Enzymatic Biofuel Cells and Biosupercapacitors

Abstract

Due to the global energy crisis and climate change, overexploitation of fossil fuels, and emission of greenhouse gases, developing sustainable and environmentally friendly energy conversion technologies are becoming a feasible and efficient solution. Biological power sources (BPSs) that can produce electrical power from renewable fuels by catalysis or conversion from solar energy using biodegradable biocatalysts, have therefore captured researchers’ interest. The aim of this Ph.D. project is to fabricate BPSs by employing enzymes or proteins as catalysts to convert chemical energy directly to electrical energy or to store electrical charge. In this project, we have synthesized graphene paper (GP) electrodes by assembling graphene oxide (GO) nanosheets into paper-like architecture, followed by reduction to form layered and cross-linked networks of reduced GO with good mechanical strength, high conductivity and little dependence on the degree of mechanical bending. Subsequently, the GP electrodes served as both a current collector and an enzyme loading substrate that can be used directly as bioanode and biocathode in two kinds of electrochemical systems: EBFCs and hybrid bioelectrochemical systems. The first application of the as-prepared GP electrodes was to assemble glucose/dioxygen EBFCs. Pyrroloquinoline quinone dependent glucose dehydrogenase (PQQ-GDH) and bilirubin oxidase (BOx) adsorbed on the GP electrodes both retain their biocatalytic activities. Electron transfer (ET) at the bioanode required Meldola blue (MB) as an ET mediator to shuttle electrons between PQQ-GDH and electrode, but direct electron transfer (DET) at the biocathode was achieved. The resulting EBFC displayed notable mechanical flexibility, with a wide open circuit voltage range up to 0.665 V and maximum power density of approximately 4 μW cm-2. These are both fully competitive with reported values for related EBFCs, and with the mechanical flexibility and facile enzyme immobilization as novel merits. The second application of the as-prepared GP electrodes was in a membrane-free hybrid bioelectrochemical system that integrated an energy converting part, viz. a glucose/oxygen EBFC, with a charge-storing component, in which the redox properties of the immobilized redox protein cytochrome c (cyt c) were utilized. BOx and PQQ-GDH were employed as the biocatalysts for dioxygen reduction and glucose oxidation, respectively. A bi-protein PQQ-GDH/cyt c signal chain was created that facilitated electron transfer between the enzyme and the electrode surface. The assembled supercapacitor/biofuel cell hybrid biodevice displayed 15 times higher the power density tested in the pulse mode compared to the performance achieved from the continuously operating regime (4.5 and 0.3 μW cm-2, respectively) with an 80% residual activity after 50 charge/discharge pulses. This can be considered as a notable step forward in the field of glucose/oxygen membranefree, biocompatible hybrid power sources.