Emory University inorganic chemist Craig Hill, who led the laboratory effort said, “(The result) has really upped the standard from the other known homogeneous water oxidation catalysts. It’s like a home run compared to a base hit.”  There’s some excitement here.

The claim the Emory team with their collaborators at Paris Institute of Molecular Chemistry is for development of the most potent homogeneous catalyst now known for water oxidation, considered a crucial component for generating clean hydrogen fuel using only water and sunlight. The breakthrough’s paper was published March 11 in the journal Science.

Water Oxidation Catalyst In Action. Click image for more info.

In order for water oxidation catalyst (WOC) to be viable, a WOC needs selectivity, stability and speed. Homogeneity is also a desired trait, since it boosts efficiency and makes the WOC easer to study and optimize. The Emory team’s new WOC has all of these qualities, and it is based on the cheap and abundant element cobalt, adding to its potential to help solar powered hydrogen production go mainstream.

The project was funded by The U.S. Department of Energy. Benjamin Yin, an undergraduate student in Hill’s lab, was the lead author for the Science paper. Emory chemists who are co-authors include Hill, Yurii Gueletii, Jamal Musaev, Zhen Luo and Ken Hardcastle.

A homogeneous catalyst is considered a crucial component for generating clean hydrogen fuel using only water for the input and sunlight for the energy.  Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions.  The team built the complex [Co4(H2O)2(PW9O34)2]10- comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, which is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst.  The catalyst self-assembles in water from salts of earth abundant elements (Co, W, and P).

With [Ru(bpy)3]3+ (bpy = 2,2’-bipyridine) as the oxidant, the team observed catalytic turnover frequencies for O2 production 5 s-1 at a pH of 8.  The rate’s pH sensitivity reflects the pH dependence of the 4-electron H2O/O2 couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that the preparation is stable under catalytic turnover conditions.  Also neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.

The long-term goal is to use sunlight to split water into oxygen and hydrogen. Hydrogen becomes the fuel. Its combustion produces the by-product of water – which flows back into a clean, green, renewable cycle.  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.

Hill says, “We are aiming for a WOC that is free of organic structure, because organic components will combine with oxygen and self-destruct. You’ll wind up with a lot of gunk.”

By comparison 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 explains.  “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 been trying for decades to imitate Mother Nature and create a WOC for artificial photosynthesis. Nearly all of the more than 40 homogeneous WOCs developed so far have had significant limitations, such as containing organic components that burn up quickly during the water oxidation process.

Hill’s lab and collaborators developed the first prototype of a stable, homogenous, carbon-free WOC two years ago, which also worked faster than others known at the time. That prototype was based on ruthenium, a relatively rare and expensive element.  From this point the research team began experimenting with the cheaper and more abundant element cobalt. The cobalt-based WOC has proved even faster than the ruthenium version for light-driven water oxidation.

The Emory University press release isn’t saying what the production could be from a square meter of exposure, just that the new catalyst is a marked improvement from the existing catalyst stable.  The light collection seems to be a simple enough engineering project when the conditions and concentration needs are better known.  Cleaving out the hydrogen from the deoxidized water remains a question in search of a low cost solution.  More data in the new catalyst’s productivity might offer more incentive to others.

Still, its not like this technology would be an energy salvation.  But at low enough cost it could store hydrogen for use later, preferably in just hours or days.  That would make hydrogen fuel a contender for a marginal part of the fuel needs, maybe even a large marginal part.  Cheap hydrogen would be welcome, even in small amounts at low cost it would make a difference replicated millions of times, we will find worthwhile uses for it.


5 Comments so far

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