Team demonstrates solar water-splitting technology

September 4, 2015
Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. Credit: I. Thomann/Rice University

Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules.

The technology, which is described online in the American Chemical Society journal Nano Letters, relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer solar energy to highly excited electrons, which scientists sometimes refer to as "hot electrons."

"Hot electrons have the potential to drive very useful chemical reactions, but they decay very rapidly, and people have struggled to harness their energy," said lead researcher Isabell Thomann, assistant professor of electrical and computer engineering and of chemistry and materials science and nanoengineering at Rice. "For example, most of the energy losses in today's best photovoltaic solar panels are the result of hot electrons that cool within a few trillionths of a second and release their energy as wasted heat."

Capturing these high-energy electrons before they cool could allow solar-energy providers to significantly increase their solar-to-electric power-conversion efficiencies and meet a national goal of reducing the cost of solar electricity.

In the light-activated nanoparticles studied by Thomann and colleagues at Rice's Laboratory for Nanophotonics (LANP), light is captured and converted into plasmons, waves of electrons that flow like a fluid across the metal surface of the nanoparticles. Plasmons are high-energy states that are short-lived, but researchers at Rice and elsewhere have found ways to capture plasmonic energy and convert it into useful heat or light. Plasmonic nanoparticles also offer one of the most promising means of harnessing the power of hot electrons, and LANP researchers have made progress toward that goal in several recent studies.

Thomann and her team, graduate students Hossein Robatjazi, Shah Mohammad Bahauddin and Chloe Doiron, created a system that uses the energy from hot electrons to split molecules of water into oxygen and hydrogen. That's important because oxygen and hydrogen are the feedstocks for fuel cells, electrochemical devices that produce electricity cleanly and efficiently.

To use the hot electrons, Thomann's team first had to find a way to separate them from their corresponding "electron holes," the low-energy states that the hot electrons vacated when they received their plasmonic jolt of energy. One reason hot electrons are so short-lived is that they have a strong tendency to release their newfound energy and revert to their low-energy state. The only way to avoid this is to engineer a system where the hot electrons and electron holes are rapidly separated from one another. The standard way for electrical engineers to do this is to drive the hot electrons over an energy barrier that acts like a one-way valve. Thomann said this approach has inherent inefficiencies, but it is attractive to engineers because it uses well-understood technology called Schottky barriers, a tried-and-true component of electrical engineering.

"Because of the inherent inefficiencies, we wanted to find a new approach to the problem," Thomann said. "We took an unconventional approach: Rather than driving off the hot electrons, we designed a system to carry away the electron holes. In effect, our setup acts like a sieve or a membrane. The holes can pass through, but the hot electrons cannot, so they are left available on the surface of the plasmonic nanoparticles."

The setup features three layers of materials. The bottom layer is a thin sheet of shiny aluminum. This layer is covered with a thin coating of transparent nickel-oxide, and scattered atop this is a collection of plasmonic gold nanoparticles—puck-shaped disks about 10 to 30 nanometers in diameter.

When sunlight hits the discs, either directly or as a reflection from the aluminum, the discs convert the light into hot electrons. The aluminum attracts the resulting electron holes and the nickel oxide allows these to pass while also acting as an impervious barrier to the , which stay on gold. By laying the sheet of material flat and covering it with water, the researchers allowed the to act as catalysts for water splitting. In the current round of experiments, the researchers measured the photocurrent available for water splitting rather than directly measuring the evolved hydrogen and oxygen gases produced by splitting, but Thomann said the results warrant further study.

"Utilizing hot electron solar water-splitting technologies we measured photocurrent efficiencies that were on par with considerably more complicated structures that also use more expensive components," Thomann said. "We are confident that we can optimize our system to significantly improve upon the results we have already seen."

Explore further: Plasmonics study suggests how to maximize production of 'hot electrons' for cheap, efficient metal-based solar cells

More information: "Direct Plasmon-Driven Photoelectrocatalysis," pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b02453

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11 comments

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grondilu
3 / 5 (2) Sep 04, 2015
I have the feeling a similar article shows up every week or something.
docile
Sep 04, 2015
This comment has been removed by a moderator.
antialias_physorg
5 / 5 (10) Sep 04, 2015
I have the feeling a similar article shows up every week or something.

Well, renewables - in all their forms - are currently a hot research topic. So there's bound to be quite a bit of progress.

You can never say which avenue will pan out beforehand. But if you put a lot of smart people on a problem then you can expect, on average, an increase in findings.
howhot2
5 / 5 (4) Sep 04, 2015
Combustion of fossil fuels is killing us and is going to destroy the World. Projects like these need to be developed rapidly and in haste. Solar to hydrogen helps in that it's less expensive to store hydrogen than it is electrons and positive ions. Interesting article.
Mike_Massen
2.6 / 5 (5) Sep 05, 2015
howhot2 suggested
Solar to hydrogen helps in that it's less expensive to store hydrogen than it is electrons and positive ions
Having studied an "end to end" thermodynamic analysis re H2, its generation, storage & usage, evidence still shows its a net loss unless produced for extraterrestrial applications or as might be the case here as localised conversion medium.

ie. If there's plenty sunlight & water & cannot already use *that* solar power for local needs then in that particular local environment H2 "might" be of use - especially in chemical engineering as feedstock

Eg if you have CO, (waste) heat & various catalysts you can generate H2 to produce methanol & thus feed into existing liquid fuel infrastructure (LFI)

Unfortunately anything else shows up H2's combinatorial issues re trying to fit it into a LFI or worse trying to (mass) change that LFI to a gaseous equivalent..

An ideal & minimal impact would be to efficiently vector energy into our common LFI
MR166
not rated yet Sep 05, 2015
Mike you and I are usually on opposite sides of an argument but you are 100% correct with this line of thinking.
Mike_Massen
2 / 5 (4) Sep 05, 2015
MR166 says
Mike you and I are usually on opposite sides of an argument but you are 100% correct..
Its logic & no argument, deal with Physics & (eg) dispense with your anti-AGW idle propaganda re political/strawman retorts implying action necessitates crippling economies - it doesn't !

Its issue of managed change as AGW confirmed, no-one sensible/intelligent is suggesting massive costly step change, history affirms this & especially given the established inertia re LFI

Elaborating re H2, there's the other side, often ignored - what to do with the O2

Related: Energy management essentially=economic management, most don't notice that exchange rate is posted on placards in most cities & arises from Physics extended to human activity

Eg. Sewage treatment; can be assisted by injecting O2 speeding aerobic digestion, byproducts of which can be consumed by anerobic then to produce (useful) methane & thus use H2 mainly as byproduct vector into LFI :-)
gkam
1 / 5 (5) Sep 05, 2015
Ah, yes, . . nothing like "activated sludge".
howhot2
not rated yet Sep 05, 2015
Basically, besides injecting the idea that catalyzed hydrogen production could provide a CO2-less source of energy and help mankind reduce it's global warming problem, Its cheaper to store and transport Hydrogen than it is to store charge in the form of +/-ions in batteries. That was the point I wanted to make. I'm thinking the cost of battery production, recycling and disposal, even for Li-ion batteries, compared to some tank or storage system for hydrogen. Hydrogen wins.

MR166
not rated yet Sep 06, 2015
Here is a good link to H2 storage data.

https://www.googl...;cad=rja
gkam
1.8 / 5 (5) Sep 06, 2015
166, I did not read it, but know Doctor Burke. I worked with one of his PhDs on fuel cell applications. Those folk are some of the ones who gave us hybrids. His work will be good.

The storage will still be the problem, with the low energy density of Hydrogen and the very small molecular size.

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