NREL’s collaboration with Purdue University’s School of Mechanical Engineering has yielded new insights for lithium-ion (Li-ion) battery electrodes at the microstructural level, which can lead to improvements in electric vehicle (EV) battery performance and lifespan.


NREL LI Batt 1 2018018-thsc-micromodelElectrochemical simulation within a 3D nickel manganese cobalt electrode microstructure during a 20-minute fast charge. Streamlines represent Li-ion current in the electrolyte phase as ions travel through pores between the solid active material particles. Colors represent current magnitude. Illustration by Francois Usseglio-Viretta and Nicholas Brunhart-Lupo, NREL.

NREL’s collaboration with Purdue University’s School of Mechanical Engineering has yielded new insights for lithium-ion (Li-ion) battery electrodes at the microstructural level, which can lead to improvements in electric vehicle (EV) battery performance and lifespan. A stochastic algorithm developed by Purdue University, as part of NREL’s Advanced Computer-Aided Battery Engineering Consortium, is prominently displayed on the cover of the 10th anniversary issue of American Chemical Society’s Applied Materials and Interfaces. The NREL/Purdue team’s corresponding article, “Secondary-Phase Stochastics in Lithium-Ion Battery Electrodes” detailing the research and resulting discoveries, is showcased inside.

This work builds on earlier phases of the U.S. Department of Energy’s Computer-Aided Engineering for Electric-Drive Vehicle Batteries (CAEBAT) program. NREL’s energy storage team has led key research projects since CAEBAT’s inception in 2010, resulting in the creation of software tools for cell and battery design, as well as advancements in crash simulations used by many automakers.

This next phase of CAEBAT focuses on Li-ion electrode microstructure applications (accurately simulating the physics and geometric complexity of a battery) to better understand the impact materials and manufacturing controls have on cell performance. Li-ion batteries represent a complex non-linear system and considering EVs use larger batteries with more complex configurations, it is imperative to understand the interplay between electrochemical, thermal, and mechanical physics.

Says Kandler Smith, NREL co-author on the article, “Batteries are an exceedingly complex system—both in terms of their physics and geometry. In a real battery, it’s difficult to get a clear view of what’s going on inside, because so few measurements are possible. Models are a place where all physics can come together and the advantage of the model is that everything can be measured and probed. As we build an increasingly accurate physical understanding of batteries, we can expect that technological advances will follow.”

The secondary phase in Li-ion electrodes, comprised of inert binder and electrical conductive additives, has been found to critically influence various forms of microstructural resistances. This phase has benefits for improved electronic conductivity and mechanical integrity but may block access to electrochemical active sites and introduce additional transport resistances in the pore (electrolyte) phase, thus, canceling out its original advantages.

Because the secondary phase is important for electrode mechanical integrity and electronic conductivity, its recipe and morphology will have a strong impact on battery kinetics and transport. The algorithm created and explained in the journal article explores morphologies for this phase. Stochastics comes into play as each microstructure variant is numerically generated multiple times using random seeds to ensure statistically relevant conclusions. By simulating battery electrochemistry on the various microstructure geometries, researchers can calculate the pore size of an electrode’s microstructure geometry as well as the lithium displacement within an electrode to evaluate the difficulty of movement. Finding ways to overcome resistances via electrode microstructural modifications can greatly improve overall Li-ion battery performance.

The value of this work is that improvements to Li-ion batteries—the most expensive and complex component in EVs—is helping to overcome the concerns consumers have that limit EV adoption, including restricted driving range and high costs.

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NREL: New Study Indicates Shorter-Term Storage Can Reduce Variable-Generation Curtailments


Storage devices, like this one-megawatt battery at the National Wind Technology Center, can reduce variable generation curtailments. Photo by Dennis Schroeder / NREL 47219

Increasing the penetration of variable energy sources such as solar and wind energy in the grid—without introducing heavy curtailment—does not require costly, very-long-duration storage, says a new study by National Renewable Energy Laboratory researchers Paul Denholm and Trieu Mai.

