Bringing Chemistry to Life

The electrifying chemistry of the nitrogen cycle

Episode Summary

In this episode we revisit a topic from Season 1 with a fresh guest, Matthew Liu. Specifically, we take a hard look at the complex topic of nitrogen reduction. The conversation dives into the fundamental importance of this process for life, and how we have to become better at finding sustainable ways to recover nitrogen to create a circular economy. Matthew tells us about his work using electrocatalysis toward this goal.

Episode Notes

Visit to access the extended video version of this episode and the episode summary sheet, which contains links to recent publications and additional content recommendations for our guest. You can also access the extended video version of this episode via our YouTube channel to hear, and see, more of the conversation!

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Moving from a linear economy, where things are made, used and discarded, to a circular one, based on recycling and reuse, is one of the most important and difficult challenges for our society. Cracking this problem and moving to a more sustainable way of living, while maintaining or even improving living standards, is key for the future of our planet.

With Matthew Liu, we go back to topics discussed in Episode 6 of Season 1 to look at one of the most important chemical elements, nitrogen. Reducing atmospheric nitrogen to nitrates is fundamental to our modern world. Nitrogen reduction makes possible to feed billions of people globally and it provides some of the most fundamental building blocks of modern chemistry. At the same time, it is one of the most energy-intense industrial processes, and its products, while essential and beneficial, eventually become environmental pollutants at the end of their lifecycle. 

An old technology might be the key to change this landscape. Electrochemistry is going through a renaissance and it’s a very promising tool to recover nitrogen and put it back into the economic circle. In our discussion with Matthew we discuss some breakthrough and novel electrochemical approaches, electrocatalysis in particular, and how they can impact the economy of developed and under-developed countries.

Episode Transcription


Matthew Liu00:06

Having the space to really think about catalysis so deeply and explore it in all these different ways, was what ultimately allowed me to call that one of my strongest suits of research and where I have the most ideas.


Paolo  00:23

You may remember our past conversation with Dr. Will Tarpeh. That episode is one of the most downloaded in our show's history. Today, we speak with one of his most talented graduate students, Matthew Liu, who is bringing lots of new ideas on the table. With students like him, the future of electrochemistry is certainly a bright one. I'm Paolo Braiuca, from Thermo Fisher Scientific and host of the show. We began by asking Matthew about his early college research career, and how it's influenced his work today.


Matthew Liu00:58

My freshman year I applied for an energy internship through the school. And so how the internship worked was you were paired up with one of several sites. And I happen to be paired up at Lawrence Berkeley National Lab, which was just right up the hill from the College of Chemistry where I studied. So, I spent that first summer after my freshman year researching there. And I was actually formally trained for the first two years of research, doing nothing but modeling rather than experiments. So I did the kinetic modeling of how aerosols age in atmosphere. But after those two years, I moved on to experiments, and that was on electrochemistry. So particularly electrochemical carbon dioxide production. And that's really what got me into my current research. That's when I got into my, my current interests in electrochemistry and experiments. But I still very much keep those initial beginnings I had with modeling and I find myself always pairing the two, whenever I approach a research problem. How can I think of some base case model that I think might describe my system? And approach the experiments that way? Or having completed experiments? How can I then propose a few models that might validate those results? I have found that because I think both like an experimentalist and a modeler, I've been able to seek a lot of collaborations because I feel like there's a common language when it comes to experiments. A common language when it comes to modeling and kind of understanding the difficulties with each helps you at least practically speak to someone in that in that expertise. And so, recently, in grad school, I've had a lot of collaborations with different departments in the school, with national labs outside of the school and those often crossed the boundaries of experiments and modeling.


Paolo  03:09

You work with Will Tarpeh, he's really, you know, a hot name, right, in the field. He's a great guy, we interviewed him for the podcast, I think it's already nearly a couple of years ago. Were you attracted by his personality in the first place when you applied, and you got into his system? What got you there?


