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Bio-inspired molybdenum sulfide catalyst offers low-cost and efficient photo-electrochemical water splitting to produce hydrogen

2 May 2011

Slac
The optimized photo-electrochemical water splitting device uses light absorbers made of silicon arranged in closely packed pillars, dotted with tiny clusters of the new molybdenum sulfide catalyst. Image courtesy of Christian D. Damsgaard, Thomas Pedersen and Ole Hansen, Technical University of Denmark. Click to enlarge.

Researchers from the US and Denmark have engineered a bio-inspired molybdenum sulfide catalyst as an inexpensive, abundant alternative to platinum and coupled it with a light-absorbing electrode to create a photo-electrochemical water splitting device to make hydrogen fuel from sunlight and water.

The discovery was published last week in the journal Nature Materials by theorist Jens Nørskov of the US Department of Energy’s SLAC National Accelerator Laboratory and Stanford University and a team of colleagues led by Ib Chorkendorff and Søren Dahl at the Technical University of Denmark (DTU).

We show that bio-inspired molecular clusters based on molybdenum and sulphur evolve hydrogen at rates comparable to that of platinum. The incomplete cubane-like clusters (Mo3S4) efficiently catalyze the evolution of hydrogen when coupled to a p-type Si semiconductor that harvests red photons in the solar spectrum. The current densities at the reversible potential match the requirement of a photo-electrochemical hydrogen production system with a solar-to-hydrogen efficiency in excess of 10%. The experimental observations are supported by density functional theory calculations of the Mo3S4 clusters adsorbed on the hydrogen-terminated Si(100) surface, providing insights into the nature of the active site.

—Hou et al.

Today, most hydrogen is produced via steam methane reforming (SMR), resulting in large emissions of CO2. An alternative, clean method is to make hydrogen fuel from sunlight and water via a photo-electrochemical (PEC, or water-splitting) process. When sun hits the PEC cell, the solar energy is absorbed and used for splitting water molecules into its components, hydrogen and oxygen.

“If we can find new ways of rationally designing catalysts, we can speed up the development of new catalytic materials enormously”
—Jens Nørskov

Progress has so far been limited in part by a lack of cheap catalysts that can speed up the generation of hydrogen and oxygen. A critical component of the American-Danish effort was combining theory and advanced computation with synthesis and testing to accelerate the process of identifying new catalysts. This is a new development in a field that has historically relied on trial and error.

The team first tackled the hydrogen half of the problem. The DTU researchers created a device to harvest the energy from part of the solar spectrum and used it to power the conversion of single hydrogen ions into hydrogen gas. However, the process requires a catalyst to facilitate the reaction. Platinum is already known as an efficient catalyst, but platinum is too rare and too expensive for widespread use. The collaborators turned to nature for inspiration.

They investigated hydrogen-producing enzymes, using a theoretical approach Nørskov’s group has been developing to describe catalyst behavior. These studies led them to related compounds, which eventually took them to molybdenum sulfide. Molybdenum is an inexpensive solution for catalyzing hydrogen production, Chorkendorff said.

The team also optimized parts of the device, introducing a “chemical solar cell” designed to capture as much solar energy as possible. The experimental researchers at DTU designed light absorbers that consist of silicon arranged in closely packed pillars, and dotted the pillars with tiny clusters of the molybdenum sulfide. When they exposed the pillars to light, hydrogen gas bubbled up—as quickly as if they’d used platinum.

The hydrogen gas-generating device is only half of a full photo-electrochemical cell. The other half of the PEC would generate oxygen gas from the water; though hydrogen gas is the goal, without the simultaneous generation of oxygen, the whole PEC cell shuts down.

Many groups—including Chorkendorff, Dahl and Nørskov and their colleagues—are working on finding catalysts and sunlight absorbers to do this well.

This is the most difficult half of the problem, and we are attacking this in the same way as we attacked the hydrogen side.

—Søren Dahl

Resources

  • Yidong Hou, Billie L. Abrams, Peter C. K. Vesborg, Mårten E. Björketun, Konrad Herbst, Lone Bech, Alessandro M. Setti, Christian D. Damsgaard, Thomas Pedersen, Ole Hansen, Jan Rossmeisl, Søren Dahl, Jens K. Nørskov & Ib Chorkendorff (2011) Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nature Materials doi: 10.1038/nmat3008

  • Supplementary movie

May 2, 2011 in Catalysts, Hydrogen Production, Solar | Permalink | Comments (17) | TrackBack (0)

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Comments

I don't understand how 10% efficiency is very good. If you take a solar panel with 20% efficiency and use the electricity for water hydrolysis which can yield a 50-80% efficiency, you should reap more than %10 efficiency.

