Nanotrees Harvest the Sun’s Energy to Turn Water into Hydrogen Fuel


University of California, San Diego electrical engineers are building a forest of tiny nanowire trees in order to cleanly capture solar energy without using fossil fuels and harvest it for hydrogen fuel generation. Reporting in the journal Nanoscale, the team said nanowires, which are made from abundant natural materials like silicon and zinc oxide, also offer a cheap way to deliver hydrogen fuel on a mass scale.

“This is a clean way to generate clean fuel,” said Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, according to Wang. That’s because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it, Wang said, adding that it is also similar to retinal photoreceptor cells in the human eye. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.
Wang’s team has mimicked this structure in their “3D branched nanowire array” which uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to be used as fuel. This process uses clean energy with no green-house gas byproduct. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels.

Schematic shows the light trapping effect in nanowire arrays. Photons on are bounced between single nanowires and eventually absorbed by them (R). By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts where they are reflected off the surface (L). Image Credit: Wang Research Group, UC San Diego Jacobs School of Engineering.
“Hydrogen is considered to be clean fuel compared to fossil fuel because there is no carbon emission, but the hydrogen currently used is not generated cleanly,” said Ke Sun, a PhD student in electrical engineering who led the project.
By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts. Wang is also affiliated with the California Institute of Telecommunications and Information Technology and the Material Science and Engineering Program at UC San Diego.
The vertical branch structure also maximizes hydrogen gas output, said Sun. For example, on the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, very small gas bubbles of hydrogen can be extracted much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” said Sun.

In this experiment, nanotree electrodes are submersed in water and illuminated by simulated sun light to measure electricity output of the device. Photo credit: Joshua Knoff, UC San Diego Jacobs School of Engineering.
In the long run, what Wang’s team is aiming for is even bigger: artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.
“We are trying to mimic what the plant does to convert sunlight to energy,” said Sun. “We are hoping in the near future our ‘nanotree’ structure can eventually be part of an efficient device that functions like a real tree for photosynthesis.”
The team is also studying alternatives to zinc oxide, which absorbs the sun’s ultraviolet light, but has stability issues that affect the lifetime usage of the nanotree structure.


Rechargeable Flow Batteries: Solution to Cheap Renewable Energy Storage

imagesCAMR5BLR Einstein Judging a FishResearchers at MIT have developed a battery that could bring us reliable and cheap large scale energy storage. Based on flow battery technology, the researchers took out the costly membrane and created a battery that has a power density that is an order of magnitude higher than lithium-ion batteries and three times greater than other membrane-less systems.



MIT reports, “The device stores and releases energy in a device that relies on a phenomenon called laminar flow: Two liquids are pumped through a channel, undergoing electrochemical reactions between two electrodes to store or release energy. Under the right conditions, the solutions stream through in parallel, with very little mixing. The flow naturally separates the liquids, without requiring a costly membrane.”

The reactants used are liquid bromine and hydrogen fuel, which is cheap, but also has had issues with breaking down the membrane in other flow batteries. By taking out the membrane they were able to speed up energy storage and extend the life of the battery.

“Here, we have a system where performance is just as good as previous systems, and now we don’t have to worry about issues of the membrane,” says Martin Bazant, a professor of chemical engineering. “This is something that can be a quantum leap in energy-storage technology.”

As we bring more renewable technologies like wind and solar into the grid, affordable and reliable energy storage is increasingly important. While solar and wind energy output varies based on weather conditions, large scale energy storage systems can smooth out the power delivery from those technologies by storing any excess energy when it’s produced and using it when the output is lower or demand is higher.

Energy storage is the key enabling technology for renewables,” says Cullen Buie, an assistant professor of mechanical engineering. “Until you can make [energy storage] reliable and affordable, it doesn’t matter how cheap and efficient you can make wind and solar, because our grid can’t handle the intermittency of those renewable technologies.”

MIT says, “Braff built a prototype of a flow battery with a small channel between two electrodes. Through the channel, the group pumped liquid bromine over a graphite cathode and hydrobromic acid under a porous anode. At the same time, the researchers flowed hydrogen gas across the anode. The resulting reactions between hydrogen and bromine produced energy in the form of free electrons that can be discharged or released.

