Year

2014

Degree Name

Doctor of Philosophy

Department

Intelligent Polymer Research Institute, Department of Chemistry

Abstract

A bioelectric battery provides a promising alternative to commercial lithium batteries to drive active implantable medical devices (AIMD) due to its ease of miniaturization. Bioelectric batteries use body fluid as electrolyte. The selected electrode materials themselves and the reaction products are both safe for the implantation of these electrodes into the human body. The bioabsordable and high energy density Mg alloy is used as anode. However, most electrocatalysts could not meet the requirements for cathode materials in a bioelectric battery, e.g. possessing electrocatalytic activity as well as biocompatibility. Thus this thesis aims to synthesize novel cathode materials specifically for bioelectric batteries. Conducting polymers (CPs) and graphene show electrocatalytic activity and biocompatibility in previous studies, satisfying the requirements for their use in bioelectric batteries. Furthermore, the low cost compared to noble metals (like Pt) makes them promising cathode materials. Herein, the application of CPs and graphene in bioelectric batteries was intensively studied in this thesis.

The introduction of graphene into PPy is expected to greatly improve the electrochemical properties of PPy due to its high conductivity, large surface area and excellent mechanical properties. In this work, a novel PPy/reduced graphene oxide (r-GO) composite was fabricated by inducing an electrochemical reduction of graphene oxide incorporated into PPy as the dopant. The incorporation of r-GO significantly improved the electrical conductivity of the composite. The incorporation and the subsequent reduction of GO in the PPy matrix can be confirmed by Raman and spectroelectrochemistry. The GO concentration affected the composite properties. The PPy/r-GO composites synthesized from 1 mg ml-1 GO exhibited the best electrochemical performance and battery performance. In a bioelectric battery, PPy/r-GO displayed a discharge voltage of 1.03 V, 380 mV higher than that of PPy/p-toluenesulfonate (pTS) at 100 μA cm-2. Furthermore, the PPy/pTS composite exhibited a considerable increase of charge transfer resistance in electrochemical impedance spectra (EIS) when it was almost reduced. In contrast, a slight increase in resistance was obtained for PPy/r-GO, demonstrating that the resistivity in PPy is a determinant of cell performance.

Dextran sulphate (DS) is an anti-coagulant and its introduction into PPy/r-GO is expected to not only improve the biocompatibility, but also enhance the doping level of the composite, benefitting its use in bioelectric batteries. Thus, the PPy/r-GO/DS composites were successfully prepared by employing a co-dopant system. The effect of different feed ratios of r-GO and DS was studied by SEM, FTIR, UV-Vis and Raman. PPy/r-GO1DS2 that was electro-synthesized from an aqueous solution containing 1 mg ml-1 GO and 2 mg ml-1 DS exhibited a porous structure. It also possessed the highest effective conjugation length as evidenced from UV-vis and FTIR spectra, which results in the best electrochemical performance among PPy/r- GO/DS composites as suggested in CVs. As a result, PPy/r-GO1DS2 showed a superior performance at low current density in bioelectric batteries. However the PPy/r-GO1DS1 outperformed PPy/r-GO1DS2 at a high density of 100 μA cm-2. The results in EIS show a smaller resistance increase obtained for PPy/r-GO1DS1, than for PPy/r-GO1DS2 in their nearly reduced state, indicating that the conductivity, rather than redox property, was the more important factor that determines the battery performance of PPy at a high current density.

Due to the excellent electrical conductivity and stability of Poly (3, 4- ethylenedioxythiophene) (PEDOT), its application in bioelectric batteries was investigated as well. The PEDOT/biocompatible molecule composites were electropolymerized by using biomolecules dopants, hyaluronic acid (HA), chondroitin sulphate A, sodium salt (CS) and dextran sulfate (DS). Poly (2-methoxyaniline-5- sulfonic acid) is also used as dopant due to its biocompatibility. It is believed that all these dopants could improve the biocompatibility of the composites. The PEDOT/DS exhibited the best electro-activity as confirmed by CVs. The results from previous chapters demonstrated that the electrical conductivity is the most important factor that determines the battery performance of CPs at a high current density. Thus the highly conductive r-GO sheets were co-incorporated into PEDOT with DS via an electrochemical route. The synthesis of PEDOT/r-GO/DS composites was optimized by varying the feed ratio of r-GO and DS. It was found that the doping level of PEDOT can be tuned by adjusting the feed ratio of r-GO and DS as evidenced in Raman spectra. The PEDOT/r-GO/DS composites with increased doping level showed improved electrochemical properties in CVs and higher cell voltage in bioelectric batteries, compared with pure PEDOT/r-GO or PEDOT/DS.

Graphene is another promising electrode material in bioelectric batteries because of its biocompatibility and electrocatalysis towards oxygen reduction. In this work, graphene foam (GF) was successfully synthesized with the use of a sacrificial Ni foam template. Graphene foam exhibits a three-dimensional porous structure resembling that of Ni foam. The graphene nanosheets assembly could provide sufficient mechanical support itself by forming a free-standing 3D structure without a nickel skeleton support. The as-prepared GF exhibited a superior battery performance to that of close compacted graphene paper in bioelectric batteries, due to it affording easier access of ion and oxygen through the electrode. Furthermore, GF could serve as a biocompatible and conductive substrate for the electropolymerization of PPy instead of a metal substrate, which is expected to improve the biocompatibility of the whole electrode. The as-prepared PPy-coated GF exhibited an improved electrochemical performance compared to GF.

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