Doctor of Philosophy
Department of Chemistry
Little, Shannon, Improving the performance metrics of enzymatic biofuel cells, Doctor of Philosophy thesis, Department of Chemistry, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3225
The shift towards renewable energy solutions in order to meet the demands of an exponentially growing populace has resulted in significant amounts of research into new, environmentally friendly, power generating technologies. Among these innovations, biofuel cells capable of utilising non-toxic, non-flammable and renewable fuel substrates have begun to gain momentum due to the portability and projected low cost of such devices. However, the commercialisation and mass adoption of biofuel cells has been hampered by their less than ideal performance characteristics when compared to inorganic based fuel cell technologies. For example, low power outputs due to potential losses within the cell arising from complex reaction pathways, low current densities resulting from low surface area electrodes, and enzyme immobilisation strategies that are difficult to scale up to practical sizes remain factors that need to be addressed. In addition, the stability of such devices is often an inhibiting factor to widespread adoption since the poor lifetime of the enzymatic species utilised within a biofuel cell often limits their practical usage to a time scale well below that of their inorganic counterparts.
This thesis aimed to address the above problems through several different strategies. These were:
1) Employing a high surface area electrode material comprised of an interwoven network of nano-sized carbon fibres (named herein as NanoWeb) in order to increase the current densities able to be produced by a biofuel cell. 2) Increasing the working potential of biofuel cells by directly connecting the redox centres embedded in an enzyme with electrodes via the use of nanostructured carbon materials, and
3) Enhancing the lifetime and stability of biofuel cells through the use of a biocompatible, enzyme stabilising ionic liquid, choline dihydrogen phosphate.
Electrochemical characterisation of the NanoWeb material was conducted using Fourier transform voltammetry and cyclic voltammetry, and the results compared to those obtained using other nanostructured carbon electrode materials. The former technique allowed the separation of capacitive, pseudo-capacitive and Faradaic responses, which was found to be essential since the NanoWeb material was found to have a high capacitance of nearly 1.5 mF.cm-2. The NanoWeb material was also found to have a large electroactive surface area of 2.16 cm-2 per square centimetre of geometric surface area, which is much larger than that of other nanostructured carbon materials such as carbon nanotubes and graphene.
Direct electrical contact of the enzymes bilirubin oxidase and glucose oxidase to an electrode comprised of the NanoWeb material was investigated using cyclic voltammetry. While glucose oxidase immobilised at the NanoWeb electrode displayed redox activity attributable to its embedded cofactor, and thus appeared to be intimately connected to the electrode, only bilirubin oxidase was able to produce a catalytic current upon the introduction of its substrate, oxygen, into the cell. The current produced in the latter case was significant, reaching values as high as 270 μA.cm-2.
Immobilisation of both bilirubin oxidase and glucose oxidase in a redox mediating hydrogel at a NanoWeb modified electrode allowed large catalytic currents to be produced from relatively small amounts of reactants. Increased mass transport of substrate to the electrode surface was observed for such electrodes, with subsequent rotation of the electrode increasing the current density only marginally. Despite thicker films of redox hydrogel requiring rotational rates of up to 1500 rpm to achieve maximum current density, the NanoWeb was found to be a more effective electrode platform than others reported in the literature. Cathodes comprised of bilirubin oxidase immobilised in redox hydrogel at the surface of NanoWeb modified electrodes produced currents of up to 35.49 μA.μg-1 of enzyme or 8.28 μA.μg-1 of hydrogel. Such efficiency allowed for currents as high as 1.86 mA to be produced from an electrode of only ~ 1cm2 (geometric area). Even larger current densities of 43.55 μA.μg-1 of glucose oxidase and 29.27 μA.μg-1 of hydrogel were produced at NanoWeb modified anodes when glucose was introduced into the cell. In this case, ~ 1cm2 (geometric area) electrodes were able to produce current densities exceeding 2.3 mA.
The catalytic oxidation of glucose by glucose oxidase was performed in mixed ChDHP/phosphate buffer media with ChDHP concentrations as high as 80% (w/w). This is the highest concentration of an ionic liquid in a mixed solvent system ever used to successfully perform glucose oxidase based catalysis. Although the catalytic currents produced in ChDHP based electrolytes were lower than that obtained in purely aqueous media, this was found to be a direct consequence of the ILs low pH and high viscosity rather than any denaturing effect caused by the ionic liquid. Indeed, when data was normalised with respect to both the activity of the enzyme at low pH, as well as diffusional impedances caused by the high viscosity media, it was found that the catalytic currents produced in ChDHP based electrolytes were in fact higher than that obtained in aqueous buffer.