Originally Posted by
BAP
MIT splits water with sunlight
Publisher: The Chemical Engineer, London, UK
Author: Claudia Flavell-White
SPLITTING WATER INTO hydrogen and oxygen at room temperature using solar power, under benign conditions and without the need for expensive catalysts: it's the holy grail of the hydrogen economy. Now, researchers at the MIT claim they found it.
Writing in the 31 July issue of Science, MIT chemistry professor Daniel Nocera and postdoctoral fellow Matthew Kanan say they have developed a practical way of generating plentiful supplies of hydrogen gas to power cars, houses etc. Their discovery hinges on a novel cobalt/phosphate catalyst that forms a film on the anode which in turn cleaves the oxygen from water when a current runs through it. A conventional platinum catalyst produces hydrogen at the cathode. The process works at room temperature, uses water at neutral pH, and is easy to set up, raising hopes that it will be straightforward to develop and scale up, and hopefully economic to run at large scale.
The researchers are still working to understand the exact mechanism of catalysis at the anode. They know that cobalt ions and phosphate ions were observed to form a thin dark film on the indium tin oxide anode, and that it was this film that appeared to act as the catalyst -- but whether it is Co3+ or Co4+ ions that eventually pull electrons from water to leave behind oxygen atoms and H+ protons is still not entirely certain. However, they have observed that the cobalt phosphate film catalyst appears to regenerate itself, which would make it infinitely more practical for everyday commercial application.
While it is of course already possible to split water into hydrogen and oxygen, current technologies consume vast amounts of power and a lot of expensive platinum to catalyse the process, and operate in a harshly alkaline environment. By contrast, Nocera and Kanan's set-up works on standard water and even appears to tolerate some impurities.
Interestingly, their breakthrough is the result of Nocera questioning some basic assumptions of catalysis. Frustrated by his lack of progress, Nocera decided to abandon the usual rule to use very stable catalysts that aren't corroded by the reactions they catalyse. By contrast, the cobalt phosphorous film that acts as catalyst breaks down as soon as the current is cut -- but it reassembles as soon as power is restored.
"This is the nirvana of what we've been talking about for years," says Nocera. "Solar power has always been a limited, far-off solution. Now we can seriously think about solar power as unlimited and soon."
Other experts concur. James Barber, biochemistry professor and photosynthesis expert at Imperial College London, says: "This is a major discovery with enormous implications for the future prosperity of humankind. The importance of [Nocera and Kanan's] discovery cannot be overstated."
Nocera says that in ten years' time, efficient, effective photovoltaic cells could provide all the necessary power for household during the day, using excess energy to generate hydrogen for household fuel cells that would provide power over night. Electricity-by-wire from a central source could be a thing of the past, he believes.
Before this happens, plenty of work remains to be done. The efficiency and speed of the oxygen-production has to be speeded up. Then the whole arrangement needs to be connected to an energy source -- preferably wind or solar -- on one end and an electricity-producing fuel cell on the other. Finally, scientists hope to find an alternative for platinum both at the cathode and to catalyse the reverse reaction of hydrogen and oxygen into water: with platinum costing over $2000 per troy ounce, a cheaper replacement would go a long way to making both the water reactor and the fuel cell affordable at scale.
The solution to at least the second half of this final challenge may be closer at hand than thought: a materials scientists in Australia says that a combination of poly(3,4-ethylenedioxythiophene) (PEDOT) and Gore-tex -- a polymer widely used for water-repellent outdoor and sports clothing -- could replace platinum as the fuel cell catalyst of choice. PEDOT is a conducting polymer which, coated onto a Gore-tex membrane, is just as effective as a fuel cell catalyst as platinum. And unlike platinum, the polymer catalyst is not subject to carbon monoxide poisoning, which can occur in platinum-catalysed fuel cells.
Doug MacFarlane, professor of chemistry at the Australian Centre for Electromaterials Science, says: "The cost of the platinum component alone of current fuel cells for a small car with a 100kW electric engine is more than the total cost of an 100kW gasoline engine. Also current annual world production of platinum is only sufficient for about 3 million 100kW vehicles, less than one-twentieth of the current annual global production of vehicles." In other words, if we ever want to replace conventional cars with fuel cell models, finding an alternative to the platinum catalyst is a must.
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