University of Texas at Arlington chemists have developed new high-performing materials for cells that harness sunlight to split carbon dioxide and water into usable fuels like methanol and hydrogen gas. These “green fuels” can be used to power cars, home appliances or even to store energy in batteries.
“Technologies that simultaneously permit us to remove greenhouse gases like carbon dioxide while harnessing and storing the energy of sunlight as fuel are at the forefront of current research,” said Krishnan Rajeshwar, UTA distinguished professor of chemistry and biochemistry and co-founder of the University’s Center of Renewable Energy, Science and Technology.
“Our new material could improve the safety, efficiency and cost-effectiveness of solar fuel generation, which is not yet economically viable,” he added.
The new hybrid platform uses ultra-long carbon nanotube networks with a homogeneous coating of copper oxide nanocrystals. It demonstrates both the high electrical conductivity of carbon nanotubes and the photocathode qualities of copper oxide, efficiently converting light into the photocurrents needed for the photoelectrochemical reduction process.
Morteza Khaledi, dean of the UTA College of Science, said Rajeshwar’s work is representative of the University’s commitment to addressing critical issues with global environmental impact under the Strategic Plan 2020.
“Dr. Rajeshwar’s ongoing, global leadership in research focused on solar fuel generation forms part of UTA’s increasing focus on renewable and sustainable energy,” Khaledi said. “Creating inexpensive ways to generate fuel from an unwanted gas like carbon dioxide would be an enormous step forward for us all.”
For the solar fuel cells project, Rajeshwar worked with Csaba Janáky, an assistant chemistry professor at the University of Szeged in Hungary and Janáky’s master’s student Egon Kecsenovity. Janaky served as a UTA Marie Curie Fellow from 2011 to 2013.
The findings are the subject of a Feb. 15 minireview, “Electrodeposition of Inorganic Oxide/Nanocarbon Composites: Opportunities and Challenges,” published in ChemElectroChem Europe and a companion article in the Journal of Materials Chemistry A on “Decoration of ultra long carbon nanotubes with Cu2O nanocrystals: a hybrid platform for photoelectrochemical CO2 reduction.”
“The performance of our hybrid has proved far superior to the properties of the individual materials,” Rajeshwar said. “These new hybrid films demonstrate five-fold higher electrical conductivity compared to their copper oxide counterparts, and generate a three-fold increase in the photocurrents needed for the reduction process.”
The new material also demonstrates much greater stability during long-term photoelectrolysis than pure copper oxide, which corrodes over time, forming metallic copper.
The research involved developing a multi-step electrodeposition process to ensure that a homogeneous coating of copper oxide nanoparticles were deposited on the carbon nanotube networks. By varying the thickness of the carbon nanotube film and the amount of electrodeposited copper oxide, the researchers were able to optimize the efficiency of this new hybrid material.
Rajeshwar also is working with Brian Dennis, a UTA associate professor of mechanical and aerospace engineering, and Norma Tacconi, a research associate professor of chemistry and biochemistry, on a project with NASA to develop improved methods for oxygen recovery and reuse aboard human spacecraft.
The team is designing, building and demonstrating a “microfluidic electrochemical reactor” to recover oxygen from carbon dioxide extracted from cabin air. The prototype will be built over the next months at the Center for Renewable Energy Science and Technology at UTA.
Rajeshwar joined the College of Science in 1983, is a charter member of the UTA Academy of Distinguished Scholars and senior vice president of The Electrochemical Society, an organization representing the nation’s premier researchers who are dedicated the advancing solid state, electrochemical science and technology.
He is an expert in photoelectrochemistry, nanocomposites, electrochemistry and conducting polymers, and has received numerous awards, including the Wilfred T. Doherty Award from the American Chemical Society and the Energy Technology Division Research Award of the Electrochemical Society.
Rajeshwar earned his Ph.D. in chemistry from the Indian Institute of Science in Bangalore, India, and completed his post-doctoral training in Colorado State University.
E. Kecsenovity, B. Endrődi, Zs. Pápa, K. Hernádi, K. Rajeshwar, C. Janáky. Decoration of ultra-long carbon nanotubes with Cu2O nanocrystals: a hybrid platform for enhanced photoelectrochemical CO2reduction. J. Mater. Chem. A, 2016; 4 (8): 3139 DOI:10.1039/C5TA10457B
Prof. Walter Mérida is working on fuel cells powered by hydrogen to allow us to replace fossil fuels with a truly zero-emission chemical fuel.
