Researchers at CalTech and Berkeley Lab Discover New materials that could turn “water into the fuel of the future”


Water for Fuel 170306151722_1_900x600New materials are created through deposition onto disks, which are then tested to determine their properties. Credit: Caltech

 

California Institute of Technology Summary: Combining computational with experimental approaches, researchers identify 12 new materials with potential use in solar fuels generators.

Researchers at Caltech and Lawrence Berkeley National Laboratory (Berkeley Lab) have — in just two years — nearly doubled the number of materials known to have potential for use in solar fuels.

They did so by developing a process that promises to speed the discovery of commercially viable solar fuels that could replace coal, oil, and other fossil fuels.

Solar fuels, a dream of clean-energy research, are created using only sunlight, water, and carbon dioxide (CO2). Researchers are exploring a range of target fuels, from hydrogen gas to liquid hydrocarbons, and producing any of these fuels involves splitting water.

Each water molecule is comprised of an oxygen atom and two hydrogen atoms. The hydrogen atoms are extracted, and then can be reunited to create highly flammable hydrogen gas or combined with CO2 to create hydrocarbon fuels, creating a plentiful and renewable energy source. The problem, however, is that water molecules do not simply break down when sunlight shines on them — if they did, the oceans would not cover most of the planet. They need a little help from a solar-powered catalyst.

To create practical solar fuels, scientists have been trying to develop low-cost and efficient materials, known as photoanodes, that are capable of splitting water using visible light as an energy source. Over the past four decades, researchers identified only 16 of these photoanode materials. Now, using a new high-throughput method of identifying new materials, a team of researchers led by Caltech’s John Gregoire and Berkeley Lab’s Jeffrey Neaton and Qimin Yan have found 12 promising new photoanodes.

A paper about the method and the new photoanodes appears the week of March 6 in the online edition of the Proceedings of the National Academy of Sciences. The new method was developed through a partnership between the Joint Center for Artificial Photosynthesis (JCAP) at Caltech, and Berkeley Lab’s Materials Project, using resources at the Molecular Foundry and the National Energy Research Scientific Computing Center (NERSC).

“This integration of theory and experiment is a blueprint for conducting research in an increasingly interdisciplinary world,” says Gregoire, JCAP thrust coordinator for Photoelectrocatalysis and leader of the High Throughput Experimentation group. “It’s exciting to find 12 new potential photoanodes for making solar fuels, but even more so to have a new materials discovery pipeline going forward.”

“What is particularly significant about this study, which combines experiment and theory, is that in addition to identifying several new compounds for solar fuel applications, we were also able to learn something new about the underlying electronic structure of the materials themselves,” says Neaton, the director of the Molecular Foundry.

Previous materials discovery processes relied on cumbersome testing of individual compounds to assess their potential for use in specific applications. In the new process, Gregoire and his colleagues combined computational and experimental approaches by first mining a materials database for potentially useful compounds, screening it based on the properties of the materials, and then rapidly testing the most promising candidates using high-throughput experimentation.

In the work described in the PNAS paper, they explored 174 metal vanadates — compounds containing the elements vanadium and oxygen along with one other element from the periodic table.

The research, Gregoire says, reveals how different choices for this third element can produce materials with different properties, and reveals how to “tune” those properties to make a better photoanode.

“The key advance made by the team was to combine the best capabilities enabled by theory and supercomputers with novel high throughput experiments to generate scientific knowledge at an unprecedented rate,” Gregoire says.


Story Source:

Materials provided by California Institute of Technology. Original written by Robert Perkins. Note: Content may be edited for style and length.


Journal Reference:

  1. Qimin Yan, Jie Yu, Santosh K. Suram, Lan Zhou, Aniketa Shinde, Paul F. Newhouse, Wei Chen, Guo Li, Kristin A. Persson, John M. Gregoire, and Jeffrey B. Neaton. Solar fuels photoanode materials discovery by integrating high-throughput theory and experiment. PNAS, March 2017 DOI: 10.1073/pnas.1619940114
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Solar Fuels: An’Artificial Leaf’ with Protective Layer for ‘Water Splitting’


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The illustration shows the structure of the sample: n-doped silicon layer (black), a thin silicon oxide layer (gray), an intermediate layer (yellow) and finally the protective layer (brown) to which the catalysing particles are applied. The acidic water is shown in green.
Credit: M. Lublow

A team at the HZB Institute for Solar Fuels has developed a process for providing sensitive semiconductors for solar water splitting (“artificial leaves”) with an organic, transparent protective layer. The extremely thin protective layer made of carbon chains is stable, conductive, and covered with catalysing nanoparticles of metal oxides. These accelerate the splitting of water when irradiated by light. The team was able to produce a hybrid silicon-based photoanode structure that evolves oxygen at current densities above 15 mA/cm2. The results have now been published in Advanced Energy Materials.