Historically, energy storage has been considered too pricey an option for integrating wind and solar into the grid, and utilities have relied on less expensive options. However, declining costs indicate that energy storage solutions may play a greater role in providing grid flexibility and storing because of their ability to decrease the amounts of curtained wind and solar generation.

When wind and solar are curtailed, grid operators cannot use all of the available renewable resources (i.e., there is more wind blowing than the grid can utilize). Often, this means that the grid must rely on a fossil-fuel energy generation source, rather than a cleaner source of energy. But with storage solutions, operators can save some of that excess energy, and thereby incorporate more renewable energy into the grid.

“Interest in very-high-renewable systems warrants further exploration into storage’s role in the future grid,” says Mai.

The team’s study, “Timescales of Energy Storage Needed for Reducing Renewable Energy CurtailmentPDF,” examines the amount and configuration of energy storage required to decrease variable-generation curtailments under high-renewable scenarios. Researchers developed a case study based on the U.S. Department of Energy’s Wind Vision report, in which variable generation provides 55% of the electricity demand in the Electricity Reliability Council of Texas (ERCOT) grid system in 2050. Analysis results showed that the amount of avoided curtailment falls off rapidly with storage durations longer than 8 hours, with the first 4 hours providing the largest benefit.

Denholm and Mai developed several energy mix scenarios based on the isolated ERCOT grid system, and found that deploying wind and solar together produced much lower levels of curtailment than when deployed individually.

“Wind and photovoltaics play well together,” Denholm says. However, the mix that produces the least amount of curtailment uses about twice as much wind as solar.

Among all energy mixes analyzed, there was little incremental benefit to deploying costly, very-long-duration or seasonal storage—at penetrations of up to 55%, the greatest benefit is found with the first 4 hours of storage.

NREL, University of Washington Scientists Elevate Quantum Dot Solar Cell World Record to 13.4 Percent



Researchers at the U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) established a new world efficiency record for quantum dot solar cells, at 13.4 percent.

Colloidal quantum dots are electronic materials and because of their astonishingly small size (typically 3-20 nanometers in dimension) they possess fascinating optical properties. 


Quantum dot solar cells emerged in 2010 as the newest technology on an NREL chart that tracks research efforts to convert sunlight to electricity with increasing efficiency. 

The initial lead sulfide quantum dot solar cells had an efficiency of 2.9 percent. Since then, improvements have pushed that number into double digits for lead sulfide reaching a record of 12 percent set last year by the University of Toronto. 

The improvement from the initial efficiency to the previous record came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.


 NREL scientists Joey Luther and Erin Sanehira are part of a team that has helped NREL set an efficiency record of 13.4% for a quantum dot solar cell.

The latest development in quantum dot solar cells comes from a completely different quantum dot material. The new quantum dot leader is cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. 

In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.

“This voltage, coupled with the material’s bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell,” said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. 

The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. 
Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today’s solar market.

This latest advance, titled “Enhanced mobility CsPbI3 quantum dot arrays for record-efficiency, high-voltage photovoltaic cells,” is published in Science Advances. The paper was co-authored by Erin Sanehira, Ashley Marshall, Jeffrey Christians, Steven Harvey, Peter Ciesielski, Lance Wheeler, Philip Schulz, and Matthew Beard, all from NREL; and Lih Lin from the University of Washington.

The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. 
The quantum dot perovskite materials developed by Luther and the NREL/University of Washington team could be paired with cheap thin-film perovskite materials to achieve similar high efficiency as demonstrated for space solar cells, but built at even lower costs than silicon technology–making them an ideal technology for both terrestrial and space applications.

“Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations,” said Erin Sanehira a doctoral student at the University of Washington who conducted research at NREL.

The NREL research was funded by DOE’s Office of Science, while Sanehira and Lin acknowledge a NASA space technology fellowship.

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.

NREL Inks Technology Agreement for High Efficiency Multijunction Solar Cells


October 24, 2017

MicroLink Devices opens the door for new multijunction solar cell applications

October 24, 2017

The U.S. Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) has entered into a license agreement with MicroLink Devices, Inc. (Niles, IL) to commercialize NREL’s patented inverted metamorphic (IMM) multijunction solar cells. 

