Nanocrystals Improve Hydrogen Production

Adding nanocrystals can make the light-driven production of hydrogen more robust and cost-effective, a team at the University of Rochester discovered.

The work advances what is sometimes considered the “Holy Grail” of energy science — efficiently using sunlight to provide clean, carbon-free energy for vehicles and anything that requires electricity.

Because it gives off no greenhouse emissions and can easily be converted into electrical energy, hydrogen is seen as an attractive fuel source for the future. But compounds essential to process are very short-lived, and the catalysts used are often rare Earth metals.

Cadmium selenide nanocrystals absorb light and transfer electrons to a nickel catalyst (blue), which subsequently generates hydrogen (white). Courtesy of Ted Pawlicki/University of Rochester.

“People have typically used catalysts made from platinum and other expensive metals,” said chemistry professor Patrick Holland. “It would be much more sustainable if we used metals that were more easily found on the Earth, more affordable and lower in toxicity. That would include metals such as nickel.”

“Organic molecules are typically used to capture light in photocatalytic systems,” said chemistry professor Todd Krauss, who has been working in the field of nanocrystals for more than 20 years. “The problem is they only last hours or, if you’re lucky, a day.”

The Rochester team replaced platinum catalysts (which typically cost $24,000/lb) with nickel-based ones ($8/lb) and substituted fragile organic molecules with more durable cadmium selenide quantum dots. The result was that “these nanocrystals performed without any sign of deterioration for at least two weeks,” Krauss said.

The nanocrystals — capped with dihydrolipoic acid to make them soluble — and the nickel catalyst (nickel nitrate) were placed in an ascorbic acid and water solution and exposed to a light source. Photons excited electrons in the nanocrystals and transferred them to the nickel catalyst. When two electrons are available, they combine on the catalyst with protons from the water, to form a hydrogen molecule.

The resulting system was so robust that it kept producing hydrogen until the source of electrons was removed, the researchers said.

“Presumably, it could continue even longer, but we ran out of patience!” Holland said.

The researchers saw turnovers — instances of hydrogen atoms being formed — in excess of 600,000 compared with the usual 10,000 instances.

They say the commercial implementation of their work, which is still in the basic research stages, is years off, but they point out that an efficient, low-cost system could have uses beyond energy.

“Any industry that requires large amounts of hydrogen would benefit, including pharmaceuticals and fertilizers,” Holland said.

For now, the team plans to look at the nature of the nanocrystal.

“Some nanocrystals are like M&Ms — they have a core with a shell around it,” said Richard Eisenberg, the Tracy H. Harris professor of chemistry. “Ours is just like the core. So we need to consider if they would work better if they were enclosed in shells.”

The work, funded by the US Department of Energy, will appear later this month in Science.

For more information, visit: www.rochester.edu