29 Jan 2015
Researchers report a high-resolution method for printing quantum dots to make light-emitting diodes (Nano Lett. 2015, DOI: 10.1021/nl503779e). With further development, the technique could be used to print pixels for richly colored, low-power displays in cell phones and other electronic devices.
Quantum dots are appealing materials for displays because engineers can finely tune the light the semiconducting nanocrystals emit by controlling their dimensions.
Electronics makers already use quantum dots in some backlit displays on the market, in which red and green quantum dots convert blue light from a light-emitting diode (LED) into white light. Quantum dots also emit light in response to voltage changes, so researchers are looking into using them in red, green, and blue pixels in displays that wouldn’t need a backlight.
Quantum dot LED displays should provide richer colors and use less power than the liquid-crystal displays (LCDs) used in many flat screens, which require filters and polarizers that reduce efficiency and limit color quality. But it’s not yet clear how quantum dot LED displays would be made commercially, says John A. Rogers, a materials scientist at the University of Illinois, Urbana-Champaign.
In 2011, researchers at Samsung made the first full-color quantum dot LED display by using a rubber stamp to pick up and transfer quantum dot inks (Nat. Photonics, DOI: 10.1038/nphoton.2011.12). As a manufacturing strategy, printing from ink nozzles would offer more flexibility to change designs on the fly, without the need for making new transfer stamps. Jet printing also would require less material, Rogers says.
Unfortunately, the resolution of conventional ink-jet printers, which use a heating element to force vapor droplets out of a nozzle, is limited. “It’s hard to get droplets smaller than about 25 µm,” Rogers says, because the smaller the nozzle diameter, the more pressure required to get the droplet out.
So for the past seven years, Rogers has been developing another method called electrohydrodynamic jet printing. This kind of printer works by pulling ink droplets out of the nozzle rather than pushing them, allowing for smaller droplets. An electric field at the nozzle opening causes ions to form on the meniscus of the ink droplet. The electric field pulls the ions forward, deforming the droplet into a conical shape. Then a tiny droplet shears off and lands on the printing surface. A computer program controls the printer by directing the movement of the substrate and varying the voltage at the nozzle to print a given pattern.
The Illinois researchers used this new method, including specialized quantum dot inks, to print lines on average about 500 nm wide. This allowed them to fabricate red and green quantum dot LEDs. They also showed they could carefully control the thickness of the printed film, which is difficult to do with stamp transfer and ink-jet printing methods.
The ultimate resolution possible with these kinds of printers is very high, says David J. Norris, a materials engineer at the Swiss Federal Institute of Technology (ETH), Zurich. Last year, Norris used a similar printing method to print spots containing as few as 10 quantum dots (Nano Lett. 2014, DOI: 10.1021/nl5026997). He says it’s even possible to place single quantum dots using electrohydrodynamic nozzles, albeit with less control and repeatability. Single-particle printing isn’t needed for making pixels for displays, but it is useful for studying other kinds of optical effects in quantum dots, he says.
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © 2015 American Chemical Society
Inspired by the unique optical and electronic property of graphene, two-dimensional layered materials have been intensively investigated in recent years, driven by their potential applications for future high speed and broadband electronic and optoelectronic devices. Layers of molybdenum disulfide (MoS2), one kind of transition metals chalcogenides, have been proven to be a very interesting material with the semiconducting property.
The basic infrastructure of molybdenum disulfide is a single-atomic layer of molybdenum sandwiched between two adjacent atomic layers of sulfide. This compound exists in nature as molybdenite, a crystal material found in rocks around the world, frequently taking the characteristic form of silver-colored hexagonal plates. For decades, molybdenite has been used in the manufacturing of lubricants and metal alloys. Like in the case of graphite, the properties of single-atom sheets of MoS2 long went unnoticed.
From the view point of applications in electronics, molybdenum disulfide sheets exhibit a significant advantage over graphene: they have an energy gap – an energy range within which no electron states can exist. By applying an electric field, the sheets can be switched between a state that conducts electricity and one that behaves like an insulator. Theoretically, a switched-off molybdenum disulfide transistor would consume even as little as several hundred thousand times less energy than a silicon transistor.
Graphene, on the other hand, has no energy gap and transistors made of graphene cannot be fully switched off. More importantly, the relatively weak absorption co-efficiency of graphene (2.3 % of incident light per layer) might significantly delimit its light modulation ability for optical communication devices such as light detector, modulator and absorber.
Molybdenum disulfide’s semiconducting ability, strong light-matter interaction and similarity to the carbon-based graphene makes it of interest to scientists as a viable alternative to graphene in the manufacture of electronics, particularly photoelectronics. Scientists have found that the physical properties of two-dimensional (2D) MoS2 change markedly when it has nanoscale properties.
