New Nanoparticles to make Solar Cells Cheaper to Manufacture


Originally posted on Great Things from Small Things .. Nanotechnology Innovation:

072613solarUniv. of Alberta researchers have found that abundant materials in the Earth’s crust can be used to make inexpensive and easily manufactured nanoparticle-based solar cells.

The U of A discovery, several years in the making, is an important step forward in making solar power more accessible to parts of the world that are off the traditional electricity grid or face high power costs, such as the Canadian North, said researcher Jillian Buriak, a chemistry professor and senior research officer of the National Institute for Nanotechnology, based on the U of A campus.

Buriak and her team have designed nanoparticles that absorb light and conduct electricity from two very common elements: phosphorus and zinc. Both materials are more plentiful than scarce materials such as cadmium and free from manufacturing restrictions imposed on lead-based nanoparticles.

“Half the world already lives off the grid, and with demand for electrical…

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Harvard Researchers Develop New System for Producing Stable, Amorphous Nanoparticles from Wide Material Range


Before Ibuprofen can relieve your headache, it has to dissolve in your bloodstream. The problem is Ibuprofen, in its native form, isn’t particularly soluble. Its rigid, crystalline structures — the molecules are lined up like soldiers at roll call — make it hard to dissolve in the bloodstream. To overcome this, manufacturers use chemical additives to increase the solubility of Ibuprofen and many other drugs, but those additives also increase cost and complexity.

David A. Weitz, Mallinckrodt Professor of Physics and Applied Physics (Photo by Eliza Grinnell, Harvard SEAS.)

The key to making drugs by themselves more soluble is not to give the molecular soldiers time to fall in to their crystalline structures, making the particle unstructured or amorphous.

Researchers from Harvard John A. Paulson School of Engineering and Applied Science (SEAS) have developed a new system that can produce stable, amorphous nanoparticles in large quantities that dissolve quickly.

But that’s not all. The system is so effective that it can produce amorphous nanoparticles from a wide range of materials, including for the first time, inorganic materials with a high propensity towards crystallization, such as table salt.

These unstructured, inorganic nanoparticles have different electronic, magnetic and optical properties from their crystalized counterparts, which could lead to applications in fields ranging from materials engineering to optics.

David A. Weitz, Mallinckrodt Professor of Physics and Applied Physics and an associate faculty member of the Wyss Institute for Biologically Inspired Engineering at Harvard, describes the research in a paper published today in Science.

“This is a surprisingly simple way to make amorphous nanoparticles from almost any material,” said Weitz. “It should allow us to quickly and easily explore the properties of these materials. In addition, it may provide a simple means to make many drugs much more useable.”

The technique involves first dissolving the substances in good solvents, such as water or alcohol. The liquid is then pumped into a nebulizer, where compressed air moving twice the speed of sound sprays the liquid droplets out through very narrow channels. It’s like a spray can on steroids. The droplets are completely dried between one to three microseconds from the time they are sprayed, leaving behind the amorphous nanoparticle.

At first, the amorphous structure of the nanoparticles was perplexing, said Esther Amstad, a former postdoctoral fellow in Weitz’ lab and current assistant professor at EPFL in Switzerland. Amstad is the paper’s first author. Then, the team realized that the nebulizer’s supersonic speed was making the droplets evaporate much faster than expected.

“If you’re wet, the water is going to evaporate faster when you stand in the wind,” said Amstad. “The stronger the wind, the faster the liquid will evaporate. A similar principle is at work here. This fast evaporation rate also leads to accelerated cooling. Just like the evaporation of sweat cools the body, here the very high rate of evaporation causes the temperature to decrease very rapidly, which in turn slows down the movement of the molecules, delaying the formation of crystals.”

These factors prevent crystallization in nanoparticles, even in materials that are highly prone to crystallization, such as table salt. The amorphous nanoparticles are exceptionally stable against crystallization, lasting at least seven months at room temperature.

The next step, Amstad said, is to characterize the properties of these new inorganic amorphous nanoparticles and explore potential applications.

