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ScienceWise - Spring 2010

Silver lining

Article Illustration
Some of the scientists involved in the research with a Silver orthophosphate solution, L to R: Hilary Stuart-Williams, Yun Liu, Zhiguo Yi and Ray Withers
Article Illustration
Zhiguo photographs the vigorous stream of oxygen bubbles rising from a silver orthophosphate solution when a bright light spot shines on it

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For centuries photographers have taken advantage of the fact some silver compounds degenerate when exposed to light, making it possible to record images on suitably prepared film and paper. But just as most people have now ditched film in favour of digital photography, the photochemistry of silver may be about to take on a new and quite unexpected role - the generation of oxygen and hydrogen from water.

Hydrogen and oxygen are the dream fuels of the environmentally conscious engineer. They can combine in a fuel cell to produce electricity and the exhaust is pure clean water. The difficulty at present is that the only practical ways to create hydrogen gas are from coal or from the electrolysis of water, which requires vast amounts of electrical energy. If you generate this electrical electricity by burning coal, then you’re no better off in environmental terms than you were extracting the hydrogen directly from the coal. But a new and exciting option for cleanly splitting water may be just around the corner.

Dr Zhiguo Yi is a scientist currently working with Professor Ray Withers at the Research School of Chemistry at ANU. One day he was thinking about the silver halide chemicals that are used in traditional photography and the way they break down in sunlight and he began to wonder if he could tailor this process to create a chemistry based system for splitting water molecules. Having considered a number of silver compounds to base this work on, the one he finally chose to work with was Silver orthophosphate.

“It’s well known that silver halides used in photography are not stable under illumination, Dr Yi says, “and the silver orthophosphate that we use in our work has similar chemistry.”

Silver orthophosphate (Ag3PO4) is a semiconductor and one of the properties of all semiconductors is that they have a band gap. What this means is that there is a range of energy values that any free electrons wandering around the crystal are not allowed to have. If the energy of photons of light hitting the semiconductor is too small to ionise an electron from an atomic orbital right across this forbidden gap, nothing happens. The photon will pass right through making the material transparent to that wavelength of light. However once the energy of the photon becomes enough to catapult a valence electron right across the energy gap, the light is strongly absorbed.

One of the things about Silver orthophosphate that makes it so suitable for harnessing sunlight is that it’s forbidden energy gap (2.36eV) corresponds almost perfectly with the wavelengths of sunlight making the process highly efficient. When sunlight hits the crystal the strong absorption process creates many free electrons within the lattice and vacant holes where the electrons came from.

Electrons and holes are the domain of electronic engineers, enabling them to create many devices from transistors to lasers. But chemists get excited about free electrons for different reasons. Electron exchange is the basis of chemical reactions.

When powdered silver orthophosphate is placed on a conductive electrode under water interesting things begin to happen. Sunlight falling on the powder creates free electrons and this drives a reaction that very efficiently oxidises the water.

4Ag3PO4 + 6H2O + 12h+ +12e-   to  12Ag + 4H3PO4 + 3O2

“The reaction is so vigorous you can see a plume of oxygen bubbles pouring from the anode.” Professor Withers says.
Of course once you’ve taken the oxygen out, your silver orthophosphate has become metallic silver and phosphoric acid and the reaction stops when the silver orthophosphate is consumed. So to keep the process going, it’s necessary to electrolyse the solution to regenerate the silver orthophosphate, a process that also liberates hydrogen gas that can then be stored.

As the driving voltage to electrolytically release the hydrogen from this solution is much lower than that for water electrolysis, the new process would greatly reduce the costs of producing hydrogen and oxygen fuel gases by electrolysis, but the group want to take it a step further. They’re investigating ways to adapt the chemistry so that it can generate both oxygen and hydrogen gas whilst at the same time regenerating the silver orthophosphate electrodes. And all with no net input of energy other than sunlight.

“It’s a really simple artificial photosynthesis,” Professor Withers explains, “At the moment it’s a very economic way to generate oxygen gas from water and more than halves the electrical energy required to then electrolyse the hydrogen out. But what we’re really working towards is a simple all-chemistry process that will convert sunlight and water directly into hydrogen and oxygen for fuel cells.”

There are many problems to be solved before hydrogen fuel could become a practical and economically viable alternative to petrol, but this work may represent a very significant step on that road.

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