A slab of MoS2 that is even a micron thick has an “indirect” bandgap while a two-dimensional sheet of molybdenum disulfide has a “direct” bandgap. It shows thickness dependent band-gap properties, allowing for the production of tunable optoelectronic devices with diversified spectral operation. In pushing towards practical optical applications of 2D MoS2, an essential gap on understanding the nonlinear optical response of 2D MoS2 and how it interacts with light, must be filled. Now, one research group on photonics based on 2D materials, from Shenzhen University, reports a breakthrough in the light-matter interaction of 2D MoS2 and fabricating a novel optical device using few layers of molybdenum disulfide (see paper in Optics Express: “Molybdenum disulfide (MoS2) as a broadband saturable absorber for ultra-fast photonics”).
Thanks to the direct-band and ultrafast response in few layer MoS2, its optical absorbance can become saturated if under high power excitation, as a result of the band filling effect in conduction band. A saturable absorber is an important element for pulse operation in a laser cavity which absorb weaker energy of light modes while get across higher energy. After millions of circulation in laser cavity, ultra-short (ps or fs in temporal duration) pulses with a high concentration power could be generated. MoS2 has indirect bandgap in bulk material with a band gap of ∼1.2 eV and direct band-gap in monolayer structure with a broader band gap of ∼1.9 eV. Although it seems that few-layer MoS2 might have limited operation bandwidth and fails to operate as a broadband saturable absorber.
However, according to their careful experimental studies, the team found that few-layer MoS2 could still possess wavelength insensitive saturable absorption responses, which is caused by the special molecular structures in few-layer MoS2. It is worth commenting on the broadband performance of graphene and MoS2. The broadband performance of graphene is intrinsic, due to its gapless nature. However, it is more complex in the exfoliated MoS2 nanoparticle sample they used (see paper in Scientific Reports: “Ytterbium-doped fiber laser passively mode locked by few-layer Molybdenum Disulfide (MoS2) saturable absorber functioned with evanescent field interaction”) due to the mixture of 1T (metallic) and 2H (semiconducting) phases present. The 1T phases usually predominate in as-exfoliated samples due to doping by impurities, giving rise to similar broadband performance as graphene. If the MoS2 can be rendered predominantly 2H, its absorption at resonance energy will be stronger.
This means that at specific wavelength that is in resonance with the band gap, we expect that MoS2 saturable absorber can potentially give stronger saturable absorption response than graphene in view of its strong bulk-like photon absorption and exciton generation owing to Van Hove singularities.
Fig. 1: The broadband saturable absorption of few-layer MoS2 and the performance of mode locked operation. (click on image to enlarge)
The enhanced, broadband and ultra-fast nonlinear optical response in 2D semiconducting transition metal disulfides (TMDs) indicates unprecedented potential for ultra-fast photonics, ranging from high speed light modulation, ultra-short pulse generation to ultra-fast optical switching. However, the stability and robustness issues of TMDs turns out to be a significant problem if exposed to high power laser illumination. Unlike graphene that has extremely high thermal conductivity, flexibility and mechanical stability, TMDs may show much lower optical damage threshold than graphene because of their poorer thermal and mechanical property, although explorations on the photonic applications are being fueled by their advantages.
It is worth mentioning that polymethacrylate (PMMA) is indispensable for protecting few-layer MoS2 from vertical transmission if under strong optical power density. In principle, MoS2 couldn’t afford even higher laser illumination than 100 mW (pure material) and 500 mW (with PMMA protection) adheres to a fiber tail with mode field diameter of several micrometers in our experiment, which might seriously limit its potential applications in practical optical devices. Taper fibers inspired us to solve this challenge, schematically shown in Fig. 2. Few layer MoS2 was coupled on the waist of the taper fiber and interacted with an evanescent field of laser illumination. In this approach, the material doesn’t need to bear high optical power.
This optical device could bear 1 W laser injection without damage and also could achieve mode locked operation in a fiber laser as a saturable absorber.
Fig. 2: Schematic diagram of the taper fiber and the ytterbium-doped fiber laser passively mode locked by the MoS2-taper-fiber-saturable absorber.
“By depositing few-layer MoS2 upon the tapered fiber, we can employ a ‘lateral interaction scheme’ of utilizing the strong optical response of 2D MoS2, through which not only the light-matter interaction can be significantly enhanced owing to the long interaction distance, but also the drawback of optical damage of MoS2 can be mitigated. This MoS2-taper-fiber device can withstand strong laser illumination up to 1 W. Considering that layered TMDs hold similar problems as MoS2, our findings may provide an effective approach to solve the optical damage problem on those layered semiconductor materials,” Prof. Han Zhang from the Key Laboratory for Micro-Nano Optoelectronic Devices at Hunan University, concludes.
“Beyond MoS2, we anticipated that a number of MoS2-like layered TMDs (such as, WSe2, MoSe2, TaS2 etc) can also be developed as promising optoelectronic devices with high power tolerance, offering inroads for more practical applications, such as large energy laser mode-locking, nonlinear optical modulation and signal processing etc.”
