MIT: A Big Leap for an Artificial Leaf: Making Liquid Fuel from Sunlight, Water and CO2: Video

A cross-disciplinary team at Harvard University has created a system that uses solar energy to split water molecules and hydrogen-eating bacteria to produce liquid fuels. The system can convert solar energy to biomass with 10 percent efficiency, far above the one percent seen in the fastest-growing plants.

 

The bionic leaf is one step closer to reality.

Daniel Nocera, a professor of energy science at Harvard who pioneered the use of artificial photosynthesis, says that he and his colleague Pamela Silver have devised a system that completes the process of making liquid fuel from sunlight, carbon dioxide, and water. And they’ve done it at an efficiency of 10 percent, using pure carbon dioxide—in other words, one-tenth of the energy in sunlight is captured and turned into fuel.

That is much higher than natural photosynthesis, which converts about 1 percent of solar energy into the carbohydrates used by plants, and it could be a milestone in the shift away from fossil fuels. The new system is described in a new paper in Science.

 

 
“Bill Gates has said that to solve our energy problems, someday we need to do what photosynthesis does, and that someday we might be able to do it even more efficiently than plants,” says Nocera. “That someday has arrived.”

 

 
In nature, plants use sunlight to make carbohydrates from carbon dioxide and water. Artificial photosynthesis seeks to use the same inputs—solar energy, water, and carbon dioxide—to produce energy-dense liquid fuels. Nocera and Silver’s system uses a pair of catalysts to split water into oxygen and hydrogen, and feeds the hydrogen to bacteria along with carbon dioxide.

 

 

The bacteria, a microörganism that has been bioengineered to specific characteristics, converts the carbon dioxide and hydrogen into liquid fuels.

 

 
Several companies, including Joule Unlimited and LanzaTech, are working to produce biofuels from carbon dioxide and hydrogen, but they use bacteria that consume carbon monoxide or carbon dioxide, rather than hydrogen. Nocera’s system, he says, can operate at lower temperatures, higher efficiency, and lower costs.

 

 
Nocera’s latest work “is really quite amazing,” says Peidong Yang of the University of California, Berkeley. Yang has developed a similar system with much lower efficiency. “The high performance of this system is unparalleled” in any other artificial photosynthesis system reported to date, he says.

 

 
The new system can use pure carbon dioxide in gas form, or carbon dioxide captured from the air—which means it could be carbon-neutral, introducing no additional greenhouse gases into the atmosphere. “The 10 percent number, that’s using pure CO2,” says Nocera. Allowing the bacteria themselves to capture carbon dioxide from the air, he adds, results in an efficiency of 3 to 4 percent—still significantly higher than natural photosynthesis.

 

 

“That’s the power of biology: these bioörganisms have natural CO2 concentration mechanisms.”

 

 
Nocera’s research is distinct from the work being carried out by the Joint Center for Artificial Photosynthesis, a U.S. Department of Energy-funded program that seeks to use inorganic catalysts, rather than bacteria, to convert hydrogen and carbon dioxide to liquid fuel.

 

 

According to Dick Co, who heads the Solar Fuels Institute at Northwestern University, the innovation of the new system lies not only in its superior performance but also in its fusing of two usually separate fields: inorganic chemistry (to split water) and biology (to convert hydrogen and carbon dioxide into fuel). “What’s really exciting is the hybrid approach” to artificial photosynthesis, says Co. “It’s exciting to see chemists pairing with biologists to advance the field.”

 

 
Commercializing the technology will likely take years. In any case, the prospect of turning sunlight into liquid fuel suddenly looks a lot closer.

 

Artificial Leaf Harnesses Sunlight for Efficient Fuel Production

A highly efficient photoelectrochemical (PEC) device uses the power of the sun to split water into hydrogen and oxygen. The stand-alone prototype includes two chambers separated by a semi-permeable membrane that allows collection of both gas products.

Credit: Lance Hayashida/Caltech

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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.

Artificial photosynthesis could help make fuels, plastics and medicine

April 29, 2015

Source: American Chemical Society Summary: The global industrial sector accounts for more than half of the total energy used every year. Now scientists are inventing a new artificial photosynthetic system that could one day reduce industry’s dependence on fossil fuel-derived energy by powering part of the sector with solar energy and bacteria. The system converts light and carbon dioxide into building blocks for plastics, pharmaceuticals and fuels — all without electricity.

The global industrial sector accounts for more than half of the total energy used every year. Now scientists are inventing a new artificial photosynthetic system that could one day reduce industry’s dependence on fossil fuel-derived energy by powering part of the sector with solar energy and bacteria. In the ACS journal Nano Letters, they describe a novel system that converts light and carbon dioxide into building blocks for plastics, pharmaceuticals and fuels — all without electricity.

