Graduation Year

2018

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Ashok Kumar, Ph.D.

Co-Major Professor

Manoj K. Ram, Ph.D.

Committee Member

Arash Takshi, Ph.D.

Committee Member

Rajiv Dubey, Ph.D.

Committee Member

Ajit Mujumdar, Ph.D.

Committee Member

Jiangfeng Zhou, Ph.D.

Keywords

electrochemical capacitors, electrode deposition, poly(3,4-ethylenedioxythiophene), polyaniline emeraldine, polymerization techniques

Abstract

The needs for energy storage devices have kindled researchers desire to explore and synthesize nanocomposite materials. Storing energy efficiently, effectively and sustainably are the science and engineering communities’ highest priorities to develop electrochemical energy storage devices. Supercapacitors have become power solution not only because supercapacitors can bridge the gap between the traditional capacitors and rechargeable batteries but also because of many other advantages which include extraordinary electrochemical properties, wide working-temperature range, cost effective, safe operation and long/stable cycle life. They have higher current pules than batteries due to the mechanism of charging and discharging. Batteries charging and discharging via chemical reactions, whereas supercapacitors utilize electrochemical double layer capacitors, in which nanoscopic charge separation is employed for energy storage at the electrode/electrolyte interface of the device.

One of the key factors that determine the performance and properties of electrochemical capacitors is the electrode material. The performance of supercapacitor relies on features such as specific surface area, electronic conductivity as well as mechanical and chemical stability of the electrode materials. Using conventional electrode materials, it is challenging to address all critical features include: toxicity, low specific capacitance and energy density, poor cycle stability, high cost and self-discharge. One of most intensive approaches of overcoming these obstacles is by introducing and developing nanocomposite materials for supercapacitors.

In this investigation, a MoS2/PEDOT nanocomposite material was chemically synthesized at various ratios of MoS2 to ethylenedioxythiophene (EDOT) to understand the charge mechanism in a symmetric supercapacitor. Whereas, previous attempts have been made to homogeneously cover MoS2 nanosheets with a PANI coating layer to obtain nanocomposite electrodes. However, emeraldine salt (ES) form of PANI is an insoluble polymer which may impede advantageous deposition techniques. Taking advantage of the processability of emeraldine base (EB) form of PANI, this approach can create an orderly distribution and homogeneous layer of PANI/N-Methyl-2-pyrrolidone (NMP) over MoS2 allowing the nanocomposite to coat over conducting substrates. The targeted nanocomposite material was finally de-doped to convert the PANI to ES, the conducting form, and enhanced the supercapacitor performance. Finally, after understanding the interesting combined effects of the material’s chemistry and capacitive properties in both two and three electrodes based electrochemical cells configuration and sponge based substrates was proposed for solid-state supercapacitor. The electrochemical deposition technique was applied, along with in-situ self-assembled polymerization of PPy and PANI, to fabricate the device. The morphological and crystal structures of the nanomaterials and nanocomposites of all projects were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyzer (PSA), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray-diffraction (XRD) techniques. In addition, the electrochemical properties of them were investigated to reveal their intriguing electrochemical and physicochemical properties using four-point probe, cyclic voltammetry (CV), constant current charging–discharging (CCCD), electrochemical impedance spectroscopy (EIS) in aqueous electrolytes. MoS2/PANI/PPy nanocomposite materials were considered for ideal supercapacitors which rendered highest specific capacitance around 631 F g-1. Owing to its superior electrochemical performance with the merits supercapacitors were produced for storing energy. This approach has revealed the possibility of using the natural or synthesized porous substrate to obtain a high surface area based supercapacitor.

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