Moving away from fossil fuels like coal and oil are an important step in making our energy consumption more sustainable. Alternative sources include hydro, solar, and wind, but once electricity is generated, it needs to be used right away because we lack a reliable method to store large amounts of power. Prof. Walter Mérida, Director of the Clean Energy Research Centre at the University of British Columbia, is looking for ways to bypass fossil fuels by using electricity to generate hydrogen as a zero-emission chemical fuel.
“The simplest possible chemical that you can imagine is hydrogen. It is the lightest element, the simplest element, and it’s one of the elements that you can make from electricity and water. So if you use electrolysis in the one hand and water in the other to produce a chemical fuel, you can really envision a truly zero emission transportation system.“
This move is driven by our increased power needs for modern services and technologies. However, to make a real change, we need a better system. “The main driver for energy system evolution is not scarcity. We didn’t abandon the stone age due to the scarcity of stones. We abandoned it because there were better things to build things with. And in the case of fossil fuel – these transitions you have seen from wood, to coal, to oil – are due to quality and convenience; the fuels are much more convenient,” explains Mérida.
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Researchers at the National Institute of Standards and Technology (NIST) have developed a fast, simple process for making platinum “nano-raspberries”—microscopic clusters of nanoscale particles of the precious metal. The berry-like shape is significant because it has a high surface area, which is helpful in the design of catalysts. Even better news for industrial chemists: the researchers figured out when and why the berry clusters clump into larger bunches of “nano-grapes.” The research could help make fuel cells more practical.
Colorized micrographs of platinum nanoparticles made at NIST. The raspberry color suggests the particles’ corrugated shape, which offers high surface area for catalyzing reactions in fuel cells. Individual particles are 3-4 nm in diameter but can clump into bunches of 100 nm or more under specific conditions discovered in a NIST study.
Credit: Curtin/NIST View hi-resolution image
Nanoparticles can act as catalysts to help convert methanol to electricity in fuel cells. NIST’s 40-minute process for making nano-raspberries, described in a new paper,* has several advantages. The high surface area of the berries encourages efficient reactions. In addition, the NIST process uses water, a benign or “green” solvent. And the bunches catalyze methanol reactions consistently and are stable at room temperature for at least eight weeks.
Although the berries were made of platinum, the metal is expensive and was used only as a model. The study will actually help guide the search for alternative catalyst materials, and clumping behavior in solvents is a key issue. For fuel cells, nanoparticles often are mixed with solvents to bind them to an electrode. To learn how such formulas affect particle properties, the NIST team measured particle clumping in four different solvents for the first time. For applications such as liquid methanol fuel cells, catalyst particles should remain separated and dispersed in the liquid, not clumped.
“Our innovation has little to do with the platinum and everything to do with how new materials are tested in the laboratory,” project leader Kavita Jeerage says. “Our critical contribution is that after you make a new material you need to make choices. Our paper is about one choice: what solvent to use. We made the particles in water and tested whether you could put them in other solvents. We found out that this choice is a big deal.”
The NIST team measured conditions under which platinum particles, ranging in size from 3 to 4 nanometers (nm) in diameter, agglomerated into bunches 100 nm wide or larger. They found that clumping depends on the electrical properties of the solvent. The raspberries form bigger bunches of grapes in solvents that are less “polar,” that is, where solvent molecules lack regions with strongly positive or negative charges. (Water is a strongly polar molecule.)
The researchers expected that. What they didn’t expect is that the trend doesn’t scale in a predictable way. The four solvents studied were water, methanol, ethanol and isopropanol, ordered by decreasing polarity. There wasn’t much agglomeration in methanol; bunches got about 30 percent bigger than they were in water. But in ethanol and isopropanol, the clumps got 400 percent and 600 percent bigger, respectively—really humongous bunches. This is a very poor suspension quality for catalytic purposes.
Because the nanoparticles clumped up slowly and not too much in methanol, the researchers concluded that the particles could be transferred to that solvent, assuming they were to be used within a few days—effectively putting an expiration date on the catalyst.
Two college students in NIST’s Summer Undergraduate Research Fellowship (SURF) program helped with the extensive data collection required for the study.
* I. Sriram, A.E. Curtin, A.N. Chiaramonti, J.H. Cuchiaro, A.D. Weidner, T.M. Tingley, L.F. Greenlee and K.M. Jeerage. Stability and phase transfer of catalytically active platinum nanoparticle suspensions. Journal of Nanoparticle Research 17:230.DOI 10.1007/s11051-015-3034-1. Published online May 22, 2015
Catalysts are used with fuels such as hydrogen or methanol to produce hydrogen ions. Platinum, which is very expensive, is the catalyst typically used in this process. Companies are using nanoparticles of platinum to reduce the amount of platinum needed, or using nanoparticles of other materials to replace platinum entirely and thereby lower costs.
