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      in Energy

      A more sustainable way to generate phosphorus

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      Phosphorus is an essential ingredient in thousands of products, including herbicides, lithium-ion batteries, and even soft drinks. Most of this phosphorus comes from an energy-intensive process that contributes significantly to global carbon emissions.

      In an effort to reduce that carbon footprint, MIT chemists have devised an alternative way to generate white phosphorus, a critical intermediate in the manufacture of those phosphorus-containing products. Their approach, which uses electricity to speed up a key chemical reaction, could reduce the carbon emissions of the process by half or even more, the researchers say.

      “White phosphorus is currently an indispensable intermediate, and our process dramatically reduces the carbon footprint of converting phosphate to white phosphorus,” says Yogesh Surendranath, an associate professor of chemistry at MIT and the senior author of the study.

      The new process reduces the carbon footprint of white phosphorus production in two ways: It reduces the temperatures required for the reaction, and it generates significantly less carbon dioxide as a waste product.

      Recent MIT graduate Jonathan “Jo” Melville PhD ’21 and MIT graduate student Andrew Licini are the lead authors of the paper, which appears today in ACS Central Science.

      Purifying phosphorus

      When phosphorus is mined out of the ground, it is in the form of phosphate, a mineral whose basic unit comprises one atom of phosphorus bound to four oxygen atoms. About 95 percent of this phosphate ore is used to make fertilizer. The remaining phosphate ore is processed separately into white phosphorus, a molecule composed of four phosphorus atoms bound to each other. White phosphorus is then fed into a variety of chemical processes that are used to manufacture many different products, such as lithium battery electrolytes and semiconductor dopants.

      Converting those mined phosphates into white phosphorus accounts for a substantial fraction of the carbon footprint of the entire phosphorus industry, Surendranath says. The most energy-intensive part of the process is breaking the bonds between phosphorus and oxygen, which are very stable.

      Using the traditional “thermal process,” those bonds are broken by heating carbon coke and phosphate rock to a temperature of 1,500 degrees Celsius. In this process, the carbon serves to strip away the oxygen atoms from phosphorus, leading to the eventual generation of CO2 as a byproduct. In addition, sustaining those temperatures requires a great deal of energy, adding to the carbon footprint of the process.

      “That process hasn’t changed substantially since its inception over a century ago. Our goal was to figure out how we could develop a process that would substantially lower the carbon footprint of this process,” Surendranath says. “The idea was to combine it with renewable electricity and drive that conversion of phosphate to white phosphorus with electrons rather than using carbon.”

      To do that, the researchers had to come up with an alternative way to weaken the strong phosphorus-oxygen bonds found in phosphates. They achieved this by controlling the environment in which the reaction occurs. The researchers found that the reaction could be promoted using a dehydrated form of phosphoric acid, which contains long chains of phosphate salts held together by bonds called phosphoryl anhydrides. These bonds help to weaken the phosphorus-oxygen bonds.

      When the researchers run an electric current through these salts, electrons break the weakened bonds, allowing the phosphorus atoms to break free and bind to each other to form white phosphorus. At the temperatures needed for this system (about 800 C), phosphorus exists as a gas, so it can bubble out of the solution and be collected in an external chamber.

      Decarbonization

      The electrode that the researchers used for this demonstration relies on carbon as a source of electrons, so the process generates some carbon dioxide as a byproduct. However, they are now working on swapping that electrode out for one that would use phosphate itself as the electron source, which would further reduce the carbon footprint by cleanly separating phosphate into phosphorus and oxygen.

      With the process reported in this paper, the researchers have reduced the overall carbon footprint for generating white phosphorus by about 50 percent. With future modifications, they hope to bring the carbon emissions down to nearly zero, in part by using renewable energy such as solar or wind power to drive the electric current required.

      If the researchers succeed in scaling up their process and making it widely available, it could allow industrial users to generate white phosphorus on site instead of having it shipped from the few places in the world where it is currently manufactured. That would cut down on the risks of transporting white phosphorus, which is an explosive material.

      “We’re excited about the prospect of doing on-site generation of this intermediate, so you don’t have to do the transportation and distribution,” Surendranath says. “If you could decentralize this production, the end user could make it on site and use it in an integrated fashion.”

      In order to do this study, the researchers had to develop new tools for controlling the electrolytes (such as salts and acids) present in the environment, and for measuring how those electrolytes affect the reaction. Now, they plan to use the same approach to try to develop lower-carbon processes for isolating other industrially important elements, such as silicon and iron.

      “This work falls within our broader interests in decarbonizing these legacy industrial processes that have a huge carbon footprint,” Surendranath says. “The basic science that leads us there is understanding how you can tailor the electrolytes to foster these processes.”

      The research was funded by the UMRP Partnership for Progress on Sustainable Development in Africa, a fellowship from the MIT Tata Center for Technology and Design, and a National Defense Science and Engineering Graduate Fellowship. More

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      in Energy

      How to pull carbon dioxide out of seawater

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      As carbon dioxide continues to build up in the Earth’s atmosphere, research teams around the world have spent years seeking ways to remove the gas efficiently from the air. Meanwhile, the world’s number one “sink” for carbon dioxide from the atmosphere is the ocean, which soaks up some 30 to 40 percent of all of the gas produced by human activities.

      Recently, the possibility of removing carbon dioxide directly from ocean water has emerged as another promising possibility for mitigating CO2 emissions, one that could potentially someday even lead to overall net negative emissions. But, like air capture systems, the idea has not yet led to any widespread use, though there are a few companies attempting to enter this area.

      Now, a team of researchers at MIT says they may have found the key to a truly efficient and inexpensive removal mechanism. The findings were reported this week in the journal Energy and Environmental Science, in a paper by MIT professors T. Alan Hatton and Kripa Varanasi, postdoc Seoni Kim, and graduate students Michael Nitzsche, Simon Rufer, and Jack Lake.

      The existing methods for removing carbon dioxide from seawater apply a voltage across a stack of membranes to acidify a feed stream by water splitting. This converts bicarbonates in the water to molecules of CO2, which can then be removed under vacuum. Hatton, who is the Ralph Landau Professor of Chemical Engineering, notes that the membranes are expensive, and chemicals are required to drive the overall electrode reactions at either end of the stack, adding further to the expense and complexity of the processes. “We wanted to avoid the need for introducing chemicals to the anode and cathode half cells and to avoid the use of membranes if at all possible” he says.