This work provides a very convenient but practical way to overcome the disadvantages (very low optical damage threshold) of 2D semiconducting TMDs, simply by adopting a ‘lateral interaction scheme’. Stimulated by this technological innovation, we anticipate that researcher might propose new types of light interaction modes with 2D materials, particularly, the integration of 2D materials with various waveguide structures, such as Silicon waveguide. It will definitely not only solve the problems concerning easily optical damage, but also lead to new physics on how light propagates along and interacts with the 2D semiconducting surface, in the present of waveguides. Eventually, it might revolutionize our viewpoints on 2D optoelectronics, and open up a new test-bed with unprecedented chances for conceptually new optoelectronic devices.
By Dr. Feng Luan, Assistant Professor, Division of Communication Engineering, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore
The solar industry is abuzz over a relative newcomer that burst onto the scene less than a decade ago and has risen rapidly through the ranks. The all-star rookie has also been published in high-impact academic journals in the last few years, but it isn’t a newly minted professor or a hot solar startup. It’s a material known as perovskite.
Materials scientists started testing perovskite’s sun-capturing qualities in the 2000s, and by 2009, a team lead by Tsutomu Miyasaka from Toin University of Yokohama in Japan had produced a solar cell that converted 3.8% of the sun’s light into electricity, a respectable amount for such a new material. Just last fall, another group lead by Henry Snaith from the University of Oxford published a breakthrough—their perovskite solar cells were 15.4% efficient.
In a world where gains of fractions of a percent are lauded, such a leap was unprecedented. “Very few come in out of the cold and have a 15% conversion efficiency.” says David Ginley, a research fellow at the National Renewably Energy Laboratory.
“It’s exciting,” says Michael McGehee, a professor of materials science at Stanford University. “It’s a new material with a lot of potential.”
That excitement is evident in recent news coverage. Even Nature, a well-respected academic journal, hailed Snaith as one of the 2013’s “ten people who mattered.” “This year, Snaith amazed materials researchers by massively boosting the efficiency of solar cells made with perovskite semiconductors,” they wrote.
Those plaudits come with a small catch—they tacitly presume that perovskite will continue its rapid ascent. If it does, the material truly could be revolutionary. Currently, photovoltaics cost between $2 and $5 per watt depending on the scale of the installation. That’s significantly lower than just five years ago, though it’s still not competitive with coal or natural gas. But if perovskite continues to gain efficiency, it could tilt the playing field solidly in favor of solar power.
The target is 25% efficiency. Very few types of cells exceed that goal, and even fewer are commercially available currently. “A lot of people think that you need the efficiency of the cells to be up near 25% because if the efficiency is lower, you need a larger area to get the power, and the larger area, the more the installation costs are,” McGehee says. Perovskite made waves with how quickly it broke 15% efficiency, and unspoken assumption in many articles is that the material could breach 25% in a matter of years, not decades.
Snaith, whose team achieved the recent perovskite milestone, seems convinced that perovskite already has commercial potential. He has founded a company that’s striving to produce perovskite solar cells in mass quantity, which he says will happen in “three to five years.”
Snaith’s compressed timeline mirrors the great strides perovskite has taken as a photovoltaic material. But the road from the laboratory to the rooftop can be filled with unexpected speed bumps, something known all too well by researchers and manufacturers of copper indium gallium selenide, or CIGS, a photovoltaic material that’s just recently become available on the market. In fact, the story of CIGS could be viewed as a cautionary tale, one that might temper some of the excitement surrounding perovskite.
CIGS began life as CIS, or copper indium selenide. It, too, is a semiconductor and was originally discovered in 1953 by Harry Hahn and his team at the University of Heidelberg. They published their discovery in Zeitschrift für anorganische und allgemeine Chemie, a German-language chemistry journal. It wasn’t uncommon at the time for chemists to publish in German, though that may have been partly why it was overlooked as a photovoltaic until 1974 when Sigurd Wagner, a young Austrian scientist and a fresh face at Bell Labs, and his team published an article on how his lab-grown crystals that could capture the sun’s rays.
CIS crystals were expensive and proved difficult to grow, though, which was part of the reason why Larry Kazmerski, then a professor at the University of Maine, started searching for a better technique. It didn’t take him long. Shortly after Wagner’s first paper came out, Kazmerski told colleagues how he deposited CIS in a thin-film on a piece of glass. His first cells were between 4-5% efficient.
It was a promising development, but work on CIS was only one part of a larger government investment in solar power. In the 1970s, the National Science Foundation was directing large investments in solar power research for the U.S. government. Much of the money was going toward developing silicon-based solar cells. “Silicon, they knew, would do well eventually. That was the known semiconductor,” says Kannan Ramanathan, head of the CIGS team at the NREL. “Yet they wanted to divest, take risks, and nurture thin films.”