Fuel cells contain membranes that allow hydrogen ions to pass through the cell but do not allow other atoms or ions, such as oxygen, to pass through. Companies are using nanotechnology to create more efficient membranes; this will allow them to build lighter weight and longer lasting fuel cells.
Small fuel cells are being developed that can be used to replace batteries in handheld devices such as PDAs or laptop computers. Most companies working on this type of fuel cell are using methanol as a fuel and are calling them DMFC’s, which stands for direct methanol fuel cell. DMFC’s are designed to last longer than conventional batteries. In addition, rather than plugging your device into an electrical outlet and waiting for the battery to recharge, with a DMFC you simply insert a new cartridge of methanol into the device and you’re ready to go.
Fuel cells that can replace batteries in electric cars are also under development. Hydrogen is the fuel most researchers propose for use in fuel cell powered cars. In addition to the improvements to catalysts and membranes discussed above, it is necessary to develop a lightweight and safe hydrogen fuel tank to hold the fuel and build a network of refueling stations. To build these tanks, researchers are trying to develop lightweight nanomaterials that will absorb the hydrogen and only release it when needed. The Department of Energy is estimating that widespread usage of hydrogen powered cars will not occur until approximately 2020.
Fuel Cells: Nanotechnology Applications
Researchers at the University of Copenhagen have demonstrated the ability to significantly reduce the amount of platinum needed as a catalyst in fuel cells. The researchers found that the spacing between platinum nanoparticles affected the catalytic behavior, and that by controlling the packing density of the platinum nanoparticles they could reduce the amount of platinum needed.
Researchers at Brown University are developing a catalyst that uses no platinum. The catalyst is made from a sheet of graphene coated with cobalt nanoparticles. If this catalyst works out for production use with fuel cells it should be much less expensive than platinum based catalysts.
Researchers at Ulsan National Institute of Science and Technology have demonstrated how to produce edge-halogenated graphene nanoplatelets that have good catalytic properties. The researchers prepared the nanoplatelets by ball-milling graphene flakes in the presence of chlorine, bromine or iodine. They believe these halogenated nanoplatelets could be used as a replacement for expensive platinum catalystic material in fuel cells.
Researchers at Cornell University have developed a catalyst using platinum-cobalt nanoparticles that produces 12 times more catalytic activity than pure platinum. In order to achieve this performance the researchers annealed the nanoparticles so they formed a crystalline lattice which reduced the spacing between platinum atoms on the surface, increasing their reactivity.
Researchers at the University of Illinois have developed a proton exchange membrane using a silicon layer with pores of about 5 nanometers in diameter capped by a layer of porous silica. The silica layer is designed to insure that water stays in the nanopores. The water combines with the acid molecules along the wall of the nanopores to form an acidic solution, providing an easy pathway for hydrogen ions through the membrane. Evaluation of this membrane showed it to have much better conductivity of hydrogen ions (100 times better conductivity was reported) in low humidity conditions than the membrane normally used in fuel cells.
Researchers at Rensselaer Polytechnic Institute have investigated the storage of hydrogen in graphene (single atom thick carbon sheets). Hydrogen has a high bonding energy to carbon, and the researchers used annealing and plasma treatment to increase this bonding energy. Because graphene is only one atom thick it has the highest surface area exposure of carbon per weight of any material. High hydrogen to carbon bonding energy and high surface area exposure of carbon gives graphene has a good chance of storing hydrogen. The researchers found that they could store14% by weight of hydrogen in graphene.
Researchers at Stony Brook University have demonstrated that gold nanoparticles can be very effective at using solar energy to generate hydrogen from water. The key is making the nanoparticles very small. They found that nanoparticles containing less than a dozen gold atoms are very effective photocatalysts for the generation of hydrogen.
Researchers at the SLAC National Accelerator Laboratory have developed a way to use less platinum for the cathode in a fuel cell, which could significantly reduce the cost of fuel cells. They alloyed platinum with copper and then removed the copper from the surface of the film, which caused the platinum atoms to move closer to each other (reducing the lattice space). It turns out that platinum with reduced lattice spacing is more a more effective catalyst for breaking up oxygen molecules into oxygen ion. The difference is that the reduced spacing changes the electronic structure of the platinum atoms so that the separated oxygen ions more easily released, and allowed to react with the hydrogen ions passing through the proton exchange membrane.
