The research benefit goes to a fuel cell catalyst that converts hydrogen into electricity that must tear open a hydrogen molecule. The PBBL results confirms previous hypotheses and provides insight into how to make the catalyst work better for alternative energy uses.
This study is the first time scientists have shown precisely where the hydrogen halves end up in the structure of a molecular catalyst that breaks down hydrogen. The team’s research paper was published online April 22 in the Angewandte Chemie International Edition.
Morris Bullock of the Department of Energy’s Pacific Northwest National Laboratory explains the background, “The catalyst shows us what likely happens in the natural hydrogenase system. The catalyst is where the action is, but the natural enzyme has a huge protein surrounding the catalytic site. It would be hard to see what we have seen with our catalyst because of the complexity of the protein.”
The research purpose is to innovate better fuel cells. Hydrogen-powered fuel cells offer an alternative to burning fossil fuels, which generates greenhouse gases. Molecular hydrogen of two hydrogen atoms linked by an energy-rich chemical bond feeds a fuel cell. Generating electricity through chemical reactions, the fuel cell produces water and power.
If very low cost or renewable power is used to store energy in molecular hydrogen, fuel cells can could be more marketable and perhaps be carbon-neutral. But today’s fuel cells aren’t cheap enough for everyday use.
To make fuel cells less expensive, the PNNL researchers have turned to natural hydrogenase enzymes for inspiration. These enzymes break hydrogen for energy in the same way a fuel cell would. But while conventional fuel cell catalysts require expensive platinum, natural enzymes use cheap iron or nickel at their core.
The PNNL researchers have been designing catalysts inspired by hydrogenase cores and testing them. In this work, an important step in breaking a hydrogen molecule so the bond’s energy can be captured as electricity is to break the bond unevenly. Instead of producing two equal hydrogen atoms, this catalyst must produce a positively charged proton and a negatively charged hydride.
The physical shape of a catalyst – along with electrochemical information – can reveal how it does that. So far, scientists have determined the overall structure of catalysts with cheap metals using X-ray crystallography, but hydrogen atoms can’t be located accurately using X-rays. Based on chemistry and X-ray methods, researchers have a best guess for the position of hydrogen atoms, but imagination is no substitute for reality.
Bullock, Tianbiao “Leo” Liu and their colleagues at the Center for Molecular Electrocatalysis at PNNL, one of DOE’s Energy Frontier Research Centers, collaborated with scientists at the Spallation Neutron Source at Oak Ridge National Laboratory in Tennessee to find the lurking proton and hydride. Using a beam of neutrons like a flashlight allows researchers to pinpoint the nucleus of atoms that form the backbone architecture of their iron-based catalyst.
To use their iron-based catalyst in neutron crystallography, the team had to modify it chemically so it would react with the hydrogen molecule in just the right way. Neutron crystallography also requires larger crystals as starting material compared to X-ray crystallography.
“We were designing a molecule that represented an intermediate in the chemical reaction, and it required special experimental techniques,” Liu said. “It took more than six months to find the right conditions to grow large single crystals suitable for neutron diffraction. And another six months to pinpoint the position of the split H2 molecule.”
Crystallizing their catalyst of interest into a nugget almost 40 times the size needed for X-rays, the team succeeded in determining the structure of the iron-based catalyst.
They found the structure confirmed theories based on chemical analyses. For example, the barbell-shaped hydrogen molecule snuggles into the catalyst core. On being split, the negatively charged hydride attaches to the iron at the center of the catalyst; meanwhile, the positively charged proton attaches to a nitrogen atom across the catalytic core. The researchers expected this set-up, but no one had accurately characterized it in an actual structure before.
In this form, the hydride and proton form a type of bond uncommonly seen by scientists – a dihydrogen bond. The energy-rich chemical bond between two hydrogen atoms in a molecule is called a covalent bond and is very strong. Another bond called a “hydrogen bond” is a weak one formed between a slightly positive hydrogen and another, slightly negative atom.