Peidong Yang, Michelle C. Y. Chang, Christopher J. Chang and colleagues note that plants use photosynthesis to convert sunlight, water and carbon dioxide to make their own fuel in the form of carbohydrates. Globally, this natural process harvests 130 Terawatts of solar energy. If scientists could figure out how to harness just a fraction of that amount to make fuels and power industrial processes, they could dramatically cut our reliance on fossil fuels. So, Yang, Michelle Chang and Christopher Chang’s teams wanted to contribute to these efforts.

 

 

The groups developed a stand-alone, nanowire array that captures light and with the help of bacteria, converts carbon dioxide into acetate. The bacteria directly interact with light-absorbing materials, which the researchers say is the first example of “microbial photo-electrosynthesis.” Another kind of bacteria then transforms the acetate into chemical precursors that can be used to make a wide range of everyday products from antibiotics to paints.

The authors acknowledge funding from the U.S. Department of Energy, the Lawrence Berkeley National Laboratory, Howard Hughes Medical Institute, the National Science Foundation and the National Institutes of Health.

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The above story is based on materials provided by American Chemical Society. Note: Materials may be edited for content and length.

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Journal Reference:

1. Chong Liu, Joseph J. Gallagher, Kelsey K. Sakimoto, Eva M. Nichols, Christopher J. Chang, Michelle C. Y. Chang, Peidong Yang. Nanowire–Bacteria Hybrids for Unassisted Solar Carbon Dioxide Fixation to Value-Added Chemicals. Nano Letters, 2015; 150407103432009 DOI: 10.1021/acs.nanolett.5b01254

Researchers discover new catalysts to generate renewable fuels

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

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

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

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

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

Source: Yale Univ.

Supersonic Electrons Could Produce Future Solar fuel

Researchers from institutions including Lund University have taken a step closer to producing solar fuel using artificial photosynthesis. In a new study, they have successfully tracked the electrons’ rapid transit through a light-converting molecule.

The ultimate aim of the present study is to find a way to make fuel from water using sunlight. This is what photosynthesis does all the time – plants convert water and carbon dioxide to energy rich molecules using sunlight. Researchers around the world are therefore attempting to borrow ideas from photosynthesis in order to find a way to produce solar fuel artificially.

Researchers from institutions including Lund University have taken a step closer to producing solar fuel using artificial photosynthesis. In a new study, they have successfully tracked the electrons’ rapid transit through a light-converting molecule. (Image courtesy of Lund University)

“Our study shows how it is possible to construct a molecule in which the conversion of light to chemical energy happens so fast that no energy is lost as heat. This means that all the energy in the light is stored in a molecule as chemical energy”, said Villy Sundström, Professor of Chemical Physics at Lund University.

Thus far, solar energy is harnessed in solar cells and solar thermal collectors. Solar cells convert solar energy to electricity and solar thermal collectors convert solar energy to heat. However, producing solar fuel, for example in the form of hydrogen gas or methanol, requires entirely different technology. The idea is that solar light can be used to extract electrons from water and use them to convert light energy to energy rich molecules, which are the constituent of the solar fuel.

“A device that can do this – a solar fuel cell – is a complicated machine with light-collecting molecules and catalysts”, said Villy Sundström.

In the present study, Professor Sundström and his colleagues have developed and studied a special molecule that can serve as a model for the type of chemical reactions that can be employed in a solar fuel cell. The molecule comprises two metal centres, one that collects the light and another that imitates the catalyst where the solar fuel is produced. The researchers have managed to track the path of the electrons through the molecule in great detail. They measured the time it took for an electron to cross the bridge between the two metal atoms in the molecule. It takes half a picosecond, or half a trillionth of a second.

“In everyday terms, this means that the electron flies through the molecule at a speed of around four kilometres a second, which is over ten times the speed of sound”, said Villy Sundström.

The researchers were surprised by the high speed. Another surprising discovery was that the speed appears to be highly dependent on the type of bridge between the atoms. In this study, the speed was 100 times higher than with another type of bridge tested.

“This is the first time anyone has managed to track such a complex and rapid reaction and to distinguish all the stages of the reaction”, said Villy Sundström about the study, which has been published in the journal Nature Communications (“Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses”).

The study is a collaboration between researchers from several departments at Lund University and from Denmark, Germany, Hungary, Japan and the USA. The measurements were performed in Japan at the SACLA X-ray FEL in Harima, Japan, one of only two operating X-ray free-electron lasers in the world.

