NREL Establishes World Record for Solar Hydrogen Production


NREL Solar to Hydrogen 20170412-42601NREL researchers Myles Steiner (left), John Turner, Todd Deutsch and James Young stand in front of an atmospheric pressure MDCVD reactor used to grow crystalline semiconductor structures. They are co-authors of the paper “Direct Solar-to-Hydrogen Conversion via Inverted Metamorphic Multijunction Semiconductor Architectures” published in Nature Energy. Photo by Dennis Schroeder.

 

Scientists at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) recaptured the record for highest efficiency in solar hydrogen production via a photo-electrochemical (PEC) water-splitting process.

The new solar-to-hydrogen (STH) efficiency record is 16.2 percent, topping a reported 14 percent efficiency in 2015 by an international team made up of researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and the California Institute of Technology. A paper in Nature Energy titled Direct Solar-to-hydrogen Conversion via Inverted Metamorphic Multijunction Semiconductor Architectures outlines how NREL’s new record was achieved. The authors are James Young, Myles Steiner, Ryan France, John Turner, and Todd Deutsch, all from NREL, and Henning Döscher of Philipps-Universität Marburg in Germany. Döscher has an affiliation with NREL.

solar-hydrogen-system-illustrationThe record-setting PEC cell represents a significant change from the concept device Turner developed at NREL in the 1990s.

Both the old and new PEC processes employ stacks of light-absorbing tandem semiconductors that are immersed in an acid/water solution (electrolyte) where the water-splitting reaction occurs to form hydrogen and oxygen gases. But unlike the original device made of gallium indium phosphide (GaInP2) grown on top of gallium arsenide (GaAs), the new PEC cell is grown upside-down, from top to bottom, resulting in a so-called inverted metamorphic multijunction (IMM) device.

This IMM advancement allowed the NREL researchers to substitute indium gallium arsenide (InGaAs) for the conventional GaAs layers, improving the device efficiency considerably. A second key distinguishing feature of the new advancement was depositing a very thin aluminum indium phosphide (AlInP) “window layer” on top of the device, followed by a second thin layer of GaInP2. These extra layers served both to eliminate defects at the surface that otherwise reduce efficiency and to partially protect the critical underlying layers from the corrosive electrolyte solution that degrades the semiconductor material and limits the lifespan of the PEC cell.

Turner’s initial breakthrough created an interesting new way to efficiently split water using sunlight as the only energy input to make renewable hydrogen. Other methods that use sunlight entail additional loss-generating steps. For example: Electricity generated by commercial solar cells can be sent through power conversion systems to an electrolyzer to decompose water into hydrogen and oxygen at an approximate STH efficiency of 12 percent. Turner’s direct method set a long-unmatched STH efficiency record of 12.4 percent, which has been surpassed by NREL’s new PEC cell.

Before the PEC technology can be commercially viable, the cost of hydrogen production needs to come down to meet DOE’s target of less than $2 per kilogram of hydrogen.solarhydrogen

 

Continued improvements in cell efficiency and lifetime are needed to meet this target. Further enhanced efficiency would increase the hydrogen production rate per unit area, which decreases hydrogen cost by reducing balance-of-system expenditures. In conjunction with efficiency improvements, durability of the current cell configuration needs to be significantly extended beyond its several hours of operational life to dramatically bring down costs. NREL researchers are actively pursuing methods of increasing the lifespan of the PEC device in addition to further efficiency gains.

While an alternative configuration where the device isn’t submerged in acidic electrolyte and instead is wired to an external electrolyzer would solve the durability challenge, a techno-economic analysis commissioned by DOE has shown that submerged devices have the potential to produce hydrogen at a lower cost.

The latest research was funded by the Energy Department’s Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

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Umea University: An Efficient Path from Carbon to Renewable Fuel Production


Nano fuel cells c2cs35307e-f1Earth-abundant materials based primarily on carbon, nitrogen and transition metal oxides can be combined into highly efficient energy conversion devices. These devices can be used in fuel cells as well as in electrolysis. This is shown experimentally by Tiva Sharifi, physicist at Umeå University, Sweden. She defends her thesis on 31 March. 