The researchers were also able to reverse the chemical reaction within the channel to capture electrons and store energy — a first for any membraneless design.”

Now that the team’s experiments have lined up with their computer models, they’re focused on scaling up the technology and seeing how it performs. They predict that the technology will be able to produce energy costing as little as $100/kWh, which would make it the cheapest large scale energy storage system built yet.

Radically new water splitting technique to produce hydrogen fuel

3adb215 D Burris(Nanowerk News) A University of Colorado Boulder team  has developed a radically new technique that uses the power of sunlight to  efficiently split water into its components of hydrogen and oxygen, paving the  way for the broad use of hydrogen as a clean, green fuel. The CU-Boulder team  has devised a solar-thermal system in which sunlight could be concentrated by a  vast array of mirrors onto a single point atop a central tower up to several  hundred feet tall. The tower would gather heat generated by the mirror system to  roughly 2,500 degrees Fahrenheit (1,350 Celsius), then deliver it into a reactor  containing chemical compounds known as metal oxides, said CU-Boulder Professor  Alan Weimer, research group leader.

imagesCAMR5BLR Einstein Judging a Fish

As a metal oxide compound heats up, it releases oxygen atoms,  changing its material composition and causing the newly formed compound to seek  out new oxygen atoms, said Weimer. The team showed that the addition of steam to  the system — which could be produced by boiling water in the reactor with the  concentrated sunlight beamed to the tower — would cause oxygen from the water  molecules to adhere to the surface of the metal oxide, freeing up hydrogen  molecules for collection as hydrogen gas.
solar thermal plant
An  artist’s conception of a commercial hydrogen production plant that uses sunlight  to split water in order to produce clean hydrogen fuel. (Image courtesy  University of Colorado Boulder)   
“We have designed something here that is very different from  other methods and frankly something that nobody thought was possible before,”  said Weimer of the chemical and biological engineering department. “Splitting  water with sunlight is the Holy Grail of a sustainable hydrogen economy.”
A paper on the subject was published in the Aug. 2 issue of  Science (“Efficient Generation of H2 by Splitting Water with an Isothermal Redox Cycle  “). The team included co-lead authors Weimer and Associate Professor Charles  Musgrave, first author and doctoral student Christopher Muhich, postdoctoral  researcher Janna Martinek, undergraduate Kayla Weston, former CU graduate  student Paul Lichty, former CU postdoctoral researcher Xinhua Liang and former  CU researcher Brian Evanko.
One of the key differences between the CU method and other  methods developed to split water is the ability to conduct two chemical  reactions at the same temperature, said Musgrave, also of the chemical and  biological engineering department. While there are no working models,  conventional theory holds that producing hydrogen through the metal oxide  process requires heating the reactor to a high temperature to remove oxygen,  then cooling it to a low temperature before injecting steam to re-oxidize the  compound in order to release hydrogen gas for collection.
“The more conventional approaches require the control of both  the switching of the temperature in the reactor from a hot to a cool state and  the introduction of steam into the system,” said Musgrave. “One of the big  innovations in our system is that there is no swing in the temperature. The  whole process is driven by either turning a steam valve on or off.”
“Just like you would use a magnifying glass to start a fire, we  can concentrate sunlight until it is really hot and use it to drive these  chemical reactions,” said Muhich. “While we can easily heat it up to more than  1,350 degrees Celsius, we want to heat it to the lowest temperature possible for  these chemical reactions to still occur. Hotter temperatures can cause rapid  thermal expansion and contraction, potentially causing damage to both the  chemical materials and to the reactors themselves.”
A  laboratory model of a multi-tube solar reactor at the University of Colorado  Boulder that can be used to split water in order to produce clean hydrogen fuel.  (Photo courtesy University of Colorado Boulder)
In addition, the two-step conventional idea for water splitting  also wastes both time and heat, said Weimer, also a faculty member at  CU-Boulder’s BioFrontiers Institute. “There are only so many hours of sunlight  in a day,” he said.
The research was supported by the National Science Foundation  and by the U.S. Department of Energy.
With the new CU-Boulder method, the amount of hydrogen produced  for fuel cells or for storage is entirely dependent on the amount of metal oxide  — which is made up of a combination of iron, cobalt, aluminum and oxygen — and  how much steam is introduced into the system. One of the designs proposed by the  team is to build reactor tubes roughly a foot in diameter and several feet long,  fill them with the metal oxide material and stack them on top of each other. A  working system to produce a significant amount of hydrogen gas would require a  number of the tall towers to gather concentrated sunlight from several acres of  mirrors surrounding each tower.
Weimer said the new design began percolating within the team  about two years ago. “When we saw that we could use this simpler, more effective  method, it required a change in our thinking,” said Weimer. “We had to develop a  theory to explain it and make it believable and understandable to other  scientists and engineers.”