Hydrogen bonds stabilize the structure of molecules by tacking down chains as they fold over within a molecule or between two independent molecules. Hydrogen bonds are also key to water surface tension, ice’s ability to float and even a snowflake’s shape.
The dihydrogen bond seen in the structure is much stronger than a single hydrogen bond. Measuring the distance between atoms reveals how tight the bond is. The team found that the dihydrogen bond was much shorter than typical hydrogen bonds but longer than typical covalent bonds. In fact, the dihydrogen bond is the shortest of its type so far identified, the researchers report.
This unusually strong dihydrogen bond likely plays into how well the catalyst balances tearing the hydrogen molecule apart and putting it back together. This balance allows the catalyst to work efficiently.
Bullock said, “We’re not too far from acceptable with its efficiency. Now we just want to make it a little more efficient and faster.”
This is the kind of research that enables progress. Fuel cells are wondrous devices that remain fearfully expensive thus simply impractical for mass market products. What remains even if the fuel cell cost problem has a solution at hand is coming up with, transporting and storing the hydrogen fuel. Perhaps this research or others will drive more to practical hydrogen sources such as the hydrogen rich alcohol fuel products that can be biologically produced or simply methane or natural gas.
April 23, 2014 | Leave a Comment
Researchers at the Energy Department’s National Renewable Energy Laboratory (NREL) are analyzing the new material, perovskite, using the lab’s unique testing capabilities and broad spectrum of expertise to uncover the secrets and potential of the semiconducting cube-like mineral.
The NREL material is synthetic version of a mineral first found in the Ural Mountains in 1839. The synthesized perovskite has the same crystal structure and is shooting up the efficiency charts faster than almost anything researchers have seen before. Perovskite is generating a lot optimism that a less expensive way of using sunlight to generate electricity may be in our future.
The NREL team has already produced three scientific papers on perovskite, reporting on the science behind the very large length of the electron pairs (or charge diffusion length) in mesostructured perovskite solar cells. The two most-studied perovskite device structures are mesostructured (of medium complexity) and planar (two-dimensional).
NREL Research Fellow David Ginley, who is a world-renowned materials scientist and winner of several R&D 100 Awards, said what makes perovskite device structures so remarkable is that when processed in a liquid solution, they have unusual abilities to diffuse photons a long distance through the cell. That makes it far less likely that the electrons will recombine with their hole pairs and be lost to make useful electricity. And that indicates a potential for low-cost, high-efficiency devices.
NREL Senior Scientist Daniel Friedman notes that the light-absorbing perovskite cells have “a diffusion length 10 times longer than their absorption length,” not only an unusual phenomenon, but a very useful one, too.
The new cells are made from a relative of the perovskite mineral found in the Ural Mountains. Small but vital changes to the material allow it to absorb sunlight very efficiently. The material is also easy to fabricate using liquids that could be printed on substrates like ink in a printing press, or made from simple evaporation. These properties suggest an easy, affordable route to solar cells.
By experimenting with the elemental composition, it is also possible to tune the perovskite material to access different parts of the sun’s spectrum. That flexibility can be crucial, because it means that the material can be changed by deliberately introducing impurities, and in such a way that it can be used in multijunction solar cells that have ultra-high efficiencies. Multijunction solar cells are an NREL invention from 1991, but because of high material costs, standard multijunctions are used mostly in outer space applications such as satellites and the Mars rovers. Cheaper multijunction cells based on perovskites could radically change this.
In four years, perovskite’s conversion efficiency – the yield at which the photons that hit the material are turned into electrons that can be used to generate electricity – has grown from 3.8% in 2009 to just north of 16%, with unconfirmed reports of even higher efficiencies arriving regularly. That’s better than a four-fold increase. By contrast, efficiencies of single-crystal solar cells grew by less than 50% during their first five years of development, and most other types of solar cells showed similar modest improvements during their first few years.
NREL materials scientists are encouraged by the possibility of further optimizing the materials. For example, replacing lead with tin in the cells could improve the efficiency of multijunction cells made from perovskite. Besides switching to a more environmentally friendly material, the change from lead to tin would also allow the finished solar cell to better withstand high humidity.