Another way to reduce the use of platinum for catalyst in fuel cell cathodes is being developed by researchers at Brown University. They deposited a one nanometer thick layer of platinum and iron on spherical nanoparticles of palladium. In laboratory scale testing they found that an catalyst made with these nanoparticles generated 12 times more current than a catalyst using pure platinum, and lasted ten times longer. The researchers believe that the improvement is due to a more efficient transfer of electrons than in standard catalysts.
Abstract: Swedish and Chinese researchers show how a unique nano-alloy composed of palladium nano-islands embedded in tungsten nanoparticles creates a new type of catalysts for highly efficient oxygen reduction, the most important reaction in hydrogen fuel cells. Their results are published in the scientific journal Nature Communications.
Unique catalysts for hydrogen fuel cells synthesized in ordinary kitchen microwave oven
Umea, Sweden | Posted on October 14th, 2014
The world’s rapidly growing demand for energy and the requirement of sustainable energy production calls for an urgent change in today’s fossil fuel based energy system. Research groups worldwide work intensively to develop novel advanced energy conversion and storage systems with high efficiency, low cost and environmental compatibility.
A schematic model of the unique morphology of the alloy. The Pd-islands (light-brown spheres) are embedded in an environment of tungsten (blue spheres). Oxygen are represented by red spheres, and hydrogen by white spheres.
Fuel cell systems represent a promising alternative for low carbon emission energy production. Traditional fuel cells are however limited by the need of efficient catalysts to drive the chemical reactions involved in the fuel cell. Historically, platinum and its alloys have frequently been used as anodic and cathodic catalysts in fuel cells, but the high cost of platinum, due to its low abundance, motivates researchers to find efficient catalysts based on earth-abundant elements.
“In our study we report a unique novel alloy with a palladium (Pd) and tungsten (W) ratio of only one to eight, which still has similar efficiency as a pure platinum catalyst. Considering the cost, it would be 40 times lower”, says Thomas Wågberg, Senior lecturer at Department of Physics, Umeå University.
The explanation for the very high efficiency is the unique morphology of the alloy. It is neither a homogeneous alloy, nor a fully segregated two-phase system, but rather something in between.
By advanced experimental and theoretical investigations, the researchers show that the alloy is composed of metallic Pd-islands embedded in the Pd-W alloy. The size of the islands are about one nanometer in diameter and are composed of 10-20 atoms that are segregated to the surface. The unique environment around the Pd-islands give rise to special effects that all together turn the islands into highly efficient catalytic hot-spots for oxygen reduction.
To stabilize the nanoparticles in practical applications, they are anchored on ordered mesoporous carbon. The anchoring keep the nanoparticles stable over long time by hindering them from fusing together in the fuel cell tests.
“The unique formation of the material is based on a synthesis method, which can be performed in an ordinary kitchen micro-wave oven purchased at the local supermarket. If we were not using argon as protective inert gas, it would be fully possible to synthesize this advanced catalyst in my own kitchen! “, says Thomas Wågberg.
Wågberg and his fellow researchers have recently received funding from the Kempe Foundation to buy a more advanced micro-wave oven, and therefore they will be able to run more advanced experiments to fine tune some of the catalyst properties.
The Artificial Leaf
The study has been conducted within the artificial leaf project, which is funded by Knut and Alice Wallenberg foundation. Physicist, chemists, and plant science researchers at Umeå University work together in the search for clean and renewable energy sources.
“The solar energy business has been trying to overcome … challenge for years. The cost of installing solar panels has fallen dramatically but storing the energy produced for later use has been problematic.”
“In a single hour, the amount of power from the sun that strikes the Earth is more than the entire world consumes in an year.” To put that in numbers, from the US Department of EnergyEach hour 430 quintillion Joules of energy from the sun hits the Earth. That’s 430 with 18 zeroes after it! In comparison, the total amount of energy that all humans use in a year is 410 quintillion Joules. For context, the average American home used 39 billion Joules of electricity in 2013.
Clearly, we have in our sun “a source of unlimited renewable energy”. But how can we best harness this resource? How can we convert and “store” this energy resource on for sun-less days or at night time … when we also have energy needs?
Now therein lies the challenge!
Would you buy a smartphone that only worked when the sun was shining? Probably not. What it if was only half the cost of your current model: surely an upgrade would be tempting? No, thought not.
The solar energy business has been trying to overcome a similar challenge for years. The cost of installing solar panels has fallen dramatically but storing the energy produced for later use has been problematic.