Despite the discovery, the commercialization of such a  solar-thermal reactor is likely years away. “With the price of natural gas so  low, there is no incentive to burn clean energy,” said Weimer, also the  executive director of the Colorado Center for Biorefining and Biofuels, or C2B2.  “There would have to be a substantial monetary penalty for putting carbon into  the atmosphere, or the price of fossil fuels would have to go way up.”
Source: University of Colorado at Boulder

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Researcher Realizes Water-Splitting Solar Cell Structure Using Nanoparticles

Published on June 18, 2013 at 6:47 AM

QDOTS imagesCAKXSY1K 8Due to the fluctuating availability of solar energy, storage solutions are urgently needed. One option is to use the electrical energy generated inside solar cells to split water by means of electrolysis, in the process yielding hydrogen that can be used for a storable fuel. Researchers at the HZB Institute for Solar Fuels have modified so called superstrate solar cells with their highly efficient architecture in order to obtain hydrogen from water with the help of suitable catalysts. This type of cell works something like an “artificial leaf.”

This complex solar cell is coated with two different catalysts and works like an “artificial leaf”, using sunlight to split water and yield hydrogen gas.

But the solar cell rapidly corrodes when placed in the aqueous electrolyte solution. Now, Ph.D. student Diana Stellmach has found a way to prevent corrosion by embedding the catalysts in an electrically conducting polymer and then mounting them onto the solar cell’s two contact surfaces, making her the first scientist in all of Europe to have come up with this solution. As a result, the cell’s sensitive contacts are sealed to prevent corrosion with a stable yield of approx. 3.7 percent sunlight.

Hydrogen stores chemical energy and is highly versatile in terms of its applicability potential. The gas can be converted into fuels like methane as well as methanol or it can generate electricity directly inside fuel cells. Hydrogen can be produced through the electrolytic splitting of water molecules into hydrogen and oxygen by using two electrodes that are coated with suitable catalysts and between which a minimum 1.23 volt tension is generated. The production of hydrogen only becomes interesting if solar energy can be used to produce it. Because that would solve two problems at once: On sunny days, excess electricity could yield hydrogen, which would be available for fuel or to generate electricity at a later point like at night or on days that are overcast.

New approach with complex thin film technologies

At the Helmholtz Centre Berlin for Materials and Energy (HZB) Institute for Solar Fuels, researchers are working on new approaches to realizing this goal. They are using photovoltaic structures made of multiple ultrathin layers of silicon that are custom-made by the Photovoltaic Competence Centre Berlin (PVcomB), another of the HZB’s institutes. Since the cell consists of a single – albeit complex – “block,” this is known as a monolithic approach. At the Institute for Solar Fuels, the cell’s electrical contact surfaces are coated with special catalysts for splitting water. If this cell is placed in dilute sulphuric acid and irradiated with sun-like light, a tension is produced at the contacts that can be used to split water. During this process, it is the catalysts, which speed up the reactions at the contacts, that are critically important.