The “artificial leaf” consists in principle of a solar cell that is combined with further functional layers. These act as electrodes and additionally are coated with catalysts. If the complex system of materials is submerged in water and illuminated, it can decompose water molecules. This causes hydrogen to be generated that stores solar energy in chemical form. However, there are still several problems with the current state of technology. For one thing, sufficient light must reach the solar cell in order to create the voltage for water splitting — despite the additional layers of material. Moreover, the semiconductor materials that the solar cells are generally made of are unable to withstand the typical acidic conditions for very long. For this reason, the artificial leaf needs a stable protective layer that must be simultaneously transparent and conductive.

Catalyst used twice

The team worked with samples of silicon, an n-doped semiconductor material that acts as a simple solar cell to produce a voltage when illuminated. Materials scientist Anahita Azarpira, a doctoral student in Dr. Thomas Schedel-Niedrig’s group, prepared these samples in such a way that carbon-hydrogen chains on the surface of the silicon were formed. “As a next step, I deposited nanoparticles of ruthenium dioxide, a catalyst,” Azarpira explains. This resulted in formation of a conductive and stable polymeric layer only three to four nanometres thick. The reactions in the electrochemical prototype cell were extremely complicated and could only be understood now at HZB.

The ruthenium dioxide particles in this new process were being used twice for the first time. In the first place, they provide for the development of an effective organic protective layer. This enables the process for producing protective layers — normally very complicated — to be greatly simplified. Only then does the catalyst do its “normal job” of accelerating the partitioning of water into oxygen and hydrogen.

Organic protection layer combines excellent stability with high current densities

The silicon electrode protected with this layer achieves current densities in excess of 15 mA/cm2. This indicates that the protection layer shows good electronic conductivity, which is by no means trivial for an organic layer. In addition, the researchers observed no degradation of the cell — the yield remained constant over the entire 24-hour measurement period. It is remarkable that an entirely different material has been favoured as an organic protective layer: graphene. This two-dimensional material has been the subject of much discussion, yet up to now could only be employed for electrochemical processes with limited success, while the protective layer developed at HZB works quite wel . Because the novel material could lend itself for the deposition process as well as for other applications, we are trying to acquire international protected property rights,” says Thomas Schedel-Niedrig, head of the group.


Story Source:

The above post is reprinted from materials provided by Helmholtz-Zentrum Berlin für Materialien und Energie. Note: Materials may be edited for content and length.


Journal Reference:

  1. Anahita Azarpira, Thomas Schedel-Niedrig, H.-J. Lewerenz, Michael Lublow. Sustained Water Oxidation by Direct Electrosynthesis of Ultrathin Organic Protection Films on Silicon. Advanced Energy Materials, 2016; DOI: 10.1002/aenm.201502314

Artificial Photo-Synthesis ‘Solar-Fuels’ – One Step Closer?


Artificial Photosynth extCaltech scientists, inspired by a chemical process found in leaves, have developed an electrically conductive film that could help pave the way for devices capable of harnessing sunlight to split water into hydrogen fuel.

When applied to semiconducting materials such as silicon, the film prevents rust buildup and facilitates an important chemical process in the solar-driven production of fuels such as methane or hydrogen.

“We have developed a new type of protective coating that enables a key process in the solar-driven production of fuels to be performed with record efficiency, stability, and effectiveness, and in a system that is intrinsically safe and does not produce explosive mixtures of hydrogen and oxygen,” says Nate Lewis, the George L. Argyros Professor and professor of chemistry at Caltech and a coauthor of a new study, published the week of March 9 in the online issue of the Proceedings of the National Academy of Sciences (PNAS), that describes the film.

The development could help lead to safe, efficient artificial photosynthetic systems—also called solar-fuel generators or “artificial leaves”—that replicate the natural process of photosynthesis that plants use to convert sunlight, water, and carbon dioxide into oxygen and fuel in the form of carbohydrates, or sugars.

The artificial leaf that Lewis’ team is developing in part at Caltech’s Joint Center for Artificial Photosynthesis (JCAP) consists of three main components: two electrodes—a photoanode and a —and a membrane. The photoanode uses sunlight to oxidize water molecules to generate oxygen gas, protons, and electrons, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane, which is typically made of plastic, keeps the two gases separate in order to eliminate any possibility of an explosion, and lets the gas be collected under pressure to safely push it into a pipeline.

George L. Argyros Professor and Professor of Chemistry Nate Lewis and postdoc Ke Sun, who together have helped develop a protective film that is rust-resistant, highly transparent, and highly catalytic. This new thin-film could help pave the …more

Scientists have tried building the electrodes out of common semiconductors such as silicon or gallium arsenide—which absorb light and are also used in solar panels—but a major problem is that these materials develop an oxide layer (that is, rust) when exposed to water.

Lewis and other scientists have experimented with creating protective coatings for the electrodes, but all previous attempts have failed for various reasons. “You want the coating to be many things: chemically compatible with the semiconductor it’s trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels,” says Lewis, who is also JCAP’s scientific director. “Creating a protective layer that displayed any one of these attributes would be a significant leap forward, but what we’ve now discovered is a material that can do all of these things at once.”

