All-in-one light-driven water splitting with a novel nanocatalyst (photocatalytic splitting of H2O molecules)


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Solar-powered water splitting is a promising means of generating clean and storable energy. A novel catalyst based on semiconductor nanoparticles has now been shown to facilitate all the reactions needed for “artificial photosynthesis”.

In the light of global climate change, there is an urgent need to develop efficient ways of obtaining and storing power from renewable energy sources. The photocatalytic splitting of water into hydrogen fuel and oxygen provides a particularly attractive approach in this context. However, efficient implementation of this process, which mimics biological photosynthesis, is technically very challenging, since it involves a combination of processes that can interfere with each other.
Now, LMU physicists led by Dr. Jacek Stolarczyk and Professor Jochen Feldmann, in collaboration with chemists at the University of Würzburg led by Professor Frank Würthner, have succeeded in demonstrating the complete splitting of water with the help of an all-in-one catalytic system for the first time.
Their new study appears in the journal Nature Energy (“All-in-one visible-light-driven water splitting by combining nanoparticulate and molecular co-catalysts on CdS nanorods”).
solar-powered-water-splitting-device-incorporating-two-separateTechnical methods for the photocatalytic splitting of water molecules use synthetic components to mimic the complex processes that take place during natural photosynthesis.
In such systems, semiconductor nanoparticles that absorb light quanta (photons) can, in principle, serve as the photocatalysts. Absorption of a photon generates a negatively charged particle (an electron) and a positively charged species known as a ‘hole’, and the two must be spatially separated so that a water molecule can be reduced to hydrogen by the electron and oxidized by the hole to form oxygen.
“If one only wants to generate hydrogen gas from water, the holes are usually removed rapidly by adding sacrificial chemical reagents,” says Stolarczyk. “But to achieve complete water splitting, the holes must be retained in the system to drive the slow process of water oxidation.”
The problem lies in enabling the two half-reactions to take place simultaneously on a single particle – while ensuring that the oppositely charged species do not recombine. In addition, many semiconductors can be oxidized themselves, and thereby destroyed, by the positively charged holes.

Nanorods with spatially separated reaction sites

“We solved the problem by using nanorods made of the semiconducting material cadmium sulfate, and spatially separated the areas on which the oxidation and reduction reactions occurred on these nanocrystals,” Stolarczyk explains.
The researchers decorated the tips of the nanorods with tiny particles of platinum, which act as acceptors for the electrons excited by the light absorption. As the LMU group had previously shown, this configuration provides an efficient photocatalyst for the reduction of water to hydrogen. The oxidation reaction, on the other hand, takes place on the sides of the nanorod.
To this end, the LMU researchers attached to the lateral surfaces a ruthenium-based oxidation catalyst developed by Würthner‘s team. The compound was equipped with functional groups that anchored it to the nanorod.
“These groups provide for extremely fast transport of holes to the catalyst, which facilitates the efficient generation of oxygen and minimizes damage to the nanorods,” says Dr. Peter Frischmann, one of the initiators of the project in Würzburg.
The study was carried out as part of the interdisciplinary project “Solar Technologies Go Hybrid” (SolTech), which is funded by the State of Bavaria.
“SolTech’s mission is to explore innovative concepts for the conversion of solar energy into non-fossil fuels,” says Professor Jochen Feldmann, holder of the Chair of Photonics and Optoelectronics at LMU.

 

“The development of the new photocatalytic system is a good example of how SolTech brings together the expertise available in diverse disciplines and at different locations. The project could not have succeeded without the interdisciplinary cooperation between chemists and physicists at two institutions,” adds Würthner, who, together with Feldmann, initiated SolTech in 2012.

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Source: CeNS Center for NanoScience
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New Material For Splitting Water: Halide double Perovskites – “All the Right Properties” for creating Fuel Cells


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MIT: Some catalysts contribute their own oxygen for reactions ~ Crucial for Chemical energy storage, Water splitting, & Electrochemical carbon dioxide reduction


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New research shows that when metal oxide (flat array of atoms at bottom) is used as a catalyst for splitting water molecules, some of the oxygen produced comes out of the metal oxide itself, not just from the surrounding water. This was proved by first using water with a heavier isotope of oxygen (oxygen 18, shown in white), and later switching to ordinary water (made with oxygen 16, shown in red). The detection of the heavier oxygen 18 in the resulting gas proves that this came out of the catalyst.

Chemical reactions that release oxygen in the presence of a catalyst, known as oxygen-evolution reactions, are a crucial part of chemical energy storage processes, including water splitting, electrochemical carbon dioxide reduction, and ammonia production. The kinetics of this type of reaction are generally slow, but compounds called metal oxides can have catalytic activities that vary over several orders of magnitude, with some exhibiting the highest such rates reported to date. The physical origins of these observed catalytic activities is not well-understood.