Protection against corrosion

The PVcomB photovoltaic cells’ main advantage is their “superstrate architecture”: Light enters through the transparent front contact, which is deposited on the carrier glass; there is no opacity due to catalysts being mounted onto the cells, because they are located on the cell’s back side and are in contact with the water/acid mixture. This mixture is aggressive, that is to say, it is corrosive, so much so that Diana Stellmach had to first replace the usual zinc oxide silver back contact with a titanium coat approximately 400 nanometers thick. In a second step, she developed a solution to simultaneously protect the cell against corrosion with the mounting of the catalyst: She mixed nanoparticles of RuO2 with a conducting polymer (PEDOT:PSS) and applied this mixture to the cell’s back side contact to act as a catalyst for the production of oxygen. Similarly, platinum nanoparticles, the sites of hydrogen production, were applied to the front contact.

Stable H2-Production

In all, the configuration achieved a degree of efficacy of 3.7 percent and was stable over a minimum 18 hours. “This way, Ms. Stellmach is the first ever scientist anywhere in Europe to have realized this kind of water-splitting solar cell structure,” explains Prof. Dr. Sebastian Fiechter. And just maybe anywhere in the World, as photovoltaic membranes with different architectures have proved far less stable.

Yet the fact remains that catalysts like platinum and RuO2 are rather expensive and will ultimately have to give way to less costly types of materials. Diana Stellmach is already working on that as well; she is currently in the process of developing carbon nanorods that are coated with layers of molybdenum sulphide and which serve as catalysts for hydrogen production.

Watch the “artificial leaf” in action:


A Magic Formula to Predict Fracture in Steel

22.11.12 – EPFL researchers have elucidated a century-old mystery: how hydrogen destroys steels. A new mathematical model predicts this failure in the presence of the destructive atoms.

A veritable gangrene for steels and other structural metals, hydrogen is one of the most important causes of ruptures in industrial parts, such as pipelines. At the slightest defect in a material, these atoms introduce themselves in the crack and weaken the structure dramatically, making it brittle. The material need only be in contact with aggressive substances or placed in an aqueous environment from which for the dangerous hydrogen atoms enter the material. This phenomenon of “hydrogen embrittlement” has been known for many years, but so far no one managed to capture the physical process or predict when hydrogen embrittlement will occur. Bill Curtin of the Laboratory of Multiscale Mechanical Modeling at EPFL and his collaborator Prof. Jun Song at McGill, tackled this problem and developed a mathematical model to understand the behavior of hydrogen atoms in iron-based steels and thus to predict steel fracture. This is revolutionary in the world of materials, and serves as the subject of an article in the journal Nature Materials.

Hydrogen Attracted by Fractures To establish their equation, the researchers studied the behavior of iron at the atomic level. They showed that the reason hydrogen weakens the materials comes from the tendency of hydrogen atoms to cluster at the tip of a crack. “In the absence of hydrogen, dislocation defects form around a crack, allowing it to relax the stress in the material and preventing the crack from growing, making the material more resilient or tougher, explained Bill Curtin. By grouping around the crack, the hydrogen atoms prevent the creation of these dislocations, and prevent the stress relaxation, allowing the crack to grow and the material becomes extremely brittle.”

A mathematical model that predicts the fracture Using their simulations, the scientists were able to establish a complex mathematical model that calculates when a material in contact with hydrogen will start to break. Several factors are taken into account, such as the concentration of hydrogen in the environment, the speed at which the hydrogen molecules move toward the crack, type of steel, and the load on the structure. If a combination of these parameters attains a critical value, computed from the simulations, then the material will break. Using the model, they predicted the breaking point for a various steels under various conditions. “Our predictions coincided with the experiments in 9 out of 10 cases, rejoiced Bill Curtin. And the 10th case was right on the border”.

This knowledge should allow scientists to tackle the problem armed with new weapons. It will become easier to identify adverse operating modes and to construct materials that are more resistant to this type of deterioration.


How does hydrogen come into contact with a material?

•When welding in damp conditions (presence of H2O). •When steels are used in the presence of hydrogen or hydrogenated gas mixtures (hydrocarbons in pipelines, for example) •Hydrogen can originate from corrosion in an aqueous environment, for example.

