Removing CO₂ from power plant exhaust

Katie Luu: From MIT, this is the Energy Initiative. I’m Katie Luu. Today we’re speaking with Betar Gallant, an assistant professor of mechanical engineering at MIT.

Betar Gallant: Hi. I’m very happy to be here.

KL: Thanks for joining us. What do you do here at MIT?

BG: I’m an assistant professor in mechanical engineering. However, my research treats electrochemistry. Different types of electrochemical devices, ranging from lithium ion batteries. We work a bit on developing advanced materials that can go inside the battery, cathode and anode materials, so that we can develop systems with higher energy density for transportation applications.

We also apply electrochemistry to try to tackle environmental challenges. Recently, we’ve been working a lot on designing new electric chemical processes to take CO₂ from a gas phase, for example, as emitted from transportation or from a power plant, capture it, and find some valuable and use for the emissions and try to sequester and get it out of the airstream.

KL: What kind of end uses do you envision for the solid CO₂?

BG: That’s a great question. This is a really big debate that has been going on for a long time. We have technologies that allow us to think about capturing CO₂, essentially filtering it out from a combustion stream that might contain other gases and water and so on. We have those technologies in the world of CO₂ capture. They’re being piloted as we speak.

Then the question is, let’s imagine we can safely and effectively capture the CO₂. Typically, at that point, we’re talking about getting it into a liquid phase. Some kind of sorbent that will absorb the CO₂ selectively. Then the question is, what do we do with it? This is where, depending on what topic you do research in, there are different opinions out there. Right now, essentially, the best we can do or the closest we are to a commercialized process is to essentially heat up that liquid and regenerate the CO₂ back to the gas phase. Now we have purified CO₂ that’s been separated from a waste stream.

Then what do you do with it? Essentially, the idea is you can pump an underground, so find a reservoir where you can storage and some kind of geological formation, hopefully safely for thousands and thousands of years. That’s known as carbon sequestration. Then there are those of us in the electrochemistry community, we’re very good at, or we like to think we’re good at, figuring out how to shuttle electrons around to drive chemical processes. That’s what we do. There are those of us in the community who are interested in saying, well, there might be some risks associated with underground storage. Also, wouldn’t it be nice if we could make some kind of useful, valuable even product out of the CO₂?

The electric chemists try to step in downstream of that capture and regeneration process and say, “Okay, we’ll take the CO₂, and we’ll figure out what to make with it.“ But it’s not so easy, because at that point, doing electrochemistry with CO₂ is also very challenging. We have a challenging capture and regeneration process. We can talk more about the challenges there. Then we’ve got a challenging electrochemistry process. It turns out what to make is not an easy question to answer.

KL: How much energy goes into current processes for turning CO₂ into solids? How does your approach differ?

BG: We can think about the CO₂ management process, let’s call it two steps in a series. You have the capture and regeneration step. This would be essentially a system integrated, for example, into a power plant. You can exothermically, or essentially spontaneously, absorb this year too. That just means that the liquid sorbent that we use in these applications really likes to bind with CO₂. That’s a so-called energetically free process or downhill process that just spontaneously occurs. That’s great. But then you only have so many sites or molecules in this orbit that can bind with CO₂. Eventually, if becomes saturated. Then the liquid is no longer useful for reacting with CO₂. At that point, you have to do a process on this liquid to what’s called regenerate the sorbent. Typically, that involves routing the CO₂ rich liquid through another part of the system where you heat it up. You can either go to higher temperatures or you can also go to lower pressures. But typically, we think in terms of a higher temperature process. You heat it up and you essentially undo the reaction that happened in the previous step and essentially release the CO₂ back to the gas phase.

The issue is that requires quite a lot of energy. You have to heat up not only the specific bonds that you want to break, but typically all of this is occurring in a water-based solution. You have to heat up all of the water as well and that doesn’t really go into driving the reaction that we want, the heating up the water doesn’t necessarily aid with the CO₂ liberation, it’s just the direct heating of the sorbent. It turns out that heating up the entire solution just to break a few specific bonds is really energetically inefficient. A typical metric that we use is, a prototypical or a prototype capture system might require something like 30% of the power plant’s rated capacity in order to clean up and regenerate the CO₂. That’s a really big problem, 30% we lose upstream just to create a separated purified stream of CO₂.