      The team came up with a reversible process consisting of membrane-free electrochemical cells. Reactive electrodes are used to release protons to the seawater fed to the cells, driving the release of the dissolved carbon dioxide from the water. The process is cyclic: It first acidifies the water to convert dissolved inorganic bicarbonates to molecular carbon dioxide, which is collected as a gas under vacuum. Then, the water is fed to a second set of cells with a reversed voltage, to recover the protons and turn the acidic water back to alkaline before releasing it back to the sea. Periodically, the roles of the two cells are reversed once one set of electrodes is depleted of protons (during acidification) and the other has been regenerated during alkalization.

      This removal of carbon dioxide and reinjection of alkaline water could slowly start to reverse, at least locally, the acidification of the oceans that has been caused by carbon dioxide buildup, which in turn has threatened coral reefs and shellfish, says Varanasi, a professor of mechanical engineering. The reinjection of alkaline water could be done through dispersed outlets or far offshore to avoid a local spike of alkalinity that could disrupt ecosystems, they say.

      “We’re not going to be able to treat the entire planet’s emissions,” Varanasi says. But the reinjection might be done in some cases in places such as fish farms, which tend to acidify the water, so this could be a way of helping to counter that effect.

      Once the carbon dioxide is removed from the water, it still needs to be disposed of, as with other carbon removal processes. For example, it can be buried in deep geologic formations under the sea floor, or it can be chemically converted into a compound like ethanol, which can be used as a transportation fuel, or into other specialty chemicals. “You can certainly consider using the captured CO2 as a feedstock for chemicals or materials production, but you’re not going to be able to use all of it as a feedstock,” says Hatton. “You’ll run out of markets for all the products you produce, so no matter what, a significant amount of the captured CO2 will need to be buried underground.”

      Initially at least, the idea would be to couple such systems with existing or planned infrastructure that already processes seawater, such as desalination plants. “This system is scalable so that we could integrate it potentially into existing processes that are already processing ocean water or in contact with ocean water,” Varanasi says. There, the carbon dioxide removal could be a simple add-on to existing processes, which already return vast amounts of water to the sea, and it would not require consumables like chemical additives or membranes.

      “With desalination plants, you’re already pumping all the water, so why not co-locate there?” Varanasi says. “A bunch of capital costs associated with the way you move the water, and the permitting, all that could already be taken care of.”

      The system could also be implemented by ships that would process water as they travel, in order to help mitigate the significant contribution of ship traffic to overall emissions. There are already international mandates to lower shipping’s emissions, and “this could help shipping companies offset some of their emissions, and turn ships into ocean scrubbers,” Varanasi says.

      The system could also be implemented at locations such as offshore drilling platforms, or at aquaculture farms. Eventually, it could lead to a deployment of free-standing carbon removal plants distributed globally.

      The process could be more efficient than air-capture systems, Hatton says, because the concentration of carbon dioxide in seawater is more than 100 times greater than it is in air. In direct air-capture systems it is first necessary to capture and concentrate the gas before recovering it. “The oceans are large carbon sinks, however, so the capture step has already kind of been done for you,” he says. “There’s no capture step, only release.” That means the volumes of material that need to be handled are much smaller, potentially simplifying the whole process and reducing the footprint requirements.

      The research is continuing, with one goal being to find an alternative to the present step that requires a vacuum to remove the separated carbon dioxide from the water. Another need is to identify operating strategies to prevent precipitation of minerals that can foul the electrodes in the alkalinization cell, an inherent issue that reduces the overall efficiency in all reported approaches. Hatton notes that significant progress has been made on these issues, but that it is still too early to report on them. The team expects that the system could be ready for a practical demonstration project within about two years.

      “The carbon dioxide problem is the defining problem of our life, of our existence,” Varanasi says. “So clearly, we need all the help we can get.”

      The work was supported by ARPA-E. More

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      in Energy

      Decarbonization amid global crises

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      A global pandemic. Russia’s invasion of Ukraine. Inflation. The first-ever serious challenge to the peaceful transfer of power in the United States.

      Forced to face a seemingly unending series of once-in-a-generation crises, how can the world continue to focus attention on goals around carbon emissions and climate change? That was the question posed by Philip R. Sharp, the former president of Resources for the Future and a former 10-term member of the U.S. House of Representatives from Indiana, during his MIT Energy Initiative Fall Colloquium address, entitled “The prospects for decarbonization in America: Will global and domestic crises disrupt our plans?”

      Perhaps surprisingly, Sharp sounded an optimistic note in his answer. Despite deep political divisions in the United States, he noted, Congress has passed five major pieces of legislation — under both presidents Donald Trump and Joseph Biden — aimed at accelerating decarbonization efforts. Rather than hampering movement to combat climate change, Sharp said, domestic and global crises have seemed to galvanize support, create new incentives for action, and even unify political rivals around the cause.

      “Almost everybody is dealing with, to some degree, the absolutely profound, churning events that we are amidst now. Most of them are unexpected, and therefore [we’re] not prepared for [them], and they have had a profound shaking of our thinking,” Sharp said. “The conventional wisdom has not held up in almost all of these areas, and therefore it makes it much more difficult for us to think we know how to predict an uncertain future, and [it causes us to] question our own ability as a nation — or anywhere — to actually take on these challenges. And obviously, climate change is one of the most important.”

      However, Sharp continued, these challenges have, in some instances, spurred action. The war in Ukraine, he noted, has upset European energy markets, but it has also highlighted the importance of countries achieving a more energy-independent posture through renewables. “In America,” he added, “we’ve actually seen absolutely stunning … behavior by the United States Congress, of all places.”

      “What we’ve witnessed is, [Congress] put out incredible … sums of money under the previous administration, and then under this administration, to deal with the Covid crisis,” Sharp added later in his talk. “And then the United States government came together — red and blue — to support the Ukrainians against Russia. It saddens me to say, it seems to take a Russian invasion or the Chinese probing us economically to get us moving. But we are moving, and these things are happening.”

