Energy is the fuel for growth and life. Despite centuries of over-reliance on fossil-based energy sources with devastating effects on the planet, more environmentally friendly and renewable options keep emerging as suitable alternatives.
Hydrogen generation by water splitting has long attracted attention due to its zero-emission advantage. Each water molecule comprises an oxygen atom and two hydrogen atoms. The hydrogen atoms are extracted and then can be reunited to create highly flammable hydrogen gas or combined with CO2 to create hydrocarbon fuels, creating a plentiful and renewable energy source. The problem, however, is that water molecules do not simply break down when sunlight shines on them, they need a little help from a solar-powered catalyst.
The process uses a special kind of solar cell that absorbs sunlight and conveys electricity to a pool of carbon dioxide dissolved in water. Catalysts spur a chemical transformation that yields oxygen and a carbon-based byproduct. The technological challenge is to develop cheap, efficient catalysts and durable solar cells. “This will require a long-lasting, worldwide research effort, probably similar to fusion energy, which does, however, not guarantee success in due time,” he said. Countries would also need to find a way to pay for it.
Water splitting or electrolysis is a technique employed in decomposing water molecules into hydrogen and oxygen by the passage of an electric current. It requires excess energy to overcome the reaction’s activation barriers; hence, it requires catalysts to increase the efficiency of the chemical process.
The theoretical minimum voltage needed to split water molecules into hydrogen and oxygen is 1.23 V. However, in real world systems, 1.5 V or more is generally needed because of the low reaction kinetics. So far, other researchers have only been able to achieve this voltage level through the use of either inefficient materials, such as titanium oxide, or very expensive semiconductors, such as gallium arsenide. Also, overcoming the corrosive degradation of these “artificial photosynthesis” systems remains a monumental challenge and has thus far eluded commercialization.
To create practical solar fuels, scientists have been trying to develop low-cost and efficient materials, known as photoanodes, that are capable of splitting water using visible light as an energy source. Over the past four decades, researchers identified only 16 of these photoanode materials. Now, using a new high-throughput method of identifying new materials, a team of researchers led by Caltech’s John Gregoire and Berkeley Lab’s Jeffrey Neaton and Qimin Yan have found 12 promising new photoanodes.
An international team of researchers has now succeeded in raising the efficiency of producing hydrogen from direct solar water-splitting to a record 19 per cent. They did so by combining a tandem solar cell of III-V semiconductors with a catalyst of rhodium nanoparticles and a crystalline titanium dioxide coating.
From Michigan State University: “Nature provides roadmap to potential breakthroughs in solar energy technology”
Sunlight, although abundant, is a low-density energy source. To collect meaningful amounts of energy you need larger amounts of space. However, the most effective materials in use today for solar energy conversion, such as Ruthenium, are some of the rarest metals on Earth. Future solar technologies must be able to scale up with more efficient and cheaper methods of energy conversion.
MSU Foundation Professor James McCusker, Department of Chemistry, believes that the future of solar energy lies in abundant, scalable materials designed to mimic and improve upon the energy conversion systems found in nature. Light-absorbing compounds in common synthetic methods for artificial photosynthesis make use of excited molecular states produced after a molecule absorbs energy from sunlight. The absorption of light energy exists long enough to be used in chemical reactions that rely on the ability to move electrons from one place to another. One possible solution is to find more commonly available materials that can achieve the same result.
“The problem with switching (from rare Earth metals) to something Earth-abundant like iron — where the scalability problem disappears — is that the processes that allow you to convert the absorbed sunlight into chemical energy are fundamentally different in these more widely available materials,” McCusker said. The excited state produced by absorbing light energy in an iron-based compound, for example, decays too quickly to enable its use in a similar manner.
In a groundbreaking new study in Nature reported in June 2020 , McCusker reveals a novel process that allows molecules to tell scientists how they should be modified to better absorb and convert solar energy. The method uses a molecular property known as quantum coherence where different aspects of a molecule are synchronous, like when your car’s turn signal blinks in unison with that of the car in front of you. Scientists believe that quantum coherence may play a role in natural photosynthesis. By hitting a molecule with a burst of light lasting less than one-tenth of one trillionth of a second, McCusker and his students could observe the interconnection between the molecule’s excited state and its structure, allowing them to visualize how the atoms of the molecule were moving during the conversion of solar to chemical energy.
“Once we had a picture of how this process occurred, the team used that information to synthetically modify the molecule in such a way as to slow the rate of the process down,” McCusker said. “This is an important goal that must be achieved if these types of chromophores — a molecule that absorbs particular wavelengths of visible light and are responsible for a material’s color — are to find their way into solar energy technologies.”
