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      in Energy

      Toward batteries that pack twice as much energy per pound

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      In the endless quest to pack more energy into batteries without increasing their weight or volume, one especially promising technology is the solid-state battery. In these batteries, the usual liquid electrolyte that carries charges back and forth between the electrodes is replaced with a solid electrolyte layer. Such batteries could potentially not only deliver twice as much energy for their size, they also could virtually eliminate the fire hazard associated with today’s lithium-ion batteries.

      But one thing has held back solid-state batteries: Instabilities at the boundary between the solid electrolyte layer and the two electrodes on either side can dramatically shorten the lifetime of such batteries. Some studies have used special coatings to improve the bonding between the layers, but this adds the expense of extra coating steps in the fabrication process. Now, a team of researchers at MIT and Brookhaven National Laboratory have come up with a way of achieving results that equal or surpass the durability of the coated surfaces, but with no need for any coatings.

      The new method simply requires eliminating any carbon dioxide present during a critical manufacturing step, called sintering, where the battery materials are heated to create bonding between the cathode and electrolyte layers, which are made of ceramic compounds. Even though the amount of carbon dioxide present is vanishingly small in air, measured in parts per million, its effects turn out to be dramatic and detrimental. Carrying out the sintering step in pure oxygen creates bonds that match the performance of the best coated surfaces, without that extra cost of the coating, the researchers say.

      The findings are reported in the journal Advanced Energy Materials, in a paper by MIT doctoral student Younggyu Kim, professor of nuclear science and engineering and of materials science and engineering Bilge Yildiz, and Iradikanari Waluyo and Adrian Hunt at Brookhaven National Laboratory.

      “Solid-state batteries have been desirable for different reasons for a long time,” Yildiz says. “The key motivating points for solid batteries are they are safer and have higher energy density,” but they have been held back from large scale commercialization by two factors, she says: the lower conductivity of the solid electrolyte, and the interface instability issues.

      The conductivity issue has been effectively tackled, and reasonably high-conductivity materials have already been demonstrated, according to Yildiz. But overcoming the instabilities that arise at the interface has been far more challenging. These instabilities can occur during both the manufacturing and the electrochemical operation of such batteries, but for now the researchers have focused on the manufacturing, and specifically the sintering process.

      Sintering is needed because if the ceramic layers are simply pressed onto each other, the contact between them is far from ideal, there are far too many gaps, and the electrical resistance across the interface is high. Sintering, which is usually done at temperatures of 1,000 degrees Celsius or above for ceramic materials, causes atoms from each material to migrate into the other to form bonds. The team’s experiments showed that at temperatures anywhere above a few hundred degrees, detrimental reactions take place that increase the resistance at the interface — but only if carbon dioxide is present, even in tiny amounts. They demonstrated that avoiding carbon dioxide, and in particular maintaining a pure oxygen atmosphere during sintering, could create very good bonding at temperatures up to 700 degrees, with none of the detrimental compounds formed.

      The performance of the cathode-electrolyte interface made using this method, Yildiz says, was “comparable to the best interface resistances we have seen in the literature,” but those were all achieved using the extra step of applying coatings. “We are finding that you can avoid that additional fabrication step, which is typically expensive.”

      The potential gains in energy density that solid-state batteries provide comes from the fact that they enable the use of pure lithium metal as one of the electrodes, which is much lighter than the currently used electrodes made of lithium-infused graphite.

      The team is now studying the next part of the performance of such batteries, which is how these bonds hold up over the long run during battery cycling. Meanwhile, the new findings could potentially be applied rapidly to battery production, she says. “What we are proposing is a relatively simple process in the fabrication of the cells. It doesn’t add much energy penalty to the fabrication. So, we believe that it can be adopted relatively easily into the fabrication process,” and the added costs, they have calculated, should be negligible.

      Large companies such as Toyota are already at work commercializing early versions of solid-state lithium-ion batteries, and these new findings could quickly help such companies improve the economics and durability of the technology.

      The research was supported by the U.S. Army Research Office through MIT’s Institute for Soldier Nanotechnologies. The team used facilities supported by the National Science Foundation and facilities at Brookhaven National Laboratory supported by the Department of Energy. More

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      in Energy

      More sensitive X-ray imaging

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      Scintillators are materials that emit light when bombarded with high-energy particles or X-rays. In medical or dental X-ray systems, they convert incoming X-ray radiation into visible light that can then be captured using film or photosensors. They’re also used for night-vision systems and for research, such as in particle detectors or electron microscopes.

      Researchers at MIT have now shown how one could improve the efficiency of scintillators by at least tenfold, and perhaps even a hundredfold, by changing the material’s surface to create certain nanoscale configurations, such as arrays of wave-like ridges. While past attempts to develop more efficient scintillators have focused on finding new materials, the new approach could in principle work with any of the existing materials.

      Though it will require more time and effort to integrate their scintillators into existing X-ray machines, the team believes that this method might lead to improvements in medical diagnostic X-rays or CT scans, to reduce dose exposure and improve image quality. In other applications, such as X-ray inspection of manufactured parts for quality control, the new scintillators could enable inspections with higher accuracy or at faster speeds.

      The findings are described today in the journal Science, in a paper by MIT doctoral students Charles Roques-Carmes and Nicholas Rivera; MIT professors Marin Soljacic, Steven Johnson, and John Joannopoulos; and 10 others.

      While scintillators have been in use for some 70 years, much of the research in the field has focused on developing new materials that produce brighter or faster light emissions. The new approach instead applies advances in nanotechnology to existing materials. By creating patterns in scintillator materials at a length scale comparable to the wavelengths of the light being emitted, the team found that it was possible to dramatically change the material’s optical properties.