      Congressional action

      Sharp cautioned against getting “caught up” in the familiar viewpoint that Congress, in its current incarnation, is fundamentally incapable of passing meaningful legislation. He pointed, in particular, to the passage of five laws over the previous two years:

      The 2020 Energy Act, which has been characterized as a “down payment on fighting climate change.”
      The Infrastructure Investment and Jobs Act (sometimes called the “bipartisan infrastructure bill”), which calls for investments in passenger rail, electric vehicle infrastructure, electric school buses, and other clean-energy measures;
      The CHIPS and Science Act, a $280 billion effort to revitalize the American semiconductor industry, which some analysts say could direct roughly one-quarter of its funding toward accelerating zero-carbon industries and conducting climate research;
      The Inflation Reduction Act (called by some “the largest climate legislation in U.S. history”), which includes tax credits, incentives, and other provisions to help private companies tackle climate change, increase investments in renewable energy, and enhance energy efficiency; and
      The Kigali Amendment to the Montreal Protocol, ratified by the Senate to little fanfare in September, under which the United States agreed to reduce the consumption and production of hydrofluorocarbons (HFCs).
      “It is a big deal,” Sharp said of the dramatic increase in federal climate action. “It is very significant actions that are being taken — more than what we would expect, or I would expect, out of the Congress at any one time.”

      Along with the many billions of dollars of climate-related investments included in the legislation, Sharp said, these new laws will have a number of positive “spillover” effects.

      “This enables state governments, in their policies, to be more aggressive,” Sharp said. “Why? Because it makes it cheaper for some of the investments that they will try to force within their state.” Another “pretty obvious” spillover effect, Sharp said, is that the new laws will enhance U.S. credibility in international negotiations. Finally, he said, these public investments will make the U.S. economy more competitive in international markets for clean-energy technology — particularly as the United States seeks to compete against China in the space.

      “[Competition with China] has become a motivator in American politics, like it or not,” Sharp said. “There is no question that it is causing and bringing together [politicians] across blue [states] and red [states].”

      Holding onto progress

      Even in an uncertain political climate in which Democrats and Republicans seem unable to agree on basic facts, recent funding commitments are likely to survive, no matter which party controls Congress and the presidency, Sharp said. That’s because most of the legislation relies on broadly popular “carrots” that reward investments in decarbonization, rather than less popular “sticks” that create new restrictions or punishments for companies that fail to decarbonize.

      “Politically, the impact of this is very significant,” Sharp said. “It is so much easier in politics to give away tax [credits] than it is to penalize or put requirements onto people. The fact is that these tax credits are more likely to be politically sustained than other forms of government intervention. That, at least, has been the history.”

      Sharp stressed the importance of what he called “civil society” — institutions such as universities, nonprofits, churches, and other organizations that are apart from government and business — in promoting decarbonization efforts. “[Those groups] can act highly independently, and therefore, they can drive for things that others are not willing to do. Now this does not always work to good purposes. Partly, this diversity and this decentralization in civil society … led to deniers and others being able to stop some climate action. But now my view is, this is starting to all move in the right direction, in a very dynamic and a very important way. What we have seen over the last few years is a big uptick in philanthropy related to climate.”

      Looking ahead

      Sharp’s optimism even extended to the role of social media. He suggested that the “Wild West” era of social platforms may be ending, pointing to the celebrities who have recently lost valuable business partnerships for spreading hate speech and disinformation. “We’re now a lot more alert to the dangers,” he said.

      Some in the audience questioned Sharp about specific paths toward decarbonization, but Sharp said that progress will require a number of disparate approaches — some of which will inevitably have a greater impact than others. “The current policy, and the policy embedded in this [new] legislation … is all about doing both,” he said. “It’s all about advancing [current] technologies into the marketplace, and at the same time driving for breakthroughs.”

      Above all, Sharp stressed the need for continued collective action around climate change. “The fact is, we’re all contributors to some degree,” he said. “But we also all can do something. In my view, this is clearly not a time for hand-wringing. This is a time for action. People have to roll up their sleeves, and go to work, and not roll them down anytime soon.” More

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      in Environment

      Ocean microbes get their diet through a surprising mix of sources, study finds

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      One of the smallest and mightiest organisms on the planet is a plant-like bacterium known to marine biologists as Prochlorococcus. The green-tinted microbe measures less than a micron across, and its populations suffuse through the upper layers of the ocean, where a single teaspoon of seawater can hold millions of the tiny organisms.

      Prochlorococcus grows through photosynthesis, using sunlight to convert the atmosphere’s carbon dioxide into organic carbon molecules. The microbe is responsible for 5 percent of the world’s photosynthesizing activity, and scientists have assumed that photosynthesis is the microbe’s go-to strategy for acquiring the carbon it needs to grow.

      But a new MIT study in Nature Microbiology today has found that Prochlorococcus relies on another carbon-feeding strategy, more than previously thought.

      Organisms that use a mix of strategies to provide carbon are known as mixotrophs. Most marine plankton are mixotrophs. And while Prochlorococcus is known to occasionally dabble in mixotrophy, scientists have assumed the microbe primarily lives a phototrophic lifestyle.

      The new MIT study shows that in fact, Prochlorococcus may be more of a mixotroph than it lets on. The microbe may get as much as one-third of its carbon through a second strategy: consuming the dissolved remains of other dead microbes.

      The new estimate may have implications for climate models, as the microbe is a significant force in capturing and “fixing” carbon in the Earth’s atmosphere and ocean.

      “If we wish to predict what will happen to carbon fixation in a different climate, or predict where Prochlorococcus will or will not live in the future, we probably won’t get it right if we’re missing a process that accounts for one-third of the population’s carbon supply,” says Mick Follows, a professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), and its Department of Civil and Environmental Engineering.

      The study’s co-authors include first author and MIT postdoc Zhen Wu, along with collaborators from the University of Haifa, the Leibniz-Institute for Baltic Sea Research, the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, and Potsdam University.

      Persistent plankton

      Since Prochlorococcus was first discovered in the Sargasso Sea in 1986, by MIT Institute Professor Sallie “Penny” Chisholm and others, the microbe has been observed throughout the world’s oceans, inhabiting the upper sunlit layers ranging from the surface down to about 160 meters. Within this range, light levels vary, and the microbe has evolved a number of ways to photosynthesize carbon in even low-lit regions.