While high-efficiency multijunction solar cells are commonly used for space satellites, researchers have continued to look for ways to improve cost and performance to enable a broader range of applications. 

The IMM technique licensed by MicroLink Devices enables multijunction III-V solar cells to be grown with both higher efficiencies and lower costs than traditional multijunction solar cells by reversing the order in which individual sub-cells are typically grown.



Two hands holding the IMM solar cellA 6-inch MicroLink Devices high-efficiency, lightweight and flexible ELO IMM solar cell wafer. Photo courtesy of MicroLink Devices

The IMM architecture enables greater power extraction from the higher-bandgap sub-cells and further allows the use of more efficient low-bandgap sub-cell materials such as Indium Gallium Arsenide. 

In contrast to traditional III-V multijunction solar cells, IMM devices are removed from their growth substrate, allowing the substrate to be reused over multiple growth runs – a significant component in reducing overall device costs. Removing the substrate also reduces the weight of the solar cell, which is important for applications such as solar-powered unmanned aerial vehicles.

MicroLink Devices is an Illinois-based ISO 9001 certified semiconductor manufacturer specializing in removing active semiconductor device layers from their growth substrate via a proprietary epitaxial liftoff (ELO) process. 

By utilizing its ELO capabilities, MicroLink will be able to make thin, lightweight, and highly flexible IMM solar cells which are ideal for use in unmanned aerial vehicles, space-based vehicles and equipment, and portable power generation applications. 

“IMM makes multijunction solar cells practical for a wide variety of weight-, geometry-, and space-constrained applications where high efficiency is critical,” said Jeff Carapella, one of the researchers in NREL’s III-V multijunction materials and devices research group that developed the technology.

“Former NREL Scientist Mark Wanlass pioneered the use of metamorphic buffer layers to form tandem III-V solar cells with three or more junctions. 

This approach is very synergistic with our ELO process technology, and MicroLink Devices is excited to now be commercializing IMM solar cells for high-performance space and UAV applications,” said Noren Pan, CEO of MicroLink Devices.

MicroLink and NREL have collaborated to evaluate the use of ELO for producing IMM solar cells since 2009, when MicroLink was the recipient of a DOE PV Incubator subcontract from NREL. 

Tests of MicroLink-produced IMM solar cells conducted at NREL have demonstrated multiple successful substrate reuses and efficiencies exceeding 30%.

NREL has more than 800 technologies available for licensing and continues to engage in advanced research and development of next-generation IMM and ultra-high-efficiency multijunction solar cells with both academic and commercial collaborators. 

Companies interested in partnering to advance research on or commercialize renewable energy technologies can visit the EERE Energy Innovation Portal, which features descriptions of all renewable energy technologies funded by the DOE’s Office of Energy Efficiency and Renewable Energy. 

Parties interested specifically in ongoing development of IMM solar cells can contact Dan Friedman, Manager of NREL’s High Efficiency Crystalline Photovoltaics Group, for more information.

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.

NREL Charges Forward to Reduce Time at EV Stations



Shortening recharge times may diminish range anxiety, increase EV market viability, however Speeding up battery charging will be crucial to improving the convenience of owning and driving an electric vehicle (EV). 




The Energy Department’s National Renewable Energy Laboratory (NREL) is collaborating with Argonne National Laboratory (ANL), Idaho National Laboratory (INL), and industry stakeholders to identify the technical, infrastructure, and economic requirements for establishing a national extreme fast charging (XFC) network.


Today’s high power EV charging stations take 20 minutes or more to provide a fraction of the driving range car owners get from 10 minutes at the gasoline pump. 

Porsche is leading the industry with the deployment of two XFC 350kW EV charging stations in Europe that will begin to approach the refueling time of gasoline vehicles. Photo courtesy of Porsche.

Drivers can pump enough gasoline in 10 minutes to carry them a few hundred miles. Most of today’s fast charging stations take 20 minutes to provide 50-70 miles of electric driving range. 

A series of articles in the current edition of the Journal of Power Sources summarizes the NREL team’s findings on how battery, vehicle, infrastructure, and economic factors impact XFC feasibility.