As the world runs short of oil, the hunt for new alternative energy resources is intensified. Hydrogen production — by splitting water to oxygen and hydrogen using sunlight as the driving force — is an interesting approach to fuel production.

Tiva Sharifi’s focus has been to address the problems of energy conversion processes, aiming mainly to fabricate durable electrode materials that allow up-scaling of electrochemical cells.

“I have created an electrocatalyst with outstanding performance and stability for several important energy conversion processes,” says Tiva Sharifi.

Since electrochemical processes are not spontaneous, they need efficient electrocatalyst materials to §run smoothly. Commonly, the electrocatalyst is synthesised separately and then loaded onto the surface of a conductive material, which plays the role of current collector. This combination works as the electrode — cathode or anode — in the electrochemical cell.

In such an approach, up-scaling is inhibited and the whole process is limited to laboratory scale. Furthermore, the fabricated electrode is not durable and the catalyst material easily peels off from the conductive substrate during the electrochemical reaction.

Tiva Sharifi has solved these problems by fabricating electrode materials in such a way that these two steps — electrocatalyst synthesis and loading onto a conductive material — are combined. She grew electrocatalyst material directly on the surface of a current collector in a bottom-up self-assembly process.

An inexpensive conductive substrate made of carbon paper was chosen as current collector. It has an acceptable conductivity and the capability of easy handling and up-scaling. Instead of using scarce and hence expensive noble metals — like platinum and ruthenium — as catalyst materials she chose all organic carbon materials, transition metal oxides — such as cobalt oxide and iron oxide — or their combination.

Tiva Sharifi also investigated nitrogen-doped carbon nanotubes (NCNT’s) as an electrocatalyst, since their properties are interesting when it comes to manufacturing all organic metal-free catalysts. She discovered that NCNT’s — grown directly on the current collector — is a highly efficient electrocatalyst.

The NCNT’s exhibit strong electrocatalytic activity for some fundamental energy conversion reactions, such as oxygen reduction reaction in fuel cells. They are formed by introducing nitrogen in the pure hexagonal carbon structure of carbon nanotubes. Interestingly, it is possible to keep track of nitrogen in the carbon framework, and depending on where the nitrogen replaces carbon in the structure, the nanotubes behave catalytically different. In her thesis, Tiva Sharifi shows that it is possible to transfer nitrogen from non-favourable sites to catalytically active sites in already synthesised NCNT’s.


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

Novel water-splitting photocatalyst (with solar energy) operable over wide range of the visible light spectrum


1-solar qdot-images-6

Clean renewable energy can be produced by photocatalytically splitting water into hydrogen and oxygen with solar energy. Most of the conventionally developed water-splitting photocatalysts, however, were only active under UV irradiation, and only a few have been demonstrated to operate under visible light, at up to 500 nm. For making high-efficiency use of solar energy, it was necessary to develop a photocatalyst that can utilize longer wavelength light.
To accomplish this, a photocatalyst that is operable under lower-energy light needed to be developed, but since the energy that can be used for the water-splitting reaction would also be smaller, more advanced material design was required, which posed a very difficult challenge.

Water Splitting Graph 150318074230-large

 Graph. A water-splitting photocatalyst that is operable at up to 600nm has been developed for the first time, using a transition-metal oxynitride whose electronic structure is suitable for long wavelength absorption.

Credit: NIMS
A research group led by Chengsi Pan, Postdoctoral Researcher, and Tsuyoshi Takata, NIMS Special Researcher, at the Global Research Center for Environment and Energy Based on Nanomaterials Science (GREEN; Director-General: Kohei Uosaki) of the National Institute for Materials Science (NIMS; President: Sukekatsu Ushioda), and Kazunari Domen, a professor of the Department of Chemical System Engineering, School of Engineering, The University of Tokyo (President: Junichi Hamada) newly developed a water-splitting photocatalyst that is operable over a wider range of the visible light spectrum than before.