      The organism has also evolved ways to consume organic compounds including glucose and certain amino acids, which could help the microbe survive for limited periods of time in dark ocean regions. But surviving on organic compounds alone is a bit like only eating junk food, and there is evidence that Prochlorococcus will die after a week in regions where photosynthesis is not an option.

      And yet, researchers including Daniel Sher of the University of Haifa, who is a co-author of the new study, have observed healthy populations of Prochlorococcus that persist deep in the sunlit zone, where the light intensity should be too low to maintain a population. This suggests that the microbes must be switching to a non-photosynthesizing, mixotrophic lifestyle in order to consume other organic sources of carbon.

      “It seems that at least some Prochlorococcus are using existing organic carbon in a mixotrophic way,” Follows says. “That stimulated the question: How much?”

      What light cannot explain

      In their new paper, Follows, Wu, Sher, and their colleagues looked to quantify the amount of carbon that Prochlorococcus is consuming through processes other than photosynthesis.

      The team looked first to measurements taken by Sher’s team, which previously took ocean samples at various depths in the Mediterranean Sea and measured the concentration of phytoplankton, including Prochlorococcus, along with the associated intensity of light and the concentration of nitrogen — an essential nutrient that is richly available in deeper layers of the ocean and that plankton can assimilate to make proteins.

      Wu and Follows used this data, and similar information from the Pacific Ocean, along with previous work from Chisholm’s lab, which established the rate of photosynthesis that Prochlorococcus could carry out in a given intensity of light.

      “We converted that light intensity profile into a potential growth rate — how fast the population of Prochlorococcus could grow if it was acquiring all it’s carbon by photosynthesis, and light is the limiting factor,” Follows explains.

      The team then compared this calculated rate to growth rates that were previously observed in the Pacific Ocean by several other research teams.

      “This data showed that, below a certain depth, there’s a lot of growth happening that photosynthesis simply cannot explain,” Follows says. “Some other process must be at work to make up the difference in carbon supply.”

      The researchers inferred that, in deeper, darker regions of the ocean, Prochlorococcus populations are able to survive and thrive by resorting to mixotrophy, including consuming organic carbon from detritus. Specifically, the microbe may be carrying out osmotrophy — a process by which an organism passively absorbs organic carbon molecules via osmosis.

      Judging by how fast the microbe is estimated to be growing below the sunlit zone, the team calculates that Prochlorococcus obtains up to one-third of its carbon diet through mixotrophic strategies.

      “It’s kind of like going from a specialist to a generalist lifestyle,” Follows says. “If I only eat pizza, then if I’m 20 miles from a pizza place, I’m in trouble, whereas if I eat burgers as well, I could go to the nearby McDonald’s. People had thought of Prochlorococcus as a specialist, where they do this one thing (photosynthesis) really well. But it turns out they may have more of a generalist lifestyle than we previously thought.”

      Chisholm, who has both literally and figuratively written the book on Prochlorococcus, says the group’s findings “expand the range of conditions under which their populations can not only survive, but also thrive. This study changes the way we think about the role of Prochlorococcus in the microbial food web.”

      This research was supported, in part, by the Israel Science Foundation, the U.S. National Science Foundation, and the Simons Foundation. More

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      in Environment

      Small eddies play a big role in feeding ocean microbes

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      Subtropical gyres are enormous rotating ocean currents that generate sustained circulations in the Earth’s subtropical regions just to the north and south of the equator. These gyres are slow-moving whirlpools that circulate within massive basins around the world, gathering up nutrients, organisms, and sometimes trash, as the currents rotate from coast to coast.

      For years, oceanographers have puzzled over conflicting observations within subtropical gyres. At the surface, these massive currents appear to host healthy populations of phytoplankton — microbes that feed the rest of the ocean food chain and are responsible for sucking up a significant portion of the atmosphere’s carbon dioxide.

      But judging from what scientists know about the dynamics of gyres, they estimated the currents themselves wouldn’t be able to maintain enough nutrients to sustain the phytoplankton they were seeing. How, then, were the microbes able to thrive?

      Now, MIT researchers have found that phytoplankton may receive deliveries of nutrients from outside the gyres, and that the delivery vehicle is in the form of eddies — much smaller currents that swirl at the edges of a gyre. These eddies pull nutrients in from high-nutrient equatorial regions and push them into the center of a gyre, where the nutrients are then taken up by other currents and pumped to the surface to feed phytoplankton.

      Ocean eddies, the team found, appear to be an important source of nutrients in subtropical gyres. Their replenishing effect, which the researchers call a “nutrient relay,” helps maintain populations of phytoplankton, which play a central role in the ocean’s ability to sequester carbon from the atmosphere. While climate models tend to project a decline in the ocean’s ability to sequester carbon over the coming decades, this “nutrient relay” could help sustain carbon storage over the subtropical oceans.

      “There’s a lot of uncertainty about how the carbon cycle of the ocean will evolve as climate continues to change, ” says Mukund Gupta, a postdoc at Caltech who led the study as a graduate student at MIT. “As our paper shows, getting the carbon distribution right is not straightforward, and depends on understanding the role of eddies and other fine-scale motions in the ocean.”

      Gupta and his colleagues report their findings this week in the Proceedings of the National Academy of Sciences. The study’s co-authors are Jonathan Lauderdale, Oliver Jahn, Christopher Hill, Stephanie Dutkiewicz, and Michael Follows at MIT, and Richard Williams at the University of Liverpool.

      A snowy puzzle

      A cross-section of an ocean gyre resembles a stack of nesting bowls that is stratified by density: Warmer, lighter layers lie at the surface, while colder, denser waters make up deeper layers. Phytoplankton live within the ocean’s top sunlit layers, where the microbes require sunlight, warm temperatures, and nutrients to grow.

      When phytoplankton die, they sink through the ocean’s layers as “marine snow.” Some of this snow releases nutrients back into the current, where they are pumped back up to feed new microbes. The rest of the snow sinks out of the gyre, down to the deepest layers of the ocean. The deeper the snow sinks, the more difficult it is for it to be pumped back to the surface. The snow is then trapped, or sequestered, along with any unreleased carbon and nutrients.