Now scientists in Sweden have found a new way to store solar energy in chemical liquids. Although still in an early phase, with niche applications, the discovery has the potential to make solar power more practical and widespread.
Until now, solar energy storage has relied on batteries, which have improved in recent years. However, they are still bulky and expensive, and they degrade over time.
Trap and release solar power on demand
A research team from Chalmers University of Technology in Gothenburg made a prototype hybrid device with two parts. It’s made from silica and quartz with tiny fluid channels cut into both sections.
The top part is filled with a liquid that stores solar energy in the chemical bonds of a molecule. This method of storing solar energy remains stable for several months. The energy can be released as heat whenever it is required.
The lower section of the device uses sunlight to heat water which can be used immediately. This combination of storage and water heating means that over 80% of incoming sunlight is converted into usable energy.
Suddenly, solar power looks a lot more practical. Compared to traditional battery storage, the new system is more compact and should prove relatively inexpensive, according to the researchers. The technology is in the early stages of development and may not be ready for domestic and business use for some time.
From the lab to off-grid power stations or satellites?
The researchers wrote in the journal Energy & Environmental Science: “This energy can be transported, and delivered in very precise amounts with high reliability(…) As is the case with any new technology, initial applications will be in niches where [molecular storage] offers unique technical properties and where cost-per-joule is of lesser importance.”
The team now plans to test the real-world performance of the technology and estimate how much it will cost. Initially, the device could be used in off-grid power stations, extreme environments, and satellite thermal control systems.
Editor’s Note: As Solomon wrote in Ecclesiastes 1:9: “What has been will be again, what has been done will be done again; there is nothing new under the sun.”
Storing Solar Energy chemically and converting ‘waste heat’ has and is the subject of many research and implementation Projects around the globe. Will this method prove to be “the one?” This writer (IMHO) sees limited application, but not a broadly accepted and integrated solution.
Solar Energy to Hydrogen Fuel
So where does that leave us? We have been following the efforts of a number of Researchers/ Universities who are exploring and developing “Sunlight to Hydrogen Fuel” technologies to harness the enormous and almost inexhaustible energy source power-house … our sun! What do you think? Please leave us your Comments and we will share the results with our readers!
We have written and posted extensively about ‘Solar to Hydrogen Renewable Energy’ – here are some of our previous Posts:
Researchers at Rice University are on to a relatively simple, low-cost way to pry hydrogen loose from water, using the sun as an energy source. The new system involves channeling high-energy “hot” electrons into a useful purpose before they get a chance to cool down. If the research progresses, that’s great news for the hydrogen […]
HyperSolar has achieved a major milestone with its hybrid technology HyperSolar, a company that specializes in combining hydrogen fuel cells with solar energy, has reached a significant milestone in terms of hydrogen production. The company harnesses the power of the sun in order to generate the electrical power needed to produce hydrogen fuel. This is […]
Rice University researchers have demonstrated an efficient new way to capture the energy from sunlight and convert it into clean, renewable energy by splitting water molecules. The technology, which is described online in the American Chemical Society journal Nano Letters, relies on a configuration of light-activated gold nanoparticles that harvest sunlight and transfer solar energy […]
NREL researchers Myles Steiner (left), John Turner, Todd Deutsch and James Young stand in front of an atmospheric pressure MDCVD reactor used to grow crystalline semiconductor structures. They are co-authors of the paper “Direct Solar-to-Hydrogen Conversion via Inverted Metamorphic Multijunction Semiconductor Architectures” published in Nature Energy. Photo by Dennis Schroeder. Scientists at the U.S. […]
Photo shows a lead sulfide quantum dot solar cell. A lead sulfide quantum dot solar cell developed by researchers at NREL. Photo by Dennis Schroeder.
Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have developed a proof-of-principle photo-electro-chemical cell capable of capturing excess photon energy normally lost to generating heat. Using quantum […]
The sun was out in full force the fall morning I arrived at Caltech to visit Professor Nate Lewis’s research laboratory. Temperatures in southern California had soared to 20 degrees above normal, prompting the National Weather Service to issue warnings for extreme fire danger and heat-related illnesses.
The weather was a fitting introduction to what I had come to see inside Nate’s lab—how we might be able to tap the sun’s tremendous energy to make fuels to power cars, trucks, ships, and airplanes.
Stepping into the lab cluttered with computer screens, jars of chemicals, beakers, and other equipment, Nate handed me a pair of safety goggles and offered some advice for what I was about to see. “Everything we do is simple in the end, even though there’s lots of complicated stuff,” he said.