In this research, a water-splitting photocatalyst that is operable at up to 600nm was developed for the first time, using a transition-metal oxynitride whose electronic structure is suitable for long wavelength absorption. As a development approach, solid solutions were formed between two existing perovskite-type compounds, LaTaON2 and LaMg2/3Ta1/3O3 (La: lanthanum, Ta: tantalum, O: oxygen, N: nitrogen, Mg: magnesium), and electronic structure was adjusted. This made LaMg1/3Ta2/3O2N solid solutions usable for water-splitting reactions by visible light irradiation, but since the degradation of the photocatalyst and the reverse reaction simultaneously occurred, a steady water-splitting reaction could not be achieved. To overcome this problem, the photocatalyst particle surface was covered with a layer of amorphous oxyhydroxide in order to inhibit the degradation of the photocatalyst and reverse reaction, and made the steady water-splitting reaction possible. This oxyhydroxide coating plays a role to control chemical reactions on the photocatalyst surface.

This research result established a new effective method in water-splitting photocatalyst development. Also, by applying this method to other photocatalyst materials, the development of photocatalysts with higher activity can be expected. At present, the quantum yield is still low, and the improvement of the yield is the challenge for the future.

This research was performed jointly with a group led by Yuichi Ikuhara, Professor of the Institute of Engineering Innovation, School of Engineering, The University of Tokyo. Also, this research was supported in part by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Specially Promoted Research, “Development of innovative water splitting photocatalysts based on photocarrier dynamics at solid/liquid interfaces,” and projects commissioned by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), “Program for Development of Environmental Technology using Nanotechnology,” “Nanotechnology Platform Japan,” and “Area of Advanced Environmental Materials, Green Network of Excellence (GRENE): Creation of the Network of Excellence for the Human Resource Development, and Advanced Environmental Materials and Devices toward Environment and Energy Technology.”


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The above story is based on materials provided by National Institute for Materials Science (NIMS). Note: Materials may be edited for content and length.

Toward a low-cost ‘artificial leaf’ that produces clean hydrogen fuel


Articicial Leaf III towardalowcoFor years, scientists have been pursuing “artificial leaf” technology, a green approach to making hydrogen fuel that copies plants’ ability to convert sunlight into a form of energy they can use. Now, one team reports progress toward a stand-alone system that lends itself to large-scale, low-cost production. They describe their nanowire mesh design in the journal ACS Nano.

Peidong Yang, Bin Liu and colleagues note that harnessing sunlight to split water and harvest hydrogen is one of the most intriguing ways to achieve clean energy. Automakers have started introducing cell vehicles, which only emit water when driven. But making hydrogen, which mostly comes from natural gas, requires electricity from conventional carbon dioxide-emitting power plants.

Articicial Leaf III towardalowco

Producing hydrogen at low cost from water using the from the sun would make this form of energy, which could also power homes and businesses, far more environmentally friendly. Building on a decade of work in this area, Yang’s team has taken one more step toward this goal.

The researchers took a page from the paper industry, using one of its processes to make a flat mesh out of light-absorbing semiconductor nanowires that, when immersed in water and exposed to sunlight, produces . The scientists say that the technique could allow their technology to be scaled up at low cost. Although boosting efficiency remains a challenge, their approach—unlike other artificial leaf systems—is free-standing and doesn’t require any additional wires or other external devices that would add to the environmental footprint.

Explore further: Harvesting hydrogen fuel from the Sun using Earth-abundant materials

More information: “All Inorganic Semiconductor Nanowire Mesh for Direct Solar Water Splitting” ACS Nano, 2014, 8 (11), pp 11739–11744. DOI: 10.1021/nn5051954

Abstract
The generation of chemical fuels via direct solar-to-fuel conversion from a fully integrated artificial photosynthetic system is an attractive approach for clean and sustainable energy, but so far there has yet to be a system that would have the acceptable efficiency, durability and can be manufactured at a reasonable cost. Here, we show that a semiconductor mesh made from all inorganic nanowires can achieve unassisted solar-driven, overall water-splitting without using any electron mediators. Free-standing nanowire mesh networks could be made in large scales using solution synthesis and vacuum filtration, making this approach attractive for low cost implementation.

