A Simpler Route to Splitting Water?
The key to the hydrogen economy could come from a common mineral better known as a black stain on rock, according to researchers at Monash University in Melbourne, Australia and UC Davis, and published online May 15 in the journal Nature Chemistry.
Chemists and materials scientists have been trying to reproduce what green plants have been doing for billions of years: split water into hydrogen and oxygen. A cheap, efficient way to split water, powered for example by sunlight, would open up production of hydrogen as a clean fuel.
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).
“The hardest part about turning water into fuel is the oxidation of water,” said Bill Casey, professor of chemistry and geology at UC Davis and a co-author on the paper. The experimental work was conducted at Monash University, but was partly inspired by conversations between Casey and senior author Leone Spiccia while Spiccia was visiting UC Davis.
Scientists around the world have been studying complex catalysts designed mimic the catalysts plants use to split water with sunlight, Casey said. But the new study shows that there might be much simpler alternatives to hand.
The Monash team developed a water-splitting cell based on a complicated manganese-based catalyst. When an electrical voltage is applied to the cell, it splits water into hydrogen and oxygen.
But when the researchers carefully examined the catalyst as it was working, they found that it had decomposed into a much simpler material called birnessite, well-known to geologists as a black stain on many rocks.
Birnessite, it turns out, is what does the work. Like other elements in the middle of the Periodic Table, manganese can exist in a number of what chemists call oxidation states. These correspond to the number of oxygen atoms with which a metal atom could be combined.
The manganese in the catalyst is cycling between two oxidation states. First, the voltage causes it to oxidize from a manganese-II state to manganese-IV state in birnessite. Then in sunlight, it goes back to the manganese-II state, oxidizing water to produce oxygen gas, protons and electrons.
The chemistry of these catalysts also looks a lot like the cycling of manganese in the oceans, Casey said.
“Scientists have been making very complicated manganese molecules to copy plants, but it turns out they convert to a very common structure found in the Earth,” Casey said. “These minerals are sufficiently robust to survive tough use.”
The work was funded by Monash University, the Australian Research Council, and the Australian Synchrotron, the U.S. National Science Foundation and the U.S. Department of Energy.