Additional information: Atomic mechanism and prediction of hydrogen embrittlement in iron

Nanotechnology Simplifies Hydrogen Production for Clean Energy

Stony Brook University· 310 Admin · Stony Brook, NY 11794-0701

SBU-Led Research Reveals Nanotechnology Simplifies Hydrogen Production for Clean Energy
Researcher says project is first ever demonstration of the potential of using metal nanoparticles to make fuel from water

Nov 20, 2012 – 3:30:00 PM

STONY BROOK, NY, November 20, 2012– In the first-ever experiment of its kind, researchers have demonstrated that clean energy hydrogen can be produced from water splitting by using very small metal particles that are exposed to sunlight. In the article, “Outstanding activity of sub-nm Au clusters for photo-catalytic hydrogen production,” published in the journal Applied Catalysis B: Environmental,  Alexander Orlov, PhD, an Assistant Professor of Materials Science & Engineering at Stony Brook University, and his colleagues from Stony Brook and Brookhaven National Laboratory, found that the use of gold particles smaller than one nanometer resulted in greater hydrogen production than other co-catalysts tested.

“This is the first ever demonstration of the remarkable potential of very small metal nanoparticles [containing fewer than a dozen atoms] for making fuel from water,” said Professor Orlov. Using nanotechnology, Professor Orlov’s group found that when the size of metal particles are reduced to dimensions below one nanometer, there is a tremendous increase in the ability of these particles to facilitate hydrogen production from water using solar light. They observed a “greater than 35 times increase” in hydrogen evolution as compared to ordinary materials.

Experimental and theory predicted optical properties of supported sub-nanometer particles.

In order to explain these fascinating results, Professor Orlov collaborated with Brookhaven National Lab computational scientist Dr. Yan Li, who found some interesting anomalies in electronic properties of these small particles.  Professor Orlov noted that there is still a tremendous amount of work that needs be done to understand this phenomenon. “It is conceivable that we are only at the beginning of an extraordinary journey to utilize such small particles [of less than a dozen atoms in size] for clean energy production,” he said.

“In order to reduce our dependence on fossil fuels it is vital to explore various sustainable energy options,” Professor Orlov said. “One possible strategy is to develop a hydrogen-based energy economy, which can potentially offer numerous environmental and energy efficiency benefits. Hydrogen can conceivably be a promising energy source in the future as it is a very clean fuel, which produces water as a final combustion product. The current challenge is to find new materials, which can help to produce hydrogen from sustainable sources, such as water.”

Professor Orlov also serves as a faculty member of the Consortium for Inter-Disciplinary Environmental Research at Stony Brook University. Members of his research team include Peichuan Shen and Shen Zhao from the Department of Materials Science and Engineering at Stony Brook and Dr. Dong Su of the Center for Functional Nanomaterials at Brookhaven National Laboratory.


Editors’ Note: This project was partially funded by an $80,500 exploratory grant from the National Science Foundation.

Light-Based Hydrogen Production

Nanocrystals and Nickel Catalyst Substantially Improve

Light-Based Hydrogen Production

November 8, 2012

Hydrogen is an attractive fuel source because it can easily be converted into electric energy and gives off no greenhouse emissions. A group of chemists at the University of Rochester is adding to its appeal by increasing the output and lowering the cost of current light-driven hydrogen-production systems.

The work was done by graduate students Zhiji Han and Fen Qiu, as part of a collaboration between chemistry professors Richard Eisenberg, Todd Krauss, and Patrick Holland, which is funded by the U.S. Department of Energy. Their paper will be published later this month (Nov. 23) in the journal Science.

The chemists say their work advances what is sometimes considered the “holy grail” of energy science—efficiently using sunlight to provide clean, carbon-free energy for vehicles and anything that requires electricity.

One disadvantage of current methods of hydrogen production has been the lack of durability in the light-absorbing material, but the Rochester scientists were able to overcome that problem by incorporating nanocrystals. “Organic molecules are typically used to capture light in photocatalytic systems,” said Krauss, who has been working in the field of nanocrystals for over 20 years. “The problem is they only last hours, or, if you’re lucky, a day. These nanocrystals performed without any sign of deterioration for at least two weeks.”