Graphene may help Fuel cell technology


Graphene-help-Fuel-cell-technologyAccording to researchers, a weak spot found in graphene, could benefit the fuel cell technology. As per a research at Britain’s Manchester University, graphene is not that impermeable as thought and will allow protons to easily pass through it.

The research was led by Andre Geim, who shared a Nobel Prize for the discovery of graphene and Professor Wu Hengan from the University of Science and Technology of China. Researchers suggest that graphene membranes could be created in future which could filter hydrogen gas directly out of the air to be used to generate electricity.

A co-researcher in Geim’s study, Marcelo Lozada-Hidalgo said that the results are really exciting and this opens a new area of graphene applications in clean energy harvesting and hydrogen-based technologies.

Graphene-help-Fuel-cell-technology

Graphene is 200 times stronger than steel and used in impermeable packaging and corrosion-proof coating, because of it is impermeablity to atoms of any gas or liquid.

A barrier is required in fuel cells and other hydrogen-based technologies, which permits only protons to pass through it. Geim and his researchers are expecting that hydrogen protons may pass through the graphene. This indicates that graphene could be used in fuel cells.

The study has been published in the journal Nature.

Currently fuel cells have a problem of leakage of fuels across their membranes and it decreases the cell’s efficiency. And the researchers are expecting graphene to solve this problem.

“When you know how it should work, it is a very simple setup. You put a hydrogen-containing gas on one side, apply small electric current and collect pure hydrogen on the other side. This hydrogen can then be burned in a fuel cell”, says Lozada-Hidalgo.

The graphene could be created in square meter sheets these days and soon it could be used in commercial fuel cells, said researcher Sheng Hu.

Semiconductor nanowire mesh produces clean hydrogen fuel


Nano Hydro ananosizedhyFor years, scientists have been pursuing “artificial leaf” technology, a green approach to making hydrogen fuel that copies plants’ ability to convert sunlight into a form of energy they can use. Now, one team reports progress toward a stand-alone system that lends itself to large-scale, low-cost production. They describe their nanowire mesh design in the journal ACS Nano (“All Inorganic Semiconductor Nanowire Mesh for Direct Solar Water Splitting”).
Peidong Yang, Bin Liu and colleagues note that harnessing sunlight to split water and harvest hydrogen is one of the most intriguing ways to achieve clean energy. Automakers have started introducing hydrogen fuel cell vehicles, which only emit water when driven. But making hydrogen, which mostly comes from natural gas, requires electricity from conventional carbon dioxide-emitting power plants. Producing hydrogen at low cost from water using the clean energy from the sun would make this form of energy, which could also power homes and businesses, far more environmentally friendly. Building on a decade of work in this area, Yang’s team has taken one more step toward this goal.
The researchers took a page from the paper industry, using one of its processes to make a flat mesh out of light-absorbing semiconductor nanowires that, when immersed in water and exposed to sunlight, produces hydrogen gas. The scientists say that the technique could allow their technology to be scaled up at low cost. Although boosting efficiency remains a challenge, their approach — unlike other artificial leaf systems — is free-standing and doesn’t require any additional wires or other external devices that would add to the environmental footprint.
Source: American Chemical Society

Read more: Semiconductor nanowire mesh produces clean hydrogen fuel

Could hydrogen vehicles take over as the “green” car of choice?


1-Toyota Hydro 1416262251402Now that car makers have demonstrated through hybrid vehicle success that consumers want less-polluting tailpipes, they are shifting even greener. In 2015, Toyota will roll out the first hydrogen fuel-cell car for personal use that emits only water. An article in Chemical & Engineering News (C&EN), the weekly newsmagazine of the American Chemical Society, explains how hydrogen could supplant hybrid and electric car technology — and someday, even spur the demise of the gasoline engine.

