The goal of the hydrogen economy involves using hydrogen fuel cells to power vehicles, buildings and portable devices. But that can be an expensive business, as both fuel cells and electrolyzers—€”devices that run a current through water to produce hydrogen—€”currently depend on the use of expensive, noble-metal catalysts such as platinum.
"Hydrogen and fuel-cell technologies address the need to both mitigate greenhouse gas emissions and develop energy alternatives to fossil fuels," says NREL scientist Michael Heben. "Hydrogen gas is scarce, however, and cheap, efficient hydrogen production technologies will be required for its wide-scale deployment."
One inexpensive approach is found in nature. Microbes have had billions of years to figure out efficient ways to catalyze hydrogen reactions. Their solutions involve enzymes called hydrogenases, which use more abundant metals such as iron and nickel to activate hydrogen. For years, scientists have searched for ways to employ hydrogenases in electrolyzers and fuel cells.
One challenge for scientists is their inability to electrically tap into the workings of the hydrogenase enzyme, but new research being conducted by the NREL team of Heben, Paul King, Drazenka Svedruzic-Chang, Tim McDonald and Jeffrey Blackburn may point the way to solving that problem. The researchers found that under certain conditions, carbon nanotubes will spontaneously combine with hydrogenases to create an electrical connection.
In these experiments, the NREL team has used photoluminescence and Raman spectroscopy to look at what happens when hydrogenase from the anaerobic bacterium Clostridium acetobutylicum interacts with single-walled carbon nanotubes. Carbon nanotubes normally absorb and re-emit light at wavelengths that can be measured using photoluminescence spectroscopy. After hydrogenase was added, the photoluminescence disappeared.
"This suggests that the enzyme is feeding electrons into the nanotubes as it catalyzes the oxidation of hydrogen," says King.
The resulting biohybrid has the catalytic properties of hydrogenase and the excellent electrical conductivity of carbon nanotubes. The team found that they could control the catalytic reaction by changing the pH balance and hydrogen partial pressure of the solutions. When they added oxygen, which inactivates hydrogenase, the nanotubes lit up again. In the absence of oxygen, the hydrogenase-nanotube connections continued to work for up to a week.
These initial hydrogenase-carbon nanotube results, in combination with advances made by NREL researchers in carbon nanotube separation and film deposition techniques, have led to the development of hydrogenase-carbon nanotube electrodes. In this configuration, the carbon nanotubes and hydrogenases are immobilized together on an electrode surface. The result is a significant enhancement in the electrocatalytic activity of the immobilized hydrogenase and fabrication of an electrode that can be directly incorporated into a photoelectrochemical device for solar hydrogen production.
"Our team's research demonstrates that combining hydrogenase with carbon nanotubes may offer an inexpensive alternative to noble-metal catalysts," says King. "The results suggest the possible construction of functional biohybrids of hydrogenase and single-walled carbon nanotubes for applications in a variety of hydrogen-production and fuel cell technologies. Such biohybrids could replace expensive precious-metal catalysts in electrolyzers and fuel cells."