Richard Eisenberg, the Tracy H. Harris Professor of Chemistry, has spent two decades working on solar energy systems. During that time, his systems have typically generated 10,000 instances—called turnovers—of hydrogen atoms being formed without having to replace any components. With the nanocrystals, Eisenberg and his colleagues witnessed turnovers in excess of 600,000.

The researchers managed to overcome other disadvantages of traditional photocatalytic systems. “People have typically used catalysts made from platinum and other expensive metals,” Holland said. “It would be much more sustainable if we used metals that were more easily found on the Earth, more affordable, and lower in toxicity. That would include metals, such as nickel.”

Holland said their work is still in the “basic research stage,” making it impossible to provide cost comparisons with other energy production systems. But he points out that nickel currently sells for about $8 per pound, while the cost of platinum is $24,000 per pound.

While all three researchers say the commercial implementation of their work is years off, Holland points out that an efficient, low-cost system would have uses beyond energy. “Any industry that requires large amounts of hydrogen would benefit, including pharmaceuticals and fertilizers,” said Holland.

The process developed by Holland, Eisenberg, and Krauss is similar to other photocatalytic systems; they needed a chromophore (the light-absorbing material), a catalyst to combine protons and electrons, and a solution, which in this case is water. Krauss, an expert in nanocrystals, provided cadmium selenide (CdSe) quantum dots (nanocrystals) as the chromophore. Holland, whose expertise lies in catalysis and nickel research, supplied a nickel catalyst (nickel nitrate). The nanocrystals were capped with DHLA (dihydrolipoic acid) to make them soluble, and ascorbic acid was added to the water as an electron donor.

Photons from a light source excite electrons in the nanocrystals and transfer them to the nickel catalyst. When two electrons are available, they combine on the catalyst with protons from water, to form a hydrogen molecule (H2).

This system was so robust that it kept producing hydrogen until the source of electrons was removed after two weeks. “Presumably, it could continue even longer, but we ran out of patience!” said Holland.

One of the next steps will be to look at the nature of the nanocrystal. “Some nanocrystals are like M&Ms – they have a core with a shell around it,” said Eisenberg. “Ours is just like the core. So we need to consider if they would they work better if they were enclosed in shells.”

A Nano Way to Store Hydrogen

October / November 2012By: Tona KunzVolume 10 Number 5
A new type of nanoscale molecular trap makes it possible for industry to store large amounts of hydrogen in small fuel cells or capture, compact and remove volatile radioactive gas from spent nuclear fuel in an affordable, easily commercialized way.

The ability to adjust the size of the trap openings to select for specific molecules or to alter how molecules are released at industrially accessible pressures makes the trap uniquely versatile.  The trap is constructed of commercially available material and made possible through collaborative work at Argonne and Sandia national laboratories.

“This introduces a new class of materials to nuclear waste remediation,” said Tina Nenoff, a Sandia chemist. “This design can capture and retain about five times more iodine that current material technologies.”

Organic molecules linked together with metal ions in a molecular-scale Tinker Toy-like lattice called a metal-organic-framework, or MOF, form the trap. Molecules of radioactive iodine or carbon dioxide or even hydrogen for use as fuel can enter through windows in the framework.
Once pressure is applied, these windows are distorted, preventing the molecules from leaving.  This creates a cage and a way of selecting what to trap based on the molecule’s shape and size.
The compression also turns the MOF from a fluffy molecular sponge taking up a lot of space into a compact pellet.  The ability to compress large amounts of gas into small volumes is a crucial step to developing hydrogen gas as an alternative fuel for engines.

But what makes this MOF, called ZIF-8, dramatically different from designs created during the past decade is its ability to distort the windows in the framework and trap large volumes of gas at relatively low pressures. ZIF-8 takes about twice the pressure of a junkyard car compactor, which is about 10 times less pressure than is needed to compress other comparable zeolite MOFs.

This creates an environmentally friendly process that is within the reach of existing industrial machinery, can be produced on a large scale, and is financially viable.