Source: Lund University

Read more: Supersonic electrons could produce future solar fuel

Mimicking photosynthesis with man-made leaves: Harvesting Light

Scientists have long been trying to emulate the way in which plants harvest energy from the sun through photosynthesis. Plants are able to absorb photons from even weak sunlight using light antennae made from chlorophyll molecules in their leaves. This absorbed energy is then transferred to reaction centers wherein the plants create the sugars they use as food. So far, artificial systems built to replicate this super-efficient natural process have been limited to a single reaction center with a few light absorbers, and have been unable to absorb enough energy from light sources with low photon levels such as sunlight.

Now, Osamu Ishitani at the Tokyo Institute of Technology, along with researchers from Toyota Central R&D Labs, Inc., has created an efficient, artificial light-harvesting system based on the natural two-step process of photosynthesis. The new system uses man-made ‘leaves’ as light absorbers, which relay energy through a metal complex to feed a final energy acceptor.

“It is difficult to make an efficient solar-energy converter using molecular devices such as so-called photocatalysts because the molecules are so small and solar light is so dilute,” explains Ishitani. “Such systems would require huge numbers of molecular devices, which are expensive and time-consuming to make. Introducing devices with the ability to harvest light into solar-energy conversion would be one possible solution.”

Ishitani and his team realized that building a system with multiple light absorbers feeding a smaller number of energy relay ‘antennae’ linked to an energy acceptor would allow more photons to be absorbed from dilute light, with less energy being lost along the way (“Efficient light harvesting via sequential two-step energy accumulation using a Ru–Re5 multinuclear complex incorporated into periodic mesoporous organosilica”).

(Left) A ruthenium complex connected to the center of the rhenium tetramer is adsorbed into the mesopores of periodic mesoporous organosilica (PMO). (Right) Photons absorbed by PMO framework are first concentrated to the rhenium oligomers, and then to the ruthenium reaction center. (click on image to enlarge)

The researchers created a device with 440 ‘leaves’ using tubes made from so-called periodic mesoporous organosilica (PMO) and light-absorbing biphenyl (Bp). The PMO-Bp complexes were linked to five connected rhenium metal sticks, which transferred the light energy harvested by PMO-Bp directly to a central ruthenium sphere. In this way, the photons from the light source were concentrated very efficiently, first through the rhenium sticks and then into the ruthenium reaction center, with little loss of energy en-route.

In a series of tests using the new system, Ishitani and his team found that the reaction center of their device was capable of emitting a strong light powered by the photonic energy from the man-made ‘leaves’.

The new system could be used to build better photocatalysts, which can be used for a number of purposes including CO2 reduction and water oxidation photocatalysis. However, Ishitani and co-workers state that it will be some time before artificial photosynthesis becomes commonplace in such systems, because the process requires considerable further research and development.

Source: Tokyo Institute of Technology

Read more: Light-harvesting: Mimicking photosynthesis with man-made leaves

Hydrogen fuel from sunlight

*** Note to Readers: I must admit it .. to me [and I have been accused of being “geeky” and “technology smitten” : -) ] … these guys are the true “Rock Stars” of our time.  “Imagine .. the possibilities!” – GNT –

“Great Things … From Small Things!”

 

DOE/Lawrence Berkeley National Laboratory

*** Note to Readers: I must admit it .. to me [and I have been accused of being “geeky” and “technology smitten” : -) ] … these guys are the true “Rock Stars” of our time.  “Imagine .. the possibilities!” – GNT –      “Great Things … From Small Things!”

DOE/Lawrence Berkeley National Laboratory

Berkeley Lab researchers at the Joint Center for Artificial Photosynthesis have developed a way to interface a molecular hydrogen-producing catalyst with a visible light absorbing semiconductor. With this approach, hydrogen fuel can be produced off a photocathode using sunlight. »

 

		IMAGE:   For more than two billion years, nature, through photosynthesis, has used the energy in sunlight to convert water and carbon dioxide into fuel (sugars) for green plants.

In the search for clean, green sustainable energy sources to meet human needs for generations to come, perhaps no technology matches the ultimate potential of artificial photosynthesis. Bionic leaves that could produce energy-dense fuels from nothing more than sunlight, water and atmosphere-warming carbon dioxide, with no byproducts other than oxygen, represent an ideal alternative to fossil fuels but also pose numerous scientific challenges.

A major step toward meeting at least one of these challenges has been achieved by researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) working at the Joint Center for Artificial Photosynthesis (JCAP).

“We’ve developed a method by which molecular hydrogen-producing catalysts can be interfaced with a semiconductor that absorbs visible light,” says Gary Moore, a chemist with Berkeley Lab’s Physical Biosciences Division and principal investigator for JCAP. “Our experimental results indicate that the catalyst and the light-absorber are interfaced structurally as well as functionally.”