      Oceanographers thought that the main source of nutrients in subtropical gyres came from recirculating marine snow. But as a portion of this snow inevitably sinks to the bottom, there must be another source of nutrients to explain the healthy populations of phytoplankton at the surface. Exactly what that source is “has left the oceanography community a little puzzled for some time,” Gupta says.

      Swirls at the edge

      In their new study, the team sought to simulate a subtropical gyre to see what other dynamics may be at work. They focused on the North Pacific gyre, one of the Earth’s five major gyres, which circulates over most of the North Pacific Ocean, and spans more than 20 million square kilometers. 

      The team started with the MITgcm, a general circulation model that simulates the physical circulation patterns in the atmosphere and oceans. To reproduce the North Pacific gyre’s dynamics as realistically as possible, the team used an MITgcm algorithm, previously developed at NASA and MIT, which tunes the model to match actual observations of the ocean, such as ocean currents recorded by satellites, and temperature and salinity measurements taken by ships and drifters.  

      “We use a simulation of the physical ocean that is as realistic as we can get, given the machinery of the model and the available observations,” Lauderdale says.

      Play video

      An animation of the North Pacific Ocean shows phosphate nutrient concentrations at 500 meters below the ocean surface. The swirls represent small eddies transporting phosphate from the nutrient-rich equator (lighter colors), northward toward the nutrient-depleted subtropics (darker colors). This nutrient relay mechanism helps sustain biological activity and carbon sequestration in the subtropical ocean. Credit: Oliver Jahn

      The realistic model captured finer details, at a resolution of less than 20 kilometers per pixel, compared to other models that have a more limited resolution. The team combined the simulation of the ocean’s physical behavior with the Darwin model — a simulation of microbe communities such as phytoplankton, and how they grow and evolve with ocean conditions.

      The team ran the combined simulation of the North Pacific gyre over a decade, and created animations to visualize the pattern of currents and the nutrients they carried, in and around the gyre. What emerged were small eddies that ran along the edges of the enormous gyre and appeared to be rich in nutrients.

      “We were picking up on little eddy motions, basically like weather systems in the ocean,” Lauderdale says. “These eddies were carrying packets of high-nutrient waters, from the equator, north into the center of the gyre and downwards along the sides of the bowls. We wondered if these eddy transfers made an important delivery mechanism.”

      Surprisingly, the nutrients first move deeper, away from the sunlight, before being returned upwards where the phytoplankton live. The team found that ocean eddies could supply up to 50 percent of the nutrients in subtropical gyres.

      “That is very significant,” Gupta says. “The vertical process that recycles nutrients from marine snow is only half the story. The other half is the replenishing effect of these eddies. As subtropical gyres contribute a significant part of the world’s oceans, we think this nutrient relay is of global importance.”

      This research was supported, in part, by the Simons Foundation and NASA. More

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      in Energy

      Cracking the carbon removal challenge

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      By most measures, MIT chemical engineering spinoff Verdox has been enjoying an exceptional year. The carbon capture and removal startup, launched in 2019, announced $80 million in funding in February from a group of investors that included Bill Gates’ Breakthrough Energy Ventures. Then, in April — after recognition as one of the year’s top energy pioneers by Bloomberg New Energy Finance — the company and partner Carbfix won a $1 million XPRIZE Carbon Removal milestone award. This was the first round in the Musk Foundation’s four-year, $100 million-competition, the largest prize offered in history.

      “While our core technology has been validated by the significant improvement of performance metrics, this external recognition further verifies our vision,” says Sahag Voskian SM ’15, PhD ’19, co-founder and chief technology officer at Verdox. “It shows that the path we’ve chosen is the right one.”

      The search for viable carbon capture technologies has intensified in recent years, as scientific models show with increasing certainty that any hope of avoiding catastrophic climate change means limiting CO2 concentrations below 450 parts per million by 2100. Alternative energies will only get humankind so far, and a vast removal of CO2 will be an important tool in the race to remove the gas from the atmosphere.

      Voskian began developing the company’s cost-effective and scalable technology for carbon capture in the lab of T. Alan Hatton, the Ralph Landau Professor of Chemical Engineering at MIT. “It feels exciting to see ideas move from the lab to potential commercial production,” says Hatton, a co-founder of the company and scientific advisor, adding that Verdox has speedily overcome the initial technical hiccups encountered by many early phase companies. “This recognition enhances the credibility of what we’re doing, and really validates our approach.”

      At the heart of this approach is technology Voskian describes as “elegant and efficient.” Most attempts to grab carbon from an exhaust flow or from air itself require a great deal of energy. Voskian and Hatton came up with a design whose electrochemistry makes carbon capture appear nearly effortless. Their invention is a kind of battery: conductive electrodes coated with a compound called polyanthraquinone, which has a natural chemical attraction to carbon dioxide under certain conditions, and no affinity for CO2 when these conditions are relaxed. When activated by a low-level electrical current, the battery charges, reacting with passing molecules of CO2 and pulling them onto its surface. Once the battery becomes saturated, the CO2 can be released with a flip of voltage as a pure gas stream.

      “We showed that our technology works in a wide range of CO2 concentrations, from the 20 percent or higher found in cement and steel industry exhaust streams, down to the very diffuse 0.04 percent in air itself,” says Hatton. Climate change science suggests that removing CO2 directly from air “is an important component of the whole mitigation strategy,” he adds.

      “This was an academic breakthrough,” says Brian Baynes PhD ’04, CEO and co-founder of Verdox. Baynes, a chemical engineering alumnus and a former associate of Hatton’s, has many startups to his name, and a history as a venture capitalist and mentor to young entrepreneurs. When he first encountered Hatton and Voskian’s research in 2018, he was “impressed that their technology showed it could reduce energy consumption for certain kinds of carbon capture by 70 percent compared to other technologies,” he says. “I was encouraged and impressed by this low-energy footprint, and recommended that they start a company.”

      Neither Hatton nor Voskian had commercialized a product before, so they asked Baynes to help them get going. “I normally decline these requests, because the costs are generally greater than the upside,” Baynes says. “But this innovation had the potential to move the needle on climate change, and I saw it as a rare opportunity.”

