Prolongation of electrode lifetime in biofuel cells by periodic enzyme renewal.

Enzymatically catalyzed biofuel cells show unique specificity and promise high power densities, but suffer from a limited lifetime due to enzyme deactivation. In the present work, we demonstrate a novel concept to extend the lifetime of a laccase-catalyzed oxygen reduction cathode in which we decouple the electrode lifetime from the limited enzyme lifetime by a regular resupply of fresh enzymes. Thereto, the adsorption behavior of laccase from Trametes versicolor to buckypaper electrode material, as well as its time-dependent deactivation characteristics, has been investigated. Laccase shows a Langmuir-type adsorption to the carbon nanotube-based buckypaper electrodes, with a mean residence time of 2 days per molecule. In a citrate buffer of pH 5, laccase does not show any deactivation at room temperature for 2 days and exhibits a half-life of 9 days. In a long-term experiment, the laccase electrodes were operated at a constant galvanostatic load. The laccase-containing catholyte was periodically exchanged against a freshly prepared one every second day to provide sufficient active enzymes in the catholyte for the replacement of desorbed inactive enzymes. Compared to a corresponding control experiment without catholyte exchange, this procedure resulted in a 2.5 times longer cathode lifetime of 19 ± 9 days in which the electrode showed a potential above 0.744 V vs. normal hydrogen electrode at 110 µA cm(-2). This clearly indicates the successful exchange of molecules by desorption and re-adsorption and is a first step toward the realization of a self-regenerating enzymatic biofuel cell in which enzyme-producing microorganisms are integrated into the electrode to continuously resupply fresh enzymes.

A highly efficient buckypaper-based electrode material for mediatorless laccase-catalyzed dioxygen reduction.

The redox enzyme laccase from Trametes versicolor efficiently catalyzes the oxygen reduction reaction (ORR) in mediatorless biofuel cell cathodes when adsorbed onto multi-walled carbon nanotubes (MWCNTs). In this work we demonstrate that the fabrication of MWCNTs in form of buckypaper (BP) results in an excellent electrode material for laccase-catalyzed cathodes. BPs are mechanically stable, self-entangling mats with high dispersion of MWCNTs resulting in easy to handle homogeneous layers with highly mesoporous structures and excellent electrical conductivities. All biocathodes have been electrochemically investigated in oxygen-saturated buffer at pH 5 by galvanostatic polarization and potentiodynamic linear sweep voltammetry. Both methods confirm an efficient direct interaction of laccase with BP with a high open circuit potential of 0.882 V vs. normal hydrogen electrode (NHE). The high oxygen reduction performance leads to high current densities of 422±71 µA cm(-2) at a typical cathode potential of 0.744 V vs. NHE. When the current density is normalized to the mass of the electrode material (mass activity), the BP-based film electrodes exhibit a 68-fold higher current density at 0.744 V vs. NHE than electrodes fabricated from the same MWCNTs in a non-dispersed agglomerated form as packed electrodes. This clearly shows that MWCNTs can act more efficiently as cathode when prepared in form of BP. This can be attributed to reduced diffusional mass transfer limitations and enhanced electrical conductivity. BP is thus a very promising material for the construction of mediatorless laccase cathodes for ORR in biofuel cells. In addition we demonstrated that these electrodes exhibit a high tolerance towards glucose, the most common bioanode fuel.

A versatile miniature bioreactor and its application to bioelectrochemistry studies.

Often, reproducible investigations on bio-microsystems essentially require a flexible but well-defined experimental setup, which in its features corresponds to a bioreactor. We therefore developed a miniature bioreactor with a volume in the range of a few millilitre that is assembled by alternate stacking of individual polycarbonate elements and silicone gaskets. All the necessary supply pipes are incorporated as bore holes or cavities within the individual elements. Their combination allows for a bioreactor assembly that is easily adaptable in size and functionality to experimental demands. It allows for controlling oxygen transfer as well as the monitoring of dissolved oxygen concentration and pH-value. The system provides access for media exchange or sterile sampling. A mass transfer coefficient for oxygen (k(L)a) of 4.3x10(-3) s(-1) at a flow rate of only 15 ml min(-1) and a mixing time of 1.5s at a flow rate of 11 ml min(-1) were observed for the modular bioreactor. Single reactor chambers can be interconnected via ion-conductive membranes to form a two-chamber test setup for investigations on electrochemical systems such as fuel cells or sensors. The versatile applicability of this modular and flexible bioreactor was demonstrated by recording a growth curve of Escherichia coli (including monitoring of pH and oxygen) saturation, and also as by two bioelectrochemical experiments. In the first electrochemical experiment the use of the bioreactor enabled a direct comparison of electrode materials for a laccase-catalyzed oxygen reduction electrode. In a second experiment, the bioreactor was utilized to characterize the influence of outer membrane cytochromes on the performance of Shewanella oneidensis in a microbial fuel cell.