Ginley said, “We can help the field, especially in areas where they need help in reliability and larger development,” including understanding transport, or moving electrons from the solar cell to a circuit. Those are all the things we do well.”
NREL Senior Scientist Joey Luther, who works with nanomaterials added, “Perovskite shows promise to be a whole lot easier to make” compared to most other solar cells. It doesn’t require high-temperature processing. You can just dip glass into two chemicals and get the material to form on it.”
The field is growing fast, but that’s because there is so much to do, Luther said. “Every technique that everyone has used for every solar cell in the past, they want to try it on perovskite solar cells to see what they can learn. Anytime you jump into a new material, you need to get a feel for how it works – you just have to play around for a while,” Luther said. “Look at the layers, see what modifications you can make with new materials, see what you can do to tune it.”
Luther predicts that researchers will approach perovskite from two different directions. One will be to make the best semiconductor possible without regard to cost, and the other will be to try to make it as cheap as possible, trying spray-on techniques, for example. “Those fields are going to merge eventually,” he said, as researchers discover the optimal trade-offs.
“What is interesting about perovskite is that all the research groups – in Korea, England, Switzerland, the United States – they’re all getting very high efficiencies,” Luther said. “It’s not as if just one person knows the secret.”
The theoretical maximum efficiency of a perovskite-based solar cell is about 31%, meaning that of all the solar energy contained in the sunlight that hits the cell, 31% is converted to useful electrical energy. But, multijunction cells based on perovskites could attain higher efficiencies still.
“The goal shouldn’t be to stop at 20% efficiency,” Luther said. “The goal should be to try to get to 28% or higher. In the lab, the best cells need to be almost perfect at small scale. Then the commercial people can stop at whatever efficiency is economical for them to deploy.”
Perovskite has come to dominate dye-sensitized solar cell research, which is about low-cost thin-film cells. NREL Senior Scientist Kai Zhu. co-organizer of a scientific conference on dye-sensitized solar cells, noted the majority of talks, posters, and papers proposed for the conference are on the subject of perovskite – so exciting is the field even though perovskite isn’t technically a dye cell.
Progress is coming fast. In 2009, Japanese scientist Tsutomu Miyasaka reported perovskite’s potential as a light absorber and possible material for a solar cell, noting a 3.8% conversion efficiency, but that was such a low rate that it didn’t spark much interest, said Zhu.
But in 2011, a Korean scientist, Nam-Gyu Park, who served as a postdoctoral researcher at NREL in the late 1990s, reported achieving 6.5% efficiency with perovskite. “Fifteen years ago, he was working in the same lab I am working in now,” Zhu said. “So I started paying attention to his work on perovskites.”
A year later, Michael Grätzel, a top solar scientist from Switzerland, teamed with Park on a paper, sparking more widespread interest. Their paper in the journal Nature Scientific Reports reported a conversion efficiency of about 10% with perovskite. “By then, I knew this was something I wanted to pursue,” Zhu said. At the beginning of 2013, the efficiency level for perovskite had climbed to 12.3%.
Ginley takes up explaining again, “And then about a year ago, when they added chlorine to the materials, the electron and hole diffusion lengths just went through the roof. The most remarkable thing is that you add a little bit of chlorine and you see how the diffusion lengths change – by a factor of 10. That really brought attention to them.” Ideally, a solar cell has a diffusion length long enough for the electron to reach the contacts both above and below it, and thus escape the possibility that it will be trapped in its layer and recombine into an electron-hole pair.
When Zhu’s proposal to examine perovskite was approved, the efficiency level had climbed to 14.1%. Now, the highest certified rate is 16.2% by Sang Il Seok of Korea. “Seeing how rapidly this field is progressing, I feel very lucky that I started on this more than a year ago,” Zhu said.
Right now Zhu is in the midst of an experiment in which he prepares a precursor solution that converts from a liquid base to an absorber in a device. “This material is so easy to work with,” Zhu said. “Working on solution processing, we can make a device in one or two days, from beginning to finish.”