Work on CIS trundled along until 1981, when Boeing scientists Reid Mickelsen and Wen Chen announced at a conference in Orlando, Florida, that they had doubled Kazmerski’s efficiency by depositing the material in a new way. Thin-films had arrived.
Though silicon remained the favored material, a handful of companies grew interested in thin-film cells and CIS in particular. They wagered that if they could get the chemistry right, thin-film cells would be vastly cheaper to produce than silicon cells, which had to be grown as crystals. Plus, CIS could be deposited on inexpensive glass, reducing weight and materials costs. For Boeing, which used solar cells on spacecraft, lightweight panels would translate into cheaper launch costs.
Meanwhile, the aerospace company’s continued investment was yielding dividends. Chen and another colleague, John Stewart, figured out in the late-1980s that they could substitute gallium for some of the indium, further raising the efficiency. (That was what put the G in CIGS.)
Earlier that decade, oil company Arco had also begun exploring CIS and other thin-film technologies. During the energy crisis in 1979, the company had become a serious player in the nascent solar power industry. After throwing its weight behind CIS research, it quickly developed an alternative to Boeing’s production technique. It wasn’t quite as efficient, but was considered easier to manufacture. By 1988, the Southern California-based Arco Solar produced a four-square-foot module with 11% efficiency. That same year, they offered to permanently light the Hollywood sign using solar power.
Despite the bravado, things weren’t going well for the Arco Solar pioneer. Development problems plagued the run-up to production, frustrating its parent company. Plus, the solar power market wasn’t growing as quickly as they had hoped. Looking to cut costs, Arco sold its solar division to Siemens in 1989.
Boeing had also lost interest, and left their work to NREL. Researchers in academia and industry had to go back to the drawing board in an attempt to resolve the issues that plagued previous manufacturing efforts. But without the major players, the material that had shown so much promise in the 1970s and 1980s stumbled. It would be almost 10 years before the CIGS industry would recover.
Out from the Shadows
By the late 1990s, Siemens was feeling confident in its progress on CIGS and spooled up a pilot production line. The results of an early run were tested at NREL and scored higher than 10% efficiency. They were the first thin-film photo-voltaics made outside of a lab to reach that landmark. But just as Arco had dropped its solar division after it made the 11% module, Siemens started looking for a buyer for the California-based division shortly thereafter. It eventually ended up with another oil company, Shell. (The division ended up being a hot potato; Shell would only own it for four years before selling it to Germany-based Solar World in 2006.)
The 2000s could have been another lost decade for CIGS, but then, in 2003, Germany began offering generous subsidies on solar power. That encouraged a number of universities and small companies to jump in the game, who, along with NREL, would end up carrying the torch when, a few years later, Shell “walked away” from their solar division, Ramanathan says.
The handful of smaller companies kept at it, encouraged by government subsidies and an influx of venture capital, fine-tuning their materials and lowering their production costs. Then, as so many times before, they ran into a series of unexpected problems. While many companies had become adept at producing cells in the lab, they couldn’t replicate that success on a large scale. Some of these delays were blamed on an incomplete scientific understanding of the CIGS material. William N. Shafarman, a professor at the University of Delaware, and Lars Stolt, a professor at Uppsala University, wrote in 2003 that the “lack of a science base has been perhaps the biggest hindrance to the maturation of Cu(InGa)Se2 solar cell technology as most of the progress has been empirical.” At many companies, the cart had gotten in front of the horse.
That lack of understanding would catch up with manufacturers a few years later when product testing company TÜV Rheinland documented a sharp spike in the number of failures among thin-film panels, including CIGS and other types, during the damp-heat test, where panels are subjected to 1,000 hours of 85˚ F and 85% humidity. Between 2005-2007, 70% of thin-film panels failed, more than double the failures for 1997-2005. They had to go back to the drawing board, again.
Meanwhile, manufacturers also had to perfect how the cells would be packaged and connected. Each wire, sheet of glass, and piece of aluminum had to be tested for durability and reliability. They had to simulate everything from snafus that might take place during installation to 20 years of heat and moisture. Thanks to accelerated testing, the process doesn’t take 20 years, but it can still take many months to several years.
Bert Haskell, the CTO at Pecan Street, oversaw these tests in an earlier job as director of product development at Heliovolt, an Austin, Texas-based CIGS company. There, he and his team would subject completed panels to a grueling regimen of abuse. They’d yank on connecting cables, drop one-and-a-half-pound ball bearings onto the glass, and fire chunks of ice at the panels at 50 mph. They’d subject them to high humidity and drastic fluctuations in temperature. They’d bake them and they’d freeze them. “Those tests, you might run those for 90 days or six months before you get results back,” Haskell says. It was quicker than waiting 20 years, but it wasn’t instantaneous.
Add it all up, and you quickly realize that just testing the non-photovoltaic part of the module took several years. Some tests could occur in parallel with work on the CIGS cells themselves, but in the end, the entire package still had to be tested and certified.