“This system offers exceptionally good control over the composition, structure, and size of particles, enabling the formation of new materials,” said Amstad. “ It allows us to see and manipulate the very early stages of crystallization of materials with high spatial and temporal resolution, the lack of which had prevented the in-depth study of some of the most prevalent inorganic biomaterials. This systems opens the door to understanding and creating new materials.”

This research was coauthored by Manesh Gopinadhan, Christian Holtze, Chinedum O. Osuji, Michael P. Brenner, the Glover Professor of Applied Mathematics and Applied Physics and Professor of Physics, and Frans Spaepen, the John C. and Helen F. Franklin Professor of Applied Physics. It was supported by the National Science Foundation, Harvard MRSEC and BASF through the North American Center for Research on Advanced Materials (NORA), headed by Dr. Marc Schroeder.

Source: http://www.harvard.edu

DOE & Argone National Laboratory: New Catalyst may make Commercialization of Fuel Cell Vehicles a Reality


Argone NL 090115 114727Scientists at the U.S. Department of Energy’s Argonne National Laboratory have developed a new fuel cell catalyst using earthly abundant materials with performance that is comparable to platinum in laboratory tests. If commercially viable, the new catalyst could replace platinum in electric cars powered by fuel cells instead of batteries, which would greatly extend the range of electric vehicles and eliminate the need for recharging.

Fuel cells generate electricity by using hydrogen from a fuel tank with oxygen in the air. The only waste product emitted to the environment is water.

But fuel cells are expensive, largely because they depend on the precious metal platinum to cause the hydrogen-oxygen reaction. Argonne’s fuel cell catalyst replaces much of the platinum with a non-precious metal.

“Platinum represents about 50 percent of the cost of a fuel cell stack, so replacing or reducing platinum is essential to lowering the price of fuel cell vehicles,” said Di-Jia Liu, who led the Argonne team. Their catalyst replaces all the platinum in the fuel cell’s cathode, which usually requires four times as much platinum as the anode, and their new electrode design also optimizes the flow of protons and electrons within the fuel cell and the removal of water.

Many automakers see sales of vehicles powered by fuel cells as eventually outpacing battery-powered electric vehicles for several reasons: fuel-cell vehicles emit only water, can travel over 300 miles between fill ups, can be refilled quickly and place no burden on the electrical grid because they don’t need recharging.

Since both technologies lack refilling or recharging infrastructures and are expensive, both are currently suitable mainly for early adopters and use in corporate fleets. But this may change, if advances made by Argonne researchers can be realized in commercial fuel-cell vehicles.

Fuel cells generate electricity to propel vehicles through electrochemical reactions between onboard hydrogen fuel and oxygen in the air. Hydrogen molecules are stripped of electrons at the fuel cell’s anode, becoming protons that travel through a polymer electrolyte membrane to the cathode, where they react with electrons and oxygen to form water.

“In order for a fuel cell to work,” Liu explained, “the catalyst must be densely packed with active sites that are uniformly distributed throughout the cathode and directly connected to the arriving protons and electrons, while maintaining easy access to oxygen. The catalyst should also have an architecture that can readily channel away the produced water.” No conventional method for preparing carbon-based platinum or non-precious metal catalysts can meet all these criteria, Liu added.

In a paper recently published in the Proceedings of the National Academy of Sciences of the United States of America, the team led by Liu reported on a new method of synthesizing a highly efficient, nanofibrous non-precious metal catalyst by electrospinning a polymer solution containing a mixture of ferrous organometallics and metal-organic frameworks. Following thermal activation, the new catalyst delivered an unprecedented level of catalytic activity in actual fuel cell tests.

“The new catalyst offers a unique carbon nano-network architecture made of microporous nanofibers interconnected through a macroporous framework,” Liu explained. “Not only do the active sites inside the micropores within individual fibers catalyze chemical reactions effectively, but the macroporous voids between the fibers transport oxygen and water efficiently to and from the active sites. The continuous nano-networks also make the catalytic electrode highly conductive in charge transfer.”

The paper, “Highly efficient nonprecious metal catalyst prepared with metal–organic framework in a continuous carbon nanofibrous network,” was published online on August 10, 2015.

The research was supported by the U.S. Department of Energy’s Office of Science and the Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation’s first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. With employees from more than 60 nations, Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America’s scientific leadership and prepare the nation for a better future. Argonne is supported by the Office of Science of the U.S. Department of Energy.