      To make what they coined “nanophotonic scintillators,” Roques-Carmes says, “you can directly make patterns inside the scintillators, or you can glue on another material that would have holes on the nanoscale. The specifics depend on the exact structure and material.” For this research, the team took a scintillator and made holes spaced apart by roughly one optical wavelength, or about 500 nanometers (billionths of a meter).

      “The key to what we’re doing is a general theory and framework we have developed,” Rivera says. This allows the researchers to calculate the scintillation levels that would be produced by any arbitrary configuration of nanophotonic structures. The scintillation process itself involves a series of steps, making it complicated to unravel. The framework the team developed involves integrating three different types of physics, Roques-Carmes says. Using this system they have found a good match between their predictions and the results of their subsequent experiments.

      The experiments showed a tenfold improvement in emission from the treated scintillator. “So, this is something that might translate into applications for medical imaging, which are optical photon-starved, meaning the conversion of X-rays to optical light limits the image quality. [In medical imaging,] you do not want to irradiate your patients with too much of the X-rays, especially for routine screening, and especially for young patients as well,” Roques-Carmes says.

      “We believe that this will open a new field of research in nanophotonics,” he adds. “You can use a lot of the existing work and research that has been done in the field of nanophotonics to improve significantly on existing materials that scintillate.”

      “The research presented in this paper is hugely significant,” says Rajiv Gupta, chief of neuroradiology at Massachusetts General Hospital and an associate professor at Harvard Medical School, who was not associated with this work. “Nearly all detectors used in the $100 billion [medical X-ray] industry are indirect detectors,” which is the type of detector the new findings apply to, he says. “Everything that I use in my clinical practice today is based on this principle. This paper improves the efficiency of this process by 10 times. If this claim is even partially true, say the improvement is two times instead of 10 times, it would be transformative for the field!”

      Soljacic says that while their experiments proved a tenfold improvement in emission could be achieved in particular systems, by further fine-tuning the design of the nanoscale patterning, “we also show that you can get up to 100 times [improvement] in certain scintillator systems, and we believe we also have a path toward making it even better,” he says.

      Soljacic points out that in other areas of nanophotonics, a field that deals with how light interacts with materials that are structured at the nanometer scale, the development of computational simulations has enabled rapid, substantial improvements, for example in the development of solar cells and LEDs. The new models this team developed for scintillating materials could facilitate similar leaps in this technology, he says.

      Nanophotonics techniques “give you the ultimate power of tailoring and enhancing the behavior of light,” Soljacic says. “But until now, this promise, this ability to do this with scintillation was unreachable because modeling the scintillation was very challenging. Now, this work for the first time opens up this field of scintillation, fully opens it, for the application of nanophotonics techniques.” More generally, the team believes that the combination of nanophotonic and scintillators might ultimately enable higher resolution, reduced X-ray dose, and energy-resolved X-ray imaging.

      This work is “very original and excellent,” says Eli Yablonovitch, a professor of Electrical Engineering and Computer Sciences at the University of California at Berkeley, who was not associated with this research. “New scintillator concepts are very important in medical imaging and in basic research.”

      Yablonovitch adds that while the concept still needs to be proven in a practical device, he says that, “After years of research on photonic crystals in optical communication and other fields, it’s long overdue that photonic crystals should be applied to scintillators, which are of great practical importance yet have been overlooked” until this work.

      The research team included Ali Ghorashi, Steven Kooi, Yi Yang, Zin Lin, Justin Beroz, Aviram Massuda, Jamison Sloan, and Nicolas Romeo at MIT; Yang Yu at Raith America, Inc.; and Ido Kaminer at Technion in Israel. The work was supported, in part, by the U.S. Army Research Office and the U.S. Army Research Laboratory through the Institute for Soldier Nanotechnologies, by the Air Force Office of Scientific Research, and by a Mathworks Engineering Fellowship. More

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      in Energy

      Nurturing human communities and natural ecosystems

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      When she was in 7th grade, Heidi Li and the five other members of the Oyster Gardening Club cultivated hundreds of oysters to help repopulate the Chesapeake Bay. On the day they released the oysters into the bay, the event attracted TV journalists and local officials, including the governor. The attention opened the young Li’s eyes to the ways that a seemingly small effort in her local community could have a real-world impact.

      “I got to see firsthand how we can make change at a grassroots level and how that impacts where we are,” she says.

      Growing up in Howard County, Maryland, Li was constantly surrounded by nature. Her family made frequent trips to the Chesapeake Bay, as it reminded them of her parent’s home in Shandong, China. Li worked to bridge the cultural gap between parents, who grew up in China, and their children, who grew up in the U.S., and attended Chinese school every Sunday for 12 years. These experiences instilled in her a community-oriented mindset, which Li brought with her to MIT, where she now majors in materials science and engineering.

      During her first year, Li pursued a microbiology research project through the Undergraduate Research Opportunities Program (UROP) in the Department of Civil and Environmental Engineering. She studied microbes in aquatic environments, analyzing how the cleanliness of water impacted immunity and behavioral changes of the marine bacteria.

      The experience led her to consider the ways environmental policy affected sustainability efforts. She began applying the problem to energy, asking herself questions such as, “How can you take this specific economic principle and apply it to energy? What has energy policy looked like in the past and how can we tailor that to apply to our current energy system?”

      To explore the intersection of policy and energy, Li participated in the Roosevelt Project, through the Center of Energy and Environmental Policy Research, during the summer after her junior year. The project used case studies targeting specific communities in vulnerable areas to propose methods for a more sustainable future. Li focused on Pittsburgh, Pennsylvania, evaluating the efficiency of an energy transition from natural gas and fossil fuels to carbon-capture, which would mean redistributing the carbon dioxide produced by the coal industry. After traveling to Pittsburgh and interviewing stakeholders in the area, Li watched as local community leaders created physical places for citizens to share their ideas and opinions on the energy transition

      “I watched community leaders create a safe space for people from the surrounding town to share their ideas for entrepreneurship. I saw how important community is and how to create change at a grassroots level,” she says.

