Enzymatic biofuel cell
Enzymatic biofuel cells, while currently confined to research facilities, are widely prized for the promise they hold in terms of their relatively inexpensive components and fuels, as well as a potential power source for bionic implants.
This offers a couple of advantages for enzymatic biofuel cells: Enzymes are relatively easy to mass-produce and so benefit from economies of scale, whereas precious metals must be mined and so have an inelastic supply.
Enzymes are also specifically designed to process organic compounds such as sugars and alcohols, which are extremely common in nature.
In that year, it was announced that researchers had managed to completely oxidize methanol using a series (or “cascade”) of enzymes in a biofuel cell.
[3] While methanol is now far less relevant in this field as a fuel, the demonstrated method of using a series of enzymes to completely oxidize the cell's fuel gave researchers a way forward, and much work is now devoted to using similar methods to achieve complete oxidation of more complicated compounds, such as glucose.
[2][3] In addition, and perhaps what is more important, 1998 was the year in which enzyme “immobilization” was successfully demonstrated, which increased the usable life of the methanol fuel cell from just eight hours to over a week.
This process had been understood since the 1980s but depended heavily on placing the enzyme as close to the electrode as possible, which meant that it was unusable until after immobilization techniques were devised.
[2] In addition, developers of enzymatic biofuel cells have applied some of the advances in nanotechnology to their designs, including the use of carbon nanotubes to immobilize enzymes directly.
One research team took advantage of the extreme selectivity of the enzymes to completely remove the barrier between anode and cathode, which is an absolute requirement in fuel cells not of the enzymatic type.
[2] While enzymatic biofuel cells are not currently in use outside of the laboratory, as the technology has advanced over the past decade non-academic organizations have shown an increasing amount of interest in practical applications for the devices.
In hydrogenase-based biofuel cells, hydrogenases are present at the anode for H2 oxidation in which molecular hydrogen is split into electrons and protons.
[12] In recent years, research on hydrogenases has grown significantly due to scientific and technological interest in hydrogen.
The bidirectional or reversible reaction catalyzed by hydrogenase is a solution to the challenge in the development of technologies for the capture and storage of renewable energy as fuel with use on demand.
This can be demonstrated through the chemical storage of electricity obtained from a renewable source (e.g. solar, wind, hydrothermal) as H2 during periods of low energy demands.
[12] In addition to the advantages previously mentioned associated with incorporating enzymes in fuel cells, hydrogenase is a very efficient catalyst for H2 consumption forming electrons and protons.
The electrodes are preferably made from carbon which is abundant, renewable and can be modified in many ways or adsorb enzymes with high affinity.
[12] To avoid inactivation by O2, a proton exchange membrane can be used to separate the anode and cathode compartments such that O2 is unable to diffuse to and destructively modify the active site of hydrogenase.
[11] Carbon nanotubes can also be used for a support for hydrogenase on the electrode due to their ability to assemble in large porous and conductive networks.
Hydrogenase isolated from D. gigas (jumbo squid) was coupled to multiwalled carbon nanotube (MWCNT) networks and produced a current ~30 times higher than the graphite-hydrogenase anode.
The electrodes were placed in a single chamber with a mixture of 3% H2 gas in air and there was no membrane due to the tolerance of the hydrogenase to oxygen.
