An air-breathing enzymatic cathode with extended lifetime by continuous laccase supply

We present a novel concept of an air-breathing enzymatic biofuel cell cathode combined with continuous supply of unpurified laccase-containing supernatant of the white-rot fungus Trametes versicolor for extended lifetime. The air-breathing cathode design obviates the need for energy-intensive active aeration. In a corresponding long-term experiment at a constant current density of 50 µA cm^-2, we demonstrated an increased lifetime of 33 days (cathode potential above 0.430 V vs. SCE), independent of enzyme degradation. The obtained data suggest that theoretically a longer lifetime is feasible. However, further engineering efforts are required to prevent clogging and fouling of the supply tubes. These results represent an important step towards the realization of enzymatic biofuel cell cathodes with extended lifetime and enhanced performance.

Using planktonic microorganisms to supply the unpurified multi-copper oxidases laccase and copper efflux oxidases at a biofuel cell cathode

The feasibility to apply crude culture supernatants that contain the multicopper oxidases laccase or copper efflux oxidase (CueO) as oxygen reducing catalysts in a biofuel cell cathode is shown. As enzyme-secreting recombinant planktonic microorganisms, the yeast Yarrowia lipolytica and the bacterium Escherichia coli were investigated. The cultivation and operation conditions (choice of medium, pH) had distinct effects on the electro-catalytic performance. The highest current density of 119 ± 23 μA cm(-2) at 0.400 V vs. NHE was obtained with the crude culture supernatant of E. coli cells overexpressing CueO and tested at pH 5.0. In comparison, at pH 7.4 the electrode potential at 100 μA cm(-2) is 0.25 V lower. Laccase-containing supernatants of Y. lipolytica yielded a maximum current density of 6.7 ± 0.4 μAcm(-2) at 0.644 V vs. NHE. These results open future possibilities to circumvent elaborate enzyme purification procedures and realize cost effective and easy-to-operate enzymatic biofuel cells.

Overcoming bottlenecks of enzymatic biofuel cell cathodes: crude fungal culture supernatant can help to extend lifetime and reduce cost

Enzymatic biofuel cells (BFCs) show great potential for the direct conversion of biochemically stored energy from renewable biomass resources into electricity. However, enzyme purification is time-consuming and expensive. Furthermore, the long-term use of enzymatic BFCs is hindered by enzyme degradation, which limits their lifetime to only a few weeks. We show, for the first time, that crude culture supernatant from enzyme-secreting microorganisms (Trametes versicolor) can be used without further treatment to supply the enzyme laccase to the cathode of a mediatorless BFC. Polarization curves show that there is no significant difference in the cathode performance when using crude supernatant that contains laccase compared to purified laccase in culture medium or buffer solution. Furthermore, we demonstrate that the oxygen reduction activity of this enzymatic cathode can be sustained over a period of at least 120 days by periodic resupply of crude culture supernatant. This is more than five times longer than control cathodes without the resupply of culture supernatant. During the operation period of 120 days, no progressive loss of potential is observed, which suggests that significantly longer lifetimes than shown in this work may be possible. Our results demonstrate the possibility to establish simple, cost efficient, and mediatorless enzymatic BFC cathodes that do not require expensive enzyme purification procedures. Furthermore, they show the feasibility of an enzymatic BFC with an extended lifetime, in which self-replicating microorganisms provide the electrode with catalytically active enzymes in a continuous or periodic manner.