      In the summer of 2021, Li pursued an internship through the energy consulting firm Wood Mackenzie, where she looked at technologies that could potentially help with the energy transition from fossil fuels to renewable energy. Her job was to make sure the technology could be implemented efficiently and cost-effectively, optimizing the resources available to the surrounding area. The project allowed Li to engage with industry-based efforts to chart and analyze the technological advancements for various decarbonization scenarios. She hopes to continue looking at both the local, community-based, and external, industry-based, inputs on how economic policy would affect stakeholders.

      On campus, Li is the current president of the Sustainable Energy Alliance (SEA), where she aims to make students more conscious about climate change and their impact on the environment. During summer of her sophomore year, Li chaired a sustainability hackathon for over 200 high school students, where she designed and led the “Protecting Climate Refugees” and “Tackling Environmental Injustice” challenges to inspire students to think about humanitarian efforts for protecting frontline communities.

      “The whole goal of this is to empower students to think about solutions for themselves. Empowering students is really important to show them they can make change and inspire hope in themselves and the people around them,” she says.

      Li also hosted and produced “Open SEAcrets,” a podcast designed to engage MIT students with topics surrounding energy sustainability and provide them with the opportunity to share their opinions on the subject. She sees the podcast as a platform to raise awareness about energy, climate change, and environmental policy, while also inspiring a sense of community with listeners.

      When she is not in the classroom or the lab, Li relaxes by playing volleyball. She joined the Volleyball Club during her first year at MIT, though she has been playing since she was 12. The sport allows her to not only relieve stress, but also have conversations with both undergrads and graduate students, who bring different their backgrounds, interests, and experiences to conversations. The sport has also taught Li about teamwork, trust, and the importance of community in ways that her other experience doesn’t.

      Looking ahead, Li is currently working on a UROP project, called Climate Action Through Education (CATE), that designs climate change curriculum for K-12 grades and aims to show how climate change and energy are integral to peoples’ daily lives. Seeing the energy transition as an interdisciplinary problem, she wants to educate students about the problems of climate change and sustainability using perspectives from math, science, history, and psychology to name a few areas.

      But above all, Li wants to empower younger generations to develop solution-minded approaches to environmentalism. She hopes to give local communities a voice in policy implementation, with the end goal of a more sustainable future for all.

      “Finding a community you really thrive in will allow you to push yourself and be the best version of yourself you can be. I want to take this mindset and create spaces for people and establish and instill this sense of community,” she says. More

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      in Environment

      Students dive into research with the MIT Climate and Sustainability Consortium

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      Throughout the fall 2021 semester, the MIT Climate and Sustainability Consortium (MCSC) supported several research projects with a climate-and-sustainability topic related to the consortium, through the MIT Undergraduate Research Opportunities Program (UROP). These students, who represent a range of disciplines, had the opportunity to work with MCSC Impact Fellows on topics related directly to the ongoing work and collaborations with MCSC member companies and the broader MIT community, from carbon capture to value-chain resilience to biodegradables. Many of these students are continuing their work this spring semester.

      Hannah Spilman, who is studying chemical engineering, worked with postdoc Glen Junor, an MCSC Impact Fellow, to investigate carbon capture, utilization, and storage (CCUS), with the goal of facilitating CCUS on a gigaton scale, a much larger capacity than what currently exists. “Scientists agree CCUS will be an important tool in combating climate change, but the largest CCUS facility only captures CO2 on a megaton scale, and very few facilities are actually operating,” explains Spilman. 

      Throughout her UROP, she worked on analyzing the currently deployed technology in the CCUS field, using National Carbon Capture Center post-combustion project reports to synthesize the results and outline those technologies. Examining projects like the RTI-NAS experiment, which showcased innovation with carbon capture technology, was especially helpful. “We must first understand where we are, and as we continue to conduct analyses, we will be able to understand the field’s current state and path forward,” she concludes.

      Fellow chemical engineering students Claire Kim and Alfonso Restrepo are working with postdoc and MCSC Impact Fellow Xiangkun (Elvis) Cao, also on investigating CCUS technology. Kim’s focus is on life cycle assessment (LCA), while Restrepo’s focus is on techno-economic assessment (TEA). They have been working together to use the two tools to evaluate multiple CCUS technologies. While LCA and TEA are not new tools themselves, their application in CCUS has not been comprehensively defined and described. “CCUS can play an important role in the flexible, low-carbon energy systems,” says Kim, which was part of the motivation behind her project choice.

      Through TEA, Restrepo has been investigating how various startups and larger companies are incorporating CCUS technology in their processes. “In order to reduce CO2 emissions before it’s too late to act, there is a strong need for resources that effectively evaluate CCUS technology, to understand the effectiveness and viability of emerging technology for future implementation,” he explains. For their next steps, Kim and Restrepo will apply LCA and TEA to the analysis of a specific capture (for example, direct ocean capture) or conversion (for example, CO2-to-fuel conversion) process​ in CCUS.

      Cameron Dougal, a first-year student, and James Santoro, studying management, both worked with postdoc and MCSC Impact Fellow Paloma Gonzalez-Rojas on biodegradable materials. Dougal explored biodegradable packaging film in urban systems. “I have had a longstanding interest in sustainability, with a newer interest in urban planning and design, which motivated me to work on this project,” Dougal says. “Bio-based plastics are a promising step for the future.”

      Dougal spent time conducting internet and print research, as well as speaking with faculty on their relevant work. From these efforts, Dougal has identified important historical context for the current recycling landscape — as well as key case studies and cities around the world to explore further. In addition to conducting more research, Dougal plans to create a summary and statistic sheet.

