Water oxidation is an essential component for generating hydrogen from water by only using sunlight. Emory University researchers, led by Craig Hill, discovered the most powerful homogeneous catalyst known for oxidizing water, which could lead to successfully replicating the way photosynthesis works, but much more efficiently and controllable.
The Water Oxidation Catalyst (WOC) needs to be selective, stable and fast in order to do its job properly. It also needs to be homogeneous, thus boosting its efficiency and making it easier for the scientists to study and optimize.
Emory chemists, funded by the U.S. D.O.E., devised a new WOC that has all the properties of the one mentioned above by using cobalt, a cheap and abundant element.
Three main technical challenges are involved: developing a light collector, a catalyst to oxidize water to oxygen and a catalyst to reduce water to hydrogen. All three components need improvement, but a viable WOC may be the most difficult scientific challenge. “We are aiming for a WOC that is free of organic structure, because organic components will combine with oxygen and self-destruct,” Hill says. “You’ll wind up with a lot of gunk.”
Enzymes are nature’s catalysts. The enzyme in the oxygen-evolving center of green plants “is about the least stable catalyst in nature, and one of the shortest lived, because it’s doing one of the hardest jobs,” Hill says.
“We’ve duplicated this complex natural process by taking some of the essential features from photosynthesis and using them in a synthetic, carbon-free, homogeneous system. The result is a water oxidation catalyst that is far more stable than the one found in nature.”
Scientists have had other experiments in trying to mimic photosynthesis, but they had them with little success. Almost all of the more then 40 homogeneous WOCs previously invented had organic components in them, that burned up quickly during the water oxidation process and didn’t yield the expected results.
The current cobalt-based WOC is based on a previous prototype working with ruthenium. As ruthenium is a rare (and thus expensive) element, they replaced it. Good enough, though, since cobalt proves itself better to do the job.