Water oxidation advance boosts potential for solar fuel

Mar 11, 2010
Emory University chemists have developed the most potent homogeneous catalyst known for water oxidation, considered a crucial component for generating clean hydrogen fuel using only water and sunlight. The breakthrough, published March 11 in the journal Science, was made in collaboration with the Paris Institute of Molecular Chemistry. Pictured are bubbles of oxygen forming from water oxidation, catalyzed by the new tetra-cobalt WOC. The fastest, carbon-free molecular water oxidation catalyst (WOC) to date “has really upped the standard from the other known homogeneous WOCs,” said Emory inorganic chemist Craig Hill, whose lab led the effort. “It’s like a home run compared to a base hit.” In order 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 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 energy go mainstream. Benjamin Yin, an undergraduate student in Hill’s lab, is the lead author on the Science paper. Emory chemists who are co-authors include Hill, Yurii Gueletii, Jamal Musaev, Zhen Luo and Ken Hardcastle. The U.S. Department of Energy funded the work. Credit: Photo by Benjamin Yin, Emory University

Emory University chemists have developed the most potent homogeneous catalyst known for water oxidation, considered a crucial component for generating clean hydrogen fuel using only water and sunlight. The breakthrough, published March 11 in the journal Science, was made in collaboration with the Paris Institute of Molecular Chemistry.

The fastest, carbon-free molecular water oxidation catalyst (WOC) to date "has really upped the standard from the other known homogeneous WOCs," said Emory inorganic chemist Craig Hill, whose lab led the effort. "It's like a home run compared to a base hit."

In order 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 new WOC has all of these qualities, and it is based on the cheap and abundant element , adding to its potential to help solar energy go mainstream.

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

The WOC research is a component of the Emory Bio-inspired Renewable Energy Center, which aims to mimic natural processes such as photosynthesis to generate clean fuel. The next step involves incorporating the WOC into a solar-driven, water-splitting system.

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. "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 that is far more stable than the one found in nature."

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

Two years ago, Hill's lab and collaborators developed the first prototype of a stable, homogenous, carbon-free WOC, which also worked faster than others known at the time. The prototype, however, was based on ruthenium, a relatively rare and expensive element.

Building on that work, the researchers 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.

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Caliban
3.7 / 5 (6) Mar 11, 2010
Again, it sounds great, but what is the output?
In order to have any useful impact as an energy source, it has to generate very large quantities of H2 and O2- but there is zero information regarding this in the article itself.
Having said that- it is still yet another step in the right direction- at some point, someone is going to hit the jackpot.
danman5000
2.3 / 5 (4) Mar 11, 2010
Funny how they stress that it's "carbon-free" - its like if they don't include some mention of how evil carbon is then their research somehow loses credibility.

Still, pretty exciting development. Anything that enables alternative energy technology is the way to go. No matter what side of the AGW debate you fall on, oil will run out eventually so it's smart to invest in renewable alternatives now.

I wonder if they use the oxygen from cracking the water for anything, or if it's just released into the air. No problem either way, but it'd be neat if they got a use out of both products.
HoboWhisperer
5 / 5 (6) Mar 11, 2010
The article stated that the 'carbon-free' aspect is important because of the propensity of carbon based compounds to rapidly degrade during use - non-carbon compounds are more stable in these applications.
PinkElephant
4 / 5 (1) Mar 11, 2010
From http://www.scienc....1185372

(I assume that's the paper being described)
With [Ru(bpy)3]3+ (bpy = 2,2’-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 production ≥ 5 s-1 at pH 8.
Two observations: first, they are still mentioning Ruthenium, for some reason. So they don't seem to have gotten rid of it completely.

Second, it seems (if I'm reading it right), each catalytic complex splits 5 water molecules per second. If so, that means 5 H2 molecules per second, for each catalytic complex. (According to the abstract, each complex contains 4 Cobalt atoms, plus a bit of Tungsten and Phosphorus.)

Of course, no information that I can access, is provided on the illumination intensity or spectrum...
googleplex
4 / 5 (1) Mar 12, 2010
The other big factor is estimated production costs.
Why is everyone mad on WOC? I would rather have a good battery than a WOC. Where are you going to store all that H2 + O2. Also O2 is a controlled substance and you can't store it at home without a permit. Probably the same for H2.
danlgarmstrong
5 / 5 (1) Mar 12, 2010
Ruthenium was mentioned because thats the element the researchers used to begin their studies. They found out Cobalt was better.

Hydrogen storage is also advancing rapidly. Graphene is now shown to store and release it quite effectively. Store it for when it is needed, then send it to a feul cell to produce electricity. Why bother storing O2?

The big thing about Solar H2 is that it is effectively FREE after an initial investment in equipment is made. Just Add Water!
Feldagast
4 / 5 (1) Mar 12, 2010
Which is better? a fuel cell using h2 for source doesn't it need Oxygen to combine with for operation? or a cumbustion engine running on h2 using normal air aspiration? or a combustion engine using a h2 / o2 mixture?
I can see using something to split water into hydrogen and oxygen either from solar (very slow and only during daylight) or the woc and using both as fuel for combustion to produce energy at night.
NeilFarbstein
1 / 5 (1) Mar 14, 2010
Solar H2 is not free after you invest in it. Its a bust
PPihkala
not rated yet Mar 15, 2010
Feldagast: Fuel cell has better efficiency than combustion engine, but latter is cheaper and available today. But when the battery tech is getting better, it should have even better efficiency than using H2 as energy storage compound. In the end the prices of individual systems determines which will win.
chemgrad
5 / 5 (1) Mar 20, 2010
No, ru(bpy)3+ (the ruthenium complex) was employed as a sacrificial reductant, meaning it "soaks up" the electrons generated when oxidizing water. The electrons are transferred quickly, so the reaction rate is limited by the WOC rate. In principle, you could have thrown in a catalyst for generating hydrogen, but then you complicate things (as catalysts that generate h2 usually turn h2 + o2 to water, and you might be limited by electron transfer). The ultimate idea is you could load the catalysts onto a photodiode and produce hydrogen at one end and oxygen at the other, with solar energy providing the driving force. This would take the electrons from the WOC, excite them to an energy where they could be used for reducing hydrogen, and transfer them to the WRC.

This is great because water reduction catalysts (platinum, hydrogenases) are well studied and have good turnover rates, so for this reaction you're typically oxidation-limited.
chemgrad
5 / 5 (1) Mar 20, 2010
Also, to add to the above, diodes like the above mentioned have been used before, but to get good water oxidation rates, you typically needed iridium catalysts (expensive, v. rare metal) to provide good turnovers.

This is pretty cool, I'd like to see some mechanistic work to see if the mechanism is similar to some of the manganese catalysts that have been widely studied.

Fuel cells are theoretically more efficient than combustion engines, but they don't last that long, and any impurities kind of kill them over time. Combustion engines are carnot-efficiency limited, and are actually less efficient because engines are run at max power output instead of max efficiency (max efficiency actually produces 0 power because a truly adiabatic process requires an infinite amount of time to complete).

Overall, h2 storage still sucks, so this isn't particularly useful yet, but its a step in the right direction.
PinkElephant
not rated yet Mar 21, 2010
@chemgrad,

Thanks for the explanation. Now it all makes sense to this particular chemical non-sophisticate =)

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