The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Artificial Leaf Harnesses Sunlight for Efficient Fuel Production


Generating and storing renewable energy, such as solar or wind power, is a key barrier to a clean-energy economy. When the Joint Center for Artificial Photosynthesis (JCAP) was established at Caltech and its partnering institutions in 2010, the U.S. Department of Energy (DOE) Energy Innovation Hub had one main goal: a cost-effective method of producing fuels using only sunlight, water, and carbon dioxide, mimicking the natural process of photosynthesis in plants and storing energy in the form of chemical fuels for use on demand. Over the past five years, researchers at JCAP have made major advances toward this goal, and they now report the development of the first complete, efficient, safe, integrated solar-driven system for splitting water to create hydrogen fuels.

“This result was a stretch project milestone for the entire five years of JCAP as a whole, and not only have we achieved this goal, we also achieved it on time and on budget,” says Caltech’s Nate Lewis, George L. Argyros Professor and professor of chemistry, and the JCAP scientific director.

The new solar fuel generation system, or artificial leaf, is described in the August 24 online issue of the journal Energy and Environmental Science. The work was done by researchers in the laboratories of Lewis and Harry Atwater, director of JCAP and Howard Hughes Professor of Applied Physics and Materials Science.

“This accomplishment drew on the knowledge, insights and capabilities of JCAP, which illustrates what can be achieved in a Hub-scale effort by an integrated team,” Atwater says. “The device reported here grew out of a multi-year, large-scale effort to define the design and materials components needed for an integrated solar fuels generator.”

The new system consists of three main components: two electrodes–one photoanode and one photocathode–and a membrane. The photoanode uses sunlight to oxidize water molecules, generating protons and electrons as well as oxygen gas. The photocathode recombines the protons and electrons to form hydrogen gas. A key part of the JCAP design is the plastic membrane, which keeps the oxygen and hydrogen gases separate. If the two gases are allowed to mix and are accidentally ignited, an explosion can occur; the membrane lets the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.

Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels. However, these materials also oxidize (or rust) on the surface when exposed to water, so cannot be used to directly generate fuel. A major advance that allowed the integrated system to be developed was previous work in Lewis’s laboratory, which showed that adding a nanometers-thick layer of titanium dioxide (TiO2)–a material found in white paint and many toothpastes and sunscreens–onto the electrodes could prevent them from corroding while still allowing light and electrons to pass through. The new complete solar fuel generation system developed by Lewis and colleagues uses such a 62.5-nanometer-thick TiO2 layer to effectively prevent corrosion and improve the stability of a gallium arsenide-based photoelectrode.

Another key advance is the use of active, inexpensive catalysts for fuel production. The photoanode requires a catalyst to drive the essential water-splitting reaction. Rare and expensive metals such as platinum can serve as effective catalysts, but in its work the team discovered that it could create a much cheaper, active catalyst by adding a 2-nanometer-thick layer of nickel to the surface of the TiO2. This catalyst is among the most active known catalysts for splitting water molecules into oxygen, protons, and electrons and is a key to the high efficiency displayed by the device.

The photoanode was grown onto a photocathode, which also contains a highly active, inexpensive, nickel-molybdenum catalyst, to create a fully integrated single material that serves as a complete solar-driven water-splitting system.

A critical component that contributes to the efficiency and safety of the new system is the special plastic membrane that separates the gases and prevents the possibility of an explosion, while still allowing the ions to flow seamlessly to complete the electrical circuit in the cell. All of the components are stable under the same conditions and work together to produce a high-performance, fully integrated system. The demonstration system is approximately one square centimeter in area, converts 10 percent of the energy in sunlight into stored energy in the chemical fuel, and can operate for more than 40 hours continuously.

“This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more ,” Lewis says.

“Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components,” Lewis adds, “Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress.”

Because the work assembled various components that were developed by multiple teams within JCAP, coauthor Chengxiang Xiang, who is co-leader of the JCAP prototyping and scale-up project, says that the successful end result was a collaborative effort. “JCAP’s research and development in device design, simulation, and materials discovery and integration all funneled into the demonstration of this new device,” Xiang says.


