(501f) Development of Biocathode Using Laccase and Redox-Polymer-Grafted Carbon

Enzymatic electrodes have been used for biosensors. Technologies of biosensors are the basis of biofuel cells which use enzymes as catalysts to convert chemical energy to electrical energy. Enzymes are able to oxidize safety fuels, such as glucose and ethanol. However, the biofuel cells have serious issues to be solved. One of the issues is low power density caused by low current density. In the biofuel cell electrode, one of the important factors is the electron transfer between electrodes and enzymes. For effective electron transfer, mediators are generally used to shuttle electrons between electrodes and enzymes. When the electrodes are constructed only from the redox-polymer in which mediators are immobilized, the current density is restricted, because a redox-polymer has low electric conductivity and the conventional redox-polymer layer is thick. To solve this problem, our group has developed a new biofuel cell electrode which consists of redox-polymer-grafted carbon black for glucose-oxidizing anode. This electrode divides conduction of electrons into the redox-polymer and carbon, and thus, the electron conduction distance in the redox-polymer was decreased. Additionally carbon black has large surface area. Effectiveness of the electrode was verified experimentally and by simulation. The model calculation shows the possibility of the further increase in current density by two orders of magnitude. The main reason of a low current density obtained in the experiment was caused by deactivation of enzymes during adsorption on carbon black. On the other hand, oxygen-reducing cathode still has the issue about rate-controlling electron transfer in the redox-polymer.

In this research, a novel biofuel cell cathode was developed using a hyperthermophilic laccase and redox-polymer-grafted carbon with the mediator for a cathode. The hyperthermophilic enzyme with the optimal temperature above 90^oC has a potential to reduce deactivation of the enzyme during adsorption on carbon black because the enzyme has rigid structure. The redox-polymer immobilized 2,2’-azinobis(3-ethylbenzotiazoline-6-sulfonate)(ABTS) as the mediator to polyacrylic acid grafted on carbon black. Enzyme activity on carbon black showed that laccase from Trametes versicolor with the optimal temperature around 60^oC has relative activity of 22% in comparison with the laccase in solution, and the hyperthermophilic laccase has relative activity of 94%. This result suggests that the hyperthermophilic laccase keeps activity during adsorption on carbon black. Immobilization of ABTS to polyacrylic acid was confirmed by NMR and IR using a linear-polymer without grafting. TG-MS of the redox-polymer-grafted carbon black showed SO and SO₂ signals from ABTS, while these peaks were not observed before immobilization of ABTS. Cyclic voltammetry of the redox-polymer-grafted carbon black showed redox peaks. Additionally, the redox potential calculated from the peaks shifted from dissolved ABTS, and the shift corresponds to the immobilization of ABTS. These results suggest that synthesis of the redox-polymer-grafted carbon black for biocathode was succeeded for the first time. The electrode using the hyperthermophilic laccase and the redox-polymer-grafted carbon with ABTS is expected to achieve improved performance on biocathode.