Published online 31 July 2008 | Nature | doi:10.1038/news.2008.996

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Catalyst heralded as solar-power breakthrough

Add simple metal salts to a beaker of water, apply a current, and watch the clean, green hydrogen bubble forth.

It sounds almost too good to be true — but chemists believe they have made a key breakthrough that would allow them to mimic photosynthesis and solve the world’s energy crisis.

Daniel Nocera and Matthew Kanan at the Massachusetts Institute of Technology in Cambridge have discovered a simple, inexpensive system that can help to split water to produce oxygen and hydrogen gas. The process could, they suggest, be powered by solar photovoltaic cells.

electrolysisA snapshot showing the new, efficient oxygen catalyst in action in Dan Nocera's laboratory at MIT.MIT/NSF

This would give a crucial boost to solar power’s potential to replace fossil fuels, because it adds the ability to both trap and then store the Sun’s energy in a readily accessible form.

Storing solar-derived electricity still largely relies on bulky, inefficient batteries. Advocates of the 'hydrogen economy' propose that the gas is an ideal chemical storehouse for solar power, because it can be piped over long distances and potentially stored in compressed form, or adsorbed onto porous, portable materials.

This is similar to nature’s method of accumulating solar power – make chemical bonds to store the energy, and then break them to release that energy whenever it is needed. Plants ultimately use sugar molecules as their chemical-energy storehouses, but they start the process by rearranging water to get oxygen, protons and electrons. They do this using a huge protein molecule called photosystem II, which has at its heart the oxygen-evolving complex (OEC) — a cluster of manganese, oxygen and calcium atoms.

Rip it up …

Copying this intricate molecular set-up has proved extremely difficult - the key problem being the lack of a suitable catalyst to generate oxygen. It's possible to tear water molecules apart with electricity in an electrolysis cell, but a sizeable voltage is needed – more than can typically be provided by a solar cell - and the process is not very efficient. Chemists can add catalysts to help the process work with a smaller voltage, but these involve rare metals such as ruthenium or platinum, and often require harsh acidic or basic conditions.

Nocera and Kanan now have a much simpler solution. They applied a current to an electrode sitting in a beaker of water that contained doubly-charged cobalt ions (Co2+) and phosphate ions (PO43-) in solution. The Co2+ ions are very soluble, but they are oxidized by the electrode into insoluble Co3+ ions, which stick to the electrode, and likely lose another electron to become Co4+.

These form the basis for an as-yet uncharacterized catalyst that helps to rip up water molecules into oxygen – seen pouring out as bubbles – along with protons and electrons. The protons are mopped up by phosphate ions and carried to the electrode on the other side of the electrolysis cell, where they combine with the electrons to create hydrogen gas.

“This study shows that we can make [an oxygen-forming] catalyst cheaply that operates under easy conditions,” says Nocera, “That was the big bottleneck to reproducing photosynthesis,” he adds.

… and start again

Nature's OEC has to rebuild itself every 30 minutes, and Nocera’s system also has an inbuilt repair mechanism. When the electrode is turned off, the catalyst starts to redissolve, as Co4+ and Co+3 turn into soluble Co2+. But turn the current on again, and the catalyst begins to re-form, triggering bubbles of oxygen. This process seems to cycle without any losses, says Nocera. The work is published in Science1.

At the moment, a platinum electrode is used to combine protons and electrons into hydrogen gas, and an input of electricity drives the process. But that doesn't worry Nocera. "This catalyst couldn’t care less where the current comes from”, he says. It may be possible to embed a photovoltaic cell in a membrane, coated on one side with the cobalt-based catalyst and on the other with an equally cheap hydrogen-producing catalyst. Nocera is already involved in a project, called Powering the Planet, which aims to do just that.

James Barber, an expert on the molecular basis of photosynthesis from Imperial College London, says the work is a real breakthrough. “I see it as a big step forward in developing technologies to get fuel from sunlight,” he says. Oxygen production is crucial for a future hydrogen economy, confusing as that might sound. “If you’re going to take hydrogen out of water then you have to make oxygen,” he says.

The exact chemical species in the catalyst hasn’t been figured out yet, nor has its mechanism of action. But Barber expects Nocera will discover that the process looks a lot like nature’s version. “It would be doing reactions something like what goes on in the leaf,” says Barber. “If the leaf can do it, we can do it. It’s just chemistry.” 

  • References

    1. Kanan, M. W. & Nocera, D. G. Science doi:10.1126/science.1162018(2008)
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