“The research demonstrates that we can use this coherence phenomenon to teach us what sorts of things we might need to incorporate into the molecular structure of a chromophore that uses more earth-abundant materials to enable us to use the energy stored in the molecule upon absorption of light for a wide range of energy conversion applications.”vFor McCusker, this breakthrough will hopefully speed up development of new technologies, “eliminating a lot of the trial and error that goes into scientific endeavors by telling us right out of the gate what kind of system we need to design.”
Artificial photosynthesis using nanoparticles produces ‘green methane’
New research describes a highly efficient and cheap device that could be used to turn waste carbon dioxide into methane. Natural gas, which mainly consists of methane, is a cleaner fuel than coal and has been characterized as a “bridge fuel” prior to transitioning to renewable energy sources, but not everyone thinks it’s a good idea to burn yet more hydrocarbons.
“Thirty percent of the energy in the U.S. comes from natural gas,” said co-author Zetian Mi in a statement. “If we can generate green methane, it’s a big deal.” Most importantly, the device makes use of low-cost and easily manufactured components, meaning that it will be scalable. The fatal flaw of many magic bullet climate change solutions is that they are expensive or difficult to make and implement, preventing them from being used at the scale necessary to combat climate change.
The device itself can be characterized as a solar panel studded with nanoparticles of iron and copper. The copper and iron nanoparticles hang onto molecules of CO2 and H2O by their carbon and hydrogen atoms. Using the sun’s energy or an electrical current, the bonds between atoms in the CO2 and H2O are broken down, enabling the water’s hydrogen atoms to connect to the carbon dioxide’s carbon atom. The end result is one carbon atom bonded with four hydrogen atoms — methane. What’s more, the new device does this work far more efficiently than other artificial photosynthesis systems.
“Previous artificial photosynthesis devices often operate at a small fraction of the maximum current density of a silicon device, whereas here we operate at 80 or 90 percent of the theoretical maximum using industry-ready materials and earth abundant catalysts,” said Baowen Zhou, a postdoctoral researcher on this project.
Methane is merely one of the more useful products this device can produce; it can also be configured to produce syngas — a fuel consisting of hydrogen, carbon monoxide, and some carbon dioxide — or formic acid, which is used as a preservative in livestock feed.
Multishelled fullerenes beat graphene at catalysing water splitting
Moving from flat to curved catalytic carbon supports could help reduce the amount and cost of platinum needed for hydrogen production in water splitting. In a new study published in Nature Energy, aimed at improving the efficiency of electrocatalytic processes, a team of scientists led by Li Song and Jun Jiang from the University of Science and Technology of China (USTC) used platinum decorated spherical onion-like carbon (OLC) catalysts with platinum atoms deposited on their outermost surface instead of flat graphene supports. This method reduced the amount of platinum atoms otherwise needed by about 75% while maintaining or improving the rate of the electrochemical hydrogen production process.
Synthesizing spherical carbon supports
In this novel approach, the researchers created OLC supports by treating surface-oxidized nanodiamonds at various temperatures to deoxygenate them. This treatment transformed the nanodiamonds into OLCs at temperatures above 900 °C. They then dispersed platinum (Pt) atoms onto the spherical carbon supports by the atomic layer deposition method, which avoids clustering. Annealing the oxidized nanodiamonds at 1500 °C resulted in multishelled spherical graphene structures – that is, fullerene. These OLC particles have a diameter of about 5 nm and interlayer distances of 0.35 nm.
Reduced amount of platinum
These optimized OLC electrocatalysts utilize only 0.27 wt% Pt to achieve a comparable amount of hydrogen production to that of commercial Pt-carbon catalysts, which have 20 wt% Pt. Moreover, they improve the hydrogen production rate compared with graphene-supported catalysts with a similar Pt loading.
Complementary computer simulations suggest that the curved carbon structure with platinum atoms deposited on it leads to strong highly localized electric fields. Other researchers have reported similar field enhancements associated with a sharp tip – akin to the effect that makes lightning rods work – in the catalysis of CO2. They suggest the highly localized enhanced electric field makes the water-splitting reaction more efficient.
New Artificial Photosynthesis Breakthrough Uses Gold to Turn CO2 Into Liquid Fuel
Scientists have developed a new way of achieving artificial photosynthesis, producing high-energy hydrocarbons by leveraging electron-rich gold nanoparticles as a catalyst. Jain’s new research builds upon previous work he led in 2018 investigating the use of gold nanoparticles as a substitute for chlorophyll – a pigment that acts as a catalyst in natural photosynthesis, helping to drive the chemical reaction.
“The goal here is to produce complex, liquefiable hydrocarbons from excess CO2 and other sustainable resources such as sunlight,” says chemist Prashant Jain from the University of Illinois at Urbana-Champaign. “Scientists often look to plants for insight into methods for turning sunlight, carbon dioxide and water into fuels,” Jain said at the time. In those experiments, the team found that tiny spherical gold particles measuring only nanometres in size could absorb visible green light and transfer photo-excited electrons and protons.
