"Photo-oxidation of water molecules into oxygen, electrons and protons (hydrogen ions) is one of the two essential half reactions of an artificial photosynthesis system — it provides the electrons needed to reduce carbon dioxide to a fuel," explains Heinz Frei, senior scientist at the lab, who worked on the development with Feng Jiao. "Effective photo-oxidation requires a catalyst that is both efficient in its use of solar photons and fast enough to keep up with the solar flux in order to avoid wasting those photons. Clusters of cobalt oxide (Co3O4) nanocrystals are sufficiently efficient and fast, and also are robust and abundant. They perfectly fit the bill."
The researchers achieved efficient oxygen generation at mild conditions — room temperature, a modest over-potential of 350 mV, and a pH of 5.8 (required for the visible light sensitizer rather than the catalyst). The tests, which ran for several hours, indicated no degradation in catalytic activity. "The yield for clusters of Co3O4 nano-sized crystals was about 1,600 times higher than for micron-sized particles, and the turnover frequency was about 1,140 oxygen molecules per second per cluster, which is commensurate with solar flux at ground level," notes Frei. (To view a video showing the evolution of oxygen, click here >> http://www.youtube.com/v/Ei4mwyjG3Vg).
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Figure 1: Artificial Photosynthesis for Fuel
Aiming to expand catalyst options, the team now is exploring use of mixed manganese oxides because manganese is more abundant and probably more environmentally friendly, explains Frei. He expects activity comparable to Co3O4 but doesn't know if the material will be as stable.
The researchers have just started working to integrate water oxidation with carbon dioxide reduction. The goal is to produce methanol by using light to split water and having the resultant oxygen and electrons react with carbon dioxide (Figure 1). This would provide both a renewable energy source and a way to deal with CO2.
The coupled system will consist of oxidation and reduction sections separated by a membrane that allows protons but not oxygen through. Work will involve membrane development as well as catalysts for CO2 reduction; it should take three to five years, says Frei.