Converting atmospheric carbon dioxide into batteries

An interdisciplinary team of scientists has worked out a way to make electric vehicles that are not only carbon neutral, but carbon negative, capable of actually reducing the amount of atmospheric carbon dioxide as they operate.

They have done so by demonstrating how the graphite electrodes used in the lithium-ion batteries that power electric automobiles can be replaced with carbon material recovered from the atmosphere.

The recipe for converting carbon dioxide gas into batteries is described in the paper titled "Carbon Nanotubes Produced from Ambient Carbon Dioxide for Environmentally Sustainable Lithium-Ion and Sodium-Ion Battery Anodes" published in the Mar. 2 issue of the journal ACS Central Science.

The unusual pairing of carbon dioxide conversion and advanced battery technology is the result of a collaboration between the laboratory of Assistant Professor of Mechanical Engineering Cary Pint at Vanderbilt University and Professor of Chemistry Stuart Licht at George Washington University.

The team adapted a solar-powered process that converts carbon dioxide into carbon so that it produces carbon nanotubes and demonstrated that the nanotubes can be incorporated into both lithium-ion batteries like those used in electric vehicles and electronic devices and low-cost sodium-ion batteries under development for large-scale applications, such as the electric grid.

"This approach not only produces better batteries but it also establishes a value for carbon dioxide recovered from the atmosphere that is associated with the end-user battery cost unlike most efforts to reuse CO2 that are aimed at low-valued fuels, like methanol, that cannot justify the cost required to produce them," said Pint.

The project builds upon a solar thermal electrochemical process (STEP) that can create carbon nanofibers from ambient carbon dioxide developed by the Licht group and described in the journal Nano Letters last August. STEP uses solar energy to provide both the electrical and thermal energy necessary to break down carbon dioxide into carbon and oxygen and to produce carbon nanotubes that are stable, flexible, conductive and stronger than steel.

"Our climate change solution is two fold: (1) to transform the greenhouse gas carbon dioxide into valuable products and (2) to provide greenhouse gas emission-free alternatives to today's industrial and transportation fossil fuel processes," said Licht. "In addition to better batteries other applications for the carbon nanotubes include carbon composites for strong, lightweight construction materials, sports equipment and car, truck and airplane bodies."

 

Joining forces with Pint, whose research interests are focused on using carbon nanomaterials for battery applications, the two laboratories worked together to show that the multi-walled carbon nanotubes produced by the process can serve as the positive electrode in both lithium-ion and sodium-ion batteries.

In lithium-ion batteries, the nanotubes replace the carbon anode used in commercial batteries. The team demonstrated that the carbon nanotubes gave a small boost to the performance, which was amplified when the battery was charged quickly. In sodium-ion batteries, the researchers found that small defects in the carbon, which can be tuned using STEP, can unlock stable storage performance over 3.5 times above that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon-nanotube batteries were exposed to about 2.5 months of continuous charging and discharging and showed no sign of fatigue.

Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40 percent of a battery could be made out of recycled CO2, Pint estimated. This does not include the outer protective packaging but he suggested that processes like STEP could eventually produce the packaging as well.

The researchers estimate that with a battery cost of $325 per kWh (the average cost of lithium-ion batteries reported by the Department of Energy in 2013), a kilogram of carbon dioxide has a value of about $18 as a battery material - six times more than when it is converted to methanol - a number that only increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronics. And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions, which is needed to put an end to the current carbon cycle that threatens future global sustainability.

Licht also proposed that the STEP process could be coupled to a natural gas powered electrical generator. The generator would provide electricity, heat and a concentrated source of carbon dioxide that would boost the performance of the STEP process. At the same time, the oxygen released in the process could be piped back to the generator where it would boost the generator's combustion efficiency to compensate for the amount of electricity that the STEP process consumes. The end result could be a fossil fuel electrical power plant with zero net CO2 emissions.

"Imagine a world where every new electric vehicle or grid-scale battery installation would not only enable us to overcome the environmental sins of our past, but also provide a step toward a sustainable future for our children," said Pint. "Our efforts have shown a path to achieve such a future."

