Creating Fuel From Water

Two research groups report new materials aimed at achieving a long-standing research goal: the ability to use fuel cells to generate H₂, a clean-burning sustainable fuel, from water.

Traditional catalysts for H₂ generation are based on platinum, but the metal is expensive, globally limited in supply, and easily poisoned. Working to find a less expensive alternative, graduate student Hemamala I. Karunadasa and chemistry professors Christopher J. Chang and Jeffrey R. Long of the University of California, Berkeley, have developed a molybdenum oxo catalyst containing the ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (Nature 2010, 464, 1329). Unlike some other catalysts, which can require organic acids or solvents to function, the molybdenum oxo complex can produce H₂ from neutral water solutions and even from seawater.

The catalyst’s ligand can also be more readily and extensively modified than those of other catalysts. That feature “will permit the relationships between catalyst geometric and electronic structure and catalyst performance to be assessed effectively,” says Craig L. Hill, a chemistry professor at Emory University. “Such assessment is key to further improvement in the catalytic rates.”

Another group is aiming to avoid metals entirely in H₂-generating systems. A team led by Bjorn Winther-Jensen, a research fellow at the Australian Center for Electromaterials Science at Monash University, prepared an electrode from a porous membrane of polytetrafluoroethylene (Gore-Tex) coated with a layer of poly(3,4-ethylenedioxythiophene) (PEDOT) and polyethylene glycol (PEG) (Adv. Mater. 2010, 22, 1727).

The researchers found that the system could generate H₂ from an acidic electrolyte solution at rates comparable with those from systems using traditional Pt-based catalysts, but the polymer composites are both less expensive and more resistant to poisoning than Pt catalysts.

Mechanistically, Winther-Jensen and colleagues believe, the PEG coordinates H₃O⁺ and traps it near the PEDOT, where the reduction to H₂ occurs. The porous membrane serves to provide a large surface area for the various components to interact. The system can run for more than 80 days without deteriorating.

The PEDOT-PEG system is “the most active and, undoubtedly, the cheapest catalyst reported so far for H₂ evolution from water,” says G. Charles Dismukes, a chemistry professor at Rutgers University. “This opens a route to totally eliminate any metal as catalyst for producing H₂ from water.”