      Santoro dove into the production angle, working on evaluating the economic viability of the startups that are creating biodegradable materials. “Non-renewable plastics (created with fossil fuels) continue to pollute and irreparably damage our environment,” he says. “As we look for innovative solutions, a key question to answer is how can we determine a more effective way to evaluate the economic viability and probability of success for new startups and technologies creating biodegradable plastics?” The project aims to develop an effective framework to begin to answer this.

      At this point, Santoro has been understanding the overall ecosystem, understanding how these biodegradable materials are developed, and analyzing the economics side of things. He plans to have conversations with company founders, investors, and experts, and identify major challenges for biodegradable technology startups in creating high performance products with attractive unit economics. There is also still a lot to research about new technologies and trends in the industry, the profitability of different products, as well as specific individual companies doing this type of work.

      Tess Buchanan, who is studying materials science and engineering, is working with Katharina Fransen and Sarah Av-Ron, MIT graduate students in the Department of Chemical Engineering, and principal investigator Professor Bradley Olsen, to also explore biodegradables by looking into their development from biomass “This is critical work, given the current plastics sustainability crisis, and the potential of bio-based polymers,” Buchanan says.

      The objective of the project is to explore new sustainable polymers through a biodegradation assay using clear zone growth analysis to yield degradation rates. For next steps, Buchanan is diving into synthesis expansion and using machine learning to understand the relationship between biodegradation and polymer chemistry.

      Kezia Hector, studying chemical engineering, and Tamsin Nottage, a first-year student, working with postdoc and MCSC Impact Fellow Sydney Sroka, explored advancing and establishing sustainable solutions for value chain resilience. Hector’s focus was understanding how wildfires can affect supply chains, specifically identifying sources of economic loss. She reviewed academic literature and news articles, and looked at the Amazon, California, Siberia, and Washington, finding that wildfires cause millions of dollars in damage every year and impact supply chains by cutting off or slowing down freight activity. She will continue to identify ways to make supply chains more resilient and sustainable.

      Nottage focused on the economic impact of typhoons, closely studying Typhoon Mangkhut, a powerful and catastrophic tropical cyclone that caused extensive damages of $593 million in Guam, the Philippines, and South China in September 2018. “As a Bahamian, I’ve witnessed the ferocity of hurricanes and challenges of rebuilding after them,” says Nottage. “I used this project to identify the tropical cyclones that caused the most extensive damage for further investigation.”She compiled the causes of damage and their costs to inform targets of supply chain resiliency reform (shipping, building materials, power supply, etc.). As a next step, Nottage will focus on modeling extreme events like Mangkunt to develop frameworks that companies can learn from and utilize to build more sustainable supply chains in the future.

      Ellie Vaserman, a first-year student working with postdoc and MCSC Impact Fellow Poushali Maji, also explored a topic related to value chains: unlocking circularity across the entire value chain through quality improvement, inclusive policy, and behavior to improve materials recovery. Specifically, her objectives have been to learn more about methods of chemolysis and the viability of their products, to compare methods of chemical recycling of polyethylene terephthalate (PET) using quantitative metrics, and to design qualitative visuals to make the steps in PET chemical recycling processes more understandable.

      To do so, she conducted a literature review to identify main methods of chemolysis that are utilized in the field (and collect data about these methods) and created graphics for some of the more common processes. Moving forward, she hopes to compare the processes using other metrics and research the energy intensity of the monomer purification processes.

      The work of these students, as well as many others, continued over MIT’s Independent Activities Period in January. More

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      Selective separation could help alleviate critical metals shortage

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      New processing methods developed by MIT researchers could help ease looming shortages of the essential metals that power everything from phones to automotive batteries, by making it easier to separate these rare metals from mining ores and recycled materials.

      Selective adjustments within a chemical process called sulfidation allowed professor of metallurgy Antoine Allanore and his graduate student Caspar Stinn to successfully target and separate rare metals, such as the cobalt in a lithium-ion battery, from mixed-metal materials.

      As they report in the journal Nature, their processing techniques allow the metals to remain in solid form and be separated without dissolving the material. This avoids traditional but costly liquid separation methods that require significant energy. The researchers developed processing conditions for 56 elements and tested these conditions on 15 elements.

      Their sulfidation approach, they write in the paper, could reduce the capital costs of metal separation between 65 and 95 percent from mixed-metal oxides. Their selective processing could also reduce greenhouse gas emissions by 60 to 90 percent compared to traditional liquid-based separation.

      “We were excited to find replacements for processes that had really high levels of water usage and greenhouse gas emissions, such as lithium-ion battery recycling, rare-earth magnet recycling, and rare-earth separation,” says Stinn. “Those are processes that make materials for sustainability applications, but the processes themselves are very unsustainable.”

      The findings offer one way to alleviate a growing demand for minor metals like cobalt, lithium, and rare earth elements that are used in “clean” energy products like electric cars, solar cells, and electricity-generating windmills. According to a 2021 report by the International Energy Agency, the average amount of minerals needed for a new unit of power generation capacity has risen by 50 percent since 2010, as renewable energy technologies using these metals expand their reach.

      Opportunity for selectivity

      For more than a decade, the Allanore group has been studying the use of sulfide materials in developing new electrochemical routes for metal production. Sulfides are common materials, but the MIT scientists are experimenting with them under extreme conditions like very high temperatures — from 800 to 3,000 degrees Fahrenheit — that are used in manufacturing plants but not in a typical university lab.

      “We are looking at very well-established materials in conditions that are uncommon compared to what has been done before,” Allanore explains, “and that is why we are finding new applications or new realities.”

      In the process of synthetizing high-temperature sulfide materials to support electrochemical production, Stinn says, “we learned we could be very selective and very controlled about what products we made. And it was with that understanding that we realized, ‘OK, maybe there’s an opportunity for selectivity in separation here.’”