Vanderbilt University

From a carpet of nanorods to a thin film solar cell absorber within a few seconds

The transformation from a layer of closely packed nanorods
(top left) to a polycrystalline semiconductor thin film (top right)
can be observed in by in-situ X-ray diffraction in real time.
The intensities of the diffraction signals are color coded in the
image at the bottom. A detailed analysis of the signals reveals
that the transformation of the nanorods into kesterite crystals
takes only 9 to 18 seconds.
Picture: R. Mainz/A. Singh

Research teams at the HZB and at the University of Limerick, Ireland, have discovered a novel solid state reaction which lets kesterite grains grow within a few seconds and at relatively low temperatures. For this reaction they exploit a transition from a metastable wurtzite compound in the form of nanorods to the more stable kesterite compound. 

At the EDDI Beamline at BESSY II, the scientists could observe this process in real-time when heating the sample: in a few seconds Kesterite grains formed. The size of the grains was found to depend on the heating rate. With fast heating they succeeded in producing a Kesterite thin film with near micrometer-sized crystal grains, which could be used in thin film solar cells. These findings have now been published in the journal “Nature Communications”.

Grain formation during growth of kesterite solar cells observed in real-time

As starting material for the formation of the kesterite film serves a “carpet of nanorods”: With the help of solution-based chemical processing, the chemists around Ajay Singh and Kevin Ryan at the University of Limerick have fabricated films of highly ordered wurtzite nanorods, which have exactly the same composition as kesterite Cu₂ZnSnS₄. With the help of real-time X-ray diffraction at the EDDI beamline of BESSY II, HZB physicists around Roland Mainz and Thomas Unold could now observe how a phase transition from the metastable wurtzite phase to the stable kesterite phase leads to a rapid formation of a thin film with large kesterite grains. “It is interesting to see that the complete formation of the kesterite film is so fast”, says Mainz. And the faster the samples are heated up, the larger the grains grow. Mainz explains that at low heating rate, the transition from wurtzite to kesterite starts at lower temperature at which many small grains form – instead of a few larger grains. Additionally, more defects are formed at lower temperatures. During fast heating, the transition takes place at higher temperature at which grains with less defects form.

Moreover, the comparison of the time-resolved evolution of the phase transition during slow and during fast heating shows that not only the grain growth is triggered by the phase transition, but also the phase transition is additionally accelerated by the grain growth. The HZB physicists have developed a model which can explain these findings. By means of numerical model calculations, they demonstrated the accordance of the model with the measured data.

Novel synthesis pathway for thin film semiconductors with controlled morphology
The work points towards a new pathway for the fabrication of thin microcrystalline semiconductor films without the need of expensive vacuum technology. Cu₂ZnSnS₄-based kesterite semiconductors have gained increasing attention in the past, since they are a promising alternative for the Cu(In,Ga)Se₂chalcopyrite solar cells which already achieved efficiencies above 20%. Kesterite has similar physical properties as the chalcopyrite semiconductors, but consist only of elements which are abundantly present in the earth crust. The new procedure could also be interesting for the fabrication of micro- and nanostructured photoelectric devices as well as for semiconductor layers consisting of other materials, says Mainz. “But we continue to focus on kesterites, because this is a really exciting topic at the moment.”

Source: http://www.helmholtz-berlin.de/pubbin/news_seite?nid=13909;sprache=en;typoid=3228

heat-resistant materials that could vastly improve solar cell efficiency

Scientists have created a heat-resistant thermal emitter that could significantly improve the efficiency of solar cells. The novel component is designed to convert heat from the sun into infrared light, which can than be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics. Unlike earlier prototypes that fell apart at temperatures below 2200 degrees Fahrenheit (1200 degrees Celsius), the new thermal emitter remains stable at temperatures as high as 2500 F (1400 C).

