- Title
- Metal oxide polymer composites for electrochemical capacitor electrodes
- Creator
- Ningsih, Purnama
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Metal oxide polymer composites, in thin film form, have been found to be promising electrode materials for electrochemical capacitors. A composite of manganese-modified polymers was prepared in this study by electrochemical polymerisation. An electrodeposition method has been used to synthesise thin films in an acidic-aqueous environment. It is shown that Mn-modified polyaniline (PAni) and Mn-modified polypyrrole (PPy) thin film electrodes can successfully be synthesised from Ani⁺Mn²⁺ and Py⁺Mn²⁺ in 0.1 M H₂SO₄ using chronoamperometry in 30 second intervals on a platinum substrate. Based on a comparison of both thin films formed through electrodeposition, it is shown that in most aspects the thin films behave very similarly electrochemically. They showed a higher capacitive performance compared to their pure polymer, in this case PAni and PPy thin films. However, Mn-modified PPy thin films exhibit the highest specific capacitance. Therefore, the Mn-modified PPy was selected for further study and prepared from different variation concentrations of electroactive species Py and Mn²⁺ in different supporting acid electrolytes including H₂SO₄, HCl, and HNO₃. The study of the electrodeposition behaviour of composite Mn-modified PPy thin films was investigated using several methods including chronoamperometry (CA), the electrochemical quartz crystal microbalance (EQCM) system, and ICP-OES. It was found that the mass transport during deposition, relative area change, charge passed, and final film colour to be largely determined by the concentration of electroactive species (Py and Mn²⁺) used. Different acid electrolytes gave a different effect in some cases; however, differences were more subtle than observations seen when changing the electroactive species concentration. In addition, it was determined that the mass of PPy in the thin films formed is dominant compared to the mass of Mn, based on EQCM and inductive coupled plasma-optical emission spectroscopy (ICP-OES) work. A combination of rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) voltammetry was also used to investigate the electrochemical behaviour of the films formed in-situ. From RDE work, it was found from linear potential sweeps that a noticeable decrease in current is observed at high potentials. This is suggested to be due to the formation of the thin film promoting thin film passivation once the bare platinum surface is covered in the polymer material. This assumption was supported by Tafel analysis result where the potential window between 0.6-1.1 V showed dual oxidation regions where the initial oxidation occurs over a bare platinum surface, followed by more limited oxidation over the polymer film formed. RRDE work further revealed that the collection efficiency (𝑁𝐷) of thin films also depends on the concentration of electroactive species. The electroactive species lost during electrodeposition relates to 𝑁𝐷 where a high 𝑁𝐷 indicates thin film is lost to solution. The 𝑁𝐷 was observed to be highest at high rotation rates and for high concentration electrodeposition solutions, suggesting a reduction in polymer growth under the conditions. Physical characterization of the electrodeposited Mn-modified PPy thin films has been carried out by fourier transform infra-red (FTIR) spectroscopy analysis, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) analysis, transmission electron microscopy (TEM), profilometry and atomic force microscopy (AFM) analysis. The different acid electrolytes used and the presence/absence of Mn did not result in any differences in the FTIR spectrum collected for thin films investigated. Similarly, the growth of films prepared in different acid electrolytes gives a similar physical structure, where a block was deposited as a base on a platinum substrate, followed by vertical growth of nucleates on the block surface, seen clearly in SEM results. However, the morphology of the thin films formed was seen to be dependent on the acid electrolytes employed and conditions used for electrodeposition (i.e., static and hydrodynamic). Based on TEM imaging, the presence of Mn can be detected on all typical thin films where Mn²⁺ is present in the electrodeposition solution. The images show a similar morphology in different acid electrolytes which is also not uniform, not flat, and semi-transparent to the electron beam. In addition, the thickness and roughness of the thin film was shown to be affected by the concentration of electroactive species Py and Mn²⁺ in the electrodeposition solution. The electrochemical characterization of the composite Mn-modified PPy thin films electrodes are also investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The CV result is affected by faradaic and non-faradaic processes. The voltammetric behaviour depends on the physical nature of the thin films, which is related to the concentration of electroactive species Py and Mn²⁺. Using different acid electrolytes for film formation is shown to give only a subtle effect in this regard. The specific capacitance of all typical thin films increased as the thickness and roughness of thin film decreased. In contrast, enlarging the surface area caused higher specific capacitance. All thin films showed electrochemical stability after an initial decrease of the specific capacitance in earlier cycling. The decreasing stability of the thin films is due to the swelling of the polymer and subsequent mechanical breakdown of material during cycling. These are marked by increased roughness in appearance in SEM images, less thickness, and more roughness in AFM measurements observed after cycling of the thin films. Impedance spectra under applied potential of the films formed from different concentration of Py and Mn²⁺ in different acid electrolytes were seen to be mostly similar, which is presumably due to mass transport phenomena and double layer processes. By applying an a modified Randles circuit model to EIS data, it was found that the four parameters; a series resistance (Rs), a charge transfer resistance (Rct), a constant phase element diffusion (CPE(D)) and double layer (CPE(DL)) with their T and P, and Cdl. It was found that Rs is dependent on the concentration of Py and Mn²⁺ electrolyte which contributed to performance of thin films. Furthermore, Rs depends on the configuration of the cell which is related to the position of the electrode to thin film substrate. The Rct of thin films are on average quite large number which means that it is dependent on intrinsic charge transfer resistance and the area over which that transfer take places. The fluctuation behaviour in CPE(D) and CPE(DL) with T and P parameters during cycling reflect to the change of electrode surface. The changes are contribution of swelling on active material which results in the mechanical breakdown on the thin films. The Cdl behaviour is again depending on the area specific term while for the individual thin film electrode the Cdl are closely related to Rct (due to parallel position in the equivalent circuit).
- Subject
- polypyrrole; manganese; electrochemical capacitor; thin films
- Identifier
- http://hdl.handle.net/1959.13/1296619
- Identifier
- uon:19284
- Rights
- Copyright 2015 Purnama Ningsih
- Language
- eng
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