Then there’s a second step, if you’re an electric chemist, because then you take that CO₂ and you want to do further reactions with it. Most typically, in the community, folks have been trying to develop what we’ll call aqueous water-based CO₂ reactions. The idea is to essentially do what we call a reduction reaction. Basically, you react CO₂ with electrons, but you also react it with protons that are available in water. You use the water as a reactant. The idea is to take CO₂ from a harmful species and convert it into something useful, for example, methane, which would be potentially usable as a fuel. There’s the value added component. The issue is that you have to put a lot of energy into the system to do an aqueous-based, a water-based CO₂ electrochemical reaction.
Herein lies the big challenge. Not only is it intrinsically somewhat energy inefficient to get the products that we want, but now you have to combine that with the upstream energy penalties that we’ve paid to separate and purify this year, too, so the two compound.

KL: What are you doing differently with your research into this?

BG: There’s a lot of really great research going on in the CO₂ community. Particularly looking at trying to catalyze the reaction that I just mentioned, the water-based CO₂ conversion to a fuel or potentially to another chemical, for example, methanol or formic acid would be useful products to make as well. There’s really a lot of excellent science going on looking at trying to lower some of the energy barriers to try to make the products we want with better selectivity, and in an efficient way. The issue, though, is that there are a lot of challenges with getting both selectivity and high energy efficiency.

When my group was looking at this challenge, we thought, “Okay, this is a very active field. But there are really a lot of hurdles. Maybe we can take a step back and challenge some of the assumptions here.” Namely, the assumption that we need to be making a fuel or a chemical at the end and thus that we need to be doing these reactions in a water-based environment to begin with. We took a step back and we said, “Are there other interesting ways to think about unlocking the reactivity of CO₂, so that we can direct it into some other kind of a useful, potentially useful product?” What we thought about was actually to go back to the technology I mentioned earlier, which is CO₂ capture. We realized, of course, that there’s this community that understands very well how to get CO₂ into a liquid environment, in an exothermic or downhill way, as I mentioned before.

There’s a particular chemical motif, it’s called an amine. Amines are widely used in CO₂ capture, typically diluted in water, because they react with CO₂ so well and so selectively. When they do that, they also electronically rearrange the CO₂. The CO₂ gets incorporated effectively into a new species in the liquid phase. We thought, what if we take that motif, which works really well, and we think about now taking this CO₂ rich capture solution and instead of regenerating CO₂ back to a gas, what if we use that solution as a starting point for further electrochemistry? This is different from what folks in the CO₂ defuels community are doing. Because, remember, they’ve already had an upstream capture and regeneration, then they start with the purified CO₂, and then they take it to a fuel.

What we thought about is, “What if we can get rid of this intermediate step? We can take CO₂ to the captured state in the liquid and then directly use that as an electrolyte, in some kind of an electric chemical system. Can we just directly discharge that solution of that electrolyte? Essentially what do we make?” This was the question that we asked.

There are a lot of reasons why scientifically, it raises a lot of interesting questions too, because we would essentially be reacting CO₂, not as CO₂, but as this new state in the new molecule that it’s reacted with.

We set about trying to do this reaction. The first step was to really see if it was viable. Meaning, if we try to discharge the CO₂ rich liquid, do we see any activity? Do we see any evidence that anything can react? That was one of the first questions that we needed to figure out. What we did a little bit differently is we looked at conducting this reaction in a water-based environment, in an aqueous electrolyte, similar to what the community is generally doing right now. For various reasons, the science was actually pretty messy, because water can actually participate in the reaction and so on. What we did differently was to say, let us isolate the amine in the CO₂ by working in a what we call a non-aqueous or a non-water containing electrolyte. We shifted the reaction space from a water-based reaction to something that actually looks a lot more like the environment that you’d find inside of your lithium ion battery.

KL: Can you tell us a bit about how your battery works?