The new study goes further with the same technique, converting CO2 into complex hydrocarbon fuel molecules – including propane and methane – which are synthesised by combining green light with the gold nanoparticles in an ionic liquid. “In this approach, plasmonic excitation of [gold] nanoparticles produces a charge-rich environment at the nanoparticle/solution interface conducive for CO2 activation,” the researchers explain in their paper, “while an ionic liquid stabilises charged intermediates formed at this interface, facilitating multi-step reduction and C–C coupling.”
In addition to propane and methane, the method also enables ethylene, acetylene, and propene to be photosynthesised – complex molecular arrangements that could one day enable viable energy storage in fuel cells. “Because they are made from long-chain molecules, [liquid fuels] contain more bonds,” Jain says, “meaning they pack energy more densely.”
Still, as with other methods used to generate artificial photosynthesis, the practicality of the breakthrough will ultimately hinge on its efficiency – and its ability to be implemented in the real world. On that front, the researchers acknowledge they now need to refine the ability of gold nanoparticles to drive these chemical conversions, and investigate how potential future applications could work at scale.
“There’s still a long way to go,” Jain explained in 2018. “I think we’ll need at least a decade to find practical CO2-sequestration, CO2-fixation, fuel-formation technologies that are economically feasible. “But every insight into the process improves the pace at which the research community can move.”
UCF Professor Trigger Artificial Photosynthesis in metal–organic frameworks (MOF)
Uribe-Romo and his team of students created a way to trigger a chemical reaction in a synthetic material called metal–organic frameworks (MOF) that breaks down carbon dioxide into harmless organic materials. Think of it as an artificial photosynthesis process similar to the way plants convert carbon dioxide (CO2) and sunlight into food. But instead of producing food, Uribe-Romo’s method produces solar fuel.
It’s something scientists around the world have been pursuing for years, but the challenge is finding a way for visible light to trigger the chemical transformation. Ultraviolet rays have enough energy to allow the reaction in common materials such as titanium dioxide, but UVs make up only about 4 percent of the light Earth receives from the sun. The visible range – the violet to red wavelengths – represent the majority of the sun’s rays, but there are few materials that pick up these light colors to create the chemical reaction that transforms CO2 into fuel.
Researchers have tried it with a variety of materials, but the ones that can absorb visible light tend to be rare and expensive materials such as platinum, rhenium and iridium that make the process cost-prohibitive. Uribe-Romo used titanium, a common nontoxic metal, and added organic molecules that act as light-harvesting antennae to see if that configuration would work. The light harvesting antenna molecules, called N-alkyl-2-aminoterephthalates, can be designed to absorb specific colors of light when incorporated in the MOF. In this case he synchronized it for the color blue.
“This work is a breakthrough,” said Uribe-Romo in a UCF news release. “Tailoring materials that will absorb a specific color of light is very difficult from the scientific point of view, but from the societal point of view we are contributing to the development of a technology that can help reduce greenhouse gases.”
“The idea would be to set up stations that capture large amounts of CO2, like next to a power plant. The gas would be sucked into the station, go through the process and recycle the greenhouse gases while producing energy that would be put back into the power plant.” Perhaps someday homeowners could purchase rooftop shingles made of the material, which would clean the air in their neighborhood while producing energy that could be used to power their homes. “That would take new technology and infrastructure to happen,” Uribe-Romo said. “But it may be possible.”
New world record for direct solar water-splitting efficiency
Teams from the California Institute of Technology, the University of Cambridge, Technische Universitaet Ilmenau, and the Fraunhofer Institute for Solar Energy Systems ISE participated in the development work. One part of the experiments took place at the Institute for Solar Fuels in the Helmholtz-Zentrum Berlin.
Photovoltaics are a mainstay of renewable-energy supply systems, and sunlight is abundantly available worldwide but not around the clock. One solution for dealing with this fluctuating power generation is to store sunlight in the form of chemical energy, specifically by using sunlight to produce hydrogen. This is because hydrogen can be stored easily and safely, and used in many ways whether in a fuel cell to directly generate electricity and heat, or as feedstock for manufacturing combustible fuels. If you combine solar cells with catalysts and additional functional layers to form a monolithic photoelectrode as a single block, then splitting water becomes especially simple: the photocathode is immersed in an aqueous medium and when light falls on it, hydrogen is formed on the front side and oxygen on the back.