The team has shown that its nickel oxide film is compatible with many different kinds of semiconductor materials, including silicon, indium phosphide, and cadmium telluride. When applied to photoanodes, the nickel oxide film far exceeded the performance of other similar films—including one that Lewis’s group created just last year. That film was more complicated—it consisted of two layers versus one and used as its main ingredient titanium dioxide (TiO2, also known as titania), a naturally occurring compound that is also used to make sunscreens, toothpastes, and white paint.

“After watching the photoanodes run at record performance without any noticeable degradation for 24 hours, and then 100 hours, and then 500 hours, I knew we had done what scientists had failed to do before,” says Ke Sun, a postdoc in Lewis’s lab and the first author of the new study.

Lewis’s team developed a technique for creating the nickel oxide film that involves smashing atoms of argon into a pellet of nickel atoms at high speeds, in an oxygen-rich environment. “The nickel fragments that sputter off of the pellet react with the oxygen atoms to produce an oxidized form of nickel that gets deposited onto the semiconductor,” Lewis says.

Crucially, the team’s nickel works well in conjunction with the membrane that separates the photoanode from the photocathode and staggers the production of hydrogen and oxygen gases.

“Without a membrane, the photoanode and photocathode are close enough to each other to conduct electricity, and if you also have bubbles of highly reactive hydrogen and oxygen gases being produced in the same place at the same time, that is a recipe for disaster,” Lewis says. “With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once.”

Lewis cautions that scientists are still a long way off from developing a commercial product that can convert sunlight into fuel. Other components of the system, such as the photocathode, will also need to be perfected.

“Our team is also working on a photocathode,” Lewis says. “What we have to do is combine both of these elements together and show that the entire system works. That will not be easy, but we now have one of the missing key pieces that has eluded the field for the past half-century.”

Explore further: New method stabilizes common semiconductors for solar fuels generation

Supersonic Electrons Could Produce Future Solar fuel


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Researchers from institutions including Lund University have taken a step closer to producing solar fuel using artificial photosynthesis. In a new study, they have successfully tracked the electrons’ rapid transit through a light-converting molecule.
The ultimate aim of the present study is to find a way to make fuel from water using sunlight. This is what photosynthesis does all the time – plants convert water and carbon dioxide to energy rich molecules using sunlight. Researchers around the world are therefore attempting to borrow ideas from photosynthesis in order to find a way to produce solar fuel artificially.
electrons’ rapid transit through a light-converting molecule
Researchers from institutions including Lund University have taken a step closer to producing solar fuel using artificial photosynthesis. In a new study, they have successfully tracked the electrons’ rapid transit through a light-converting molecule. (Image courtesy of Lund University)
“Our study shows how it is possible to construct a molecule in which the conversion of light to chemical energy happens so fast that no energy is lost as heat. This means that all the energy in the light is stored in a molecule as chemical energy”, said Villy Sundström, Professor of Chemical Physics at Lund University.
Thus far, solar energy is harnessed in solar cells and solar thermal collectors. Solar cells convert solar energy to electricity and solar thermal collectors convert solar energy to heat. However, producing solar fuel, for example in the form of hydrogen gas or methanol, requires entirely different technology. The idea is that solar light can be used to extract electrons from water and use them to convert light energy to energy rich molecules, which are the constituent of the solar fuel.
“A device that can do this – a solar fuel cell – is a complicated machine with light-collecting molecules and catalysts”, said Villy Sundström.
In the present study, Professor Sundström and his colleagues have developed and studied a special molecule that can serve as a model for the type of chemical reactions that can be employed in a solar fuel cell. The molecule comprises two metal centres, one that collects the light and another that imitates the catalyst where the solar fuel is produced. The researchers have managed to track the path of the electrons through the molecule in great detail. They measured the time it took for an electron to cross the bridge between the two metal atoms in the molecule. It takes half a picosecond, or half a trillionth of a second.
“In everyday terms, this means that the electron flies through the molecule at a speed of around four kilometres a second, which is over ten times the speed of sound”, said Villy Sundström.
The researchers were surprised by the high speed. Another surprising discovery was that the speed appears to be highly dependent on the type of bridge between the atoms. In this study, the speed was 100 times higher than with another type of bridge tested.
“This is the first time anyone has managed to track such a complex and rapid reaction and to distinguish all the stages of the reaction”, said Villy Sundström about the study, which has been published in the journal Nature Communications (“Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses”).
The study is a collaboration between researchers from several departments at Lund University and from Denmark, Germany, Hungary, Japan and the USA. The measurements were performed in Japan at the SACLA X-ray FEL in Harima, Japan, one of only two operating X-ray free-electron lasers in the world.
Source: Lund University

Read more: Supersonic electrons could produce future solar fuel