It wasn’t until the mid-2000s that CIGS-based solar panels began to trickle into the market, more than 30 years after the material’s initial discovery as a photovoltaic. Today, CIGS cells remain costly relative to silicon cells and have captured just a few percent of the market. The future could still be bright, but it will require many more years of sustained funding, research and development.
A Long Road Ahead
Judging by the challenges CIGS confronted, it’s likely that perovskite solar cells have a long road in front of them. Though the material has shown great promise, moving out of the lab and into production isn’t the same as producing high-efficiency cells in the lab. It takes time. “The development time for most technologies is 20 to 30 years,” says Ginley, the NREL scientist. “That’s pretty damn canonical.”
Haksell agrees. “When a scientist discovers a new material in the lab that has some kind of unique property, going from that to the point where it’s applied in a useful product, it just takes a long time.” (I followed up with Snaith regarding his three-to-five-year commercial timeline for perovskite solar cells, but haven’t heard back.)
Perovskite’s biggest stumbling block could be water. While most solar cells don’t react well to water, perovskite’s current formulation is an ionic salt, which means it’s highly susceptible to water damage, both McGehee and Ginley tell me. Solar manufacturers work hard to keep their products sealed, but water has a tendency to work its way into the smallest of gaps, including those cracks that happen during installation or any of the many heating and cooling cycles solar panels endure. Reformulating the material while keeping the basic chemical structure could reduce the potential for water damage, but that would require years more research.
“There’s still a lot of questions that need to be answered,” McGehee says of perovskite. “It is exciting and I don’t want to take away from it in any way, but we still need to have a wait and see attitude before we’ll know if this is going to be a commercial success.”
Scientists from around the world are working together at SOFI to create a clean liquid solar fuel using sunlight, water, and carbon taken from the atmosphere.
By mimicking one of nature’s most fundamental processes – photosynthesis – SOFI scientists and engineers aim to fuel the planet sustainably using the ultimate energy source: the sun. We’ll convert the sun’s energy into clean fuels by following nature’s photosynthetic blueprint.
LIGHT CAPTURE + WATER SPLITTING + CO2 CATALYST
To Find Out More About SOFI and the PEOPLE Behind the Mission – Go Here:
11 Nov 2014
The agency predicts in two new reports that solar will meet nearly a third of the world’s electricity demand, with photovoltaic panels like those found on residential rooftops generating 16 percent of the planet’s power. Solar thermal power plants, which use the sun’s heat to create steam that drives a turbine, will supply another 11 percent.
Replacing fossil fuels with all that solar energy would avoid the release of 6 billion tons of carbon dioxide by mid-century. That’s nearly equivalent to the carbon emissions of all planes, trains, and automobiles worldwide, according to the Solar Energy Industries Association, a Washington, D.C.–based trade group.
“This report reiterates what we already know: Clean, renewable solar energy is poised to meet our electricity needs,” SEIA spokesperson Ken Johnson said in an email.
What’s driving the solar boom?
China, largely. The country is expected to account for 37 percent of the world’s solar capacity by 2050. The rapid expansion of the Chinese photovoltaic panel industry has helped make solar electricity increasingly affordable and competitive with fossil fuels. Solar panel prices, for instance, have fallen 80 percent in recent years, and by 2050, the IEA predicts the retail cost of solar electricity will drop by 65 percent.
In the last four years alone, the world has deployed more photovoltaic systems than over the past four decades. Global capacity stands at 150 gigawatts.
Solar thermal power plants, on the other hand, generate only four gigawatts of electricity today, and the IEA predicts slower growth for that technology.
The agency’s sunny predictions, however, depend on the continuation of policies that have propelled the solar industry. For instance, in the U.S., the world’s third-largest solar market after Japan, a 30 percent tax credit for solar systems is set to fall to 10 percent at the end of 2016.
The SEIA is pushing for a legislative tweak in the tax credit that would make projects that break ground before the expiration date eligible for the subsidy.
“As with any type of prediction, there are many moving parts, but the IEA’s call for ‘clear, credible and consistent signals from policymakers’ is key to transitioning away from polluting energy sources,” Johnson wrote.
Chemical engineers at Stanford have designed a catalyst that could help produce vast quantities of pure hydrogen through electrolysis – the process of passing electricity through water to break hydrogen loose from oxygen in H2O.
Today, pure hydrogen, or H2, is a major commodity chemical that is generally derived from natural gas. Tens of millions of tons of hydrogen are produced each year; industrial hydrogen is important in petroleum refining and fertilizer production.
Chemical engineering Professor Thomas Jaramillo and research associate Jakob Kibsgaard want to use electrolysis to do things such as producing H2 from water and using the process to store solar energy. But to industrialize water-splitting they must find a more cost-effective process.