BG: Absolutely. What happens in the lab is we build what is essentially a lithium-based battery. Lithium is the anode material and then we have a carbon cathode. The carbon is the surface on which the CO₂ conversion reaction happens. It’s basically there to make sure electrons can get around to the surface of the electrode in a favorable way. But it’s not acting as a catalyst. That’s a key point. Our system doesn’t require any metals to catalyze the reaction. Then in between the two electrodes, you have a non-aqueous or organic electrolyte that contains lithium ions. In the laboratory, we assemble a cell like this and then we can bubble CO₂ or purge CO₂ through the cell’s head space and create a CO₂ rich environment which simulates what, for example, a system in a power plant might be exposed to. Then CO₂ gets incorporated into the cell. It also very readily dissolves into the electrolyte because the electrolyte really likes to react with CO₂.

Now we have CO₂ available in fairly high concentration in our electrolyte. Then we hook it up to our electric chemical equipment. We essentially just directly discharge the battery. In doing so, what we’re doing is we’re reacting the CO₂ in the liquid state with the lithium ions that are available, and electrons, and we’re doing what’s called a reduction reaction. Essentially what we learned is, during that process, we can undo the bond that originally formed between the amine and the CO₂. We can essentially nicely selectively pluck off the CO₂ from the electrolyte molecule and react just the CO₂ and not the amine.

This is the key point, just the CO₂ to another product. Essentially, what the special electrolyte is doing is, it’s acting as a medium to have a very high concentration of CO₂ around and it also activates the CO₂ and makes it a lot easier to cycle it to some type of a product.
The question is, what product do we make? Because we’ve got CO₂ around and we’ve got a lot of lithium around, indeed, what we make is a lithium carbonate species. We discharge the battery and we make from initially a liquid, and even before that a gas phase CO₂, in the end, we make a solid phase, which is a lithium carbonate solid that grows inside the electrode as we discharge it.
Then we have two options as to what we do with that lithium carbonate. The first option is we can, potentially. down the road devise a process where we remove it from the cell. The reason that’s potentially attractive is we’ve now taken CO₂ and in an energetically favorable way to a solid phase. A solid phase is really easy to handle. You can remove it from the system and clean it up a little bit, but essentially, pretty readily dispose of it. Now CO₂ is in a solid form. It’s not compressed underground, stored underground where there’s a potential risk of release. Now it’s essentially sequestered for good in a solid. That’s one of the potential end uses. In that case, you would really think about just discharging the battery potentially even once. That starts to look like what we would describe as a primary battery.

Another thing you can do is decompose the lithium carbonate electric chemically. Once you incorporate the CO₂ into the battery, you can seal it off, discharge it, make lithium carbonate. Then using electrochemistry, you could decompose the lithium carbonate and it would go back to CO₂ and liberate the lithium ions that were consumed in the process. That CO₂ would then stay within the closed system, so within the battery. It turns out, you can actually cycle the system like this, between lithium carbonate and then back to CO₂, for a number of cycles. Now, it doesn’t cycle extremely well. This is not a battery that one would potentially drop into replace lithium ion batteries and their Tesla anytime soon. But it definitely does cycle and with some more research may one day get to a point where it can really function well as a rechargeable system. There are a lot of hurdles that need to be figured out or jumped over before that really looks viable. But that’s another potential and end use of the system.

KL: What made you pick lithium and not another material?

BG: I’m so glad you asked. Lithium is really not a good choice. Lithium is expensive. Even more importantly, it’s not Earth abundant. We essentially long term need to be working with materials that are sustainable. We have a great interest in moving beyond lithium, potentially looking at sodium, for example. This is something that we’re working on now.

The reason we started with lithium is that we know as scientists and as electrochemists, we’re very comfortable integrating electrolytes with lithium and cathode materials with lithium because we can take a page from the battery community. I mentioned earlier that my group works a lot on lithium ion and lithium battery technology. We had a lot of lithium around. But we also have the know-how for how to safely construct an electrochemical system that would essentially let us answer the question that we raised in the beginning, is it even possible, does such a reaction work? Once we could answer that question as a proof of concept, we thought, then we can really start looking into the best material to replace lithium.