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The above post is reprinted from materials provided by California Institute of Technology. Note: Materials may be edited for content and length.

Solar Energy – Quantum Dot Powered Windows: Los Alamos National Laboratory


Quantum Dot Window 082515 id41125A luminescent solar concentrator is an emerging sunlight harvesting technology that has the potential to disrupt the way we think about energy; It could turn any window into a daytime power source.
“In these devices, a fraction of light transmitted through the window is absorbed by nanosized particles (semiconductor quantum dots) dispersed in a glass window, re-emitted at the infrared wavelength invisible to the human eye, and wave-guided to a solar cell at the edge of the window,” said Victor Klimov, lead researcher on the project at the Department of Energy’s Los Alamos National Laboratory. “Using this design, a nearly transparent window becomes an electrical generator, one that can power your room’s air conditioner on a hot day or a heater on a cold one.”
This is what becomes possible with new devices – quantum dot LSCs –which will be available in the journal Nature Nanotechnology in the study “Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots”. The work was performed by researchers at the Center for Advanced Solar Photophysics (CASP) of Los Alamos, led by Klimov and the research team coordinated by Sergio Brovelli and Francesco Meinardi of the Department of Materials Science of the University of Milan-Bicocca (UNIMIB) in Italy.
quantum dot window
The luminescent solar concentrator could turn any window into a daytime power source.
In April 2014, using special composite quantum dots, the American-Italian collaboration demonstrated the first example of large-area luminescent solar concentrators free from reabsorption losses of the guided light by the nanoparticles. This represented a fundamental advancement with respect to the earlier technology, which was based on organic emitters that allowed for the realization of concentrators of only a few centimeters in size.
However, the quantum dots used in previous proof-of-principle devices were still unsuitable for real-world applications, as they were based on the toxic heavy metal cadmium and were capable of absorbing only a small portion of the solar light. This resulted in limited light-harvesting efficiency and strong yellow/red coloring of the concentrators, which complicated their application in residential environments.
Klimov, CASP’s director, explained how the updated approach solves the coloring problem: “Our new devices use quantum dots of a complex composition which includes copper (Cu), indium (In), selenium (Se) and sulfur (S). This composition is often abbreviated as CISeS. Importantly, these particles do not contain any toxic metals that are typically present in previously demonstrated LSCs.”
“Furthermore,” Klimov noted, “the CISeS quantum dots provide a uniform coverage of the solar spectrum, thus adding only a neutral tint to a window without introducing any distortion to perceived colors. In addition, their near-infrared emission is invisible to a human eye, but at the same time is ideally suited for most common solar cells based on silicon.”
Francesco Meinardi, professor of Physics at UNIMIB, described the emerging work, noting, “In order for this technology to leave the research laboratories and reach its full potential in sustainable architecture, it is necessary to realize non-toxic concentrators capable of harvesting the whole solar spectrum.”
“We must still preserve the key ability to transmit the guided luminescence without reabsorption losses, though, so as to complement high photovoltaic efficiency with dimensions compatible with real windows. The aesthetic factor is also of critical importance for the desirability of an emerging technology,” Meinardi said.
Hunter McDaniel, formerly a Los Alamos CASP postdoctoral fellow and presently a quantum dot entrepreneur (UbiQD founder and president), added, “with a new class of low-cost, low-hazard quantum dots composed of CISeS, we have overcome some of the biggest roadblocks to commercial deployment of this technology.”
“One of the remaining problems to tackle is reducing cost, but already this material is significantly less expensive to manufacture than alternative quantum dots used in previous LSC demonstrations,” McDaniel said.
A key element of this work is a procedure comparable to the cell casting industrial method used for fabricating high optical quality polymer windows. It involves a new UNIMIB protocol for encapsulating quantum dots into a high-optical quality transparent polymer matrix. The polymer used in this study is a cross-linked polylaurylmethacrylate, which belongs to the family of acrylate polymers. Its long side-chains prevent agglomeration of the quantum dots and provide them with the “friendly” local environment, which is similar to that of the original colloidal suspension. This allows one to preserve light emission properties of the quantum dots upon encapsulation into the polymer.
Sergio Brovelli, the lead researcher on the Italian team, concluded: “Quantum dot solar window technology, of which we had demonstrated the feasibility just one year ago, now becomes a reality that can be transferred to the industry in the short to medium term, allowing us to convert not only rooftops, as we do now, but the whole body of urban buildings, including windows, into solar energy generators.”
“This is especially important in densely populated urban area where the rooftop surfaces are too small for collecting all the energy required for the building operations,” he said. He proposes that the team’s estimations indicate that by replacing the passive glazing of a skyscraper such as the One World Trade Center in NYC (72,000 square meters divided into 12,000 windows) with our technology, it would be possible to generate the equivalent of the energy need of over 350 apartments.
“Add to these remarkable figures, the energy that would be saved by the reduced need for air conditioning thanks to the filtering effect by the LSC, which lowers the heating of indoor spaces by sunlight, and you have a potentially game-changing technology towards “net-zero” energy cities,” Brovelli said.
Source: Los Alamos National Laboratory

