Solar cells made of artificial metallic crystalline structures called perovskites have shown great promise in recent years. Now scientists at Stanford University have found that applying pressure can change the properties of these inexpensive materials and how they respond to light. "Our results suggest that we can increase the voltages of perovskite solar cells by applying external pressure," said Hemamala Karunadasa, an assistant professor of chemistry at Stanford. "We also observed a dramatic increase in the electronic conductivity of these promising materials at high pressures." Karunadasa and Stanford colleague Wendy Mao report their findings in a paper in ACS Central Science. Perovskites come in several crystalline structures, including hybrid perovskites made of lead, iodine or bromine with organic compounds. These inexpensive materials have potential applications in advanced LEDs and lasers, but one of the hottest areas of research involves solar cells. Recent studies have shown that hybrid perovskites can efficiently absorb sunlight and convert it to electricity; several labs have achieved light-to-electric power efficiencies above 20%, rivaling commercially-available silicon solar cells. In this new study, Karunadasa and Mao sought to assess how pressure affects the way hybrid perovskites respond to light. To find out, the researchers loaded perovskite samples in a diamond-anvil cell, a high-pressure device consisting of two opposing diamonds. Each tiny sample was placed between the diamonds and then squeezed at very high pressures. The results were visible. One sample, which is normally orange, turned lighter in color under compression, providing an indication that the perovskite was absorbing higher-energy light waves. But as the pressure increased, the sample darkened, indicating that lower-energy light was also being absorbed. "Our findings suggest that compression can allow us to tailor the wavelength of absorbed light," explained Mao, an associate professor of geological sciences at Stanford and of photon science at the SLAC National Accelerator Laboratory. "This compression may be attained through either mechanical or chemical means." Several research groups have been developing low-cost tandem solar cells made of perovskite stacked on top of silicon. But obtaining the high voltages required for high-efficiency tandem cells has proven to be difficult. The results of the new Stanford study suggest that applying pressure could offer a simple way to increase the voltages of perovskite solar cells and should be investigated further. "By tracking the positions of atoms upon compression using X-ray diffraction, we can explain exactly how the materials' structure responds to pressure," Karunadasa said. "Overall, this work shows that pressure is a tuning knob for improving the properties of perovskite absorbers in a predictable way." This story is adapted from material from Stanford University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
News Article | August 29, 2016
If you’ve never heard of covalent organic frameworks before, that could be about to change. To the eye, these delicate, super lightweight crystalline materials look like nothing more than a pile of fluffy powder, but they are beginning to emerge as a force to be reckoned with in the energy storage field. In the latest development, a research team based at Northwestern University has figured out how to get one such material to behave like a battery and a supercapacitor, all in one. That could be an important development for the EV market. An improved battery means greater range without adding more weight, and a more efficient supercapacitor translates into faster charging and more powerful discharging. For a quick primer on COF’s you can check out Lawrence Berkeley National Laboratory, which provides this rundown: “COFs and their cousin materials, metal organic frameworks (MOFs), are porous three-dimensional crystals with extraordinarily large internal surface areas that can absorb and store enormous quantities of targeted molecules.” If that sounds a little vague, CleanTechnica offered up the origin story last year: “COFs were first developed at the University of Michigan back in 2005, when researchers ‘coaxed’ rigid plastics to organize into predictable crystal structures — something that had never been done before. The research team did it by slowing down the reaction time involved in synthesizing rigid plastics.” Like their MOF cousins, COFs benefit from a hub-and-strut structure (think Tinker Toys and you’re on the right track) that in effect loads the material with nanoscale pores, like a sponge. The pores can be altered by adjusting the angles at which the struts connect to the hubs, to provide a COF with unique properties. As for why COFs instead of MOFs, for one thing, they contain no metal, so they are much lighter than MOFs (COFs are typically made with hydrogen, boron, carbon, nitrogen, and oxygen). As an organic (aka plastic) material, COFs also have the potential to come in at a far lower cost than other materials. MOFs have previously been investigated for energy storage, but COFs are at a huge disadvantage in that field because they lack sufficient conductivity. One key problem is the disordered nature of COFs as applied to electrodes. So far, the body of research has arrived at one solution, which is to apply them to an electrode as a two-dimensional material: “Two-dimensional covalent organic frameworks (2D COFs) address these limitations by predictably and deliberately organizing redox-active groups into insoluble, high-surface area polymer networks with uniform micropores.” That’s all well and good, but the problem is that the charge/discharge rate is very slow, so you can kiss any high-performance applications goodbye. That’s where the new Northwestern University research comes in. Instead of just fiddling with the angles of their COF, they modded it out with the conducting polymer PEDOT, which is short for poly(3,4-ethylenedioxythiophene). Northwestern provides a rundown on the process: “… Two organic molecules self-assembled and condensed into a honeycomb-like grid, one 2-D layer stacked on top of the other. Into the grid’s holes, or pores, the researchers deposited the conducting polymer.” The pores in the new material are 2.3 nanometers wide, and each pore translates into increased surface area: “A small amount of the fluffy COF powder, just enough to fill a shot glass and weighing the same as a dollar bill, has the surface area of an Olympic swimming pool.” I know right? Here’s the result in plain language: “The modified COF showed a dramatic improvement in its ability to both store energy and to rapidly charge and discharge the device. The material can store roughly 10 times more electrical energy than the unmodified COF, and it can get the electrical charge in and out of the device 10 to 15 times faster.” The new device is also quite durable. According to the study, it can withstand 10,000 charge/discharge cycles. You can get all the details from the American Chemical Society journal Central Science under the title, “Superior Charge Storage and Power Density of a Conducting Polymer-Modified Covalent Organic Framework.” The EV market is slowly gathering steam, and if the Northwestern research pans out, it looks like you ain’t seen nothing yet. The researchers are confident that this is a breakthrough for EV battery technology: “This work represents the first time that electroactive COFs or crystalline framework materials have shown volumetric energy and power densities comparable with other porous carbon-based electrodes, thereby demonstrating the promise of redox-active COFs for EES [electrical energy storage] devices.” In other words, if the EV of tomorrow is equipped with a COF battery (or some similar future technology), it will go significantly farther, cost less, and charge faster than the current crop. Make that the day after tomorrow. So far, the Northwestern research has been tested out on a coin-cell-sized prototype that can light up an LED for 30 seconds. So, don’t hold your breath. But, when that affordable, sporty, 500-mile range EV of the future does come rolling into your garage, the US Army (yes, the Army) might take partial credit. Among other funders, the Northwestern team is supported by a US Army Research Office Multidisciplinary University Research Initiatives grant. Follow me on Twitter and Google+. Drive an electric car? Complete one of our short surveys for our next electric car report. Keep up to date with all the hottest cleantech news by subscribing to our (free) cleantech newsletter, or keep an eye on sector-specific news by getting our (also free) solar energy newsletter, electric vehicle newsletter, or wind energy newsletter.