The ZIF-8 is composed of zinc cations and organic imidazolate-based linkers. The topology of the framework is analogous to sodalite – a well-known zeolite.

The use of other available porous MOFs is limited to small batches because specialized scientific equipment is needed to apply the large amount of pressure they require to compress to a position that will maintain the new shape that traps the gas. This makes them not commercially viable.

Chapman and her colleagues at Argonne used X-rays from the Advanced Photon Source to perfect the low-pressure technique of making the MOFs into dense pellets. The distortion of the molecular framework that occurs during the process does not significantly reduce the gas storage capacity.

“These MOFs have wide-reaching applications,” said Karena Chapman, an Argonne scientist, who was inspired to explore low-pressure treatments for MOFs by her experiences working with flexible MOFs for hydrogen storage. Prior to this work, most high –pressure science research, such as the development of MOFs, took its cue from earth studies were extensive pressures cause transitions in geological materials.

With the pellet process worked out, the scientists tapped Nenoff at Sandia, to find a just the right type of molecule for the MOF’s structure to expand its use from hydrogen and carbon dioxide capture. Nenoff and her team had identified the ZIF-8 MOF as being ideally suited to separate and trap radioactive iodine molecules from a stream of spent nuclear fuel based on its pore size and high surface area.

This marks one of the first attempts to use MOFs in this way.  This presents opportunities for cleaning up nuclear reactor accidents and for reprocessing fuel.  Countries such as France, Russia and India recover fissile materials from radioactive components in used nuclear fuel to provide fresh fuel for power plants. This reduces the amount of nuclear waste that must be stored.  Radioactive iodine has a half-life of 16 million years.

The research team is continuing to look at different MOF structures to increase the amount of iodine storage and better predict how environmental conditions such as humidity will affect the storage lifetime.

Tona Kunz is a writer at Argonne National Laboratory.

Researchers Determine Critical Factors for Improving Performance of a Solar Fuel Catalyst

October 3, 2012

Contact: Veronika Szalai

False-color SEMs of cross-sectioned hematite films. 
False-color scanning electron micrographs of cross-sectioned hematite films grown by sputter deposition and then annealed at two different temperatures.  The physical structure and the tin dopant atom distributions in the hematite films differ depending on the annealing temperature.  Hematite annealed at higher temperatures has better catalytic performance for splitting water.

Hydrogen gas that is created using solar energy to split water into hydrogen and oxygen has the potential to be a cost-effective fuel source if the efficiency of the catalysts used in the water-splitting process can be improved. By controlling the placement of key additives (dopant atoms) in an iron oxide catalyst, researchers from the NIST Center for Nanoscale Science and Technology have found that the final location of the dopants and the temperature at which they are incorporated into the catalyst crystal lattice determine overall catalytic performance in splitting water.* The iron oxide hematite is a promising catalyst for water splitting because it is stable in water and absorbs a large portion of the solar spectrum. It is also abundant in the earth’s crust, making it inexpensive. Unfortunately, pure hematite has only modest catalytic activity, falling well short of its predicted theoretical maximum efficiency. Incorporating dopants such as tin atoms into hematite’s lattice improves performance, but it is a challenge to accurately measure the dopant concentration, making it difficult to understand and optimize their effects on catalyst performance. Using thin films of hematite doped with tin, the researchers produced highly active samples that enabled them to measure and characterize the spatial distribution of dopants in the material and their role in catalysis. The researchers determined that as a result of the sample preparation protocol they followed, a dopant gradient extends from the interface with the dopant source to the catalyst surface, where the measured concentration is low compared with previous estimates from similarly prepared samples. Contrary to prior results, they found that only a small dopant concentration is needed to improve catalytic activity. The researchers believe this study creates a path for improving the rational design of inexpensive catalysts for splitting water using solar energy.

*Effect of tin doping on α-Fe2O3 photoanodes for water splitting, C. D. Bohn, A. K. Agrawal, E. C. Walter, M. D. Vaudin, A. A. Herzing, P. M. Haney, A. A. Talin, and V. A. Szalai, The Journal of Physical Chemistry C 116, 15290–15296 (2012).
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