What’s simple is the idea behind all of his team’s research: The sun is the most reliable, plentiful source of renewable energy we have. In fact, more energy from the sun hits the Earth in one hour than humans use in an entire year. If we can find cheap and efficient ways to tap just a fraction of its power, we will go a long way toward finding a clean, affordable, and reliable energy source for the future.
We are all familiar with solar panels, which convert sunlight into electricity. As solar panel costs continue to fall, it’s been encouraging to see how they are becoming a growing source of clean energy around the world. Of course, there’s one major challenge of solar power. The sun sets each night and there are cloudy days. That’s why we need to find efficient ways to store the energy from sunlight so it’s available on demand.
Batteries are one solution. Even better would be a solar fuel. Fuels have a much higher energy density than batteries, making it far easier to use for storage and transportation. For example, one ton of gasoline stores the same amount of energy as 60 tons of batteries. That’s why, barring a major breakthrough in battery technology, it’s hard to imagine flying from Seattle to Tokyo on a plug-in airplane.
I’ve written before about the need for an energy miracle to halt climate change and provide access to electricity to millions of the poorest families who live without it. Making solar fuel would be one of those miracles. It would solve the energy storage problem for when the sun isn’t shining. And it would provide an easy-to-use power source for our existing transportation infrastructure. We could continue to drive the cars we have now. Instead of running on fossil fuels from the ground, they would be powered by fuel made from sunlight. And because it wouldn’t contribute additional greenhouses gases to the atmosphere, it would be carbon neutral.
Imagining such a future is tantalizing. Realizing it will require a lot of hard work. No one knows if there’s a practical way to turn sunlight into fuel. Thanks to the U.S. Department of Energy, Nate and a group of other researchers around the U.S. are receiving research support to find out if it is possible.
We live in a time when new discoveries and innovations are so commonplace that it’s easy to take the cutting-edge research I saw at Caltech for granted. But most breakthroughs that improve our lives—from new health interventions to new clean energy ideas—get their start as government-sponsored research like Nate’s. If successful, that research leads to new innovations, that spawn new industries, that create new jobs, that spur economic growth. It’s impossible to overemphasize the importance of government support in this process. Without it, human progress would not come as far as it has.
Nate and his team are still at the first stage of this process. But they have reason to be optimistic about what lies ahead. After all, turning sunlight into chemical energy is what plants do every day. Through the process of photosynthesis, plants combine sunlight, water, and carbon dioxide to store solar energy in chemical bonds. At Nate’s lab, his team is working with the same ingredients. The difference is that they need to figure out how to do it even better and beat nature at its own game.
“We want to create a solar fuel inspired by what nature does, in the same way that man built aircraft inspired by birds that fly,” Nate said. “But you don’t build an airplane out of feathers. And we’re not going to build an artificial photosynthetic system out of chlorophylls and living systems, because we can do better than that.”
One of Nate’s students showed me how light can be used to split water into oxygen and hydrogen—a critical first step in the path to solar fuels. The next step would involve combining hydrogen with carbon dioxide to make fuels. Using current technologies, however, it is too costly to produce a fuel from sunlight. To make it cheaper, much more research needs to be done to understand the materials and systems that could create a dependable source of solar fuel.
One idea his team is working on is a kind of artificial turf made of plastic cells that could be easily rolled out to capture sunlight to make fuel. Each plastic cell would contain water, light absorbers, and a catalyst. The catalyst helps accelerate the chemical reactions so each cell can produce hydrogen or carbon-based fuels more efficiently. Unfortunately, the best catalysts are among the rarest and most expensive elements, like platinum. A key focus of Nate’s research is finding other catalysts that are not only effective and durable, but also economical.
Nate’s interest in clean energy research started during the oil crisis in the 1970s, when he waited for hours in gas lines with his dad. He says he knew then that he wanted to dedicate his life to energy research. Now, he is helping to train a new generation of scientists to help solve our world’s energy challenge. Seeing the number of young people working in Nate’s lab was inspiring. The pace of innovation for them is now much faster than ever before. “We do experiments now in a day that would once take a year or an entire Ph.D. thesis to do,” Nate said.
Still, I believe we should be doing a lot more. We need thousands of scientists following all paths that might lead us to a clean energy future. That’s why a group of investors and I recently launched Breakthrough Energy Ventures, a fund that will invest more than $1 billion in scientific discoveries that have the potential to deliver cheap and reliable clean energy to the world.