News Article | April 27, 2016
In the alphabets of our genomes, a single typo can mean the difference between health and disease. The "words" are molecules, like enzymes, and one group of these called kinases are particularly important. However, it's difficult to pick apart the exact roles of misspellings in various kinases in a disease process. This week in ACS Central Science, researchers report development of a new way to determine this, both in a lab dish and in mice.
An interdisciplinary team of scientists has worked out a way to make electric vehicles not just carbon neutral, but carbon negative, capable of actually reducing the amount of atmospheric carbon dioxide as they operate. They have done this 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 (CO ) gas into batteries is described in a paper in ACS Central Science. The unusual pairing of CO conversion and advanced battery technology is the result of a collaboration between the laboratory of Cary Pint, assistant professor of mechanical engineering at Vanderbilt University, and Stuart Licht, professor of chemistry at George Washington University. Pint, Licht and their colleagues adapted a solar-powered process that converts CO into carbon so that it would produce carbon nanotubes. They then demonstrated that these nanotubes can be incorporated into the lithium-ion batteries used in electric vehicles and electronic devices and also into the low-cost sodium-ion batteries under development for large-scale energy storage applications. "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 CO that are aimed at low-valued fuels, like methanol, that cannot justify the cost required to produce them," said Pint. This project builds upon a solar thermal electrochemical process (STEP) for creating carbon nanofibers from ambient CO developed by the Licht group and reported in the journal Nano Letters in August 2015. STEP uses solar energy to provide both the electrical and thermal energy needed to break down CO into carbon and oxygen, and then 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 STEP 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 battery performance, with this boost 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 a stable storage performance over three and a half times above that of sodium-ion batteries with graphite electrodes. Most importantly, both carbon nanotube-based batteries were exposed to about two and a half months of continuous charging and discharging without showing any signs of fatigue. Depending on the specifications, making one of the two electrodes out of carbon nanotubes means that up to 40% of a battery could be made out of recycled CO , 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 US Department of Energy in 2013), a kilogram of CO has a value of about $18 as a battery material – six times more than when it is converted to methanol. This value only increases when moving from large batteries used in electric vehicles to the smaller batteries used in electronic devices. And unlike methanol, combining batteries with solar cells provides renewable power with zero greenhouse emissions. Licht also proposed that the STEP process could be coupled with 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 by 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 CO 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." This story is adapted from material from Vanderbilt University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
A new study has demonstrated an environmentally friendly approach to converting atmospheric carbon dioxide into batteries, improved technology that could lead to the development of electric vehicles that are carbon-neutral and also carbon-negative, therefore lessening the amount of atmospheric CO2 produced as they operate. The study, a collaboration between Vanderbilt University and George Washington University, published in the journal ACS Central Science [Licht et al. ACS Cent. Sci. (2016) DOI: 10.1021/acscentsci.5b00400], showed how the graphite electrodes used could be replaced by carbon material from the atmosphere. The team has developed a new solar thermal electrochemical process (STEP) to convert atmospheric or smokestack CO2 into carbon nanotubes (CNTs), which can then be incorporated into advanced batteries such as lithium-ion used in electric vehicles and electronic devices as well as the more inexpensive sodium-ion batteries being developed for larger applications. STEP uses solar energy to provide the electrical and thermal energy required to break down carbon dioxide into carbon and oxygen, as well as provide stable, flexible, conductive and robust CNTs. The technology helps transform the greenhouse gas CO2 into valuable products, and offers an alternative to greenhouse gas emissions from industrial and transportation fossil fuel processes. For commercial lithium-ion batteries, the CNTs could improve upon the performance of the carbon anode; for sodium-ion batteries, minor defects in the carbon (which can be tuned using STEP) were shown to offer stable and much better storage performance than those with graphite electrodes. Both CNT batteries were exposed to the same amount of continuous charging and discharging without displaying any fatigue. They estimated that CO2 as a battery material is up to six times more valuable than when it is converted to methanol. In addition, unlike methanol, combining batteries with solar cells can offer renewable energy that involves no greenhouse emissions. It is claimed the STEP process could be combined with a natural gas powered electrical generator to produce electricity and heat, as well as a concentrated source of CO2 to boost the performance of the STEP process. The oxygen released during the process could be returned to the generator to boost its combustion efficiency, as this would compensate for the amount of electricity that the STEP process consumes, and potentially result in a fossil fuel electrical power plant with zero net CO2 emissions. The team will also explore applications of the STEP CNTs made from CO2, including carbon composites for strong and lightweight construction materials, sports equipment and vehicle bodies.