Matthew Liu03:29

Yeah, so I got interested in electrochemistry. But there's a lot of labs at Stanford that do electrochemistry. So there are a lot of choices there. Will and I actually started in the same month of the same year. So when I started as a PhD student, that was also his day one as assistant professor in the department. And so we have this convocation that opens up each academic year. And as the newest professor at the time, Will spoke at that convocation. I actually didn't know he was a professor, because during my visiting weekend, he couldn't make it. And so I didn't, you know, meet with him. And it wasn't until convocation, he started speaking and I was just amazed. Not only by of course, the vision he set out for circular economies and wastewater treatment and refining. But also, like you said, his personality was so uplifting and hopeful and empathetic and compassionate. And it didn't take long for me to realize that he is the professor I want to work with. I felt like I had so much to learn in addition to the science that he was proposing. And I will say five years later that it was the correct decision.


Paolo  04:52

I don't doubt that explains it. Right. So, you two have been together since his beginning as an assistant professor.  You won the 2023 Graduate Student Award for the environmental chemistry at the American Chemical Society, so yeah, that's, that's the big deal. Tell me more about it.


Matthew Liu 05:12

Yeah, I'm round 2020, 2021, Will was very supportive and he knew that I had an interest in going onto academia after graduating. So, we decided it was time to start expanding some of the awards and grants that I could talk about and put on the CV. And so around that time, we started looking for opportunities, just through some of the chemical societies like ACS, Electrochemical Society, ECS, and so forth. And preparing those applications was a good exercise, I think in just writing about myself, kind of little about my story, and how I wanted to approach science in the future. So, sending a lot of those applications later, eventually led to, you know, this year's receipt of the Environmental Chemistry Award, through the American Chemical Society. And that was one of the divisions that seemed to hit closer to home for our research, which really touches on wastewater remediation. So, it seemed like a very fitting division to apply to.


Paolo  06:40

It makes sense. Well, congratulations for that. I'd like to dig a bit further into this.  You know, Will strikes me as a very kind person, right, he's patient, he's composed.  He's probably very thoughtful by, I guess, 100% of the time, right? Is he? Is he challenging in any way? You know, how does he push you?


Matthew Liu06:40

His is challenging, and I would say his raw intellect, kind of his processing speed, if he was like a computer, in that you'll present results to him, whether that's like a presentation or just informally. He will catch things in the corner of slides from 30 minutes ago that he wants to circle back to. And as the person that made the slides, you're amazed that, “How does he remember that?” And so often, at first glance, when you put something on the screen, he's done reading it, and he already has his thoughts, I think, sometimes the speed at which he's able to process information and then give very pointed feedback is challenging to keep up with, um because when you're still wanting to talk about point A, he's kind of moved on to point Z, and you just have to kind of keep up pace, otherwise, you're not going to get all of the advice and ideas that he has to offer. So, that was certain, certainly a challenge for me in the beginning to kind of understand like, this is his pace, and we got to keep up.



Paolo  08:09

Of course, he's obviously a very strong, visionary person, right? It's got a very clear vision in mind, and he knows where he wants to go, you know. Do you feel like you have enough space, I think space is the right word, to kind of develop your own kind of angle and perspective?


Matthew Liu08:25

Substantial. When we are, yeah, when we first started, the lab Will described the space as a wild west. He said it was, you know, so many places to claim still so many ideas to still explore. And everyone was still in electrochemistry, still trying to kind of get organized somehow. So, there is so much freedom and there still is so much freedom in what we explore. And I will say that's not for everybody, because some students prefer more structure, or having a very well-defined project and knowing that in six months, I'm still going to be working on this specific grant or this project. Whereas some students really appreciate this flexibility knowing that I don't feel so boxed in by, you know, this one project that's been passed down from student to student. I get to come up with something myself that I think can contribute to, to this vision of, you know, wastewater refining.


Paolo  09:27

Have you've got a moment where you felt like your vision or something, an idea or something a perspective from you gain a slot in his own sort of puzzle, you know, have you ever felt that?