      The chemical reaction exploited by the researchers reacts a material containing a mix of metal oxides to form new metal-sulfur compounds or sulfides. By altering factors like temperature, gas pressure, and the addition of carbon in the reaction process, Stinn and Allanore found that they could selectively create a variety of sulfide solids that can be physically separated by a variety of methods, including crushing the material and sorting different-sized sulfides or using magnets to separate different sulfides from one another.

      Current methods of rare metal separation rely on large quantities of energy, water, acids, and organic solvents which have costly environmental impacts, says Stinn. “We are trying to use materials that are abundant, economical, and readily available for sustainable materials separation, and we have expanded that domain to now include sulfur and sulfides.”

      Stinn and Allanore used selective sulfidation to separate out economically important metals like cobalt in recycled lithium-ion batteries. They also used their techniques to separate dysprosium — a rare-earth element used in applications ranging from data storage devices to optoelectronics — from rare-earth-boron magnets, or from the typical mixture of oxides available from mining minerals such as bastnaesite.

      Leveraging existing technology

      Metals like cobalt and rare earths are only found in small amounts in mined materials, so industries must process large volumes of material to retrieve or recycle enough of these metals to be economically viable, Allanore explains. “It’s quite clear that these processes are not efficient. Most of the emissions come from the lack of selectivity and the low concentration at which they operate.”

      By eliminating the need for liquid separation and the extra steps and materials it requires to dissolve and then reprecipitate individual elements, the MIT researchers’ process significantly reduces the costs incurred and emissions produced during separation.

      “One of the nice things about separating materials using sulfidation is that a lot of existing technology and process infrastructure can be leveraged,” Stinn says. “It’s new conditions and new chemistries in established reactor styles and equipment.”

      The next step is to show that the process can work for large amounts of raw material — separating out 16 elements from rare-earth mining streams, for example. “Now we have shown that we can handle three or four or five of them together, but we have not yet processed an actual stream from an existing mine at a scale to match what’s required for deployment,” Allanore says.

      Stinn and colleagues in the lab have built a reactor that can process about 10 kilograms of raw material per day, and the researchers are starting conversations with several corporations about the possibilities.

      “We are discussing what it would take to demonstrate the performance of this approach with existing mineral and recycling streams,” Allanore says.

      This research was supported by the U.S. Department of Energy and the U.S. National Science Foundation. More

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      A tool to speed development of new solar cells

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      In the ongoing race to develop ever-better materials and configurations for solar cells, there are many variables that can be adjusted to try to improve performance, including material type, thickness, and geometric arrangement. Developing new solar cells has generally been a tedious process of making small changes to one of these parameters at a time. While computational simulators have made it possible to evaluate such changes without having to actually build each new variation for testing, the process remains slow.

      Now, researchers at MIT and Google Brain have developed a system that makes it possible not just to evaluate one proposed design at a time, but to provide information about which changes will provide the desired improvements. This could greatly increase the rate for the discovery of new, improved configurations.

      The new system, called a differentiable solar cell simulator, is described in a paper published today in the journal Computer Physics Communications, written by MIT junior Sean Mann, research scientist Giuseppe Romano of MIT’s Institute for Soldier Nanotechnologies, and four others at MIT and at Google Brain.

      Traditional solar cell simulators, Romano explains, take the details of a solar cell configuration and produce as their output a predicted efficiency — that is, what percentage of the energy of incoming sunlight actually gets converted to an electric current. But this new simulator both predicts the efficiency and shows how much that output is affected by any one of the input parameters. “It tells you directly what happens to the efficiency if we make this layer a little bit thicker, or what happens to the efficiency if we for example change the property of the material,” he says.

      In short, he says, “we didn’t discover a new device, but we developed a tool that will enable others to discover more quickly other higher performance devices.” Using this system, “we are decreasing the number of times that we need to run a simulator to give quicker access to a wider space of optimized structures.” In addition, he says, “our tool can identify a unique set of material parameters that has been hidden so far because it’s very complex to run those simulations.”

      While traditional approaches use essentially a random search of possible variations, Mann says, with his tool “we can follow a trajectory of change because the simulator tells you what direction you want to be changing your device. That makes the process much faster because instead of exploring the entire space of opportunities, you can just follow a single path” that leads directly to improved performance.

      Since advanced solar cells often are composed of multiple layers interlaced with conductive materials to carry electric charge from one to the other, this computational tool reveals how changing the relative thicknesses of these different layers will affect the device’s output. “This is very important because the thickness is critical. There is a strong interplay between light propagation and the thickness of each layer and the absorption of each layer,” Mann explains.

      Other variables that can be evaluated include the amount of doping (the introduction of atoms of another element) that each layer receives, or the dielectric constant of insulating layers, or the bandgap, a measure of the energy levels of photons of light that can be captured by different materials used in the layers.

      This simulator is now available as an open-source tool that can be used immediately to help guide research in this field, Romano says. “It is ready, and can be taken up by industry experts.” To make use of it, researchers would couple this device’s computations with an optimization algorithm, or even a machine learning system, to rapidly assess a wide variety of possible changes and home in quickly on the most promising alternatives.

      At this point, the simulator is based on just a one-dimensional version of the solar cell, so the next step will be to expand its capabilities to include two- and three-dimensional configurations. But even this 1D version “can cover the majority of cells that are currently under production,” Romano says. Certain variations, such as so-called tandem cells using different materials, cannot yet be simulated directly by this tool, but “there are ways to approximate a tandem solar cell by simulating each of the individual cells,” Mann says.

      The simulator is “end-to-end,” Romano says, meaning it computes the sensitivity of the efficiency, also taking into account light absorption. He adds: “An appealing future direction is composing our simulator with advanced existing differentiable light-propagation simulators, to achieve enhanced accuracy.”

