Biofuel cells that produce electricity from ordinary air spiked with small amounts of hydrogen could hold the key to the hydrogen economy.
Researchers at Oxford University in England have built a biofuel cell with hydrogenases — enzymes from natural bacteria that use or oxidize hydrogen. The cell consists of two electrodes coated with the enzymes, inside a container of ordinary air with 3% hydrogen added. (Hydrogen becomes an explosion hazard at the 4% level.)
Prototypes of the cell generated enough electricity to power a wristwatch. Fraser Armstrong, who heads the research, foresees advanced versions as power sources for various electronics that use little power. Biofuel cells, which convert chemical energy in fuels into electricity without combustion or platinum catalysts, have a number of advantages over conventional fuel cells, says Armstrong. Hydrogen fuel cells, for example, produce only water as the waste product. And platinum, the most commonly used catalyst in conventional (proton exchange membrane) fuel cells, is costly: platinum costs over $1,000/oz. Platinum catalysts are also easily poisoned or inactivated by carbon monoxide, an impurity often found in industrially produced hydrogen.
Hydrogenase enzymes can be produced at lower cost, without concern for carbon-monoxide poisoning. And since they are chemically selective and tolerant, they work in mixtures of hydrogen and oxygen, eliminating the need for expensive fuel-separation membranes. Hydrogenases also work at about the same rate as platinum-based catalysts.
The biofuel cell uses enzymes from Ralstonia metallidurans (R. metallidurans), an ancient bacterium that evolved 2.5 billion years ago. It survived by metabolizing hydrogen when there was no oxygen in the atmosphere.
One focus of the research is to understand how the active site of the bacteria's hydrogenase developed the ability to cope with oxygen as Earth's atmosphere changed. That could help scientists make biofuel cells more tolerant of oxygen. In the current cell, the enzyme is not securely attached to the electrode and the cell only runs for about two days. Researchers are also investigating enzymes from other organisms.