Moore is the corresponding author, along with Junko Yano and Ian Sharp, who also hold joint appointments with Berkeley Lab and JCAP, of a paper describing this research in the Journal of the American Chemical Society (JACS). The article is titled “Photofunctional Construct That Interfaces Molecular Cobalt-Based Catalysts for H2 Production to a Visible-Light-Absorbing Semiconductor.” Co-authors are Alexandra Krawicz, Jinhui Yang and Eitan Anzenberg.

Earth receives more energy in one hour’s worth of sunlight than all of humanity uses in an entire year.

Through the process of photosynthesis, green plants harness solar energy to split molecules of water into oxygen, hydrogen ions (protons) and free electrons. The oxygen is released as waste and the protons and electrons are used to convert carbon dioxide into the carbohydrate sugars that plants use for energy. Scientists aim to mimic the concept but improve upon the actual process.

		IMAGE:   Gary Moore is a chemist with Berkeley Lab’s Physical Biosciences Division and principal investigator for the Joint Center for Artificial Photosynthesis.

Click here for more information.     

JCAP, which has a northern branch in Berkeley and a southern branch on the campus of the California Institute of Technology (Caltech), was established in 2010 by DOE as an Energy Innovation Hub.

Operated as a partnership between Caltech and Berkeley Lab, JCAP is the largest research program in the United States dedicated to developing an artificial solar-fuel technology. While artificial photosynthesis can be used to generate electricity, fuels can be a more effective means of storing and transporting energy. The goal is an artificial photosynthesis system that’s at least 10 times more efficient than natural photosynthesis.

To this end, once photoanodes have used solar energy to split water molecules, JCAP scientists need high performance semiconductor photocathodes that can use solar energy to catalyze fuel production. In previous efforts to produce hydrogen fuel, catalysts have been immobilized on non-photoactive substrates. This approach requires the application of an external electrical potential to generate hydrogen. Moore and his colleagues have combined these steps into a single material.

“In coupling the absorption of visible light with the production of hydrogen in one material, we can generate a fuel simply by illuminating our photocathode,” Moore says. “No external electrochemical forward biasing is required.”

The new JCAP photocathode construct consists of the semiconductor gallium phosphide and a molecular cobalt-containing hydrogen production catalyst from the cobaloxime class of compounds. As an absorber of visible light, gallium phosphide can make use of a greater number of available solar photons than semiconductors that absorb ultraviolet light, which means it is capable of producing significantly higher photocurrents and rates of fuel production. However, gallium phosphide can be notoriously unstable during photoelectrochemical operations.

		IMAGE:   Grafting molecular cobalt-containing hydrogen production catalysts to a visible-light-absorbing semiconductor exploits the UV-induced immobilization chemistry of vinylpyridine to p-type (100) gallium phosphide (GaP).

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Moore and his colleagues found that coating the surface of gallium phosphide with a film of the polymer vinylpyridine alleviates the instability problem, and if the vinylpyridine is then chemically treated with the cobaloxime catalyst, hydrogen production is significantly boosted.

“The modular aspect of our method allows independent modification of the light-absorber, linking material and catalyst, which means it can be adapted for use with other catalysts tethered over structured photocathodes as new materials and discoveries emerge,” Moore says. “This could allow us, for example, to replace the precious metal catalysts currently used in many solar-fuel generator prototypes with catalysts made from Earth-abundant elements.”

Despite its promising electronic properties, gallium phosphide features a mid-sized optical band gap which ultimately limits the total fraction of solar photons available for absorption.

Moore and his colleagues are now investigating semiconductors that cover a broader range of the solar spectrum, and catalysts that operate faster at lower electrical potentials. They also plan to investigate molecular catalysts for carbon dioxide reduction.

“We look forward to adapting our method to incorporate materials with improved properties for converting sunlight to fuel,” Moore says. “We believe our method provides researchers at JCAP and elsewhere with an important tool for developing integrated photocathode materials that can be used in future solar-fuel generators as well as other technologies capable of reducing net carbon dioxide emissions.”

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This research was funded by the DOE Office of Science.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit http://www.lbl.gov.

DOE’s 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 the Office of Science website at science.energy.gov/.

Researcher Realizes Water-Splitting Solar Cell Structure Using Nanoparticles

Published on June 18, 2013 at 6:47 AM

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

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

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

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

New approach with complex thin film technologies

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

Protection against corrosion

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

Stable H₂-Production

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

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

Watch the “artificial leaf” in action: http://www.helmholtz-berlin.de/aktuell/pr/mediathek/video/energieversorgung/superstratzelle_de.html

Source: http://www.helmholtz-berlin.de/