      Moving forward, Romano says, because this is an open-source code, “that means that once it’s up there, the community can contribute to it. And that’s why we are really excited.” Although this research group is “just a handful of people,” he says, now anyone working in the field can make their own enhancements and improvements to the code and introduce new capabilities.

      “Differentiable physics is going to provide new capabilities for the simulations of engineered systems,” says Venkat Viswanathan, an associate professor of mechanical engineering at Carnegie Mellon University, who was not associated with this work. “The  differentiable solar cell simulator is an incredible example of differentiable physics, that can now provide new capabilities to optimize solar cell device performance,” he says, calling the study “an exciting step forward.”

      In addition to Mann and Romano, the team included Eric Fadel and Steven Johnson at MIT, and Samuel Schoenholz and Ekin Cubuk at Google Brain. The work was supported in part by Eni S.p.A. and the MIT Energy Initiative, and the MIT Quest for Intelligence. More

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      Making catalytic surfaces more active to help decarbonize fuels and chemicals

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      Electrochemical reactions that are accelerated using catalysts lie at the heart of many processes for making and using fuels, chemicals, and materials — including storing electricity from renewable energy sources in chemical bonds, an important capability for decarbonizing transportation fuels. Now, research at MIT could open the door to ways of making certain catalysts more active, and thus enhancing the efficiency of such processes.

      A new production process yielded catalysts that increased the efficiency of the chemical reactions by fivefold, potentially enabling useful new processes in biochemistry, organic chemistry, environmental chemistry, and electrochemistry. The findings are described today in the journal Nature Catalysis, in a paper by Yang Shao-Horn, an MIT professor of mechanical engineering and of materials science and engineering, and a member of the Research Lab of Electronics (RLE); Tao Wang, a postdoc in RLE; Yirui Zhang, a graduate student in the Department of Mechanical Engineering; and five others.

      The process involves adding a layer of what’s called an ionic liquid in between a gold or platinum catalyst and a chemical feedstock. Catalysts produced with this method could potentially enable much more efficient conversion of hydrogen fuel to power devices such as fuel cells, or more efficient conversion of carbon dioxide into fuels.

      “There is an urgent need to decarbonize how we power transportation beyond light-duty vehicles, how we make fuels, and how we make materials and chemicals,” says Shao-Horn, emphasizing the pressing call to reduce carbon emissions highlighted in the latest IPCC report on climate change. This new approach to enhancing catalytic activity could provide an important step in that direction, she says.

      Using hydrogen in electrochemical devices such as fuel cells is one promising approach to decarbonizing fields such as aviation and heavy-duty vehicles, and the new process may help to make such uses practical. At present, the oxygen reduction reaction that powers such fuel cells is limited by its inefficiency. Previous attempts to improve that efficiency have focused on choosing different catalyst materials or modifying their surface compositions and structure.

      In this research, however, instead of modifying the solid surfaces, the team added a thin layer in between the catalyst and the electrolyte, the active material that participates in the chemical reaction. The ionic liquid layer, they found, regulates the activity of protons that help to increase the rate of the chemical reactions taking place on the interface.

      Because there is a great variety of such ionic liquids to choose from, it’s possible to “tune” proton activity and the reaction rates to match the energetics needed for processes involving proton transfer, which can be used to make fuels and chemicals through reactions with oxygen.

      “The proton activity and the barrier for proton transfer is governed by the ionic liquid layer, and so there’s a great tuneability in terms of catalytic activity for reactions involving proton and electron transfer,” Shao-Horn says. And the effect is produced by a vanishingly thin layer of the liquid, just a few nanometers thick, above which is a much thicker layer of the liquid that is to undergo the reaction.

      “I think this concept is novel and important,” says Wang, the paper’s first author, “because people know the proton activity is important in many electrochemistry reactions, but it’s very challenging to study.” That’s because in a water environment, there are so many interactions between neighboring water molecules involved that it’s very difficult to separate out which reactions are taking place. By using an ionic liquid, whose ions can each only form a single bond with the intermediate material, it became possible to study the reactions in detail, using infrared spectroscopy.

      As a result, Wang says, “Our finding highlights the critical role that interfacial electrolytes, in particular the intermolecular hydrogen bonding, can play in enhancing the activity of the electro-catalytic process. It also provides fundamental insights into proton transfer mechanisms at a quantum mechanical level, which can push the frontiers of knowing how protons and electrons interact at catalytic interfaces.”

      “The work is also exciting because it gives people a design principle for how they can tune the catalysts,” says Zhang. “We need some species right at a ‘sweet spot’ — not too active or too inert — to enhance the reaction rate.”

      With some of these techniques, says Reshma Rao, a recent doctoral graduate from MIT and now a postdoc at Imperial College, London, who is also a co-author of the paper, “we see up to a five-times increase in activity. I think the most exciting part of this research is the way it opens up a whole new dimension in the way we think about catalysis.” The field had hit “a kind of roadblock,” she says, in finding ways to design better materials. By focusing on the liquid layer rather than the surface of the material, “that’s kind of a whole different way of looking at this problem, and opens up a whole new dimension, a whole new axis along which we can change things and optimize some of these reaction rates.”

      The team also included Botao Huang, Bin Cai, and Livia Giordano in the MIT’s Research Laboratory of Electronics, and Shi-Gang Sun at Xiamen University in China. The work was supported by the Toyota Research Institute, and used the National Science Foundation’s Extreme Science and Engineering Environment. More

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      in Energy

      Using aluminum and water to make clean hydrogen fuel — when and where it’s needed

      by

      As the world works to move away from fossil fuels, many researchers are investigating whether clean hydrogen fuel can play an expanded role in sectors from transportation and industry to buildings and power generation. It could be used in fuel cell vehicles, heat-producing boilers, electricity-generating gas turbines, systems for storing renewable energy, and more.