KL: Do you think power plants would be more apt to help this technology advance by using a different material? Because lithium is expensive. I’m trying to think how to make this more of a scalable reality.

BG: It’s interesting. It depends who the customer is. Our battery, once packaged up, once there’s CO₂ contained in the electrolyte, from the user’s point of view, everything after that, when you discharge the battery, the user gets power out. You can envision a system where the power plant operators are utilizing the system to sequester CO₂. On site at least, they’re also getting some power back in the process. That’s in contrast to the case I mentioned in the beginning where you need to pay about 30% of the power plant’s capacity to take care of the CO₂. On site that’s potentially a win. But in the grand scheme of things, there are upstream costs. In the end, what we really care about is the overall impact on the planet. There are upstream energetic costs and also economic costs that had to go into acquiring the lithium and also making the electrolyte in the first place.

The question is, who really pays for that, and in the end, can the gains that we think our technology may deliver with more work, can they really offset these upstream costs? That requires drawing a bigger box around the problem, which we’re doing right now. We’re doing the technoeconomic analysis and the lifecycle analysis to try to really pinpoint the answer to your question earlier, what do you do with the CO₂? We have shown scientifically that this is a really viable pathway. But now we need to figure out, do the economics work out? Ultimately, is it really a win for the planet? We hope so.

KL: Out of all the hurdles you’ve mentioned in this whole process, which do you think is the biggest?

BG: I could give you a list of hurdles that we haven’t even talked about yet. It’s been really fun working on this system because we identified a new class of reactions involving CO₂. It’s not that easy to do. Chemists and engineers and material scientists and electrochemists have been working on and studying CO₂ for decades and decades. We stumbled into this really interesting new series of reactions. We’re at a point where we’re just understanding all of the variables that really affect how the reaction proceeds and what products you get out and how that affects the performance of the battery.

We’ve opened up this really great scientific landscape. That’s really exciting. It also means that we are still too early on to really have the metrics to be able to say, here’s what the file system design is going to look like, whether it’s better used as an open or a closed system, whether you continuously have CO₂ flowing through it or whether it’s something where you flow a fixed amount of CO₂ and you seal it up and you use it as a rechargeable battery. We’re still figuring those things out. That’s occurring in parallel with the science. Both are going to inform what the best end use for this technology is going to be. It’s very exciting. We need more people to work on this to help us answer that question.

KL: How long have you been working on this problem to get to where you are right now?

BG: We started working on the CO₂ project about when I started at MIT. That was in 2016. My very first PhD student started to tackle this project and I think she didn’t know what she was getting into. I think I didn’t either, and that’s probably to the benefit of all of us because it turned out to be really challenging. Once you know as simple as a battery or an electric chemical cell, and you see that something works, so you’re getting some current out of it, that’s great. But then you have to really dig into it and figure out and identify what reaction is going on. Because electrons can really go any number of places in a new environment, and you have to really do the science to figure out that they’re going to the place you wanted them to go. It was a long endeavor to really test our first hypothesis, that we think that this can work, but also to prove that we understood the science inside the cell. That was a multi-year effort.

We published our first paper on that in 2018. Since then, it’s opened all kinds of doors. We’re moving ahead with more fundamentals of the science right now. It’s going to be a long term project. Eventually, in the end, I’m a mechanical engineer. What we want to be doing are designing at the systems level as well.

KL: What is one thing that you hope listeners will take away from hearing about your research?

BG: I think it’s good to take risks and try to think outside of the box. Try to find the spaces where you perceive people are not looking. If you are willing to explore those areas, then you can find delightful things, and also interesting work to do. I think we need, more and more, we need students, researchers, people who are willing to have one foot in one community or one research topic and another foot in another and say, hey, I’ve identified some interesting and unique overlap here and I think it’s worth exploring.

KL: Where can we keep up with your research?

BG: You can go to my website where we publish our papers as they’re coming out, gallant.mit.edu. That’s going to have the latest research findings.

KL: Thank you so much for coming to chat with us. It was such a pleasure.

BG: Thanks. It was great to be here.