“You can charge an EV today at one of 44,000 stations across the country, but if you can’t leave your car plugged in for a few hours, you may only get enough juice to travel across town a few times,” says NREL Senior Engineer and XFC Project Lead Matthew Keyser

“We’re working to match the time, cost, and distance that generations of drivers have come to expect—with the additional benefits of clean, energy-saving technology.”

While XFC can help overcome real (and perceived) EV driving range limitations, the technology also introduces a series of new challenges. More rapid and powerful charging generates higher temperatures, which can lead to battery degradation and safety issues. 

Power electronics found in commercially available EVs are built for slower overnight charging and may not be able to withstand the stresses of higher voltage battery systems which are expected for higher power charging systems. XFC’s extreme, intermittent demands for electricity could also pose challenges to grid stability.

The XFC research team is exploring solutions for these issues, examining factors related to vehicle technology, gaps in existing technology, new demands on system design, and additional thermal management requirements. Researchers are also looking beyond vehicle systems to consider equipment and station design and potential impact on the grid.


NREL’s intercity travel analysis revealed that recharge times comparable to the time it takes to pump gas will require charge rates of at least 400 kW. 

Current DC Fast Charging rates are limited to 50-120 kW, and most public charging stations are limited to 7kW. 




XFC researchers have concluded that this will necessitate increases in battery charging density and new designs to minimize potential related increases in component size, weight, and cost. 

It appears that a more innovative battery thermal management system will be needed if XFC is to become a reality, and new strategies and materials will be needed to improve battery cell and pack cooling, as well as the thermal efficiency of cathodes and anodes.




“Yes, this substantial increase in charging rate will create new technical issues, but they are far from insurmountable—now that we’ve identified them,” says NREL Engineer Andrew Meintz.

Development of a network of XFC stations will depend on cost, market demand, and management of intermittent power demands. 
The team’s research revealed a need for more extensive analysis of potential station siting, travel patterns, grid resources, and business cases. 

At the same time, it is clear that any XFC network will call for new infrastructure technology and operational practices, along with cooperation and standardization across utilities, station operators, and manufacturers of charging systems and EVs.

These studies provide an initial framework for effectively establishing XFC technology. The initiative has attracted keen interest from industry members, who realize that faster charging will ultimately lead to wider market adoption of EV technologies.

This research is supported by the DOE Vehicle Technologies Office. Learn more about NREL’s energy storage and EV grid integration research.

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.

NREL Reports: Plug-In EV’s and the ‘Charging Infrastructure’ Needed to Support Them


How much vehicle charging infrastructure is needed in the United States to support broader adoption scenarios for various types of plug-in electric vehicles?

 

 
A new report by NREL for the U.S. Department of Energy takes a look, providing guidance to public and private stakeholders seeking a nationwide network of non-residential (public and workplace) vehicle charging infrastructure.

See the full report at: http://bit.ly/2xWVfjh

 

 

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NREL: Demonstrating and Advancing Benefits of Hydrogen Technology



by Bryan S. Pivovar, Ph.D, H2@Scale Lead/Group Manager, Chemistry and Nanosciences Center, National Renewable Energy Laboratory

Over the past several decades, technological advancements and cost reductions have dramatically changed the economic potential of hydrogen in our energy system. 
Fuel cell electric vehicles are now available for commercial sale and hydrogen stations are open to the public (more than 2,000 fuel cell vehicles are on the road and more than 30 fueling stations are open to the public in California). 

Low-cost wind and solar power are quickly changing the power generation landscape and creating a need for technologies that enhance the flexibility of the grid in the mid- to long-term.

The vision of a clean, sustainable energy system with hydrogen serving as the critical centerpiece is the focus of H2@Scale, a major initiative involving multiple U.S. Department of Energy (DOE) program offices, led by DOE’s Fuel Cell Technologies Office in the Office of Energy Efficiency and Renewable Energy, and 14 DOE national laboratories. 

H2@Scale expands the focus of hydrogen technologies beyond power generation and transportation, to grid services and industrial processes that use hydrogen.