      But while using hydrogen doesn’t generate carbon emissions, making it typically does. Today, almost all hydrogen is produced using fossil fuel-based processes that together generate more than 2 percent of all global greenhouse gas emissions. In addition, hydrogen is often produced in one location and consumed in another, which means its use also presents logistical challenges.

      A promising reaction

      Another option for producing hydrogen comes from a perhaps surprising source: reacting aluminum with water. Aluminum metal will readily react with water at room temperature to form aluminum hydroxide and hydrogen. That reaction doesn’t typically take place because a layer of aluminum oxide naturally coats the raw metal, preventing it from coming directly into contact with water.

      Using the aluminum-water reaction to generate hydrogen doesn’t produce any greenhouse gas emissions, and it promises to solve the transportation problem for any location with available water. Simply move the aluminum and then react it with water on-site. “Fundamentally, the aluminum becomes a mechanism for storing hydrogen — and a very effective one,” says Douglas P. Hart, professor of mechanical engineering at MIT. “Using aluminum as our source, we can ‘store’ hydrogen at a density that’s 10 times greater than if we just store it as a compressed gas.”

      Two problems have kept aluminum from being employed as a safe, economical source for hydrogen generation. The first problem is ensuring that the aluminum surface is clean and available to react with water. To that end, a practical system must include a means of first modifying the oxide layer and then keeping it from re-forming as the reaction proceeds.

      The second problem is that pure aluminum is energy-intensive to mine and produce, so any practical approach needs to use scrap aluminum from various sources. But scrap aluminum is not an easy starting material. It typically occurs in an alloyed form, meaning that it contains other elements that are added to change the properties or characteristics of the aluminum for different uses. For example, adding magnesium increases strength and corrosion-resistance, adding silicon lowers the melting point, and adding a little of both makes an alloy that’s moderately strong and corrosion-resistant.

      Despite considerable research on aluminum as a source of hydrogen, two key questions remain: What’s the best way to prevent the adherence of an oxide layer on the aluminum surface, and how do alloying elements in a piece of scrap aluminum affect the total amount of hydrogen generated and the rate at which it is generated?

      “If we’re going to use scrap aluminum for hydrogen generation in a practical application, we need to be able to better predict what hydrogen generation characteristics we’re going to observe from the aluminum-water reaction,” says Laureen Meroueh PhD ’20, who earned her doctorate in mechanical engineering.

      Since the fundamental steps in the reaction aren’t well understood, it’s been hard to predict the rate and volume at which hydrogen forms from scrap aluminum, which can contain varying types and concentrations of alloying elements. So Hart, Meroueh, and Thomas W. Eagar, a professor of materials engineering and engineering management in the MIT Department of Materials Science and Engineering, decided to examine — in a systematic fashion — the impacts of those alloying elements on the aluminum-water reaction and on a promising technique for preventing the formation of the interfering oxide layer.

      To prepare, they had experts at Novelis Inc. fabricate samples of pure aluminum and of specific aluminum alloys made of commercially pure aluminum combined with either 0.6 percent silicon (by weight), 1 percent magnesium, or both — compositions that are typical of scrap aluminum from a variety of sources. Using those samples, the MIT researchers performed a series of tests to explore different aspects of the aluminum-water reaction.

      Pre-treating the aluminum

      The first step was to demonstrate an effective means of penetrating the oxide layer that forms on aluminum in the air. Solid aluminum is made up of tiny grains that are packed together with occasional boundaries where they don’t line up perfectly. To maximize hydrogen production, researchers would need to prevent the formation of the oxide layer on all those interior grain surfaces.

      Research groups have already tried various ways of keeping the aluminum grains “activated” for reaction with water. Some have crushed scrap samples into particles so tiny that the oxide layer doesn’t adhere. But aluminum powders are dangerous, as they can react with humidity and explode. Another approach calls for grinding up scrap samples and adding liquid metals to prevent oxide deposition. But grinding is a costly and energy-intensive process.

      To Hart, Meroueh, and Eagar, the most promising approach — first introduced by Jonathan Slocum ScD ’18 while he was working in Hart’s research group — involved pre-treating the solid aluminum by painting liquid metals on top and allowing them to permeate through the grain boundaries.

      To determine the effectiveness of that approach, the researchers needed to confirm that the liquid metals would reach the internal grain surfaces, with and without alloying elements present. And they had to establish how long it would take for the liquid metal to coat all of the grains in pure aluminum and its alloys.

      They started by combining two metals — gallium and indium — in specific proportions to create a “eutectic” mixture; that is, a mixture that would remain in liquid form at room temperature. They coated their samples with the eutectic and allowed it to penetrate for time periods ranging from 48 to 96 hours. They then exposed the samples to water and monitored the hydrogen yield (the amount formed) and flow rate for 250 minutes. After 48 hours, they also took high-magnification scanning electron microscope (SEM) images so they could observe the boundaries between adjacent aluminum grains.

      Based on the hydrogen yield measurements and the SEM images, the MIT team concluded that the gallium-indium eutectic does naturally permeate and reach the interior grain surfaces. However, the rate and extent of penetration vary with the alloy. The permeation rate was the same in silicon-doped aluminum samples as in pure aluminum samples but slower in magnesium-doped samples.

      Perhaps most interesting were the results from samples doped with both silicon and magnesium — an aluminum alloy often found in recycling streams. Silicon and magnesium chemically bond to form magnesium silicide, which occurs as solid deposits on the internal grain surfaces. Meroueh hypothesized that when both silicon and magnesium are present in scrap aluminum, those deposits can act as barriers that impede the flow of the gallium-indium eutectic.

      The experiments and images confirmed her hypothesis: The solid deposits did act as barriers, and images of samples pre-treated for 48 hours showed that permeation wasn’t complete. Clearly, a lengthy pre-treatment period would be critical for maximizing the hydrogen yield from scraps of aluminum containing both silicon and magnesium.