To boost efficiency levels even further will take more effort, Zhu concedes. “But this new material can probably be processed at a much lower cost” than rival materials, he said. It doesn’t have to deal with the problem of the substrate not matching with the material above it, or with the delicate deposition process necessary with many alternative solar materials.
NREL is already receiving queries about forming cooperative research and development agreements so they can work with NREL on perovskite. “At NREL, we have this depth and breadth of understanding of materials, devices, transport, and, really, all aspects of solar cells that should help us make an important contribution to this new material,” Zhu said.
Go folks, 30%+ efficiency at low cost would be a real and grand disruptive tech.
April 22, 2014 | Leave a Comment
Northwestern University (NU) scientists have discovered a surprising material that is the best in the world at converting waste heat to useful electricity. An interdisciplinary team led by inorganic chemist Mercouri G. Kanatzidis found the crystal form of the chemical compound tin selenide conducts heat so poorly through its lattice structure that it is the most efficient thermoelectric material known.
The world’s energy problem could be reduced by stopping the wasting of so much energy when producing and using it. Thermal losses happen in very high proportions in power generating plants and transportation by cars, trucks, and planes where about two-thirds of the energy input is simply lost as waste heat.
The NU material has an outstanding property that could be exploited in solid-state thermoelectric devices in a variety of industries, with potentially enormous energy savings. Unlike most thermoelectric materials, tin selenide has a simple structure, much like that of an accordion, which provides the key to its exceptional properties.
Thermoelectrics are built on thin blocks of semiconductor with a useful property: heating them on one side generates an electric voltage that can be used to drive a current and power devices. To obtain that voltage, thermoelectrics must be good electrical conductors but poor conductors of heat.
The efficiency of waste heat conversion in thermoelectrics is reflected by its figure of merit, called ZT. Tin selenide exhibits a ZT of 2.6, the highest reported to date at around 650º Celsius. The material’s extremely low thermal conductivity boosts the ZT to this high level, while still retaining good electrical conductivity.
The ZT metric represents a ratio of electrical conductivity and thermoelectric power in the numerator (which needs to be high) and thermal conductivity in the denominator (which needs to be low).
Potential areas of application for the high-temperature thermoelectric material include the automobile industry (a significant amount of gasoline’s potential energy goes out of a vehicle’s tailpipe), heavy manufacturing industries (such as glass and brick making, refineries, coal- and gas-fired power plants) and places where large combustion engines operate continuously (such as in large ships and tankers).
Vinayak P. Dravid, a senior researcher on the team said, “A good thermoelectric material is a business proposition — as much commercial as it is scientific. You don’t have to convert much of the world’s wasted energy into useful energy to make a material very exciting. We need a portfolio of solutions to the energy problem, and thermoelectric materials can play an important role.”
Kanatzidis, the Charles E. and Emma H. Morrison Professor of Chemistry in the Weinberg College of Arts and Sciences said, “The inefficiency of current thermoelectric materials has limited their commercial use. We expect a tin selenide system implemented in thermoelectric devices to be more efficient than other systems in converting waste heat to useful electricity.”
Two years have passed since the same research group broke the world record with another thermoelectric material they developed in the lab with a ZT of 2.2. The material, despite having a very simple structure, conducts heat so poorly that even moderate thermoelectric power and electrical conductivity are enough to provide high thermoelectric performance at high temperature.
The researchers did not expect to find tin selenide to be such a good thermoelectric material.
Lidong Zhao, a postdoctoral fellow in Kanatzidis’ research group, grew crystals of tin selenide and measured the crystal in three directions, along each axis. He found that the thermal conductivity was “ridiculously low” along the a-axis but also along the other two axes.
Dravid said, “The results are eye-opening because they point in a direction others would not look. This material has the potential to be applied to other areas, such as thermal barrier coatings.”