Electrolysis in classroom experiments is simple: lower two metal electrodes into water; when electricity is passed through these electrodes they act as catalysts to break water molecules into bubbles of hydrogen and oxygen gas.
Graphic shows how electrolysis could produce hydrogen as a way to store renewable energy. During the day, solar panels supply surplus electricity for electrolysis, producing hydrogen. At night, hydrogen would be combined with oxygen from the air to generate electricity.
Platinum is the best catalyst for producing hydrogen through water electrolysis. But to make electrolysis an industrial process a cheaper electrode must be found. “We’re trying to make H2 in the most efficient way possible without using precious metals,” Jaramillo said.
In the German scientific journal Angewandte Chemie, Jaramillo and Kibsgaard describe a cheap, durable and efficient catalyst that could take the place of platinum.
Their ambitions go beyond using electrolysis merely to replace the current market demand for hydrogen.
Right now there is no cost-effective, large-scale way to store solar energy. The Stanford researchers believe that electrolysis could turn tanks of water into batteries for storing solar energy. During the day, electricity from solar cells could be used to break apart water into hydrogen and oxygen. Recombining these gases would generate electricity for use at night.
Electrolysis uses electricity to crack the chemical bonds that hold H2O together.
Cracking the chemical bonds of water produces a hydrogen ion – a proton with no electron to balance it out. A good H2 catalyst gives the proton a place to stick until it can pick up an electron to form a hydrogen atom on the catalyst surface and then pair up with a neighboring hydrogen atom to bubble off as H2.
The trick is finding a catalyst with the right stickiness.
“If the binding is too weak, the ions don’t stick,” Jaramillo said. “If it’s too strong, they never get released.”
Platinum is perfect but pricey. Last year the Stanford engineers discovered that a version of molybdenum sulfide, a catalyst widely used in petrochemical processing, had some of the right properties to serve as a cheap but efficient alternative to platinum.
Jaramillo explained that petrochemical processing has similarities to electrolysis. That’s because petroleum feed stocks, such as tar sands, contain a significant fraction of heavy molecules. Petroleum refineries use catalytic reactions that involve hydrogen to crack these heavy molecules into lighter molecules like gasoline.
Similarly, electrolysis involves cracking water molecules, or breaking apart their chemical bonds. As the Stanford engineers sought to improve on their own discovery they found an even better way to produce hydrogen from water by taking yet another page from the petrochemical playbook.
Petroleum processing often involves scrubbing sulfur out of fuels to reduce acid rain. During this scrubbing process, some of the sulfur atoms get incorporated into petroleum processing catalysts, increasing the activity of these catalysts.
This gave the Stanford engineers an idea: If they laced an already good catalyst with sulfur atoms, would it become an even better electrode for producing pure hydrogen?
They chose to add sulfur atoms to a catalyst called molybdenum phosphide, which is known to speed up hydrogen production though electrolysis.
Adding the sulfur atoms created a new catalyst – molybdenum phosphosulfide– that was more effective at producing hydrogen than its predecessor.
The new sulfur-laced catalyst was more durable, which is vital in an industrial process where the electrode must function day in, day out, without degrading, just like the noble metal platinum.
The molybdenum phosphosulfide catalysts developed by Kibsgaard and Jaramillo are a major advance. As electrodes they are remarkably stable with an efficiency approaching that of platinum.
Now, members of Jaramillo’s group are working to improve this new catalyst. For instance they are engineering the material at nano-scale dimensions to catalyze the reaction more effectively. Other research initiatives include incorporating this catalyst into bench-top prototypes of future energy storage systems. The idea would be to use water electrolysis to store solar energy by day in the form of H2 and then, at night, to recombine hydrogen and oxygen into water, generating electricity in the process.
Jaramillo noted that the findings in this and the prior scientific paper pursue environmentally friendly energy strategies, but they are based on ideas borrowed from petrochemical plants.
“It’s exciting to make these connections between really different areas of technology,” he said, “and aim to operate at the meta-level of science.”