      Meroueh cites several benefits to the process they used. “You don’t have to apply any energy for the gallium-indium eutectic to work its magic on aluminum and get rid of that oxide layer,” she says. “Once you’ve activated your aluminum, you can drop it in water, and it’ll generate hydrogen — no energy input required.” Even better, the eutectic doesn’t chemically react with the aluminum. “It just physically moves around in between the grains,” she says. “At the end of the process, I could recover all of the gallium and indium I put in and use it again” — a valuable feature as gallium and (especially) indium are costly and in relatively short supply.

      Impacts of alloying elements on hydrogen generation

      The researchers next investigated how the presence of alloying elements affects hydrogen generation. They tested samples that had been treated with the eutectic for 96 hours; by then, the hydrogen yield and flow rates had leveled off in all the samples.

      The presence of 0.6 percent silicon increased the hydrogen yield for a given weight of aluminum by 20 percent compared to pure aluminum — even though the silicon-containing sample had less aluminum than the pure aluminum sample. In contrast, the presence of 1 percent magnesium produced far less hydrogen, while adding both silicon and magnesium pushed the yield up, but not to the level of pure aluminum.

      The presence of silicon also greatly accelerated the reaction rate, producing a far higher peak in the flow rate but cutting short the duration of hydrogen output. The presence of magnesium produced a lower flow rate but allowed the hydrogen output to remain fairly steady over time. And once again, aluminum with both alloying elements produced a flow rate between that of magnesium-doped and pure aluminum.

      Those results provide practical guidance on how to adjust the hydrogen output to match the operating needs of a hydrogen-consuming device. If the starting material is commercially pure aluminum, adding small amounts of carefully selected alloying elements can tailor the hydrogen yield and flow rate. If the starting material is scrap aluminum, careful choice of the source can be key. For high, brief bursts of hydrogen, pieces of silicon-containing aluminum from an auto junkyard could work well. For lower but longer flows, magnesium-containing scraps from the frame of a demolished building might be better. For results somewhere in between, aluminum containing both silicon and magnesium should work well; such material is abundantly available from scrapped cars and motorcycles, yachts, bicycle frames, and even smartphone cases.

      It should also be possible to combine scraps of different aluminum alloys to tune the outcome, notes Meroueh. “If I have a sample of activated aluminum that contains just silicon and another sample that contains just magnesium, I can put them both into a container of water and let them react,” she says. “So I get the fast ramp-up in hydrogen production from the silicon and then the magnesium takes over and has that steady output.”

      Another opportunity for tuning: Reducing grain size

      Another practical way to affect hydrogen production could be to reduce the size of the aluminum grains — a change that should increase the total surface area available for reactions to occur.

      To investigate that approach, the researchers requested specially customized samples from their supplier. Using standard industrial procedures, the Novelis experts first fed each sample through two rollers, squeezing it from the top and bottom so that the internal grains were flattened. They then heated each sample until the long, flat grains had reorganized and shrunk to a targeted size.

      In a series of carefully designed experiments, the MIT team found that reducing the grain size increased the efficiency and decreased the duration of the reaction to varying degrees in the different samples. Again, the presence of particular alloying elements had a major effect on the outcome.

      Needed: A revised theory that explains observations

      Throughout their experiments, the researchers encountered some unexpected results. For example, standard corrosion theory predicts that pure aluminum will generate more hydrogen than silicon-doped aluminum will — the opposite of what they observed in their experiments.

      To shed light on the underlying chemical reactions, Hart, Meroueh, and Eagar investigated hydrogen “flux,” that is, the volume of hydrogen generated over time on each square centimeter of aluminum surface, including the interior grains. They examined three grain sizes for each of their four compositions and collected thousands of data points measuring hydrogen flux.

      Their results show that reducing grain size has significant effects. It increases the peak hydrogen flux from silicon-doped aluminum as much as 100 times and from the other three compositions by 10 times. With both pure aluminum and silicon-containing aluminum, reducing grain size also decreases the delay before the peak flux and increases the rate of decline afterward. With magnesium-containing aluminum, reducing the grain size brings about an increase in peak hydrogen flux and results in a slightly faster decline in the rate of hydrogen output. With both silicon and magnesium present, the hydrogen flux over time resembles that of magnesium-containing aluminum when the grain size is not manipulated. When the grain size is reduced, the hydrogen output characteristics begin to resemble behavior observed in silicon-containing aluminum. That outcome was unexpected because when silicon and magnesium are both present, they react to form magnesium silicide, resulting in a new type of aluminum alloy with its own properties.

      The researchers stress the benefits of developing a better fundamental understanding of the underlying chemical reactions involved. In addition to guiding the design of practical systems, it might help them find a replacement for the expensive indium in their pre-treatment mixture. Other work has shown that gallium will naturally permeate through the grain boundaries of aluminum. “At this point, we know that the indium in our eutectic is important, but we don’t really understand what it does, so we don’t know how to replace it,” says Hart.

      But already Hart, Meroueh, and Eagar have demonstrated two practical ways of tuning the hydrogen reaction rate: by adding certain elements to the aluminum and by manipulating the size of the interior aluminum grains. In combination, those approaches can deliver significant results. “If you go from magnesium-containing aluminum with the largest grain size to silicon-containing aluminum with the smallest grain size, you get a hydrogen reaction rate that differs by two orders of magnitude,” says Meroueh. “That’s huge if you’re trying to design a real system that would use this reaction.”

      This research was supported through the MIT Energy Initiative by ExxonMobil-MIT Energy Fellowships awarded to Laureen Meroueh PhD ’20 from 2018 to 2020.

      This article appears in the Spring 2021 issue of Energy Futures, the magazine of the MIT Energy Initiative.  More