      The Verdox team has no illusions about the challenge ahead. “The scale of the problem is enormous,” says Voskian. “Our technology must be in a position to capture mega- and gigatons of CO2 from air and emission sources.” Indeed, the International Panel on Climate Change estimates the world must remove 10 gigatons of CO2 per year by 2050 in order to keep global temperature rise under 2 degrees Celsius.

      To scale up successfully and at a pace that could meet the world’s climate challenge, Verdox must become “a business that works in a technoeconomic sense,” as Baynes puts it. This means, for instance, ensuring its carbon capture system offers clear and competitive cost benefits when deployed. Not a problem, says Voskian: “Our technology, because it uses electric energy, can be easily integrated into the grid, working with solar and wind on a plug-and-play basis.” The Verdox team believes their carbon footprint will beat that of competitors by orders of magnitude.

      The company is pushing past a series of technical obstacles as it ramps up: enabling the carbon capture battery to run hundreds of thousands of cycles before its performance wanes, and enhancing the polyanthraquinone chemistry so that the device is even more selective for CO2.

      After hurtling past critical milestones, Verdox is now working with its first announced commercial client: Norwegian aluminum company Hydro, which aims to eliminate CO2 from the exhaust of its smelters as it transitions to zero-carbon production.

      Verdox is also developing systems that can efficiently pull CO2 out of ambient air. “We’re designing units that would look like rows and rows of big fans that bring the air into boxes containing our batteries,” he says. Such approaches might prove especially useful in locations such as airfields, where there are higher-than-normal concentrations of CO2 emissions present.

      All this captured carbon needs to go somewhere. With XPRIZE partner Carbfix, which has a decade-old, proven method for mineralizing captured CO2 and depositing it in deep underground caverns, Verdox will have a final resting place for CO2 that cannot immediately be reused for industrial applications such as new fuels or construction materials.

      With its clients and partners, the team appears well-positioned for the next round of the carbon removal XPRIZE competition, which will award up to $50 million to the group that best demonstrates a working solution at a scale of at least 1,000 tons removed per year, and can present a viable blueprint for scaling to gigatons of removal per year.

      Can Verdox meaningfully reduce the planet’s growing CO2 burden? Voskian is sure of it. “Going at our current momentum, and seeing the world embrace carbon capture, this is the right path forward,” he says. “With our partners, deploying manufacturing facilities on a global scale, we will make a dent in the problem in our lifetime.” More

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      A lasting — and valuable — legacy

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      Betar Gallant, MIT associate professor and Class of 1922 Career Development Chair in Mechanical Engineering, grew up in a curious, independently minded family. Her mother had multiple jobs over the years, including in urban planning and in the geospatial field. Her father, although formally trained in English, read textbooks of all kinds from cover to cover, taught himself numerous technical fields including engineering, and worked successfully in them. When Gallant was very young, she and her father did science experiments in the basement.

      It wasn’t until she was in her teenage years, though, that she says she got drawn into science. Her father, who had fallen ill five years before, died when Gallant was 16, and while grieving, “when I was missing him the most,” she started to look at what had captivated her father.

      “I started to take a deeper interest in the things he had spent his life working on as a way to feel closer to him in his absence,” Gallant says. “I spent a few long months one summer looking through some of the things he had worked on, and found myself reading physics textbooks. That was enough, and I was hooked.”

      The love for independently finding and understanding solutions, that she had apparently inherited from her parents, eventually took her to the professional love of her life: electrochemistry.

      As an undergraduate at MIT, Gallant did an Undergraduate Research Opportunities Program project with Professor Yang Shao-Horn’s research group that went from her sophomore year through her senior thesis. This was Gallant’s first official exposure to electrochemistry.

      “When I met Yang, she showed me very quickly how challenging and enriching electrochemistry can be, and there was real conviction and excitement in how she and her group members talked about research,” Gallant says. “It was totally eye-opening, and I’m fortunate that she was a (relatively rare) electrochemist in a mechanical engineering department, or else I likely would not have been able to go down that road.”

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      Gallant earned three degrees at MIT (’08, SM ’10, and PhD ’13). Before joining the MIT faculty in 2016, she was a Kavli Nanoscience Institute Prize Postdoctoral Fellow at Caltech in the Division of Chemistry and Chemical Engineering.

      Her passion for electrochemistry is enormous. “Electrons are just dazzling — they power so much of our everyday world, and are the key to a renewable future,” she says, explaining that despite electrons’ amazing potential, isolated electrons cannot be stored and produced on demand, because “nature doesn’t allow excessive amounts of charge imbalances to accumulate.”

      Electrons can, however, be stored on molecules, in bonds and in metal ions or nonmetal centers that are able to lose and gain electrons — as long as positive charge transfers occur to accommodate the electrons.

      “Here’s where chemistry rears its head,” Gallant says. “What types of molecules or materials can behave in this way? How do we store as much charge as possible while making the weight and volume as low as possible?”

      Gallant points out that early battery developers using lithium and ions built a technology that “has arguably shaped our modern world more than any other.

      “If you look at some early papers, the concepts of how a lithium-ion battery or a lithium metal anode worked were sketched out by hand — they had been deduced to be true, before the field even had the tools to prove all the mechanisms were actually occurring — yet even now, those ideas are still turning out to be right!”

      Gallant says, “that’s because if you truly understand the basic principles of electrochemistry, you can start to intuit how systems will behave. Once you can do that, you can really begin to engineer better materials and devices.”

      Truly her father’s daughter, Gallant’s emphasis is on independently finding solutions.

      “Ultimately, it’s a race to have the best mental models,” she says. “A great lab and lots of funding and personnel to run it are very nice, but the most valuable tools in the toolbox are solid mental models and a way of thinking about electrochemistry, which is actually very personalized depending on the researcher.”

      She says one project with immediate impact that’s coming out of her Gallant Energy and Carbon Conversion Lab relates to primary (non-rechargeable) battery work that she and her team are working to commercialize. It involves injecting new electrochemically active electrolytes into leading high-energy batteries as they’re being assembled. Replacing a conventional electrolyte with the new chemistry decreases the normally inactive weight of the battery and boosts the energy substantially, Gallant says. One important application of such batteries would be for medical devices such as pacemakers.

      “If you can extend lifetime, you’re talking about longer times between invasive replacement surgeries, which really affects patient quality of life,” she says.