While we won’t be filling up our cars with solar fuels next week or next year, Nate’s team has already made valuable contributions to our understanding of how we might achieve this bold goal. With increased government and private sector support, we will make it possible for them to move ahead with their research at full speed.
In a discovery that could have profound implications for future energy policy, Columbia scientists have demonstrated it is possible to manufacture solar cells that are far more efficient than existing silicon energy cells by using a new kind of material, a development that could help reduce fossil fuel consumption.
The team, led by Xiaoyang Zhu, a professor of Chemistry at Columbia University, focused its efforts on a new class of solar cell ingredients known as Hybrid Organic Inorganic Perovskites (HOIPs).
Their results, reported in the prestigious journal Science, also explain why these new materials are so much more efficient than traditional solar cells—solving a mystery that will likely prompt scientists and engineers to begin inventing new solar materials with similar properties in the years ahead.
“The need for renewable energy has motivated extensive research into solar cell technologies that are economically competitive with burning fossil fuel,” Zhu says.
“Among the materials being explored for next generation solar cells, HOIPs have emerged a superstar. Until now no one has been able to explain why they work so well, and how much better we might make them. We now know it’s possible to make HOIP-based solar cells even more efficient than anyone thought possible.”
Solar cells are what turn sunlight into electricity. Also known as photovoltaic cells, these semiconductors are most frequently made from thin layers of silicon that transmit energy across its structure, turning it into DC current.
Silicon panels, which currently dominate the market for solar panels, must have a purity of 99.999 percent and are notoriously fragile and expensive to manufacture. Even a microscopic defect—such as misplaced, missing or extra ions—in this crystalline structure can exert a powerful pull on the charges the cells generate when they absorb sunlight, dissipating those charges before they can be transformed into electrical current.
In 2009, Japanese scientists demonstrated it was possible to build solar cells out of HOIPs, and that these cells could harvest energy from sunlight even when the crystals had a significant number of defects. Because they don’t need to be pristine, HOIPs can be produced on a large scale and at low cost. The Columbia team has been investigating HOIPs since 2014. Their findings could help boost the use of solar power, a priority in the age of global warming.
Over the last seven years, scientists have managed to increase the efficiency with which HOIPs can convert solar energy into electricity, to 22 percent from 4 percent. By contrast, it took researchers more than six decades to create silicon cells and bring them to their current level, and even now silicon cells can convert no more than about 25 percent of the sun’s energy into electrical current.
This discovery, Zhu said, meant that “scientists have only just begun to tap the potential of HOIPs to convert the sun’s energy into electricity.”
Theorists long ago demonstrated that the maximum efficiency silicon solar cells might ever reach— the percentage of energy in sunlight that might be converted to electricity we can use—is roughly 33 percent. It takes hundreds of nanoseconds for energized electrons to move from the part of a solar cell that infuses them with the sun’s energy, to the part of the cell that harvests the energy and converts it into electricity that can ultimately be fed into a power grid. During this migration across the solar cell, the energized electrons quickly dissipate their excess energy.
But those calculations assume a specific rate of energy loss.
The Columbia team discovered that the rate of energy loss is slowed down by over three-orders of magnitude in HOIPs – making it possible for the harvesting of excess electronic energy to increase the efficiency of solar cells.
“We’re talking about potentially doubling the efficiency of solar cells,” says Prakriti P. Joshi, a Ph.D. student in Zhu’s lab who is a coauthor on the paper. “That’s really exciting because it opens up a big, big field in engineering.” Adds Zhu, “This shows we can push the efficiencies of solar cells much higher than many people thought possible.”
After demonstrating this, the team then turned to the next question: what is it about the molecular structure of HOIPs that gives them their unique properties? How do electrons avoid defects? They discovered that the same mechanism that slows down the cooling of electron energy also protects the electrons from bumping into defects.
This “protection” makes the HOIPs turn a blind eye to the ubiquitous defects in a material developed from room-temperature and solution processing, thus allowing an imperfect material to behave like a perfect semiconductor.
HOIPs contain lead, and are also water soluble, meaning the solar cells could begin to dissolve and leach lead into the environment around them if not carefully protected from the elements.
With the explanation of the mysterious mechanisms that give HOIPs their remarkable efficiencies, Zhu knew, material scientists would likely be able to mimic them with more environmentally-friendly materials.
“Now we can go back and design materials which are environmentally benign and really solve this problem everybody is worried about,” Zhu says. “This principle will allow people to start to design new materials for solar energy.”