Read more: Capture sunlight with your quantum dot window

NREL: Genetically Modified Algae Could Replace Oil for Plastic


Algae for Ethelyne 9D305D6D-C3FB-4C68-A116B9AD02D05513_articleTweaked cyanobacteria can churn out a plastic precursor, potentially replacing fossil fuels.

From polyester shirts, plastic milk jugs and PVC pipes to the production of high-grade industrial ethanol, the contribution of the chemical feedstock ethylene can be found just about everywhere around the globe.

But ethylene’s ubiquity as a building block in plastics and chemicals masks an underlying environmental cost. The cheap hydrocarbon is made using petroleum and natural gas, and the way it is produced emits more carbon dioxide than any other chemical process. As concerns about levels of CO2 in the atmosphere have grown, some scientists have been experimenting with ways to make ethylene production more green. At the Department of Energy’s National Renewable Energy Laboratory (NREL), researchers are finding unexpected success with the help of cyanobacteria, or blue-green algae.

Jianping Yu, a research scientist with NREL’s Photobiology Group, is leading a team of researchers who are working with these organisms. In his lab, they have been able to make ethylene directly from genetically modified algae.

The researchers were able to accomplish this by introducing a gene that coded for an ethylene-producing enzyme—effectively altering the cyanobacteria’s metabolism. This allows the organisms to convert some of the carbon dioxide normally used to make sugars and starches during photosynthesis into ethylene. Because ethylene is a gas, it can easily be collected.

Making ethylene doesn’t require many inputs, either. The basic requirements for cyanobacteria are water, some minerals and light, and a carbon source. In a commercial setting, CO2 could come from a point source like a power plant, Yu said.

If this alternative production method becomes efficient enough, it could potentially replace steam cracking, the energy-intensive method currently used to break apart petrochemicals into ethylene and other compounds. Because the algae take in three times the CO2 to produce a single ton of ethylene, the process acts as a carbon sink. That would be a significant improvement over steam cracking, which generates between 1 ½ and 3 tons of carbon dioxide per ton of ethylene, according to the researchers’ own analysis. The captured ethylene gas can then be transformed for use in a wide range of fuels and products.

“I think it’s better to turn CO2 into something useful,” Yu said, comparing the approach to other methods of carbon capture. “You don’t have to pump CO2 into the ground, and [the products] will last for many years.”

Engineering genes to suck up carbon
Yu and his colleagues weren’t the first to come up with the idea of using cyanobacteria to make ethylene. The process was first attempted by researchers in Japan more than a decade ago. At the time, the researchers were not able to produce ethylene reliably. When Yu read the study years later, he thought that by genetically altering a different strain with which he had worked closely (Synechocystis sp. PCC6803), he might be able to make ethylene production more consistent.

Algae Ethelyne II safe_imageThe researchers are able to make ethylene from algae by altering a part of the organism’s metabolism called the tricarboxylic acid (TCA) cycle, which is involved in biosynthesis and energy production. In genetically unaltered blue-green algae, the cycle can only take in a relatively small fraction, or 13 percent, of the 2 to 3 percent of fixed CO2. But in Yu’s lab, the algae are able to send three times more carbon to the TCA cycle and emit 10 percent of the fixed carbon dioxide as ethylene—at a rate of 35 milligrams per liter per hour. That might not sound like very much, but it represents a thousandfold increase in productivity since he first began working with the cyanobacteria in 2010. By the end of this year, Yu is aiming to increase that productivity to 50 milligrams.