For the monolithic photocathode investigated here, the research teams combined additional functional layers with a highly efficient tandem cell made of III-V semiconductors developed at Fraunhofer ISE. This enabled them to reduce the surface reflectivity of the cell, thereby avoiding considerable losses caused by parasitic light absorption and reflection. “This is also where the innovation lies,” explains Prof. Hans-Joachim Lewerenz, Caltech, USA: “Because we had already achieved an efficiency of over 14 per cent for an earlier cell in 2015, which was a world record at the time. Here we have replaced the anti-corrosion top layer with a crystalline titanium dioxide layer that not only has excellent anti-reflection properties, but to which the catalyst particles also adhere.” And Prof. Harry Atwater, Caltech, adds: “In addition, we have also used a new electrochemical process to produce the rhodium nanoparticles that serve to catalyse the water-splitting reaction. These particles are only ten nanometres in diameter and are therefore optically nearly transparent, making them ideally suited for the job.”
Under simulated solar radiation, the scientists achieved an efficiency of 19.3 per cent in dilute aqueous perchloric acid, while still reaching 18.5 per cent in an electrolyte with neutral pH. These figures approach the 23 per cent theoretical maximum efficiency that can be achieved with the inherent electronic properties for this combination of layers.
“The crystalline titanium-dioxide layer not only protects the actual solar cell from corrosion, but also improves charge transport thanks to its advantageous electronic properties,” says Dr. Matthias May, who carried out part of the efficiency determination experiments at the HZB Institute for Solar Fuels in the forerunner laboratory to the Solar-Fuel Testing Facility of the Helmholtz Energy Materials Foundry (HEMF). The record figure now published is based on work that May had already begun as a doctoral student at the HZB and for which he was awarded the Helmholtz Association Doctoral Prize for the field of energy research in 2016. “We were able to increase the operating life to almost 100 hours. This is a major advance compared to previous systems that had already corroded after 40 hours. Nevertheless, there is still a lot to be done,” May explains.
That is because it is still fundamental research on small, high-priced systems in the laboratory. However, the researchers are optimistic: “This work shows that tailor-made tandem cells for direct solar water-splitting have the potential to achieve efficiencies beyond 20 per cent. One approach for this is to choose even better band-gap energies for the two absorber materials in the tandem cell. And one of the two could even be silicon,” explains Prof. Thomas Hannappel, TU Ilmenau. Teams at Fraunhofer ISE and TU Ilmenau are working to design cells that combine III-V semiconductors with lower-priced silicon, which could considerably reduce costs.
Swiss Researchers achieved Record efficiency for converting solar energy to hydrogen without rare metals
The scientists from Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland have achieved a solar energy to hydrogen conversion efficiency of 12.3% using abundant and common materials. Researchers used nickel and iron catalysts for the electrodes in their electrolyzer, and solar absorbers made of perovskite – another abundant material – in the solar cells.
The perovskite cells generate open circuit voltages of 1 V compared with the 0.7 V of a silicon cell, hence only two perovskite cells are needed instead of three silicon cells, to generate the required 1.7 V needed for water electrolysis. One challenge that needs to be solved is the instability of the perovskite PVs, which results in a degradation of the photocurrent over a period of hours.
A low-cost alternative is the use of a semiconductor in a photoelectrochemical (PEC) cell. When illuminating a semiconductor that is immersed in water, oxygen bubbles can evolve from one side, while hydrogen bubbles are formed at the counter electrode (or vice versa). The challenge is to find low cost materials that combine good visible light absorption with a high stability against photocorrosion. Moreover, charge carriers should easily move through the bulk of the material, and the surface of the material should have a high catalytic activity for water splitting. Researchers have struggled for years with the tradeoff between light absorption and the separation and collection of photogenerated charge carriers before they die out by recombination.
Researchers at Monash University in Melbourne claim new record energy efficiency for artificial photosynthesis
Now researchers at Monash University in Melbourne claim to have created a solar-powered device that produces hydrogen at world-record 22 percent efficiency, which is a significant step towards making cheap, efficient hydrogen production a reality.
To help achieve the required solar-input efficiencies, the team utilized the very best commercial-grade multi-junction (indium gallium phosphide, gallium arsenide, and germanium) solar cells available to ensure the maximum sunlight to electricity conversion.
However, an even greater contribution to efficiency was on the material side, where the use of expanded foam nickel electrodes increased the available electrolysis surface area with such efficiency that the electrolyte in which they were immersed was simply local river water with the addition of a standard pH buffer (generally a salt solution containing sodium phosphate and sodium chloride).
“Electrochemical splitting of water could provide a cheap, clean and renewable source of hydrogen as the ultimately sustainable fuel.” said Professor Leone Spiccia from the School of Chemistry at Monash who led the research. “This latest breakthrough is significant in that it takes us one step further towards this becoming a reality.”
“Hydrogen can be used to generate electricity directly in fuel cells,” said Doug MacFarlane, co-author of the research. “Cars driven by fuel cell electric engines are becoming available from a number of car manufacturers. Hydrogen could even be used as an inexpensive energy storage technology at the household level to store energy from roof-top solar cells.”
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