Melody M. Bomgardner, a senior editor at C&EN, notes that the first fuel-cell vehicles will be sold in Japan, then California to start. Although Toyota is the only one poised to sell fuel-cell vehicles very soon, other companies are also investing billions of dollars in the technology. Hyundai, General Motors, Honda and Daimler have all announced plans to offer their own hydrogen models in the near future. The first cars will set customers back about $70,000, but this marks a 95 percent cut in system costs in less than 10 years. As they improve the technology further, car manufacturers expect that prices will come down to more affordable levels.

1-Toyota Hydro 1416262251402

In 2015, Toyota will be the first car maker to bring a personal, hydrogen fuel-cell vehicle to the market.

Credit: Toyota            

 But does that translate into a practical edge for consumers? With a hydrogen vehicle, filling up only takes about three minutes, compared to an overnight charge for an all-electric car. Fuel-cell vehicles can go 400 miles on one fill-up, which is fewer than a hybrid but with no polluting emissions. Although electrics also boast zero tailpipe emissions, they will have a tough time competing with that kind of range. Given these advantages, some experts suggest hydrogen fuel cells could someday overtake hybrid, electric and even internal combustion technologies.

                  

Source: American Chemical Society

Cheap Hydrogen Fuel from the Sun without Rare Metals: Video


CheapHydrogenThe race is on to optimize solar energy’s performance. More efficient silicon photovoltaic panels, dye-sensitized solar cells, concentrated cells and thermodynamic solar plants all pursue the same goal: to produce a maximum amount of electrons from sunlight. Those electrons can then be converted into electricity to turn on lights and power your refrigerator.

At the Laboratory of Photonics and Interfaces at EPFL, led by Michael Grätzel, where scientists invented dye solar cells that mimic photosynthesis in plants, they have also developed methods for generating fuels such as hydrogen through solar water splitting. To do this, they either use photoelectrochemical cells that directly split water into hydrogen and oxygen when exposed to sunlight, or they combine electricity-generating cells with an electrolyzer that separates the water molecules.

By using the latter technique, Grätzel’s post-doctoral student Jingshan Luo and his colleagues were able to obtain a performance so spectacular that their achievement is being published in the journal Science. Their device converts into hydrogen 12.3% of the energy diffused by the sun on perovskite absorbers—a compound that can be obtained in the laboratory from common materials, such as those used in conventional car batteries, eliminating the need for rare-earth metals in the production of usable hydrogen fuel.

Bottled sun

This high efficiency provides stiff competition for other techniques used to convert solar energy. But this method has several advantages over others:

“Both the perovskite used in the cells and the nickel and iron catalysts making up the electrodes require resources that are abundant on Earth and that are also cheap,” explained Jingshan Luo. “However, our electrodes work just as well as the expensive platinum-based models customarily used.”

Published on Sep 25, 2014

Science published on September 25, 2014 the latest developments in Michael Grätzel’s laboratory at EPFL in the field of hydrogen production from water. By combining a pair of perovskite solar cells and low price electrodes without using rare metals, scientists have obtained a 12.3% conversion efficiency from solar energy to hydrogen, a record with earth-abundant materials. Jingshan Luo, post-doctoral researcher, explains how.
For more info, please find our press kit here: http://bit.ly/GraetzelHydrogenScience…

On the other hand, the conversion of solar energy into hydrogen makes its storage possible, which addresses one of the biggest disadvantages faced by renewable electricity—the requirement to use it at the time it is produced.

“Once you have hydrogen, you store it in a bottle and you can do with it whatever you want to, whenever you want it,” said Michael Grätzel. Such a gas can indeed be burned—in a boiler or engine—releasing only water vapor. It can also pass into a fuel cell to generate electricity on demand. And the 12.3% conversion efficiency achieved at EPFL “will soon get even higher,” promised Grätzel.

More powerful cells

These high efficiency values are based on a characteristic of perovskite cells: their ability to generate an open circuit voltage greater than 1 V (silicon cells stop at 0.7 V, for comparison).