I think it has to do with cost, solar panels are expensive and so are electrolyzers. I guess the metric would be efficiency at cost. If this costs half as much but both end up with the same overall efficiency, we then this would be preferred.

Most commercial solar cells are only 10% to 12% efficient.

Combine this with the good news about cheap Hydrogen fuel cells and the future is looking bright indeed.
http://www.lanl.gov/news/releases/cheaper-hydrogen-fuel-cells.html

The work of Daniel Nocera at MIT is related, and he has published a lot of discussion about the rationale and challenges for solar produced H2. See for example an account of his recent work on an "artificial leaf": http://www.sciencedaily.com/releases/2011/03/110327191042.htm The basic idea is to couple a semiconductor (to absorb photons and produce electrons and holes) with O2 and H2 evolving catalysts (to use the electrons and holes to produce O2 and H2). Developing an economical system will be a challenge, but a variety of research groups are making some progress.

The MIT device was still plugged into the wall. They said they might make PV part of the overall idea, but it was the oxygen catalyst that was the big deal. This show what can happen when the press, which does not understand science, gets a hold of a press release and runs with it.

@SJC - please provide a reference for your comment above. All reports of Nocera's "artificial leaf" claim the device uses sunlight and has no wires. See
http://blogs.nature.com/news/thegreatbeyond/2011/03/scientists_announce_first_prac.html
and
http://www.cleanenergyauthority.com/solar-energy-news/mit-solar-leaf-project-ready-to-roll-out-033111/

I am not going to dig it up, you do it. That is what I read and that is all.

"The crux of the matter is to exploit solar panels to power the electrolyzer to produce hydrogen."

http://www.alternative-energy-news.info/splitting-water-to-store-solar-energy/

Look around yourself, it uses external electricity, it just uses sunlight and the oxygen catalyst to make it more efficient.

"He applies a voltage to the electrode, and cobalt, potassium, and phosphate accumulate on the electrode, forming the catalyst."

http://www.technologyreview.com/Energy/21155/?nlid=1247

It seems the good doctor is participating in a bit of fine print hype himself. They talk about solar panels, but it is external electrical energy that gets it going and keeps it going.

"The crux of the matter is to exploit solar panels to power the electrolyzer to produce hydrogen."

http://www.technologyreview.com/Energy/21155/?nlid=1247

What they are implying is this makes a better electrolyzer because it uses sunlight energy and produces an oxygen catalyst.

I do not care if you believe me or not. If you choose not to that is your business, but I will not do a lot of work to satisfy your skepticism, that is just rewarding laziness.

"To produce oxygen, Nocera and Kanan added cobalt and phosphates to neutral water and then inserted a conductive-glass electrode. As soon as the researchers APPLIED a current, a dark film began to form on the electrode from which tiny pockets of oxygen began to appear, eventually building into a stream of bubbles."

http://www.nsf.gov/news/news_summ.jsp?cntn_id=111975&org=NSF&from=news

"When electricity — whether from a photovoltaic cell, a wind turbine or any other source — runs through the electrode, the cobalt and phosphate form a thin film on the electrode, and oxygen gas is produced."

http://www.sciencedaily.com/releases/2008/07/080731143345.htm

That should suffice for now....

Granted, the March announcement in the link:

http://blogs.nature.com/news/2011/03/scientists_announce_first_prac.html

Says that they combined the two, but that is not much different than using solar panels. They also say:

"There's got to be some tricky engineering to collect the gases as they're coming off the silicon," he says. "We don't know how to do that yet."

Not exactly ready for prime time.

@Cautious,
SJC did your homework for you, can't you at least say "thank you"?

:-)

My apologies, I should have read his link before reacting. In March of this year they coated a solar cell to produce the electricity. Why it took them 3 years to do this I do not know.

The impression I got from reading the articles before March 2011 was that you needed water, the catalyst and sunlight, it was only reading further where it talked about solar panels that it was apparent that electricity was required as well.

It turns out that these guys may actually have some day what the MIT guy inferred what they had, but did not. These people are working on an oxygen catalyst to go with their hydrogen catalyst by actually knowing how catalysts work rather than using trial and error. If they succeed, they WILL have something where water, catalyst and sun light will produce hydrogen without any external electrons.

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