poly(3,4-ethylenedioxy-thiophene) (PEDOT) and CNT nanocomposite electrodes based supercapacitors in the flexible solid-state format with ionic liquid gel electrolyte.  In this work nanocomposite electrode was formed by co-deposition of PEDOT and CNT over the flexible graphite substrates using the galvanic pulsed electro-polymerization. The method involves ultrasonic dispersion of 1 wt% anionic CNT for 2 h in acetonitrile to which 0.1 M EDOT monomer and 0.2 M LiClO4 dopant was added. The PEDOT-CNT composite deposition is initiated as unipolar anodic current pulses of 4 mA.cm-2 amplitude with 10 ms ON time interspaced with 100 ms OFF time were sequentially applied for 20k cycles providing a unique way to control the electrode morphology.  The symmetrical supercapacitor device was fabricated using ionic liquid 1-butyl-3-methylimidazolium tetrafluoro-borate ([BMIm]BF4) immobilized in poly(vinylidenefluoride -co-hexafluoropropylene (PVdF-HFP) forming a gel which was applied uniformly as ~50-70 μ layer sandwiched between the two PEDOT-CNT composite electrodes. With such stackable flat plate- supercapacitor device design, enhanced energy density is obtained and direct charge storage performance was realized when connected to solar cell module while mounted at the rear without affecting the form factor.

The electrode morphology show randomly dispersed CNT of average diameter of 20-30 nm embedded within and encased by copolymerized PEDOT forming a 3-D network structure. The CNT is observed to enhance the PEDOT polymerization by facilitating charge transport at the substrate and also increase the electrode conductivity. Raman shifts in the 1425 cm−1 band  assigned to  symmetric Cα=Cβ (−O) stretching show in the formation of nanocomposite, the CNTs interact with the ethylenedioxy group of PEDOT resulting in  high degree of  conjugation and enhancrd charge transfer between CNTs and PEDOT molecular chains. Conjugated interaction is further confirmed by shift in disordered A1g mode of CNT. The cyclic voltammetry curves recorded over -1 to +1 V range showed fast and near vertical current switching at end potentials over wide 10-100 mV s-1 scan rates signifying highly capacitive property. Supercapacitor showed areal capacitance density of 357 mF.cm-2 yielding high single electrode specific capacitance of 297 F.g-1. The high areal specific capacitance in the PEDOT-CNT composite is due to dopant anion aggregation along the CNTs and enhanced conduction of dopant ions in ionic liquid through embedded CNT nanopores providing rapid pathways to delocalize the electrons from along the PEDOT molecular chain. Impedance data as Nyquist plots feature near vertical trend in the low frequency region and lower real impedance cutoff suggestive of highly capacitive property and low electrode-electrolyte resistance. Occurrence of high knee frequency of > 10 Hz suggests compared to PEDOT, the CNT nanocomposite has minimal diffusion limitation. Charge-discharge (CD) data at various currents cycled between 0.05-1.0V are triangular with low ESR of 20 Ohm.cm2 and Coulomb efficiency > 90%. Long-term CD cycling for 2000 cycles showed overall degradation of 21% with a nominal aggravation of merely 7% occurring in initial 400 cycles. These data show comparatively higher stability of CNT nanocomposite compared to PEDOT.  The supercapacitor device functional properties were evaluated in terms of energy and power density parameters and presented as Ragone plots. Energy density of 18.3 Wh.kg-1 and corresponding power density of 5.1kW.kg-1 was recorded from CD data at 0.5 mA.cm-2 for 1V operation which is reasonably high for a gel electrolyte supercapacitor device.  

This paper would present detailed analysis of the pulse polymerization synthesis mechanism, microstructure and electrochemical properties of PEDOT-CNT nanocomposite. The supercapacitor device data on CV, CD, impedance and Ragone plots will be presented along with its efficacy in storing the solar electricity in combination with the solar cells.