The Energy Systems Integration Facility (ESIF) at the National Renewable Energy Laboratory (NREL) serves as a world-class, sophisticated testbed to evaluate and advance the H2@Scale concept. 

The ESIF is a DOE user facility interacting with multiple industrial stakeholders to accelerate the adoption of clean energy, including hydrogen-based technologies. Many of the barriers for making the H2@Scale vision a reality are being addressed today within ESIF by NREL researchers along with other industrial and national laboratory collaborators. 

The unique testbed capabilities at NREL and collaborating national labs are now available for use by industry and several partnerships are currently in development.
Within the ESIF, NREL researchers use electrons and water to produce hydrogen at rates of up to 100 kg/day (enough to fuel ~6,000 miles of travel in today’s fuel cell electric vehicles or more than 20 cars) with plans to expand capacity to four times this level. 

The hydrogen produced is compressed and stored in the 350 kg of on-site storage available at pressures as high as 12,500 psi. The hydrogen is used in multiple applications at the ESIF, including fueling fuel cell electric vehicles, testing and validating hydrogen infrastructure components and systems, producing renewable natural gas (through biological reaction with carbon dioxide), and as a feedstock for fuel cell power generation and research and development efforts.

To accelerate the H2@Scale concept, the cost, performance, and durability of hydrogen production, infrastructure (distribution and storage), and end use technologies need to be improved. NREL researchers, along with other labs, are actively demonstrating and advancing hydrogen technology in a number of areas including low-temperature electrolysis, biological production of renewable natural gas, and infrastructure.

Renewable hydrogen via low-temperature electrolysis




Today’s small-scale electrolysis systems are capable of producing several kilograms (kg) of hydrogen per day, but can cost as much as $10 per watt. At larger scale, megawatt (MW) systems producing more than 400 kg per day can cost under $2 per watt. However, for low-temperature electrolyzer systems to compete with the established steam methane reforming process for hydrogen production, the capital cost needs to be reduced to far below $1 per watt.

NREL has ongoing collaborations with Idaho National Laboratory (INL) to demonstrate control of a 250-kW electrolyzer system in a real-time grid simulation using a hardware-in-the-loop (HIL)-based approach to verify the performance of electrolyzer systems in providing grid support. HIL couples modeling and hardware in real-time simulations to better understand the performance of complex systems. 

The electrolyzer system, a building block for megawatt-scale deployment, was remotely controlled based on simulations of signals from a power grid. NREL and INL engineers demonstrated the ability of an electrolyzer to respond to grid signals in sub-seconds, making electrolyzers a viable candidate for “demand response” technologies that help control frequency and voltage on the grid by adjusting their power intake based on grid signals. 

A key enabler of low-cost electrolysis will be for electrolyzer technologies to respond dynamically to grid signals, such that they access low-cost power when available. The potential performance and durability implications of such dynamic operation are being elucidated in ongoing tests. Such experiments are essential to assess the potential for electrolyzers to support grid resiliency and to identify remaining R&D needs toward this value proposition.
NREL’s scientists are developing and exploring new materials for electrolysis systems, including advanced catalysts based on nanowire architecture and alkaline membranes, and approaches for integrating these materials into low-cost, durable membrane electrode assemblies.  

NREL, Swiss Scientists Power Past Solar Efficiency Records


NREL scientist Adele Tamboli, co-author of a recent article on silicon-based multijunction solar cells, stands in front of an array of solar panels. Credit: Dennis Schroeder

August 25, 2017




Second collaborative effort proves silicon-based multijunction cells that reach nearly 36% efficiency

Collaboration between researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL), the Swiss Center for Electronics and Microtechnology (CSEM), and the École Polytechnique Fédérale de Lausanne (EPFL) shows the high potential of silicon-based multijunction solar cells.

The research groups created tandem solar cells with record efficiencies of converting sunlight into electricity under 1-sun illumination. The resulting paper, “Raising the One-Sun Conversion Efficiency of III–V/Si Solar Cells to 32.8% for Two Junctions and 35.9% for Three Junctions,” appears in the new issue of Nature Energy. Solar cells made solely from materials in Groups III and V of the Periodic Table have shown high efficiencies, but are more expensive.