"This is a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics," said Shanhui Fan, a professor of electrical engineering at Stanford University. Fan and his colleagues at the University of Illinois-Urbana Champaign (Illinois) and North Carolina State University collaborated on the project. Their results are published in the October 16 edition of the journal Nature Communications.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy. But silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel.

"In theory, conventional single-junction solar cells can only achieve an efficiency level of about 34 percent, but in practice they don't achieve that," said study co-author Paul Braun, a professor of materials science at Illinois. "That's because they throw away the majority of the sun's energy."
Thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

"Essentially, we tailor the light to shorter wavelengths that are ideal for driving a solar cell," Fan said. "That raises the theoretical efficiency of the cell to 80 percent, which is quite remarkable."

		This cross-section micrographs of a tungsten thermal emitter used in the experiment. Figure 2a shows how unprotected tungsten degrades after heating to 1200 degrees Celsius. Fig. 2b demonstrates how the ceramic-coated tungsten retained structural integrity after being subjected to 1400 C heat for an hour....

So far, thermophotovoltaic systems have only achieved an efficiency level of about 8 percent, Braun noted. The poor performance is largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs.
"Our thermal emitters have a complex, three-dimensional nanostructure that has to withstand temperatures above 1800 F (1000 C) to be practical," Braun explained. "In fact, the hotter the better."

In previous experiments, however, the 3D structure of the emitter was destroyed at temperatures of around 1800 F (1000 C). To address the problem, Braun and his Illinois colleagues coated tungsten emitters in a nanolayer of a ceramic material called hafnium dioxide.

The results were dramatic. When subjected to temperatures of 1800 F (1000 C), the ceramic-coated emitters retained their structural integrity for more than 12 hours. When heated to 2500 F (1400 C), the samples remained thermally stable for at least an hour.

The ceramic-coated emitters were sent to Fan and his colleagues at Stanford, who confirmed that devices were still capable of producing infrared light waves that are ideal for running solar cells.
"These results are unprecedented," said former Illinois graduate student Kevin Arpin, lead author of the study. "We demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage."

Braun and Fan plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working solar cell.

"We've demonstrated that the tailoring of optical properties at high temperatures is possible," Braun said. "Hafnium and tungsten are abundant, low-cost materials, and the process used to make these heat-resistant emitters is well established. Hopefully these results will motivate the thermophotovoltaics community to take another look at ceramics and other classes of materials that haven't been considered."

Source: https://energy.stanford.edu/

Which makes a better solar collector, quantum dot or nanowire

A trio of researchers at North Dakota State University, Fargo and the University of South Dakota have turned to computer modeling to help decide which of two competing materials should get its day in the sun as the nanoscale energy-harvesting technology of future solar panels - quantum dots or nanowires.

 

Andrei Kryjevski and his colleagues, Dimitri Kilin and Svetlana Kilina, report in AIP Publishing's Journal of Renewable and Sustainable Energy that they used computational chemistry models to predict the electronic and optical properties of three types of nanoscale (billionth of a meter) silicon structures with a potential application for solar energy collection: a quantum dot, one-dimensional chains of quantum dots and a nanowire. The ability to absorb light is substantially enhanced in nanomaterials compared to those used in conventional semiconductors. Determining which form - quantum dots or nanowire -maximizes this advantage was the goal of the numerical experiment conducted by the three researchers.

 

"We used Density Functional Theory, a computational approach that allows us to predict electronic and optical properties that reflect how well the nanoparticles can absorb light, and how that effectiveness is affected by the interaction between quantum dots and the disorder in their structures," Kryjevski said. "This way, we can predict how quantum dots, quantum dot chains and nanowires will behave in real life even before they are synthesized and their working properties experimentally checked."

 

The simulations made by Kryjevski, Kilin and Kilina indicated that light absorption by silicon quantum dot chains significantly increases with increased interactions between the individual nanospheres in the chain. They also found that light absorption by quantum dot chains and nanowires depends strongly on how the structure is aligned in relation to the direction of the photons striking it. Finally, the researchers learned that the atomic structure disorder in the amorphous nanoparticles results in better light absorption at lower energies compared to crystalline-based nanomaterials.