“A voltage of 1.7 V or more is required for water electrolysis to occur and to obtain exploitable gases,” explained Jingshan Luo. To get these numbers, three or more silicon cells are needed, whereas just two perovskite cells are enough. As a result, there is more efficiency with respect to the surface of the light absorbers required. “This is the first time we have been able to get hydrogen through electrolysis with only two cells!” Luo adds.

The profusion of tiny bubbles escaping from the electrodes as soon as the solar cells are exposed to light say it better than words ever could: the combination of sun and water paves a promising and effervescent way for developing the energy of the future.

Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts

Source: EPFL

Hydrogen fuel from sunlight


3adb215 D Burris*** 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).

Click here for more information.     

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.”

###

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/.

Nanotrees Harvest the Sun’s Energy to Turn Water into Hydrogen Fuel


 

University of California, San Diego electrical engineers are building a forest of tiny nanowire trees in order to cleanly capture solar energy without using fossil fuels and harvest it for hydrogen fuel generation. Reporting in the journal Nanoscale, the team said nanowires, which are made from abundant natural materials like silicon and zinc oxide, also offer a cheap way to deliver hydrogen fuel on a mass scale.

“This is a clean way to generate clean fuel,” said Deli Wang, professor in the Department of Electrical and Computer Engineering at the UC San Diego Jacobs School of Engineering.
The trees’ vertical structure and branches are keys to capturing the maximum amount of solar energy, according to Wang. That’s because the vertical structure of trees grabs and adsorbs light while flat surfaces simply reflect it, Wang said, adding that it is also similar to retinal photoreceptor cells in the human eye. In images of Earth from space, light reflects off of flat surfaces such as the ocean or deserts, while forests appear darker.
Wang’s team has mimicked this structure in their “3D branched nanowire array” which uses a process called photoelectrochemical water-splitting to produce hydrogen gas. Water splitting refers to the process of separating water into oxygen and hydrogen in order to extract hydrogen gas to be used as fuel. This process uses clean energy with no green-house gas byproduct. By comparison, the current conventional way of producing hydrogen relies on electricity from fossil fuels.

Schematic shows the light trapping effect in nanowire arrays. Photons on are bounced between single nanowires and eventually absorbed by them (R). By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts where they are reflected off the surface (L). Image Credit: Wang Research Group, UC San Diego Jacobs School of Engineering.
“Hydrogen is considered to be clean fuel compared to fossil fuel because there is no carbon emission, but the hydrogen currently used is not generated cleanly,” said Ke Sun, a PhD student in electrical engineering who led the project.
By harvesting more sun light using the vertical nanotree structure, Wang’s team has developed a way to produce more hydrogen fuel efficiently compared to planar counterparts. Wang is also affiliated with the California Institute of Telecommunications and Information Technology and the Material Science and Engineering Program at UC San Diego.
The vertical branch structure also maximizes hydrogen gas output, said Sun. For example, on the flat wide surface of a pot of boiling water, bubbles must become large to come to the surface. In the nanotree structure, very small gas bubbles of hydrogen can be extracted much faster. “Moreover, with this structure, we have enhanced, by at least 400,000 times, the surface area for chemical reactions,” said Sun.

In this experiment, nanotree electrodes are submersed in water and illuminated by simulated sun light to measure electricity output of the device. Photo credit: Joshua Knoff, UC San Diego Jacobs School of Engineering.
In the long run, what Wang’s team is aiming for is even bigger: artificial photosynthesis. In photosynthesis, as plants absorb sunlight they also collect carbon dioxide (CO2) and water from the atmosphere to create carbohydrates to fuel their own growth. Wang’s team hopes to mimic this process to also capture CO2 from the atmosphere, reducing carbon emissions, and convert it into hydrocarbon fuel.
“We are trying to mimic what the plant does to convert sunlight to energy,” said Sun. “We are hoping in the near future our ‘nanotree’ structure can eventually be part of an efficient device that functions like a real tree for photosynthesis.”
The team is also studying alternatives to zinc oxide, which absorbs the sun’s ultraviolet light, but has stability issues that affect the lifetime usage of the nanotree structure.