Matthew Liu09:40

Yeah, I think it really came down to electrocatalysis. So, my first year of research with Will I extended his PhD work on electrochemical stripping, which was a process he came up with to recover ammonium from wastewaters. And after that project wrapped up, I started to dive pretty deep into electrocatalysis. And expanding in that space, I think has been a contribution to the lab that Will didn't initially foresee he would go into so deeply because within electrocatalysis, I started to explore three distinct fronts to that field of study. So heterogeneous catalysis, where you have reaction occurring at a solid liquid interface. I looked at homogeneous catalysis, so reactions purely occurring in the liquid phase. And single atom catalysis, kind of merging the two and taking the best of both worlds to immobilize molecularly or atomically, precise catalyst active sights on to your electrode surface and performing catalysis that way. So, I think that deep dive really necessitated that we had more team members working on this area of research that we needed to collaborate with multiple national labs. So we've started collaborations with SLAC National Lab, Lawrence Berkeley National Lab, and Oak Ridge National Lab, to characterize the surface properties of our catalysts. And really collaborating within Stanford as well in the chemistry department in the chemical engineering department. So that's become a cornerstone of one of the subgroups in our lab, which is the electrochemistry subgroup. I think that having the space to really think about catalysis so deeply and explore it in all these different ways, was what ultimately allowed me to now call that one of my strongest suits of research and where I have the most research ideas.


Paolo  11:44

Let's get more into it, right for the benefit of the audience. Why don't you describe, you know, electrocatalysis, actually electrochemistry, first, right at a general level. Then move on to, you know, the use of catalysis, specifically into it, and then maybe we can jump into some of the details of the reactions you're exploring. It's so powerful, it's such a hot topic in chemistry in general at the moment, plus coupled with the sort of circular economy and environmental chemistry landscape, you know, it's, I can't think of anything which is hotter at the moment in the market, isn't it?


Matthew Liu12:17

Right now electrochemistry is kind of going through a renaissance right now. The Wall Street Journal recently wrote an opinion paper about electrochemistry. And how electrochemists kind of feel like it's the beginning of the dot com, internet age. Really, everyone's kind of finally reaching this maturity in the technology where they feel like they can commercialize, whether that's in a large company that wants to adopt electrochemical technologies, or a lot of startups in the Bay Area are starting to use electrochemistry in some way for their business. So electrochemistry really is using electricity to rearrange chemical bonds rather than temperature, pressure, or other chemical initiators. And the use of electricity is so attractive because renewable energy is increasingly populating the energy grid. And if we can use electricity to produce and manufacture chemicals and goods in more sustainable ways, then, of course, this is a mode of manufacturing that we would want to go down in the future. But electricity is of course distributed. And so rather than requiring highly centralized places to manufacture, chemicals and goods, electricity allows you to do that in very distributed fashions. So that's why electrochemistry is getting very popular these days, especially in the context of chemical manufacturing. Now, electrocatalysis, is the study of using catalysts that are activated by electricity. And this can increase the chemical kinetics of your reaction, this can lower the energy needed for your reaction. And overall, this increases the efficiency of the process. So finding materials that are able to catalyze the specific reactions you want to do is a very intense area of research. Because as you can imagine, there are millions of materials and millions of reactions, and that combination really quickly becomes a very difficult space to navigate.


Paolo  14:33

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Paolo  15:07

Can you tell us a bit more about the reactions? You know the ones that tend to be more popular because a electricity you transfer electrons. So, this is a fundamental redox reaction happening at some point right but that could enable a variety of reactions. So, what are particularly hot areas at the moment, and can you describe some of you know the reactions that you have focused on during your study so far.


Matthew Liu15:35

So, I specifically look at nitrogen electrochemistry because there's so many oxidation states of nitrogen in the inorganic form. And some of the most common pollutants in wastewaters are ammonium, which is on the one end of nitrogen is oxidation state, and nitrate, which is on the complete other end of nitrogen is oxidation state. And they both are pollutants to waste waters, but converted in the right form, they can actually become a very valuable chemical, which is ammonia. Ammonium, for example, just needs to go through a pH swing to alkaline conditions and that will produce ammonia. Whereas nitrate needs to be reduced all the way to the other end of its oxidation state to ammonia before it's recovered. Which poses another challenge because wastewaters are so complex in this variable matrix of constituents. And so if you only want to get constituent A out of constituents A through Z, you need to develop some type of separation process or create a novel material that is able to sift through all of those parts and really get you just the target product that you want.