Stephanie Essig, a former NREL post-doctoral researcher now working at EPFL in Switzerland, is lead author of the newly published research that details the steps taken to improve the efficiency of the multijunction cell. While at NREL, Essig co-authored “Realization of GaInP/Si Dual-Junction Solar Cells with 29.8% 1-Sun Efficiency,” which was published in the IEEE Journal of Photovoltaics a year ago.

In addition to Essig, authors of the new research paper are Timothy Remo, John F. Geisz, Myles A. Steiner, David L. Young, Kelsey Horowitz, Michael Woodhouse, and Adele Tamboli, all with NREL; and Christophe Allebe, Loris Barraud, Antoine Descoeudres, Matthieu Despeisse, and Christophe Ballif, all from CSEM.

“This achievement is significant because it shows, for the first time, that silicon-based tandem cells can provide efficiencies competing with more expensive multijunction cells consisting entirely of III-V materials,” Tamboli said. “It opens the door to develop entirely new multijunction solar cell materials and architectures.”

In testing silicon-based multijunction solar cells, the researchers found that the highest dual-junction efficiency (32.8%) came from a tandem cell that stacked a layer of gallium arsenide (GaAs) developed by NREL atop a film of crystalline silicon developed by CSEM. An efficiency of 32.5% was achieved using a gallium indium phosphide (GaInP) top cell, which is a similar structure to the previous record efficiency of 29.8% announced in January 2016. 

A third cell, consisting of a GaInP/GaAs tandem cell stacked on a silicon bottom cell, reached a triple-junction efficiency of 35.9%—just 2% below the overall triple-junction record.

The existing photovoltaics market is dominated by modules made of single-junction silicon solar cells, with efficiencies between 17% and 24%. 

The researchers noted in the report that making the transition from a silicon single-junction cell to a silicon-based dual-junction solar cell will enable manufacturers to push efficiencies past 30% while still benefiting from their expertise in making silicon solar cells.

The obstacle to the adoption of these multijunction silicon-based solar cells, at least in the near term, is the cost. Assuming 30% efficiency, the researchers estimated the GaInP-based cell would cost $4.85 per watt and the GaAs-based cell would cost $7.15 per watt. 

But as manufacturing ramps up and the efficiencies of these types of cells climbs to 35%, the researchers predict the cost per watt could fall to 66 cents for a GaInP-based cell and to 85 cents for the GaAs-based cell. 

The scientists noted that such a precipitous price drop is not unprecedented; for instance, the cost of Chinese-made photovoltaic modules fell from $4.50 per watt in 2006 to $1 per watt in 2011.

The cost of a solar module in the United States accounts for 20% to 40% of the price of a photovoltaic system. Increasing cell efficiency to 35%, the researchers estimated, could reduce the system cost by as much as 45 cents per watt for commercial installations. 

However, if the costs of a III-V cell cannot be reduced to the levels of the researchers’ long-term scenario, then the use of cheaper, high-efficiency materials for the top cell will be needed to make them cost-competitive in general power markets.

The funding for the research came from the Energy Department’s SunShot Initiative—which aims to make solar energy a low-cost electricity source for all Americans through research and development efforts in collaboration with public and private partners—and from the Swiss Confederation and the Nano-Tera.ch initiative.


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.

NREL: Semiconducting Single-Walled Carbon Nanotubes in Solar Energy Harvesting


National Renewable Energy Laboratory, Golden, Colorado 

Semiconducting single-walled carbon nanotubes (s-SWCNTs) represent a tunable model one-dimensional system with exceptional optical and electronic properties. 

High-throughput separation and purification strategies have enabled the integration of s-SWCNTs into a number of optoelectronic applications, including photovoltaics (PVs). In this Perspective, we discuss the fundamental underpinnings of two model PV interfaces involving s-SWCNTs. 

We first discuss s-SWCNT–fullerene heterojunctions where exciton dissociation at the donor–acceptor interface drives solar energy conversion. Next, we discuss charge extraction at the interface between s-SWCNTs and a photoexcited perovskite active layer. 

