Secrets of artificial leaf revealed

Richard Van Noorden

Sun CatalytixIt isn't green and it doesn't grow, but the wafer sitting in a beaker of water in Dan Nocera's laboratory is remarkably like a leaf.

Using a silicon solar cell coated with cheap and abundant catalysts, the device uses sunlight to rip apart molecules of water, just like a photosynthesizing leaf. This produces hydrogen and oxygen gases, which bubble up on either side of the wafer (see video). The details are published today in Science1.

As with photosynthesis, the wafer ultimately stores sunlight energy as chemical bonds in a fuel: hydrogen gas, which can be piped and stored, and its energy released when required.

The parallel with nature is not exact: a real leaf does start by ripping up water, but does not end by breathing out hydrogen. Instead, it diverts the hydrogen into reactions with carbon dioxide, eventually creating sugar molecules. Nevertheless, Nocera, who works at the Massachusetts Institute of Technology in Cambridge, says that his team's concept will aid in efforts to produce clean, cheap hydrogen from sunlight and water — perhaps even allowing houses in poor but sunny countries to produce their own fuel on demand. He has founded a company, Sun Catalytix, also in Cambridge, to commercialize his electrolytic device; its influential backers include the multinational conglomerate Tata Group.

James Durrant, a chemist who researches solar fuels at Imperial College London, describes the device as "elegant". However, "it hasn't suddenly solved the problem of turning solar energy into fuels," he adds. "But the idea of the artificial leaf gives people an inspirational vision; it is undeniably cool."

Others are unconvinced of the device's broader utility. If hydrogen is wanted at all, there may be better ways to make it from solar energy. One simple alternative would be to use the electricity generated by more expensive, but more efficient, solar cells to split water.

Look, no wires!
Splitting water with sunlight to make hydrogen is not a new trick. Chemists have long used solar cells to generate electricity and sent it through wires to catalyst-covered electrodes that split water, producing hydrogen. But such devices used either expensive catalysts (such as ruthenium or platinum) or harsh acidic or basic conditions from which the solar cell had to be separated, or protected with expensive glass2,3.

Sun CatalytixNocera and his co-worker's device is the first to couple a solar cell and catalysts into one device, with no wires in between, and the first to work in conditions as mild as tap water. It even works with seawater, Nocera says; he has been using samples from the nearby Charles River.

The system's wireless nature is more than a marketing gimmick. Sun Catalytix hopes that it will allow the team to produce tiny particles of catalyst-coated solar cell, to be scattered as a slurry through flowing water. According to an analysis commissioned by the US Department of Energy, this geometry could, in theory, produce solar hydrogen more cheaply than an array of photovoltaic panels hooked up to catalyst-coated electrodes, because the glass-backed panels are so expensive.

The whole system relies on the cheap catalyst that rips electrons from water to produce oxygen, protons and electrons, which Nocera published details of in 20084 (see 'Catalyst heralded as solar-power breakthrough'). This catalyst covers one side of the silicon solar cell, with a layer of indium tin oxide or fluorine tin oxide in between to protect the cell from the oxygen generated.

Crucially, the catalyst is self-healing, spontaneously reforming its active core of cobalt-oxide clusters whenever it is degraded in the reaction. Its structure is remarkably similar to that of the manganese-oxide clusters that form the oxygen-evolving complex, the active centre of the giant photosystem II protein that real leaves use to split water.

In the research published today, Nocera has added a triple-alloy catalyst — made of nickel, molybdenum and zinc — to the other side of the silicon wafer. Protons released when water is split make their way to this face, where the nickel helps to reunite them with electrons to form hydrogen.

Are catalysts enough?
Nobody disputes the beauty of the chemistry. But whether the system is actually useful will come down to how expensive the hydrogen is to make, and how efficiently the system can use the available energy from sunlight.

A plant leaf converts, on average, just 1% of the energy it gets from sunlight into chemical bonds. Nocera reports 2.5% efficiency for his device; by including wires that rises to 4.7%. The bulk of the losses come from the semiconducting solar cell, not the electrolysing catalysts.

But John Turner, an expert on hydrogen production at the National Renewable Energy Laboratory in Golden, Colorado, says that much higher efficiencies — somewhere in the teens – are needed to make best use of the limited sunlight that falls on a roof. "Nocera's catalysts certainly open new areas," he says, "but, ultimately, without good semiconducting materials they will have no impact in terms of photoelectrochemical hydrogen production." The hitch, Turner points out, is that better solar cells are also more costly. For one application that Nocera suggests, however — lightweight, portable production of hydrogen for the military — high costs might not be a problem.

Nocera says that laboratory tests with higher-quality crystalline silicon are improving the system's efficiency. Increasing the conductance of the surrounding solution or punching small holes in the semiconductor, through which protons can flow, may also help.

Some researchers feel that solar power will ultimately be best used to produce electricity during the day and store energy in batteries at night; hydrogen, they maintain, will always be too expensive to make and hard to store. But Nocera maintains that these naysayers do not appreciate the new vistas opened up by his wireless system, such as the proposal to disperse particles as a slurry through large bodies of water.

And the sheer appeal of the artificial leaf is hard to beat. As James Barber, who researches photosynthesis at Imperial College London, says: "Imagine having a catalyst that you can drop into a glass of water on your windowsill, generating a fuel. Can you imagine such a world?"

References

1.Reece, S. R. et al. Science http://dx.doi.org/10.1126/science.1209816 (2011).
2.Rocheleau, R. E. , Miller, E. L. & Misra, A. Energy Fuels 12, 3-10 (1998).
3.Khaselev, O. & Turner, J. A. Science 280, 425-427 (1998).
4.Kanan, M. W. & Nocera D. G. Science 321, 1072-1075 (2008).

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