11 Nov 2014
|Researchers have developed lightweight “supercapacitors” that can be combined with regular batteries to dramatically boost the power of an electric car.|
|The discovery was made by Postdoctoral Research Fellow Dr Jinzhang Liu, Professor Nunzio Motta and PhD researcher Marco Notarianni, from QUT’s Science and Engineering Faculty – Institute for Future Environments, and PhD researcher Francesca Mirri and Professor Matteo Pasquali, from Rice University in Houston, in the United States.|
|QUT’s Professor Nunzio Motta with one of the university’s powerful nanotechnology microscopes.|
|The supercapacitors – a “sandwich” of electrolyte between two all-carbon electrodes – were made into a thin and extremely strong film with a high power density.|
|The film could be embedded in a car’s body panels, roof, doors, bonnet and floor – storing enough energy to turbocharge an electric car’s battery in just a few minutes.|
|The findings, published in the Journal of Power Sources (“High performance all-carbon thin film supercapacitors”) and the Nanotechnology journal (“Graphene-based supercapacitor with carbon nanotube film as highly efficient current collector”), mean a car partly powered by its own body panels could be a reality within five years, Mr Notarianni said.|
|“Vehicles need an extra energy spurt for acceleration, and this is where supercapacitors come in. They hold a limited amount of charge, but they are able to deliver it very quickly, making them the perfect complement to mass-storage batteries,” he said.|
|“Supercapacitors offer a high power output in a short time, meaning a faster acceleration rate of the car and a charging time of just a few minutes, compared to several hours for a standard electric car battery.”|
|Dr Liu said currently the “energy density” of a supercapacitor is lower than a standard lithium ion (Li-Ion) battery, but its “high power density”, or ability to release power in a short time, is “far beyond” a conventional battery.|
|“Supercapacitors are presently combined with standard Li-Ion batteries to power electric cars, with a substantial weight reduction and increase in performance,” he said.|
|“In the future, it is hoped the supercapacitor will be developed to store more energy than a Li-Ion battery while retaining the ability to release its energy up to 10 times faster – meaning the car could be entirely powered by the supercapacitors in its body panels.|
|“After one full charge this car should be able to run up to 500km – similar to a petrol-powered car and more than double the current limit of an electric car.”|
|Dr Liu said the technology would also potentially be used for rapid charges of other battery-powered devices.|
|“For example, by putting the film on the back of a smart phone to charge it extremely quickly,” he said.|
|The discovery may be a game-changer for the automotive industry, with significant impacts on financial, as well as environmental, factors.|
|“We are using cheap carbon materials to make supercapacitors and the price of industry scale production will be low,” Professor Motta said.|
|“The price of Li-Ion batteries cannot decrease a lot because the price of Lithium remains high. This technique does not rely on metals and other toxic materials either, so it is environmentally friendly if it needs to be disposed of.”|
|Source: Queensland University of Technology|
11 Nov 2014
Rechargeable battery manufacturers may get a jolt from research performed at NIST and several other institutions, where a team of scientists has discovered a safe, inexpensive, sodium-conducting material that significantly outperforms all others in its class.
The team’s discovery is a sodium-based, complex metal hydride, a material with potential as a much cheaper alternative to the lithium-based conductors used in many rechargeable batteries. Because lithium is a comparatively rare commodity near the earth’s surface, the industry would prefer to build reusable batteries out of common ingredients that are both economical and inexhaustible.
The novel hydride—which has the formula Na2B10H10—might fit the bill, and not only because it is formed of the three easily obtainable elements of sodium, boron and hydrogen. There are other practical reasons as well: It is a stable inorganic solid, meaning it would pose fewer of the risks carried by many flammable liquids in traditional batteries, such as the potential for leaking or exploding. And compared to other sodium-based solids, it can enable more power output.
This last advantage stems from its unusual ability to conduct sodium ions exceptionally well when heated. At room temperature, the hydride’s atoms are tightly packed together. But when heated to near water’s boiling point, they repack to create numerous corridors through which the sodium ions can flow easily. Because charged ions are what carry electricity in a battery, this “phase change,” as physicists call it, allows the team’s material to outperform others.
“It’s more than 20 times better at doing its job than other known sodium-based complex hydrides in this temperature range,” says Terrence Udovic of the NIST Center for Neutron Research (NCNR). “It’s also as good as the best solid lithium-based hydride that has been measured, so it’s quite promising.”
Udovic had been exploring metal hydride materials as candidates for hydrogen storage, and while this particular compound performed poorly at that task, he hit upon the idea of testing it as an ion conductor. NCNR research hinted at its abilities, but clarifying them took an international effort among collaborators from Japan’s Tohoku Univ., Russia’s Institute of Metal Physics, the Univ. of Maryland and Sandia National Laboratories.
Udovic says that future work will involve chemically tweaking the hydride’s properties in order to optimize its performance. At this point, it would be necessary to operate a battery above the phase transition temperature, so one goal will be to bring the transition temperature down to as close to room temperature as possible—a goal he is confident is within reach.
“You could probably use this material in a battery right now,” he says. “But the lower the temperature required to make it work, the more useful it will be.”
11 Nov 2014
The Rice lab of chemist James Tour has turned molybdenum disulfide’s two-dimensional form into a nanoporous film that can catalyze the production of hydrogen or be used for energy storage.
The versatile chemical compound classified as a dichalcogenide is inert along its flat sides, but previous studies determined the material’s edges are highly efficient catalysts for hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.
Tour and his colleagues have found a cost-effective way to create flexible films of the material that maximize the amount of exposed edge and have potential for a variety of energy-oriented applications.
The Rice research appears in the journal Advanced Materials.