Those properties gave the material a ZT of 2.6, the best value ever measured. The key to the ultralow thermal conductivity, Kanatzidis says, appears to be the pleated arrangement of tin and selenium atoms in the material, which looks like an accordion. The pattern seems to help the atoms flex when hit by heat-transmitting vibrations called phonons, thus dampening SbSe’s ability to conduct heat.
The threshold for practical marketable thermoelectrics looks to be a ZT of 3. The new NU material offers lessons on how to get there.
Pacific Northwest National Laboratory (PNNL) researchers have developed a nickel-based metal organic framework to hold onto polysulfide molecules in the cathodes of lithium-sulfur batteries and extend the batteries’ life spans.
A promising new battery chemistry is the lithium-sulfur battery, which can hold as much as four times more energy in a given mass than typical lithium-ion batteries. Lithium sulfur chemistry would enable far more available energy from a single charge, as well as help store more renewable energy. The down side of lithium-sulfur batteries, however, is they have a much shorter lifespan because they can’t currently be charged as many times as lithium-ion batteries.
The PNNL researchers added the nickel based powder, a kind of nanomaterial called a metal organic framework, to the battery’s cathode to capture problematic polysulfides that usually cause lithium-sulfur batteries to fail after a few charges. A paper describing the material and its performance was published online April 4 in the American Chemical Society journal Nano Letters.
Materials chemist Jie Xiao of the Department of Energy’s Pacific Northwest National Laboratory said, “Lithium-sulfur batteries have the potential to power tomorrow’s electric vehicles, but they need to last longer after each charge and be able to be repeatedly recharged. Our metal organic framework may offer a new way to make that happen.”
Of particular interest is today’s electric vehicles that are typically powered by lithium-ion batteries. But the native chemistry of lithium-ion batteries limits how much energy they can store. As a result, electric vehicle drivers are often anxious about how far they can go before needing to recharge. Metal organic frameworks in the cathodes would enable electric vehicles to drive farther on a single charge, as well as help store more renewable energy.
How the metal frameworks would improve lithium sulfur comes from how batteries work. Most batteries have two electrodes: one is positively charged and called a cathode, while the second is negative and called an anode. Electricity is generated when electrons flow through a wire that connects the two. At the same time controlling the electrons, positively charged atoms shuffle from one electrode to the other through another path inside the battery: the electrolyte solution in which the electrodes are mounted.
The lithium-sulfur battery’s main problem comes from unwanted side reactions that cut the battery’s life short. The side reactions start on the battery’s sulfur-containing cathode, which slowly disintegrates and forms molecules, called polysulfides, that dissolve into the liquid electrolyte. Some of the sulfur – an essential part of the battery’s chemical reactions – never returns to the cathode. As a result, the cathode has less material to keep the reactions going and the battery quickly dies.
Researchers worldwide are trying to improve materials for each battery component to increase the lifespan and mainstream the use of lithium-sulfur batteries. For this research, Xiao and her colleagues honed in on the cathode to stop polysulfides from moving through the electrolyte.
Many materials with tiny holes have been examined to physically trap polysulfides inside the cathode. Metal organic frameworks are porous, but the added strength of PNNL’s material is its ability to strongly attract the polysulfide molecules.
The framework’s positively charged nickel center tightly binds the polysulfide molecules to the cathodes. The result is a coordinate covalent bond that, when combined with the framework’s porous structure, causes the polysulfides to stay put.
PNNL electrochemist Jianming Zheng explains, “The metal organic framework’s highly porous structure is a plus that further holds the polysulfide tight and makes it stay within the cathode.”
Metal organic frameworks nanomaterial- also called MOFs – are crystal-like compounds made of metal clusters connected to organic molecules, or linkers. Together, the clusters and linkers assemble into porous 3-D structures. The MOFs can contain a number of different elements. PNNL researchers chose the transition metal nickel as the central element for this particular MOF because of its strong ability to interact with sulfur.