Explore further: New plastic solar cell minimizes loss of photon energy
More information: H. Zhu et al. Screening in crystalline liquids protects energetic carriers in hybrid perovskites, Science (2016). DOI: 10.1126/science.aaf9570
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 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
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.
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
Fuel cell electric vehicles have a long way to go before they can compete with their battery EV cousins, and energy storage is a key sticking point when the fuel is hydrogen. Hydrogen is light, plentiful, and fabulously energy dense, but energy storage in a personal mobility unit racing down a crowded highway is a different kind of chicken. Safety, cost, and performance are critical sticking points, and a research team at Lawrence Berkeley Laboratory is on to a solution for at least one of those.
Energy Storage Challenges For Hydrogen Fuel Cell EVs
The US Energy Department’s 2015 annual report provides a birds-eye view of the array of energy storage solutions that are emerging for hydrogen fuel cells, including advancements in hydrogen tank technology as well as solids-based storage.
Despite the progress, according to the Energy Department, challenges still remain for stationary and portable fuel cells in terms of raising the energy storage density, and there are “significant challenges” for fuel cell EVs. The problem is this:
Hydrogen has the highest energy per mass of any fuel; however, its low ambient temperature density results in a low energy per unit volume, therefore requiring the development of advanced storage methods that have potential for higher energy density.
The Energy Department has set a goal of 2020 for achieving verifiable hydrogen storage systems for light duty fuel cell EVs that meet the driving public’s thirst for range, comfort, refueling convenience, and performance. Here are the targets:
However, the Energy Department is already looking beyond the current state of on-road technology to meet its 2020 goal. According to the agency, the 300-mile range is being met by using compressed gas, high pressure energy storage technology, and the problem is that competing technology on the market today — primarily gasmobiles and hybrids — already exceeds that range.
To compete for consumers on the open market, the agency is pursuing a near-term goal of improving compressed gas storage, primarily by deploying fiber reinforced composites that enable 700 bar pressure.
Where were we? Oh right, Berkeley Lab. Berkeley Lab has been tackling the metal hydride pathway.
Metal hydrides are compounds that consist of a transition metal bonded to hydrogen. They are believed to be the most “technologically relevant” class of materials for storing hydrogen, partly due to the broad range of applications.
That’s the theory. The problem is that when it comes to real world performance, metal hydrides are highly sensitive to contamination and they degrade somewhat rapidly unless properly shielded.
The Berkeley Lab energy storage solution consists of a graphene “filter” encasing nanocrystals of magnesium. With the addition of the graphene layer, the magnesium crystals act as a sort of sponge for absorbing hydrogen, providing both safety and compactness without causing performance issues:
The graphene shields the nanocrystals from oxygen and moisture and contaminants, while tiny, natural holes allow the smaller hydrogen molecules to pass through. This filtering process overcomes common problems degrading the performance of metal hydrides for hydrogen storage.
Berkeley Lab has provided this photo to show off how stable the crystals are when exposed to air (for scale, the bottle cap is about the size of a thumbnail):
At one atom thick (yes, one atom), graphene is known to be an incredibly finicky material to work with. It is extremely difficult to synthesize it without defects, but that’s not a problem for this energy storage solution. The defects are actually desirable in this case. The tiny gaps enable molecules of hydrogen gas to wriggle through, but oxygen, water, and other contaminants are too large to penetrate the shield.
The new energy formula also solves another key challenge for metal hydrides. They tend to take in and dispense hydrogen at a relatively slow pace, but the Berkeley Lab solution has sped up the intake-outflow cycle significantly. That effect is attributed to the nanoscale size of the graphene-shielded crystals, which provide a greater surface area.
Energy Department Gets The Last Word?
We’ve been having a lively debate about fuel cell electric EVs over here at CleanTechnica, so let’s hear from the Berkeley Lab team:
A potential advantage for hydrogen-fuel-cell vehicles, in addition to their reduced environmental impact over standard-fuel vehicles, is the high specific energy of hydrogen, which means that hydrogen fuel cells can potentially take up less weight than other battery systems and fuel sources while yielding more electrical energy.
However, the team also makes it clear that:
More R&D is needed to realize higher-capacity hydrogen storage for long-range vehicle applications that exceed the performance of existing electric-vehicle batteries…
Among other issues, the next step for a sustainable fuel cell EV future is to develop sustainable and renewable sources for hydrogen fuel. Currently the main source of hydrogen is natural gas, which puts fuel cell EVs in the same boat as battery EVs that draw electricity from a coal or natural gas-fired grid.