“This is by no means close to the upper limit,” he said, explaining that the ultimate goal will be to convert 90 percent of fixed carbon to ethylene. “I cannot see why it cannot go higher; I haven’t run into a brick wall yet. I don’t know what would prevent that from happening, but of course it could.”

Surprisingly, even though the cyanobacteria are producing more ethylene, the organisms are still growing at the same rate as non-ethylene-producing algae. The results demonstrate that the cyanobacteria’s metabolism was much more flexible than previously thought, according to Yu.

“It’s like a person that’s losing blood all the time but appears healthy,” he said.

Yu and his colleagues aren’t certain how this is happening, but the mutation that enabled ethylene production has also stimulated photosynthesis.

“This system gives us a new insight into photosynthesis and gives us hope that we can learn from this and increase photosynthetic activity,” he said.

That insight into cyanobacteria’s metabolism is as important a finding as the creation of organisms that can consistently produce ethylene, said Robert Burnap, a professor of microbiology and molecular genetics at Oklahoma State University. He was not involved with the study, but did provide a reference for Yu’s application to this year’s R&D 100 Awards. Yu is now a finalist in the Mechanical Devices/Material category.

“It’s surprising how adaptive the metabolism is. It’s producing something it’s not evolved to make. There was a lot of controversy over whether or not that was even possible to have consistent ethylene production. It shows it is flexible,” he said.

The research could help other scientists better understand metabolic pathways in other plants and even in humans. The TCA cycle is even active in our cells’ mitochondria, Burnap said.

“What makes this study really special is the depths of analysis that they went into,” he said, describing the research as a whole as a “seminal piece of work.”

Manufacturing centers … in ponds?
It’s still much too early to say when or even if these algae will produce ethylene at a commercial scale. Yu estimates that development to that stage could take more than 10 years.

“It will take a lot of work to improve carbon efficiency to 50 percent or higher,” Yu said.

Philip Pienkos, principal manager of the Bioprocess R&D Group at NREL’s National Bioenergy Center, said the project is beginning to focus more on the development side, even as Yu continues to work to achieve higher ethylene volumes.

“How do you recover ethylene? What do you do with the biomass? This project is poised to answer these important questions,” Pienkos said.

Sometime next year, the researchers plan to move their work outdoors to see how the algae behave in an environment that more closely resembles how they would be grown commercially.

“We have to get a real scalable ethylene process so we have a better sense of what this will look like,” Pienkos said.

Yu envisions the cyanobacteria growing either in ponds, or possibly vertically, on newspaper-like sheets. In either case, the solid or liquid cultures would have to be enclosed to capture the ethylene, he said.

There are also some safety concerns associated with producing large quantities of the gas. The hydrocarbon and oxygen that are also produced by the algae are flammable, and certain safety precautions would have to be put in place to safely collect ethylene.

Even if the cyanobacteria can create large volumes of ethylene, their success will depend on whether the product can become cost-competitive. That won’t be easy because petrochemical-based ethylene is cheap and widely available. According to the researchers’ economic analysis, ethylene made from petrochemicals cost $600 to $1,300 per ton, while the gas coming from the algae is estimated to be about $3,240 per ton.

Proving the system’s economic viability down the road will also help maintain research funding from the Department of Energy, Peinkos said.

“Algae is not the primary focus of DOE; they’ve spent decades supporting work in cellulosics. Algae is a much smaller portfolio, and most of the work is in conversion directly to liquid fuels,” he said. “Ethylene stands out a little bit because it’s not a fuel, but it can be a fuel feedstock.”

Making hydrogen fuel from water and visible light highly efficient


Hydrogen from Light image126285-scolx250Mimicking photosynthesis is not easy. The bottleneck of artificial photosynthesis is visible light, because converting it into other forms of energy is not efficient. Researchers at Michigan Technological Univ. have found a way to solve this issue, leading to an efficient technique to produce hydrogen fuel. Their work was published in the Journal of Physical Chemistry.

