Using sunlight to split water in a cheap, efficient way is the goal of true renewable energy that won't involve ghastly wind vanes or porkbarrelled government funding of ethanol. The obstacle is splitting water into hydrogen and oxygen and researchers have been looking into complex catalysts that mimic what plants use - but a new study says that there might be much simpler alternatives.
Birnessite, an oxide mineral of manganese along with calcium, potassium and sodium, is a very simple mineral commonly seen as a black stain on rocks Like other elements in the middle of the Periodic Table, manganese can exist in a number of oxidation states. These correspond to the number of oxygen atoms with which a metal atom could be combined
The manganese in the catalyst cycles between two oxidation states. First, the voltage is applied to oxidize from the manganese-II state to manganese-IV state in birnessite. Then in sunlight, birnessite goes back to the manganese-II State. This cycling process is responsible for the oxidation of water to produce oxygen gas, protons and electrons.
Professor Leone Spiccia from the School of Chemistry at Monash University said, "When an electrical voltage is applied to the cell, it splits water into hydrogen and oxygen and when the researchers carefully examined the catalyst as it was working, using advanced spectroscopic methods they found that it had decomposed into a much simpler material called birnessite, well-known to geologists as a black stain on many rocks."
The reaction has two steps. First, two molecules of water are oxidized to form one molecule of oxygen gas (O2), four positively-charged hydrogen nuclei (protons) and four electrons. Second, the protons and electrons combine to form two molecules of hydrogen gas (H2).
Co-author Dr. Rosalie Hocking, Research Fellow in the Australian Centre for Electromaterials Science who explained that what was interesting was the operation of the catalyst, which follows closely natures biogeochemical cycling of manganese in the oceans. "This may provide important insights into the evolution of Nature's water splitting catalyst found in all plants which uses manganese centres.
"Scientists have put huge efforts into making very complicated manganese molecules to copy plants, but it turns out that they convert to a very common material found in the Earth, a material sufficiently robust to survive tough use."
The experimental work was conducted using the Australian Synchrotron, the Australian National Beam-line Facility in Japan and the Monash Centre for Electron Microscopy, and involved collaboration with Professor Bill Casey, a geochemist at UC Davis.
"The research highlights the insight obtainable from the synchrotron based spectroscopic techniques – without them the important discovery linking common earth materials to water oxidation catalysts would not have been made," Hocking said.
The findings were published in Nature Chemistry.
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