 

"Based on our findings, we believe that putting the amorphous quantum dots in an array or merging them into a nanowire are the best assemblies for maximizing the efficiency of silicon nanomaterials to absorb light and transport charge throughout a photovoltaic system," Kryjevski said. "However, our study is only a first step in a comprehensive computational investigation of the properties of semiconductor quantum dot assemblies.The next steps are to build more realistic models, such as larger quantum dots with their surfaces covered by organic ligands and simulate the processes that occur in actual solar cells," he added.

 

Source and top image: American Institute of Physics

Top image shows: Amorphous Silicon nanowire (yellow network) facilitates harvesting of solar energy in the form of a photon (wavy line). In the process of light absorption a pair of mobile charge carriers is created (red clouds depict an electron smeared in space, while the blue clouds visualize the so-called hole which is a positively charged carrier). The energy of their directed motion is then transformed into electricity. Electron and hole charge distributions are often located in different regions of space due to multiple structural defects in amorphous silicon nanowires.

 

For more read Energy Harvesting and Storage for Electronic Devices 2012-2022 

 

And attend Energy Harvesting & Storage USA 20-21 November 2013, Santa Clara, USA 

Source: http://www.energyharvestingjournal.com/articles/which-makes-a-better-solar-collector-quantum-dot-or-nanowire-00005858.asp?sessionid=1

NRL Achieves Highest Open-Circuit Voltage for Quantum Dot Solar Cells

Schematic of metal-lead sulfide quantum dot Schottky junction
solar cells (glass/ITO/PbS QDs/LiF/Al). Novel Schottky junction
solar cells developed at NRL are capable of achieving
the highest open-circuit voltages ever reported for colloidal
QD based solar cells. 
(Photo: U.S. Naval Research Laboratory) 

U.S. Naval Research Laboratory (NRL) research scientists and engineers in the Electronics Science and Technology Division have demonstrated the highest recorded open-circuit voltages for quantum dot solar cells to date. Using colloidal lead sulfide (PbS) nanocrystal quantum dot (QD) substances, researchers achieved an open-circuit voltage (VOC) of 692 millivolts (mV) using the QD bandgap of a 1.4 electron volt (eV) in QD solar cell under one-sun illumination. 

"These results clearly demonstrate that there is a tremendous opportunity for improvement of open-circuit voltages greater than one volt by using smaller QDs in QD solar cells," said Woojun Yoon, Ph.D., NRC postdoctoral researcher, NRL Solid State Devices Branch. "Solution processability coupled with the potential for multiple exciton generation processes make nanocrystal quantum dots promising candidates for third generation low-cost and high-efficiency photovoltaics."

Despite this remarkable potential for high photocurrent generation, the achievable open-circuit voltage is fundamentally limited due to non-radiative recombination processes in QD solar cells. To overcome this boundary, NRL researchers have reengineered molecular passivation in metal-QD Schottky junction (unidirectional metal to semiconductor junction) solar cells capable of achieving the highest open-circuit voltages ever reported for colloidal QD based solar cells.

Experimental results demonstrate that by improving the passivation of the PbS QD surface through tailored annealing of QD and metal-QD interface using lithium fluoride (LiF) passivation with an optimized LiF thickness. This proves critical for reducing dark current densities by passivating localized traps in the PbS QD surface and metal-QD interface close to the junction, therefore minimizing non-radiative recombination processes in the cells.

Over the last decade, Department of Defense (DoD) analyses and the department's recent FY12 Strategic Sustainability Performance Plan, has cited the military's fossil fuel dependence as a strategic risk and identified renewable energy and energy efficiency investments as key mitigation measures. Research at NRL is committed to supporting the goals and mission of the DoD by providing basic and applied research toward mission-ready renewable and sustainable energy technologies that include hybrid fuels and fuel cells, photovoltaics, and carbon-neutral biological microorganisms.