During lab tests, a lithium-sulfur battery with PNNL’s MOF cathode maintained 89% of its initial power capacity after 100 charge-and discharge cycles. Having shown the effectiveness of their MOF cathode, PNNL researchers now plan to further improve the cathode’s mixture of materials so it can hold more energy. The team also needs to develop a larger prototype and test it for longer periods of time to evaluate the cathode’s performance for real-world, large-scale applications.
“MOFs are probably best known for capturing gases such as carbon dioxide,” Xiao said. “This study opens up lithium-sulfur batteries as a new and promising field for the nanomaterial.”
Eighty nine percent at 100 cycles is a huge improvement even though not being a truly marketable solution. But the PNNL team may be closer than we know for now. Back in January, a Nature Communications paper by Xiao and some of her PNNL colleagues described another possible solution for lithium-sulfur batteries other side: developing a hybrid anode that uses a graphite shield to block the polysulfides.
More research sure to come.
Microbes can be highly efficient, versatile and sophisticated manufacturing tools, and have the potential to form the basis of a vibrant economic sector. The report is based on the deliberations of experts who were gathered by the American Academy of Microbiology to discuss the potential contributions of a microbe-powered industry and the human elements needed for this emerging sector to thrive. It can be found online at: http://academy.asm.org/images/stories/documents/MicrobePoweredJobs.pdf. However the contents of the report reflect the discussions of the colloquium and are not intended to reflect official positions of the American Academy of Microbiology or the American Society for Microbiology.
In making a basis for the report Doran-Peterson said, “Industrial microbiology is experiencing a Renaissance; microorganisms make products ranging from the tightly regulated pharmaceuticals industry to large-scale production of commodity chemicals and biofuels. Educating and training the next generation of employees for these rapidly expanding industries is critically important to their survival.”
For thousands of years humans have harnessed the power of microbes to make products such as bread, cheese, beer and wine. In the early 20th century scientists discovered how to use mold to produce antibiotics. It has only been in the past few decades, with the advent of DNA-based technologies, that our understanding of the vast diversity of microbial capabilities has exploded.
“If there is a chemical you want to break down, there is probably a microbe that can do it. If there is a compound you wish to synthesize, a microbe can probably help,” says the report, entitled Microbe-Powered Jobs: How Microbiologists Can Help Build the Bioeconomy. The report provides a litany of examples of potential biological products including bioenergy, biofuels, environmentally friendly industrial chemicals, and bioenzymes (the production of which already fuels a nearly $4 billion market).
To take full advantage of the potential the bioeconomy offers, the report suggests academia needs to re-think and take a broader approach to teaching microbiology at the undergraduate level. According to the report, the future growth of a microbial-based industry sector depends on two crucial elements: expansion of the fundamental understanding of microbiology and translation of that understanding into viable products.
Current microbiology education primarily trains scientists with an eye toward academic research, which is what is needed to continue the expansion of knowledge. Most undergraduates that take microbiology, though, have an eye on a medical career, so many undergraduate microbiology curricula focus on the biomedical aspects of microbiology, according to the report.
Here is the point that could raise cheers among employers and entrepreneurs. The report said, “One can imagine that instead of the current situation where pre-medicine is virtually the only undergraduate program with a microbiology component, there could be a series of majors with microbiology at their cores.”
One specific major, which the report outlines, could be an industrial microbiology track, with a focus towards translation. Not only would it emphasize microbiology, but it would also include quantitative skills important for success in industry. This type of curriculum could also be made available to engineering students in the form of a bioengineering track.
In addition to the traditional degree programs, the report also recommends other formats be used to teach specialized skills or offer intensive introductions to new fields of study.
The report might seem to be an appeal to department chairs and curriculum designers to build out majors for students, and it is. But the report is much more, offering a wealth of information in narrative and graphics that make the case that we are overlooking an enormous opportunity.
The American Academy of Microbiology has a self interest, obviously. But for improving the future consumers, businesses and academics could do well to take notice of the points raised in the report.
For now the scientific understanding and technological capacity to put microbes to work continues to advance at an impressive pace. It will take a lot more well educated students and graduates to discover and produce the products of the future.