Muck Tech: Natural enzyme displaces precious metal in fuel cell

Honed by billions of years of evolution, many microbial enzymes are champions at stripping electrons from hydrogen molecules and shunting the charged particles into biochemical reactions. Now, a team of scientists in England and Germany has tapped that molecular machinery to create a new type of fuel cell.

HYDROGEN HELPERS. Microbes such as these Ralstonia eutropha soil bacteria produce hydrogen-processing enzymes that might someday replace expensive metals, such as platinum, in fuel cells. F. Mayer/Univ. of Göttingen

Like most fuel cells, this one steals electrons from hydrogen molecules and bestows them on oxygen atoms and hydrogen ions to yield water and an electric current (SN: 6/11/05, p. 374: Micropower Heats Up: Propane fuel cell packs a lot of punch). Yet it makes that transfer in an atypical manner that could lead to a new class of fuel cells, says Fraser A. Armstrong of the University of Oxford in England, a coleader of the team that created a prototype cell.

He and his colleagues describe their patented prototype in an upcoming Proceedings of the National Academy of Sciences.

In oxygen-depleted mud, some microbes tap hydrogen in their vicinity for energy. Wielding enzymes called hydrogenases, the organisms split hydrogen molecules and commandeer their electrons.

That hydrogen-processing task, known as hydrogen oxidation, is usually carried out in fuel cells by platinum or other rare, expensive metals. But cells’ hydrogenases contain metals no more exotic than common iron and nickel, comments chemist Marcetta Y. Darensbourg of Texas A&M University in College Station.

Although hydrogenases might seem like promising components for fuel cells, they are typically plagued by a fatal flaw: Oxygen usually disables the molecules, rendering them useless for most fuel cells, which have to work in ordinary air.

Yet not every microbe with a hydrogenase lives completely cut off from the atmosphere. The soil bacterium Ralstonia eutropha is one of them, so its hydrogenase is more tolerant of oxygen than is that of other microbes. With colleagues at the Technical University of Berlin and Humboldt University, also in Berlin, Armstrong and his group investigated the unusual hydrogenase’s potential use in fuel cells.

The team genetically engineered R. eutropha to mass-produce the substance. After harvesting the molecules, the scientists coated a graphite electrode with them and used the electrode to build a fuel cell.

Despite being exposed to air, the apparatus produced electricity, albeit a modest amount, the researchers report. On the other hand, the power output was 25 times as great as it was when the researchers equipped the fuel cell with a different electrode coated with a bacterial hydrogenase known to have a more typical vulnerability to oxygen.

Pointing out other attractions of a hydrogenase-based fuel cell, Armstrong notes that the device dispenses with an expensive and often troublesome membrane usually needed in fuel cells and that it is unhindered by the carbon monoxide contamination that plagues most fuel cell designs.

Even so, he says that the R. eutropha enzyme isn’t likely to be the ultimate choice of fuel cell makers.

Darensbourg says that the new findings could provide design clues to researchers striving to develop fuel cells and hydrogen-generation devices based on cheap and abundant metals rather than on rare and precious ones.

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