Source: http://www.nrl.navy.mil/media/news-releases/2013/nrl-achieves-highest-open-circuit-voltage-for-quantum-dot-solar-cells#sthash.8luqmdmj.dpuf

Plastic – the new energy source

QUT's research to develop cheap plastic solar cells to charge mobile phones and other electronic devices has been boosted with the installation of one of the most powerful nanotechnology microscopes in the world.

The only one if its kind in Australia, the Zeiss Orion NanoFab enables researchers to examine natural or manmade structures in incredible detail, and will create new insights wherever it is applied.

By increasing the microscope beam current, researchers are able to etch away material to create patterns or structures with features of only a few nanometres. This is a tool that can write lines 100,000 times finer than the text on a printed page. Imagine War and Peace etched on the head of a pin - 200 times over.

QUT nanotechnology expert, Professor Nunzio Motta, said the new microscope complemented QUT's existing tunnelling microscope, the only one of its kind in Queensland, and would cement the university's place at the cutting edge of Australian nanotechnology research.

He said the super microscopes would be used to create new nanostructures which could be used in electronic devices, solar cells, gas sensors and for a range of other uses.

"At the moment plastic solar cells are quite inefficient and researchers around the world are trying to determine how to make the cells efficient and able to be commercialised," Professor Motta said.

"The advantages cheap solar cells would produce would be enormous.

"In the future plastic solar cells could generate enough energy not only to recharge the batteries of laptops and mobiles, but even to obtain power from canopies on parking areas which could be fed back into grids.

"They could even be developed as a clear film on glass windows to produce power."

Professor Motta is currently using the tunnelling microscope to improve plastic solar cells by mixing them with graphene, an atomic-scale honeycomb lattice made of carbon atoms. He has found that adding gold nano-particles traps light and improves efficiency.

"While it's difficult to put a timeframe on the development of efficient plastic solar cells, a five to ten year goal is probably not unrealistic," he said.

Professor Motta said his research team also hoped to create a new class of solar-powered nano-sensors capable of detecting pollution and monitoring the environment in remote areas.

He said nanoscale science was critical to the world's future economy as advances would transform a range of scientific and engineering disciplines.

Professor Motta said QUT was organizing NanoS-E3, an International Workshop and School on nanotechnology at Airlie Beach in September.

The initiative, in partnership with the Italian and Australian governments, will build on existing nanotechnology networks and foster new collaborations. In 2013, the workshop will also welcome scientists from France, Germany, Japan, Canada and the USA.

http://www.qut.edu.au/about/news/news?news-id=62135

New nanomaterial increases yield of solar cells

Researchers from the FOM Foundation, Delft University of Technology, Toyota Motor Europe and the University of California have developed a nanostructure with which they can make solar cells highly efficient. The researchers published their findings on 23 August 2013 in the online edition of Nature Communications. 

Smart nanostructures can increase the yield of solar cells. An international team of researchers including physicists from the FOM Foundation, Delft University of Technology and Toyota, have now optimised the nanostructures so that the solar cell provides more electricity and loses less energy in the form of heat.

Solar cells

A conventional solar cell contains a layer of silicon. When sunlight falls on this layer, electrons in the silicon absorb the energy of the light particles (photons). Using this energy the electrons jump across a 'band gap', as a result of which they can freely move and electricity flows.

The yield of a solar cell is optimised if the photon energy is equal to the band gap of silicon. Sunlight, however, contains many photons with energies greater than the band gap. The excess energy is lost as heat, which limits the yield of a conventional solar cell.

Nanospheres

Several years ago the researchers from Delft University of Technology, as well as other physicists, demonstrated that the excess energy could still be put to good use. In small spheres of a semiconducting material the excess energy enables extra electrons to jump across the band gap. These nanospheres, the so-called quantum dots, have a diameter of just one ten thousandth of a human hair.

If a light particle enables an electron in a quantum dot to cross the band gap, the electron moves around in the dot. That ensures that the electron collides with other electrons that subsequently jump across the band gap as well. As a result of this process a single photon can mobilise several electrons thereby multiplying the amount of current produced.

