|Schematic representation of a bio fuel cell involving hydrogenase and laccase enzymes. Click to enlarge. Source: Armstrong Research Group|
Researchers at Oxford University have developed an enzyme-based fuel cell that produces electricity from ordinary air spiked with small amounts of hydrogen. The new type of cell would be an inexpensive alternative to the costly platinum-based fuel cells that are a primary focus today.
The “bio fuel cell” uses an anode coated with oxygen-tolerant hydrogenases—enzymes from a naturally occurring bacterium that oxidize hydrogen in its metabolism—coupled with a cathode modified with the fungal oxygen reductase, laccase. Laccase catalyses the reduction of oxygen to water.
The enzyme-coated electrodes are placed inside a container of ordinary air with 3% added hydrogen.
The research established for the first time that it is possible to generate electricity from such low levels of hydrogen in air, according to Fraser Armstrong, the research leader.
Prototype versions of the cell produced enough electricity to power a wristwatch and other electronic devices. Armstrong foresees advanced versions of the device as potential power sources for an array of other electronic products that only require low amounts of power.
The technology is immensely developable. We are at the tip of a large iceberg, with important consequences for the future, but there is still much to do before this generation of enzyme-based fuel cells becomes commercially viable. The idea of electricity from hydrogen in air, using an oxygen-tolerant hydrogenase is new, although other scientists have been investigating enzymes as electrocatalysts for years. Most hydrogenases have fragile active sites that are destroyed by even traces of oxygen, but oxygen tolerant hydrogenases have evolved to resist attack.—Fraser Armstrong
Platinum-based fuel cell technology is hampered by the cost of the metal—platinum is more costly than gold, with recent prices topping $1,000 per ounce. In addition, platinum catalysts are easily poisoned or inactivated by carbon monoxide that often exists as an impurity in industrially produced hydrogen. Carbon monoxide can be removed, but that further increases the cost of conventional fuel cells.
By contrast, naturally occurring hydrogenase enzymes can be produced at lower cost, with carbon-monoxide poisoning not being a problem. Since the hydrogenases are chemically selective and tolerant, they work in mixtures of hydrogen and oxygen, avoiding the need for expensive fuel-separation membranes required in other types of fuel cells. Hydrogenases are able to oxidize H2 at rates that are comparable to or higher than platinum-based catalysts, according to the researchers.
The biofuel cell uses enzymes from Ralstonia metallidurans, an ancient bacterium believed to have been one of the first forms of life on Earth. It evolved 2.5 billion years ago, when there was no oxygen in Earth’s atmosphere, and survived by metabolizing hydrogen.
One focus of Armstrong’s research is understanding how the active site of the R. metallidurans hydrogenase developed the ability to cope with oxygen as Earth’s atmosphere changed. That could enable scientists to adapt the chemistry in the active site into bio fuel cells that are more tolerant of oxygen. In the current version of the cell, the enzyme is not attached tightly to the electrode and the cell runs for only about two days. The researchers also are investigating the use of enzymes from other organisms.
The Armstrong Research Group at Oxford is working closely with Prof. Bärbel Friedrich, of the Humboldt University in Berlin, who is investigating the molecular biology of hydrogenases that can tolerate oxygen.
A paper on this research was presented at the 233rd national meeting of the American Chemical Society.
ACS 233 INOR 484: “Hydrogenases: Electrocatalysis, reactions and inspiration”
“Electricity from low-level H2 in still air—an ultimate test for an oxygen tolerant hydrogenase”; Kylie A. Vincent, James A. Cracknell, Jeremy R. Clark, Marcus Ludwig, Oliver Lenz, Bärbel Friedrich and Fraser A. Armstrong; Chem. Commun., 2006, 5033 - 5035, DOI: 10.1039/b614272a
“Hydrogen cycling by enzymes: electrocatalysis and implications for future energy technology”; Kylie A. Vincent, James A. Cracknell, Alison Parkin and Fraser A. Armstrong; Dalton Trans., 2005, 3397 - 3403, DOI: 10.1039/b508520a