Paolo  16:52

I guess catalysis helps you because it might give you a chemical selectivity, right?


Matthew Liu16:56

Yes, and that that's a huge thing. So, some catalysts are beyond 90, 95%, selective for ammonia. And that really takes away from the challenge of all the side reactions that can occur when you're trying to go from nitrate to ammonia, you can produce five, six different side products just from trying.


Paolo  17:14

Can you use system where you have been asked sort of a series of catalysts so you can actually kind of isolate some of the different pollutants in your complex waste system, rather than having to isolate them and then process them separately?


Matthew Liu17:29

Because nitrate to ammonia is a challenging reaction, there are tandem catalysts that take you from point A to point B and point B to point C, which is the end. So you can go from nitrate to nitrite, which there are bacteria that do this, for example, in their, you know, their active sites inspire catalysts. But then going from nitrate to ammonia is another challenge. And so those can catalysts are increasingly being used to accomplish this transformation. And it's true the other way around, too, if you want to oxidize ammonia back to other products, hydrazine, nitrogen gas, nitrite, nitrate, a very similar game applies.


Paolo  18:11

What kind of catalysts you use, what are the natures?


Matthew Liu18:14

So one of the catalysts that we've worked with for a while is on titanium foil, just commercially available titanium foil, it's very commercially available. It's a very abundant metal, it's not expensive. It's corrosion resistant, and it's not very good at hydrogen evolution, which is the side reaction that could occur to nitrate reduction. So, we investigated titanium. But a key challenge of this material is that titanium is known to form a hydride phase, titanium hydride, and it's water stable, and it will form it under acidic reducing conditions, which is when we do nitrate reduction. And so the formation of that surface species was really kind of kind of a convoluting factor when we thought about what's really the active catalyst in our system, because as reaction is occurring, a new species is forming, and its interaction with nitrate, its interaction with titanium zero, or titanium oxide, became very puzzling question that we set out to investigate through synchrotron characterization. Through a combination of X-ray diffraction and X-ray absorption spectroscopy, we were able to understand how applied potential and reaction duration would affect that near surface formation of titanium hydride, and its impact on nitrate reduction.


Paolo  19:44

Do you study the behavior of your system with sort of different types of wastes? Or do you try and simplify your model first to study the, you know, whatever surface effects or whatever other parameters, you know, you might identify as read when or how does how does it work, you know, sort of conceptually in this sense?


Matthew Liu20:02

So we will sometimes go from either direction. We'll build up from the simplest case possible, which is, let's say, potassium nitrate and some supporting salt, try nitrate reduction, see what happens. But obviously, this isn't representative of any wastewater that we would eventually want to treat in many ways. I mean, we're missing a lot of constituents in the electrolyte. We're overestimating the concentration of nitrate, which is usually very dilute in, let's say, agricultural runoff groundwaters, few millimolar. Start off with a very simple electrolyte just so we can understand the fundamentals. And then very inductively, one by one, add another species, and add second species, a third species, and we start to simulate that wastewater, and understand by layering each new species, how's that affecting our reaction performance. But sometimes we go the other way because we need to treat a wastewater immediately. And so we have this wastewater that we've collected from a water treatment center or reverse osmosis facility nearby. And we just go right in, we put that real wastewater in our system, we run electrochemistry. And we see the performance where we are able to reduce nitrate with what selectivity’s, could we recover the ammonia, and then we start backtracking and say, okay, like, why do we think reaction performance was the way it was? What constituents can we measure it in the water that we think are responsible for either promoting or inhibiting catalysis? And so sometimes we'll approach a question that way. And I've definitely gone both ways throughout my PhD, depending on the context of what are we trying to really answer here.


Paolo  21:45

So making ammonia is one of the most important things that humans do. Right? You know, it literally is the most important parameter to feed billions of people. And yet, what we end up doing is, you know, wasting a lot of these precious material rights as discarding as a waste in the in the sort of linear economy we're living in. We're speaking about potentially recovering ammonia or putting it in the system and using it as our sort of viable material rather than being a waste. But this somehow has to fit sort of an economy, an economic structure, because making ammonium via the Bosch Haber Process, you know, as benefits from a huge economy of scale. Right. So, whatever you guys can come up with, it's going to compete. Is there any way that in your models or in your studies, you know, you put the cost factor in place? Or how far do you guys see the final applications?