      Gallant’s team is also leading efforts to enable higher-energy rechargeable lithium-ion batteries for electric vehicles. Key to a step-change in energy, and therefore driving range, is to use a lithium metal anode in place of graphite. Lithium metal is highly reactive, however, with all battery electrolytes, and its interface needs to be stabilized in ways that still elude researchers. Gallant’s team is developing design guidelines for such interfaces, and for next-generation electrolytes to form and sustain these interfaces. Gallant says that applying the technology to that purpose and commercializing it would be “a bit longer-term, but I believe this change to lithium anodes will happen, and it’s just a matter of when.”

      About six years ago, when Gallant founded her lab, she and her team started introducing carbon dioxide into batteries as a way to experiment with electrochemical conversion of the greenhouse gas. She says they realized that batteries do not present the best practical technology to mitigate CO2, but their experimentation did open up new paths to carbon capture and conversion. “That work allowed us to think creatively, and we started to realize that there is tremendous potential to manipulate CO2 reactions by carefully designing the electrochemical environment.” That led her team to the idea of conducting electrochemical transformations on CO2 from a captured state bound to a capture sorbent, replacing the energy-intense regeneration step of today’s capture processes and streamlining the process.  

      “Now we’re seeing other researchers working on that, too, and taking this idea in exciting directions — it’s a very challenging and very rich topic,” she says.

      Gallant has won awards including an MIT Bose Fellowship, the Army Research Office Young Investigator Award, the Scialog Fellowship in Energy Storage and in Negative Emissions Science, a CAREER award from the National Science Foundation, the Ruth and Joel Spira Award for Distinguished Teaching at MIT, the Electrochemical Society (ECS) Battery Division Early Career award, and an ECS-Toyota Young Investigator Award.

      These days, Gallant does some of her best thinking while brainstorming with her research group members and with her husband, who is also an academic. She says being a professor at MIT means she has “a queue of things to think about,” but she sometimes gets awarded with a revelation.

      “My brain gets overloaded because I can’t think through everything instantaneously; ideas have to get in line! So there’s a lot going on in the background at all times,” she say. “I don’t know how it works, but sometimes I’ll be going for a walk or doing something else, and an idea breaks through. Those are the fun ones.” More

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      in Energy

      Turning carbon dioxide into valuable products

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      Carbon dioxide (CO2) is a major contributor to climate change and a significant product of many human activities, notably industrial manufacturing. A major goal in the energy field has been to chemically convert emitted CO2 into valuable chemicals or fuels. But while CO2 is available in abundance, it has not yet been widely used to generate value-added products. Why not?

      The reason is that CO2 molecules are highly stable and therefore not prone to being chemically converted to a different form. Researchers have sought materials and device designs that could help spur that conversion, but nothing has worked well enough to yield an efficient, cost-effective system.

      Two years ago, Ariel Furst, the Raymond (1921) and Helen St. Laurent Career Development Professor of Chemical Engineering at MIT, decided to try using something different — a material that gets more attention in discussions of biology than of chemical engineering. Already, results from work in her lab suggest that her unusual approach is paying off.

      The stumbling block

      The challenge begins with the first step in the CO2 conversion process. Before being transformed into a useful product, CO2 must be chemically converted into carbon monoxide (CO). That conversion can be encouraged using electrochemistry, a process in which input voltage provides the extra energy needed to make the stable CO2 molecules react. The problem is that achieving the CO2-to-CO conversion requires large energy inputs — and even then, CO makes up only a small fraction of the products that are formed.

      To explore opportunities for improving this process, Furst and her research group focused on the electrocatalyst, a material that enhances the rate of a chemical reaction without being consumed in the process. The catalyst is key to successful operation. Inside an electrochemical device, the catalyst is often suspended in an aqueous (water-based) solution. When an electric potential (essentially a voltage) is applied to a submerged electrode, dissolved CO2 will — helped by the catalyst — be converted to CO.

      But there’s one stumbling block: The catalyst and the CO2 must meet on the surface of the electrode for the reaction to occur. In some studies, the catalyst is dispersed in the solution, but that approach requires more catalyst and isn’t very efficient, according to Furst. “You have to both wait for the diffusion of CO2 to the catalyst and for the catalyst to reach the electrode before the reaction can occur,” she explains. As a result, researchers worldwide have been exploring different methods of “immobilizing” the catalyst on the electrode.

      Connecting the catalyst and the electrode

      Before Furst could delve into that challenge, she needed to decide which of the two types of CO2 conversion catalysts to work with: the traditional solid-state catalyst or a catalyst made up of small molecules. In examining the literature, she concluded that small-molecule catalysts held the most promise. While their conversion efficiency tends to be lower than that of solid-state versions, molecular catalysts offer one important advantage: They can be tuned to emphasize reactions and products of interest.

      Two approaches are commonly used to immobilize small-molecule catalysts on an electrode. One involves linking the catalyst to the electrode by strong covalent bonds — a type of bond in which atoms share electrons; the result is a strong, essentially permanent connection. The other sets up a non-covalent attachment between the catalyst and the electrode; unlike a covalent bond, this connection can easily be broken.

      Neither approach is ideal. In the former case, the catalyst and electrode are firmly attached, ensuring efficient reactions; but when the activity of the catalyst degrades over time (which it will), the electrode can no longer be accessed. In the latter case, a degraded catalyst can be removed; but the exact placement of the small molecules of the catalyst on the electrode can’t be controlled, leading to an inconsistent, often decreasing, catalytic efficiency — and simply increasing the amount of catalyst on the electrode surface without concern for where the molecules are placed doesn’t solve the problem.

      What was needed was a way to position the small-molecule catalyst firmly and accurately on the electrode and then release it when it degrades. For that task, Furst turned to what she and her team regard as a kind of “programmable molecular Velcro”: deoxyribonucleic acid, or DNA.

      Adding DNA to the mix

      Mention DNA to most people, and they think of biological functions in living things. But the members of Furst’s lab view DNA as more than just genetic code. “DNA has these really cool physical properties as a biomaterial that people don’t often think about,” she says. “DNA can be used as a molecular Velcro that can stick things together with very high precision.”

