Enzymatic biofuel cells

Enzymatic biofuel cells are a specific type of fuel cell that exploit electrically-wired enzymes as electrocatalysts to convert chemical energy from natural and abundant fuels and oxidants into electrical energy. Oxidoreductase enzymes contain redox centres made of abundant metals or organic cofactors that endow attractive catalytic properties as alternatives to metal and molecular catalysts at electrodes. Owing to their wide diversity and narrow specificity, enzymes can efficiently and cleanly oxidize many types of fuels (H2, glucose, methanol, lactate, etc.) and can also be combined at electrodes for cascade reactions for deep fuel oxidation. The development of enzyme wiring techniques for protein electrodes is necessary to establish efficient mediated or direct electron transfer reactions between the enzyme(s) and the electrode. The open circuit potential at the bioelectrodes is just as important as the current density to obtain suitable cell voltages between the anode and the cathode for device powering. The performances are improved by using highly porous carbon materials as electron collectors such as carbon nanotubes, porous templated carbons, or carbon blacks. These materials allow high loading of biocatalysts, provide supportable conductivity and mass transport behaviour, and can offer a protecting environment for enhanced operational stabilities. Furthermore, highly capacitive bioelectrodes can also be charged and discharged to provide higher power on short time scales.

Glucose/O2 biofuel cells have attracted considerable interest as implantable power sources that harvest energy autonomously from “sugar” and oxygen continuously present in the body. However, such implanted technologies still face important challenges ranging from the limited power output and stability in complex media to the limited biocompatibility, the cost of enzymes, and the need for enzyme-compatible sterilisation protocols.
Nonetheless, state of the art enzymatic biofuel cell setups are capable of producing enough energy to power disposable medical devices, electronic point-of-care tests, and wearable electrochemical sensors. This technology offers therefore an eco-friendly and sustainable alternative for toxic and unrecyclable button batteries, currently used in such electronic devices.

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