Matthew Liu22:43

Yeah, so there's, I feel like there's a lot to unpack here. So there's so many inequalities in that Haber Bosch Process in that it is highly centralized process. And so there's only a few dozen of these facilities in the world, and they're concentrated in North America, Europe, and East Asia. But in parts of the world that don't have Haber Bosch facilities, let's say sub-Saharan Africa, the price of fertilizer is exorbitantly high compared to where you might purchase it in, let's say, the U.S. or Europe. If you look at the numbers, sometimes it's as much as 10, 50 times as more expensive for the same product, just because the distribution and the shipping is so far away from where it's actually being produced. And that that really exacerbates the inequality that's in play for such a crucial chemical that that people need simply just to farm food. So in that sense, electrochemistry will pose a benefit of allowing a more distributed fashion of ammonia to be produced, because you're not relying on these specially designed reactors that allow for, you know, 800 degrees Celsius, 500 Bar pressure, in only a few dozen places in the world can do it. If we can get to a place where we have modular reactors that use electricity to do the same thing, while you might not be able to achieve the same scale, but you can get the location of production much more widespread. And so this will certainly play a role in the economics eventually, of ammonia production. But I also want to get into the second point you brought up, which is the dollar amount, like how much are you paying just to produce this ammonia? Now, hydrogen evolution is at an interesting point, because recently, hydrogen has hit a point where the most expensive part of producing that hydrogen electrochemically is the maintenance cost actually. It's no longer the reactor or the catalyst or the electrolyte that you need. It's just maintaining the equipment. Making sure the parts are good for 5, 10 years, what's going to break down next. If we can reach a similar point for ammonia production now would be a good place to head to because right now, there are a lot of catalysts being explored. They're not necessarily cheap. And so finding catalysts that will lower the capital costs of production, and kind of bringing the costs to maintenance would be a way we need to head towards. Now we've looked at the economics preliminarily, for our electrochemical stripping process and ammonia production. And there are there's a lot going on at play. Because we're pumping electrolyte that accounts for energy, we need to input chemicals into the system that has some embedded energy into the processes that were needed to produce those chemicals. But ultimately, electricity was still the driving force, or for the cost that we had to pay to run the process. And electricity is already pretty cheap, so this is surprising that electricity was the most expensive thing in the process. But the reason is, we're not exactly efficient with the full cell potential that we can run our processes at, yet. And so even though at one side of the electrode, you're able to reduce nitrate at let's say, 1 volt versus some reference, the full cell potential can often be 5, 10, 20 volts. Because just to maintain such a high reaction rate, you need to be splitting an awful amount of water or driving some type of oxidation process to an enormous extent. And that voltage is really what's driving costs up despite a relatively cheap price of electricity.


Paolo  26:47

You know, once you start seeing investor money going into an area that's very positive, so there will not be shortage of funding for you guys.


Matthew Liu26:57

Yeah, I mean, government, for example, the U.S. National Academy of Engineering has identified for this century 12 grand challenges that need to be solved. And one of them is balancing the nitrogen cycle. But even in the public eye, this is starting to become a more kind of visible problem. Here in the Bay Area, recently, these algal blooms had just started appearing in the bay, and this was all over the newspaper, you could go to the waterfront and see them. And that's really because too much nitrogen was being discharged into the water and not being removed. And that promoted an overgrowth of these algae. And that starts to skew how the ecosystem works. And this actually costs the U.S. government billions of dollars a year as a holistic problem just to remediate. And so, with newspapers writing about it, with, for example, Bill Gates, thinking about clean water and sanitation, it's really coming into just out of just a purely academic sense, and really into the public eye as well. So, I think more and more people are starting to become conscientious of security to clean drinking water, and the future of access to clean water and energy, and even food security.


Paolo  28:12

It's fantastic, right? Is the science put to good use, right, and it's a very exciting field. It's also bad news for you, because you've started, you've started in such an exciting, you know, field it's going to be hard for you to keep the excitement at the same level, you know, will you be able to ever find like, a more a hotter and more exciting?