Contact between quantum dots

However, up until now the problem was that the electrons remained trapped in their quantum dots and so could not contribute to the current in the solar cell. That was due to the large molecules that stabilise the surface of quantum dots. These large molecules hinder the electrons jumping from one quantum dot to the next and so no current flows.

In the new design, the researchers replaced the large molecules with small molecules and filled the empty space between the quantum dots with aluminium oxide. This led to far more contact between the quantum dots allowing the electrons to move freely.

Yield

Using laser spectroscopy the physicists saw that a single photon indeed caused the release of several electrons in the material containing linked quantum dots. All of the electrons that jumped across the band gap moved freely around in the material. As a result of this the theoretical yield of solar cells containing such materials rises to 45%, which is more than 10% higher than a conventional solar cell.

This more efficient type of solar cell is easy to produce: the structure of linked nanospheres can be applied to the solar cell as a type of layered paint. Consequently the new solar cells will not only be more efficient but also cheaper than conventional cells.

The Dutch researchers now want to work with international partners to produce complete solar cells using this design.

http://www.tudelft.nl/en/current/latest-news/article/detail/nieuw-nanomateriaal-verhoogt-rendement-zonnecellen/

3D Graphene: Solar Power's Next Platinum?

One of the most promising types of solar cells has a few drawbacks. A scientist at Michigan Technological University may have overcome one of them.

Dye-sensitized solar cells are thin, flexible, easy to make and very good at turning sunshine into electricity. However, a key ingredient is one of the most expensive metals on the planet: platinum. While only small amounts are needed, at $1,500 an ounce, the cost of the silvery metal is still significant.

Yun Hang Hu, the Charles and Caroll McArthur Professor of Materials Science and Engineering, has developed a new, inexpensive material that could replace the platinum in solar cells without degrading their efficiency: 3D graphene.

Regular graphene is a famously two-dimensional form of carbon just a molecule or so thick. Hu and his team invented a novel approach to synthesize a unique 3D version with a honeycomb-like structure. To do so, they combined lithium oxide with carbon monoxide in a chemical reaction that forms lithium carbonate (Li2CO3) and the honeycomb graphene. The Li2CO3 helps shape the graphene sheets and isolates them from each other, preventing the formation of garden-variety graphite.  Furthermore, the Li2CO3 particles can be easily removed from 3D honeycomb-structured graphene by an acid.

The researchers determined that the 3D honeycomb graphene had excellent conductivity and high catalytic activity, raising the possibility that it could be used for energy storage and conversion. So they replaced the platinum counter electrode in a dye-sensitized solar cell with one made of the 3D honeycomb graphene. Then they put the solar cell in the sunshine and measured its output.

The cell with the 3D graphene counter electrode converted 7.8 percent of the sun’s energy into electricity, nearly as much as the conventional solar cell using costly platinum (8 percent).

Synthesizing the 3D honeycomb graphene is neither expensive nor difficult, said Hu, and making it into a counter electrode posed no special challenges.

The research has been funded by the American Chemical Society Petroleum Research Fund (PRF-51799-ND10) and the National Science Foundation (NSF-CBET-0931587). The article describing the work, “3D Honeycomb-Like Structured Graphene and Its High Efficiency as a Counter-Electrode Catalyst for Dye-Sensitized Solar Cells,” coauthored by Hu, Michigan Tech graduate student Hui Wang, Franklin Tao of the University of Notre Dame, Dario J. Stacchiola of Brookhaven National Laboratory and Kai Sun of the University of Michigan, was published online July 29 in the journal Angewandte Chemie, International Edition.

http://www.mtu.edu/news/stories/2013/august/story94626.html

Michigan Technological University (www.mtu.edu) is a leading public research university developing new technologies and preparing students to create the future for a prosperous and sustainable world. Michigan Tech offers more than 130 undergraduate and graduate degree programs in engineering; forest resources; computing; technology; business; economics; natural, physical and environmental sciences; arts; humanities; and social sciences