The technique was developed by Yun Hang Hu, the Charles and Carroll McArthur professor of Materials Science and Engineer, and his PhD student, Bing Han, at Michigan Tech.

“Hydrogen is the future of cars,” says Hu. “And if you want to power hydrogen cars, you have to make hydrogen fuels.”

In this new hydrogen production process, the key is the interactions of a catalyst, light and a sacrificial molecule.

Playbook of black titanium dioxide, methanol and light
As if in a complex sports game, the exchanges and counters between the materials used to split water look like a chemistry playbook. The players are black titanium dioxide (TiO2) and methanol (CH3OH) pitted against electron-hole recombination.

The goal of the game is to produce hydrogen molecules. Basically, that’s done by moving an electron from one place to another, like kicking a football to get a field goal. To make that score, a water molecule captures an electron excited within a material. When excited, electrons move up and down in different bands; the lower one here is the valence band and the higher one is the conduction band. The valence band and the conduction band are like goal posts, and between them is the band gap, which is like the playing field. The excited electron is the ball being passed around.

Solar energy, with both ultraviolet (UV) and visible light energy, is what gets this ball rolling. Light energy bounces off the first player, titanium dioxide, which is the material where the valence band and conduction band are in play. That excites an electron, making it a photo-excited charge that shoots up towards the conduction band. For UV light, the playing field is pretty big, and the band gap stretches 3.2 electron volts wide.

Here’s where the game gets more complicated, Harry Potter Quidditch-style, with balls that move quickly on their own. Electrons tend to zip like a golden snitch, and they sometimes misbehave, scurrying back to their starting place in the valence band, which negates the goal score and H2 production.

Enter player two: methanol. When the photo-excited electron shoots toward the conduction band, the methanol donates an electron via an oxidation reaction to swoop in and block the open spot left in the valence band. This sacrificial agent acts as a defense against the photo-excited electron snitching and scurrying its way back.

Playing in the visible light spectrum
The play between titanium dioxide, methanol and UV light serves up hydrogen (H2) molecules, However, UV is only a very small part (4%) of solar energy. It is important to use visible light, since it constitutes about 45% of solar energy. Hu’s team has been able to increase the yield and energy efficiency up to two magnitudes greater than previously reported results using visible light instead of UV. But they had to change the playing field to do so.

“It’s a two-part process,” Hu says, explaining that the catalyst is black titanium dioxide (with 1 percent platinum), which attaches to a silicon dioxide substrate in their set-up. Hu’s team first generated a “light-diffuse-reflected surface” for the silicon dioxide, making it bumpy, which traps light waves and bounces them around. “We put the catalyst on the scattered surface, and it can increase the light absorption by one hundred times,” he explains.

Light absorption is an important step that generates photo-excited electrons, but it’s only part of what’s needed. Using visible light shortens the playing field, altering the band gap to only 1.3 electron volts in black titanium oxide. Specifically, that’s because an electron is excited to conduction band from Ti3+ energy level instead of the valence band, which is like starting at the 10-yard line instead of the 50-yard line.

The photo-excited electron has less distance to travel to the conduction band, but methanol doesn’t like to play along. The shortened distance from methanol to Ti3+ energy level greatly reduces methanol’s oxidation reaction, giving it less motivation to fill the leftover hole in the Ti3+ energy level. So, it needed a little heat in the second step of the process to get that reaction going again.

“We used temperature to increase the energy for methanol oxidation to donate electrons,” Hu says, adding that in the experiment they found 280 degrees Celsius to be the sweet spot.

With the modified silicon dioxide substrate and heat, Hu’s team also had to design a different set-up for the experiment. The water they used was steam, pushed into a chamber where it collided with a disk of roughened silicon dioxide studded with black titanium dioxide-platinum catalyst. Visible light then excited the electrons, and the hydrogen could then be syphoned off.

“The set-up is not complicated,” Hu says. “It’s actually convenient for scaling up commercially.”

Furthermore, this work created a hybrid process using both light and heat, which has opened a new door for visible light photosynthesis. The method is a big step closer to mimicking photosynthesis.

Source: Michigan Technological Univ.