Matthew Liu28:35

Yeah, it's it certainly is going to be a challenge. I think with electrochemistry, there's still so much to explore that I, I'm feeling optimistic about, you know, different areas of research within electrochemistry as well.


Paolo  28:50

So you're now finishing grad school, right? Hopefully, at the end of this year, you'll be out with, you know, what you mentioned you'd like to pursue an academic career. What's your future? You know,what's Matthew science look like in the future?


Matthew Liu29:05

Yeah, certainly staying on the electrochemistry route, and staying with that combination of experimental work and modeling. But I think one area of research that I would really be interested to get into is using electrochemistry for organic synthesis. There's just so many chemicals that say, pharmaceuticals that that could be produced electrochemistry I think just such a key advantage of electrochemistry is molecular precision that ultimately allows for bulk change. And so with synthesis, you often need, let's say, stereo selectivity or atomically precise configurations in your reactants, your products, and the catalyst. And so, I think electrochemistry is definitely poised to be contributing to the synthesis of so many different useful products and this is still a relatively new idea. I think it has a lot of promise.


Paolo  30:04

I'm a chemist in my heart and so I completely understand what you're saying. This is so exciting, right.  Chemistry is now felt like steady for a number of years. But now with all these new, you know, advances in sort of adjacent fields, it has the potential to become very, very different in the coming years. I'm glad, I'm glad you mentioned it. And I look forward to see how you guys can change the landscape. Let me ask you this question, because I'm just interested in your, in your thinking process, you know, as you as you look at what you're doing now, but also how, you know, you've grown over the last few years, and maybe even thinking further, you know, obviously, you're cracking a lot of problems you mentioned in the modeling, you know, you're a very thorough way of looking at the problem is it, is the sort of problem solving aspect the, you know, the overcoming the challenge that takes you off to bed in the morning? Or is it more thinking that what you do as a potential strong impact on normal people's life?


Matthew Liu31:02

Believing in what your research represents, and what it can do is certainly, one of the most I can ask for out of my job in the first place. I think what really keeps me drawn to science are two things. One is that science has always been awe inspiring to me, in the sense that it humbles you to a truth that's greater than your own opinion, that's greater than what to want to believe or what you think it's something greater than that. I think, pursuing that has always given me this sense of awe that I really enjoy. Secondly,  science has always been this never-ending pursuit to me of mastery. It's really never going to end with your career or all the generations after you it's, we're just building after each generation that's come before us. And I think that kind of this never-ending pursuit of mastery is both inspiring and very part of human nature, it kind of represents this curiosity and fundamental optimism to improve that we've just carried on as a discipline. And so, these really get me out of bed every day to think about science and continue working on it.


Paolo  32:22

You know, and this, this is a perfect segue to my last question, which is always the same, right.  And, and it might feel like a bit out of place, because you're still probably considering yourself early in your career, and you are, but you know, you've done more than you might think, right? You know, what would be your suggestion for someone who's just starting right now, maybe, maybe undergrad, or is just just picking they're just making their first early choices for what their career might look like in the future.


Matthew Liu32:50

I guess just two things. One is to stay flexible with what they think they want or where they think they're headed, just to kind of keep options open and not be too sold on one idea that they might have had developed prior. Because there are so many opportunities that come in grad school, and I think staying flexible is going to help you identify those and kind of roll with them as they come. Secondly, would just to really take care of yourself and look after your well-being both physical and mental health are too important to let go just for the sake of extra hours in the lab or to produce results. Really, one of the greatest gifts you can have is trying to maintain a good physical and mental health that can allow you to overcome challenges that come your way.


Paolo  33:50

Thanks for joining us for this episode of Bringing Chemistry to Life and keep an ear out for more. As usual, if you enjoyed this conversation, I'd encourage you to check out Matthew's book, video, and podcast recommendations. Look in the Episode Notes for instructions and for a link to register for a free Bringing Chemistry to Life t-shirt. And if you will, please consider sharing the episode with a curious friend or colleague. This episode was produced by Sarah Briganti, Matt Ferris, and Matthew Stock.