Method utilises sunlight to create carbon-free energy

Chemists at the University of Rochester, New York, claim to have increased the output and lowered the cost of current light-driven hydrogen-production systems.

The work, funded by the US Department of Energy, was led by chemistry professors Richard Eisenberg, Todd Krauss, and Patrick Holland, and included graduate students Zhiji Han and Fen Qiu. Their paper will be published later this month in the journal Science.

In a statement, the chemists said their work efficiently uses sunlight to provide carbon-free energy for vehicles or anything requiring electricity in order to operate.

One disadvantage of current methods of hydrogen production has been the lack of durability in the light-absorbing material, but the Rochester scientists were able to overcome that problem by incorporating nanocrystals.

‘Organic molecules are typically used to capture light in photocatalytic systems,’ said Krauss. ‘The problem is they only last hours, or, if you’re lucky, a day. These nanocrystals performed without any sign of deterioration for at least two weeks.’

Eisenberg, the Tracy H Harris professor of chemistry, has spent two decades working on solar energy systems. During that time, his systems have typically generated 10,000 instances (turnovers) of hydrogen atoms being formed without having to replace any components. With the nanocrystals, Eisenberg and his colleagues witnessed turnovers in excess of 600,000.

Dealing with photocatalytic disadvantages
The researchers managed to overcome other disadvantages of traditional photocatalytic systems.

‘People have typically used catalysts made from platinum and other expensive metals,’ Holland said. ‘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.’

Holland said their work is still in the basic research stage, making it difficult to provide cost comparisons with other energy production systems. But he said that nickel currently sells for approximately $8 (£5) per pound, while the cost of platinum is $24,000 per pound.

While all three researchers say the commercial implementation of their work is years off, Holland pointed out that an efficient, low-cost system would have uses beyond energy.

‘Any industry that requires large amounts of hydrogen would benefit, including pharmaceuticals and fertilisers,’ he said.

Nanocrystal method
The process developed by Holland, Eisenberg, and Krauss is similar to other photocatalytic systems; they needed a chromophore (the light-absorbing material), a catalyst to combine protons and electrons, and a solution, which in this case is water. 

Krauss provided cadmium selenide (CdSe) quantum dots (nanocrystals) as the chromophore, while Holland supplied a nickel catalyst (nickel nitrate). The nanocrystals were capped with DHLA (dihydrolipoic acid) to make them soluble, and ascorbic acid was added to the water as an electron donor.

Photons from a light source excite electrons in the nanocrystals and transfer them to the nickel catalyst. When two electrons are available, they combine on the catalyst with protons from water, to form a hydrogen molecule (H2).

This system is claimed to be so robust that it kept producing hydrogen until the source of electrons was removed after two weeks.

‘Presumably, it could continue even longer, but we ran out of patience,’ said Holland.