Now, a team at MIT has shown that in some of these catalysts oxygen doesn’t come only from the water molecules surrounding the catalyst material; some of it comes from within the crystal lattice of the catalyst material itself. The new findings are being reported this week in the journal Nature Chemistry, in a paper by recent MIT graduate Binghong Han PhD ’16, postdoc Alexis Grimaud, Yang Shao-Horn, the W.M. Keck Professor of Energy, and six others.

The research was aimed at studying how water molecules are split to generate oxygen molecules and what factors limit the reaction rate, Grimaud says. Increasing those reaction rates could lead to more efficient energy storage and retrieval, for example, so determining just where the bottlenecks may be in the reaction is an important step toward such improvements.

The catalysts used to foster the reactions are typically metal oxides, and the team wanted “to be able to explain the activity of the sites [on the surface of the catalyst] that split the water,” Grimaud says.

The question of whether some oxygen gets stored within the crystal structure of the catalyst and then contributes to the overall oxygen output has been debated before, but previous work had never been able to resolve the issue. Most researchers had assumed that only the active sites on the surface of the material were taking any part in the reaction. But this team found a way of directly quantifying the contribution that might be coming from within the bulk of the catalyst material, and showed clearly that this was an important part of the reaction.

They used a special “labeled” form of oxygen, the isotope oxygen-18, which makes up only a tiny fraction of the oxygen in ordinary water. By collaborating with Oscar Diaz-Morales and Marc T. Koper at Leiden University in the Netherlands, they first exposed the catalyst to water made almost entirely of oxygen-18, and then placed the catalyst in normal water (which contains the more common oxygen-16).

Upon testing the oxygen output from the reaction, using a mass spectrometer that can directly measure the different isotopes based on their atomic weight, they showed that a substantial amount of oxygen-18, which cannot be accounted for by a surface-only mechanism, was indeed being released. The measurements were tricky to carry out, so the work has taken some time to complete. Diaz-Morales “did many experiments using the mass spectrometer to detect the kind of oxygen that was evolved from the water,” says Shao-Horn, who has joint appointments in the departments of Mechanical Engineering and Materials Science and Engineering, and is a co-director of the MIT Energy Initiative’s Center for Energy Storage.

With that knowledge and with detailed theoretical calculations showing how the reaction takes place, the researchers say they can now explore ways of tuning the electronic structure of these metal-oxide materials to increase the reaction rate.

The amount of oxygen contributed by the catalyst material varies considerably depending on the exact chemistry or electronic structure of the catalyst, the team found. Oxides of different metal ions on the perovskite structure showed greater or lesser effects, or even none at all. In terms of the amount of oxygen output that is coming from within the bulk of the catalyst, “you observe a well-defined signal of the labeled oxygen,” Shao-Horn says.

One unexpected finding was that varying the acidity or alkalinity of the water made a big difference to the reaction kinetics. Increasing the water’s pH enhances the rate of oxygen evolution in the catalytic process, Han says.

These two previously unidentified effects, the participation of the bulk material in the reaction, and the influence of the pH level on the reaction rate, which were found only for oxides with record high catalytic activity, “cannot be explained by the traditional mechanism” used to explain oxygen evolution reaction kinetics, Diaz-Morales says. “We have proposed different mechanisms to account for these effects, which requires further experimental and computational studies.”

“I find it very interesting that the lattice oxygen can take part in the oxygen evolution reactions,” says Ib Chorkendorff, a professor of physics at the Technical University of Denmark, who was not involved in this work. “We used to think that all these basic electrochemical reactions, related to proton membrane fuel cells and electrolyzers, are all taking place at the surface,” but this work shows that “the oxygen sitting inside the catalyst is also taking part in the reaction.”

These findings, he says, “challenge the common way of thinking and may lead us down new alleys, finding new and more efficient catalysts.”

The team also included Wesley Hong PhD ’16, former postdoc Yueh-Lin Lee, research scientist Livia Giordano in the Department of Mechanical Engineering, Kelsey Stoerzinger PhD ’16, and Marc Koper of the Leiden Institute of Chemistry, in the Netherlands. The work was supported by the Skoltech Center for Electrochemical Energy, the Singapore-MIT Alliance for Research and Technology, the Department of Energy, and the National Energy Technology Laboratory.

Novel water-splitting photocatalyst (with solar energy) operable over wide range of the visible light spectrum


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Clean renewable energy can be produced by photocatalytically splitting water into hydrogen and oxygen with solar energy. Most of the conventionally developed water-splitting photocatalysts, however, were only active under UV irradiation, and only a few have been demonstrated to operate under visible light, at up to 500 nm. For making high-efficiency use of solar energy, it was necessary to develop a photocatalyst that can utilize longer wavelength light.
To accomplish this, a photocatalyst that is operable under lower-energy light needed to be developed, but since the energy that can be used for the water-splitting reaction would also be smaller, more advanced material design was required, which posed a very difficult challenge.

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 Graph. A water-splitting photocatalyst that is operable at up to 600nm has been developed for the first time, using a transition-metal oxynitride whose electronic structure is suitable for long wavelength absorption.