      Furst knew that DNA sequences had previously been used to immobilize molecules on surfaces for other purposes. So she devised a plan to use DNA to direct the immobilization of catalysts for CO2 conversion.

      Her approach depends on a well-understood behavior of DNA called hybridization. The familiar DNA structure is a double helix that forms when two complementary strands connect. When the sequence of bases (the four building blocks of DNA) in the individual strands match up, hydrogen bonds form between complementary bases, firmly linking the strands together.

      Using that behavior for catalyst immobilization involves two steps. First, the researchers attach a single strand of DNA to the electrode. Then they attach a complementary strand to the catalyst that is floating in the aqueous solution. When the latter strand gets near the former, the two strands hybridize; they become linked by multiple hydrogen bonds between properly paired bases. As a result, the catalyst is firmly affixed to the electrode by means of two interlocked, self-assembled DNA strands, one connected to the electrode and the other to the catalyst.

      Better still, the two strands can be detached from one another. “The connection is stable, but if we heat it up, we can remove the secondary strand that has the catalyst on it,” says Furst. “So we can de-hybridize it. That allows us to recycle our electrode surfaces — without having to disassemble the device or do any harsh chemical steps.”

      Experimental investigation

      To explore that idea, Furst and her team — postdocs Gang Fan and Thomas Gill, former graduate student Nathan Corbin PhD ’21, and former postdoc Amruta Karbelkar — performed a series of experiments using three small-molecule catalysts based on porphyrins, a group of compounds that are biologically important for processes ranging from enzyme activity to oxygen transport. Two of the catalysts involve a synthetic porphyrin plus a metal center of either cobalt or iron. The third catalyst is hemin, a natural porphyrin compound used to treat porphyria, a set of disorders that can affect the nervous system. “So even the small-molecule catalysts we chose are kind of inspired by nature,” comments Furst.

      In their experiments, the researchers first needed to modify single strands of DNA and deposit them on one of the electrodes submerged in the solution inside their electrochemical cell. Though this sounds straightforward, it did require some new chemistry. Led by Karbelkar and third-year undergraduate researcher Rachel Ahlmark, the team developed a fast, easy way to attach DNA to electrodes. For this work, the researchers’ focus was on attaching DNA, but the “tethering” chemistry they developed can also be used to attach enzymes (protein catalysts), and Furst believes it will be highly useful as a general strategy for modifying carbon electrodes.

      Once the single strands of DNA were deposited on the electrode, the researchers synthesized complementary strands and attached to them one of the three catalysts. When the DNA strands with the catalyst were added to the solution in the electrochemical cell, they readily hybridized with the DNA strands on the electrode. After half-an-hour, the researchers applied a voltage to the electrode to chemically convert CO2 dissolved in the solution and used a gas chromatograph to analyze the makeup of the gases produced by the conversion.

      The team found that when the DNA-linked catalysts were freely dispersed in the solution, they were highly soluble — even when they included small-molecule catalysts that don’t dissolve in water on their own. Indeed, while porphyrin-based catalysts in solution often stick together, once the DNA strands were attached, that counterproductive behavior was no longer evident.

      The DNA-linked catalysts in solution were also more stable than their unmodified counterparts. They didn’t degrade at voltages that caused the unmodified catalysts to degrade. “So just attaching that single strand of DNA to the catalyst in solution makes those catalysts more stable,” says Furst. “We don’t even have to put them on the electrode surface to see improved stability.” When converting CO2 in this way, a stable catalyst will give a steady current over time. Experimental results showed that adding the DNA prevented the catalyst from degrading at voltages of interest for practical devices. Moreover, with all three catalysts in solution, the DNA modification significantly increased the production of CO per minute.

      Allowing the DNA-linked catalyst to hybridize with the DNA connected to the electrode brought further improvements, even compared to the same DNA-linked catalyst in solution. For example, as a result of the DNA-directed assembly, the catalyst ended up firmly attached to the electrode, and the catalyst stability was further enhanced. Despite being highly soluble in aqueous solutions, the DNA-linked catalyst molecules remained hybridized at the surface of the electrode, even under harsh experimental conditions.

      Immobilizing the DNA-linked catalyst on the electrode also significantly increased the rate of CO production. In a series of experiments, the researchers monitored the CO production rate with each of their catalysts in solution without attached DNA strands — the conventional setup — and then with them immobilized by DNA on the electrode. With all three catalysts, the amount of CO generated per minute was far higher when the DNA-linked catalyst was immobilized on the electrode.

      In addition, immobilizing the DNA-linked catalyst on the electrode greatly increased the “selectivity” in terms of the products. One persistent challenge in using CO2 to generate CO in aqueous solutions is that there is an inevitable competition between the formation of CO and the formation of hydrogen. That tendency was eased by adding DNA to the catalyst in solution — and even more so when the catalyst was immobilized on the electrode using DNA. For both the cobalt-porphyrin catalyst and the hemin-based catalyst, the formation of CO relative to hydrogen was significantly higher with the DNA-linked catalyst on the electrode than in solution. With the iron-porphyrin catalyst they were about the same. “With the iron, it doesn’t matter whether it’s in solution or on the electrode,” Furst explains. “Both of them have selectivity for CO, so that’s good, too.”

      Progress and plans

      Furst and her team have now demonstrated that their DNA-based approach combines the advantages of the traditional solid-state catalysts and the newer small-molecule ones. In their experiments, they achieved the highly efficient chemical conversion of CO2 to CO and also were able to control the mix of products formed. And they believe that their technique should prove scalable: DNA is inexpensive and widely available, and the amount of catalyst required is several orders of magnitude lower when it’s immobilized using DNA.

      Based on her work thus far, Furst hypothesizes that the structure and spacing of the small molecules on the electrode may directly impact both catalytic efficiency and product selectivity. Using DNA to control the precise positioning of her small-molecule catalysts, she plans to evaluate those impacts and then extrapolate design parameters that can be applied to other classes of energy-conversion catalysts. Ultimately, she hopes to develop a predictive algorithm that researchers can use as they design electrocatalytic systems for a wide variety of applications.

      This research was supported by a grant from the MIT Energy Initiative Seed Fund.

      This article appears in the Spring 2022 issue of Energy Futures, the magazine of the MIT Energy Initiative. More