In each case, the use of highly enriched semiconducting SWCNT samples enables fundamental insights into the thermodynamic and kinetic mechanisms that drive the efficient conversion of solar photons into long-lived separated charges. 

These model systems help to establish design rules for next-generation PV devices containing well-defined organic semiconductor layers and help to frame a number of important outstanding questions that can guide future studies.

Cheap Catalysts turn Sunlight and Carbon Dioxide into Fuel – Sustainable & Abundant Energy


Photosynthesis NREL iStock-503352336_16x9Thanks to a new catalyst, sunlight has been converted into chemical energy with a record 13.4% efficiency.

Scientists have long dreamed of mimicking photosynthesis, by using the energy in sunlight to knit together hydrocarbon fuels from carbon dioxide (CO2) and water. Now, a cheap new chemical catalyst has carried out part of that process with record efficiency, using electricity from a solar cell to split CO2 into energy-rich carbon monoxide (CO) and oxygen. The conversion isn’t yet efficient enough to compete with fossil fuels like gasoline. But it could one day lead to methods for making essentially unlimited amounts of liquid fuels from sunlight, water, and CO2, the chief culprit in global warming.

A bright idea

A new catalyst made from copper and tin oxides uses electric current from a solar cell to split water (H2O) and carbon dioxide (CO2), creating energy-rich carbon monoxide (CO) that can be further refined into liquid fuels.

 

NREL I downloadThe new work is “a very nice result,” says John Turner, a renewable fuels expert at the National Renewable Energy Laboratory in Golden, Colorado.

The transformation begins when CO2 is broken down into oxygen and CO, the latter of which can be combined with hydrogen to make a variety of hydrocarbon fuels. Adding four hydrogen atoms, for example, creates methanol, a liquid fuel that can power cars. Over the last 2 decades, researchers have discovered a number of catalysts that enable that first step and split CO2 when the gas is bubbled up through water in the presence of an electric current. One of the best studied is a cheap, plentiful mix of copper and oxygen called copper oxide. The trouble is that the catalyst splits more water than it does CO2, making molecular hydrogen (H2), a less energy-rich compound, says Michael Graetzel, a chemist at the Swiss Federal Institute of Technology in Lausanne, whose group has long studied these CO2-splitting catalysts.

Last year, Marcel Schreier, one of Graetzel’s graduate students, was looking into the details of how copper oxide catalysts work. He put a layer of them on a tin oxide–based electrode, which fed electrons to a beaker containing water and dissolved CO2. Instead of splitting mostly water—like the copper oxide catalyst—the new catalyst generated almost pure CO. “It was a discovery made by serendipity,” Graetzel says.

The tin, Graetzel adds, seems to deactivate the catalytic hot spots that help split the water. As a result, almost all the electric current went into making the more desirable CO. Armed with the new insight, Graetzel’s team sought to speed up the catalyst’s work. To do so, they remade their electrode from copper oxide nanowires, which have a high surface area for carrying out the CO2-breaking reaction, and topped them with a single atom-thick layer of tin. As Graetzel’s team reports this week in Nature Energy, the strategy worked, converting 90% of the CO2 molecules into CO, with hydrogen and other byproducts making up the rest. They also hooked their setup to a solar cell and showed that a record 13.4% of the energy in the captured sunlight was converted into the CO’s chemical bonds. That’s far better than plants, which store energy with about 1% efficiency, and even tops recent hybrid approaches that combine catalysts with microbes to generate fuel.

Nate Lewis, a chemist at the California Institute of Technology in Pasadena, says the new result comes on the heels of other recent improvements that use different catalysts to turn CO2 into fuels. “Together, they show we’re making progress,” Lewis says. But he also cautions that current efforts to turn CO2into fuel remain squarely in the realm of basic research, because they can’t generate fuel at a price anywhere near to that of refining oil.

Still, exploding supplies of renewable electricity now occasionally generate more power than the grid can handle. So scientists are looking for a viable way to store the excess electricity. That’s likely to drive further progress in storing energy in chemical fuels, Graetzel says.

 

Posted in: DOI: 10.1126/science.aan6935