A molybdenum-oxo complex markedly outperforms any other molecular catalyst for electrochemical hydrogen production from neutral water, claim co-discoverers Hemamala Karunadasa, Christopher Chang and Jeffrey Long, researchers who hold joint appointments at the U.S. Department of Energy's Lawrence Berkeley National Laboratory, Berkeley, Calif., and the University of California, Berkeley.
"Our new proton reduction catalyst is based on a molybdenum-oxo metal complex that is about 70 times cheaper than platinum, today's most widely used metal catalyst for splitting the water molecule. In addition, our catalyst does not require organic additives, and can operate in neutral water, even if it is dirty, and can operate in sea water, the most abundant source of hydrogen on earth and a natural electrolyte," says Karunadasa. "I think the ability to produce pure hydrogen gas from dirty water sources is a distinct advantage of this catalyst, since most metals are deactivated by the impurities in water, which lead to an increase in overpotential over time," she adds.
The molybdenum-oxo complex is a high valence metal — (PY5Me2)Mo-oxo. The catalyst provides a turnover frequency of 2.4 moles of hydrogen per mole of catalyst per second and a turnover number of 6.1 × 105 moles of hydrogen per mole of catalyst. These values significantly exceed those reported for any other molecular catalyst for electrochemical hydrogen production from neutral water, contend the researchers. More details appear in a paper in Nature.
"We have not come across deactivation of the catalyst due to impurities in the water, though we have not conducted a thorough study of this yet. The catalyst is not stable at high pH. Hydroxide is a byproduct of generating hydrogen from water, so an extended electrolysis will require either a buffering system to keep the pH close to 7, or a flow of water so that hydroxide ions do not accumulate," says Karunadasa.
"The catalyst is already quite good in terms of rate of activity, long term performance and its ability to tolerate impurities in sea water. The only major disadvantage is the overpotential required to drive the catalysis. Reducing this will require both modification of the catalyst as well as finding optimal conditions for the electrocatalysis. The most ideal case, however, would be to drive the catalysis by visible light to give us a catalyst that can operate with sunlight and seawater," she explains.
"Our efforts are now directed towards reducing the overpotential required for catalysis and to attempt photo-catalysis where the catalytically active state can be generated by visible light. Both these approaches should lead to significant cost reduction," notes Karunadasa. "We are looking into changing electrodes and studying the pH dependence of the catalysis," she adds.
Industrial implementation likely would involve deposition of a thin layer of the catalyst on an electrode surface.
(For details on another development for producing hydrogen from water, see: "Aluminum Eases Hydrogen Generation"; some developments for producing oxygen from water are covered in: "Homogeneous Catalyst Speeds Water Oxidation" and "Light Splits Water.")