Enzymatic biofuel cells utilize the unique activity and selectivity of enzymes to convert chemical energy directly to electrical energy, and have the potential to be miniaturized for small-scale power devices. This research program seeks to study the characteristics, limitations, and potential for improvement of a mediated laccase-catalyzed electrode for the reduction of oxygen, focusing on the role of mediator redox potential in the catalytic mechanism. The bio-cathode system studied utilizes purified laccase from Trametes versicolor mediated by osmium (Os) centered redox polymers. The results of these studies can enable design of mediated electrodes for biofuel cell applications.Exposure of fuel cell cathodes to fuels like methanol can reduce fuel efficiency and cell voltage via competitive reactions, and can foul the cathode catalyst. Introduction of selective electrocatalysts such as enzymes can solve these problems, and introduces the possibility of a mixed-feed fuel cell system with reduced complexity. The effect of redox potential of a redox hydrogel mediator on the performance of the mediated bio-cathode under varying alcohol concentrations is described. This study demonstrates that the selectivity of mediated laccase oxygen cathodes can facilitate high methanol feed concentration as compared to conventional direct methanol fuel cells and under certain optimum operating conditions, the enzyme might serve as a better cathode catalyst in presence of contaminants like methanol than the conventional Pt/Ru catalysts. A non-competitive inhibition model is proposed to describe the influence of methanol on laccase-catalyzed oxygen reduction kinetics. Methanol replaces water in the enzyme and thereby affects the electron transfer environment near the enzyme active site.In collaboration with Northeastern University, we employ X-ray absorption techniques to characterize the oxygen reduction mechanism of an immobilized laccase while the electrode is operated in situ. The overall goal of this project is to map the oxygen reduction reaction mechanism by a mediated laccase electrode as a function of mediator redox potential, applied electrode potential and presence/absence of substrate (O2). We have successfully detected active Cu sites (in micro-molar concentration) and identified key relationships between oxidation state and mediator redox potential in the presence and absence of oxygen. Our collaborators at Northeastern University have applied the powerful Δμ technique to determine the exact configuration of oxygen attachment to the Cu active sites and to identify the intermediates of the oxygen reduction reaction for a mediated biocathode. Electron-conducting redox hydrogels electrically connect the redox centers of enzymes to electrodes, enabling multi-layer activation and higher current density output. The physicochemical state of these redox polymers and their electron transport mechanism depends on the swelling behavior of these hydrogels in an ionic media. We have fabricated and characterized homogeneous sub-micron sized thin redox hydrogel films with great precision and great repeatability. We have estimated the transport and kinetic parameters for mediated enzymatic systems with precision to have a better understanding of the reaction mechanisms in these complex systems.
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