Credit: NIMS
A research group led by Chengsi Pan, Postdoctoral Researcher, and Tsuyoshi Takata, NIMS Special Researcher, at the Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN; Director-General: Kohei Uosaki) of the National Institute for Materials Science (NIMS; President: Sukekatsu Ushioda), and Kazunari Domen, a professor of the Department of Chemical System Engineering, School of Engineering, The University of Tokyo (President: Junichi Hamada) newly developed a water-splitting photocatalyst that is operable over a wider range of the visible light spectrum than before.

In this research, a water-splitting photocatalyst that is operable at up to 600nm was developed for the first time, using a transition-metal oxynitride whose electronic structure is suitable for long wavelength absorption. As a development approach, solid solutions were formed between two existing perovskite-type compounds, LaTaON2 and LaMg2/3Ta1/3O3 (La: lanthanum, Ta: tantalum, O: oxygen, N: nitrogen, Mg: magnesium), and electronic structure was adjusted. This made LaMg1/3Ta2/3O2N solid solutions usable for water-splitting reactions by visible light irradiation, but since the degradation of the photocatalyst and the reverse reaction simultaneously occurred, a steady water-splitting reaction could not be achieved. To overcome this problem, the photocatalyst particle surface was covered with a layer of amorphous oxyhydroxide in order to inhibit the degradation of the photocatalyst and reverse reaction, and made the steady water-splitting reaction possible. This oxyhydroxide coating plays a role to control chemical reactions on the photocatalyst surface.

This research result established a new effective method in water-splitting photocatalyst development. Also, by applying this method to other photocatalyst materials, the development of photocatalysts with higher activity can be expected. At present, the quantum yield is still low, and the improvement of the yield is the challenge for the future.

This research was performed jointly with a group led by Yuichi Ikuhara, Professor of the Institute of Engineering Innovation, School of Engineering, The University of Tokyo. Also, this research was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Specially Promoted Research, “Development of innovative water splitting photocatalysts based on photocarrier dynamics at solid/liquid interfaces,” and projects commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), “Program for Development of Environmental Technology using Nanotechnology,” “Nanotechnology Platform Japan,” and “Area of Advanced Environmental Materials, Green Network of Excellence (GRENE): Creation of the Network of Excellence for the Human Resource Development, and Advanced Environmental Materials and Devices toward Environment and Energy Technology.”


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The above story is based on materials provided by National Institute for Materials Science (NIMS). Note: Materials may be edited for content and length.

Researchers discover new catalysts to generate renewable fuels


Yale CatalyticInnovationsx250For the last seven years, Yale Univ. graduate student Staff Sheehan has been working on splitting water. Now, a paper published in Nature Communications reveals how one of the methods he and his team have uncovered for this process—using a specific iridium species as a water oxidation catalyst—could aid in the development of renewable fuels. The process which Sheehan is investigating is known as artificial photosynthesis—storing energy from the sun as plants do, but more efficiently.

“Artificial photosynthesis has been widely researched,” Sheehan says, “but water oxidation is the bottleneck—it’s usually the most difficult reaction to perform in generating fuel from sunlight.” Yale co-authors include Julianne Thomsen, Ulrich Hintermair (currently at the Univ. of Bath), Robert Crabtree, Gary Brudvig and Charles Schmuttenmaer.

The iridium-based catalyst featured in Nature Communications represents one of two novel methods Sheehan and other Yale researchers have discovered for splitting water in artificial photosynthesis—the other utilizes cobalt. Both have been patented through the Yale Office of Cooperative Research. Sheehan is developing these technologies as a commercial venture through the Yale Entrepreneurial Institute (YEI) under the name Catalytic Innovations along with Aaron Bloomfield, a postdoctoral researcher working jointly at the Center for Green Chemistry and Green Engineering at Yale and the Energy Sciences Institute. The team is working closely with Paul Anastas, the Director of the Center for Green Chemistry and Green Engineering and the Teresa and H. John Heinz III Professor in the Practice of Chemistry for the Environment.

Last summer, Catalytic Innovations participated in the Venture Creation Program at YEI and explored commercial uses for their catalysts outside artificial photosynthesis. Sheehan says working alongside entrepreneurial peers and mentors was a great help in developing their venture. “Our peers and mentors at YEI have been our biggest resource,” he says. “Throughout the program, we met people who could offer advice from different backgrounds.”

While use in generating renewable fuels remains a long-term goal for Catalytic Innovations, Sheehan says a near-term market opportunity is using their catalysts for metal refining, which takes advantage of the iridium-based catalyst’s stability in strong acids. These catalysts can also be used to remediate chemical waste and have military fuel cell applications. The team is successfully raising research and development funds. “What we’ve discovered is a new architecture for catalysts that is highly efficient and very stable for the eventual development of renewable fuels,” Sheehan says.

Source: Yale Univ.