A new material developed at Rice University based on molybdenum disulfide exposes as much of the edge as possible, making it efficient as both a catalyst for hydrogen production and for energy storage. Credit: Tour Group/Rice University
Molybdenum disulfide isn’t quite as flat as graphene, the atom-thick form of pure carbon, because it contains both molybdenum and sulfur atoms. When viewed from above, it looks like graphene, with rows of ordered hexagons. But seen from the side, three distinct layers are revealed, with sulfur atoms in their own planes above and below the molybdenum.
This crystal structure creates a more robust edge, and the more edge, the better for catalytic reactions or storage, Tour said.
“So much of chemistry occurs at the edges of materials,” he said. “A two-dimensional material is like a sheet of paper: a large plain with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”
The new film was created by Tour and lead authors Yang Yang, a postdoctoral researcher; Huilong Fei, a graduate student; and their colleagues. It catalyzes the separation of hydrogen from water when exposed to a current. “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make,” Tour said.
While other researchers have proposed arrays of molybdenum disulfide sheets standing on edge, the Rice group took a different approach. First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodization, an electrochemical process with many uses but traditionally employed to thicken natural oxide layers on metals.
The film was then exposed to sulfur vapor at 300 degrees Celsius (572 degrees Fahrenheit) for one hour. This converted the material to molybdenum disulfide without damage to its nano-porous sponge-like structure, they reported.
The films can also serve as supercapacitors, which store energy quickly as static charge and release it in a burst. Though they don’t store as much energy as an electrochemical battery, they have long lifespans and are in wide use because they can deliver far more power than a battery. The Rice lab built supercapacitors with the films; in tests, they retained 90 percent of their capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.
“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”
Explore further: Harnessing an unusual ‘valley’ quantum property of electrons
31 Oct 2014
Big TV – Large Screen Makers are Forsaking OLED’s in Favor of Quantum Dots – Reuters October 30, 2014
(Reuters) – The world’s biggest TV makers, Samsung Electronics Co Ltd and LG Electronics Inc, are turning to quantum dot technology for their next-generation TVs as it could still be years before OLED is affordable for the mass market.
The nascent technology involves incorporating a film of tiny light-emitting crystals into regular liquid crystal displays (LCD). The manufacturing process is relatively straightforward and offers improved picture quality at much cheaper cost than using organic light-emitting diodes (OLED).
The resulting lower prices could help the technology catch on far quicker. One industry analyst estimated a 55-inch quantum dot TV could be priced 30 to 35 percent more than a current LCD TV, while an OLED TV could be 5 times more expensive. LG recently launched a 65-inch ultra-high definition OLED TV for 12 million won ($11,350) in its home market of South Korea.
The only real challenge is securing enough quantum dot material from the small pool of suppliers, including Quantum Materials Corp and Nanoco Group PLC.
Nanoco last month said a South Korean plant being built by partner Dow Chemical Co will start quantum dot production in the first half of 2015. Analysts believe the output is destined for a local client.
On Wednesday, LG, the world’s No.2 TV maker after domestic rival Samsung, said it plans to make quantum dot TVs in addition to OLED TVs. Analysts regarded that a tacit acknowledgement that OLED needs more time for prices to come down before becoming the new standard.
“We are pursuing a dual-track strategy with quantum dot and OLED,” LG Chief Financial Officer Jung Do-hyun told analysts after the company reported earnings. OLED is the fundamentally superior product, he said.
Samsung Vice President Simon Sung told analysts on Thursday that quantum dot is among many technologies under consideration.
“How a technology will match with market conditions, when a technology will emerge as the main market segment is the most critical consideration,” Sung said. “We’ll respond aggressively after identifying such a market opportunity.”
At present, Japan’s Sony Corp is the only major electronics manufacturer selling quantum dot TVs. Last month, China’s TCL Multimedia Technology Holdings Ltd unveiled a quantum dot TV at the IFA tech expo in Berlin.
LG and affiliate LG Display Co Ltd are the biggest champions of OLED TVs, in contrast to Samsung which has not released a model this year. Analysts say the lack of Samsung support could limit OLED growth prospects and keep prices high.
“In theory, OLED should become cheaper than LCD once production yields get better because OLED doesn’t need a backlight, but at this point both the scale and production yield remain low,” said CIMB analyst Lee Do-hoon.
“LG needs a product in the interim, and they seem to be saying they’ll look at market conditions and respond with quantum dot,” Lee said.
Samsung could be more aggressive than LG in pushing quantum dot as Korea’s No.1 consumer electronics maker appears less committed to a particular technology, analysts said. LG, on the other hand, risks undermining its OLED push.
“If LG focuses on quantum dot, it’d be basically the same as signaling that it will be difficult for OLED to go mainstream in the near term,” said CIMB’s Lee.
Researcher DisplaySearch forecasts 1.95 million quantum dot TV shipments next year, for just 0.8 percent of the market, growing to 25.5 million by 2020. IHS Technology sees OLED TV shipments at 7.8 million units by 2019 from 600,000 in 2015.