Contributed by Ashley Baker, Research Assistant, ACS GCI
Dr. Nathan Lewis is the George L. Argyros Professor of Chemistry at the California Institute of Technology where he has been a professor at since 1988. The CalTech Lewis Group focuses on photoelectrochemistry and chemical vapor sensing. Recently, Nate Lewis participated in an ACS Webinar, in partnership with the ACS Green Chemistry Institute® (ACS GCI), where he discussed his work on transforming water, sunlight and carbon dioxide to make fuel. He will also be one of the esteemed keynote speakers at the 20th Annual Green Chemistry & Engineering Conference this June. ACS GCI had the opportunity to interview Lewis regarding his current research.
Q: Have you noticed changing attitudes towards green and sustainable chemistry over the past twenty or so years?
A: Oh absolutely. It wasn’t really a topic of conversation 20 or 30 years ago. Now there are symposia at American Chemical Society (ACS) meetings on it, there are special forums and sessions - not just with ACS and chemists professionally but with business people, the general public and environmental groups. I think there’s really been a sea-change in the attention given to and considerations for green and sustainable chemistry.
Q: How do you imagine the concept of “design” empowering chemists to make their research and products more benign?
A: We’re doing this, for instance, in our solar fuels effort. We could already convert sunlight indirectly into fuel by using a photovoltaic array and hooking it up to an electrolyzer. But that’s too costly, and there are barriers to scalability. At some level we won’t be able to mine enough iridium – the scarcest element in the periodic table – to be able to make electrolysis that uses iridium oxide as the only really catalytically active material for water oxidation under those conditions scalable and sustainable. So we’re working on substituting those catalysts with others that are much more abundant. We’re working on substituting scarce elements like tellurium and indium in the materials used to make the light absorbers with much more abundant ones like silicon and metal oxides that are found in minerals and ores. You could argue that if we didn’t have that first and foremost as a consideration we wouldn’t have anything to do because there are already materials that do many of these functions they’re just not sustainable and simultaneously cheap, efficient, robust and safe. You have to have all four at once to have a viable system. That’s where the “materials by design” challenge comes into the forefront.
Q: What do you think of all the solar panel scale-up that’s happening right now? Is it sustainable?
A: It’s been raised as a concern for cadmium telluride thin films. The scarcity of tellurium might well be a barrier in maybe a decade or a little more to continue scalability in the future. But silicon, that’s not an issue of course because it’s abundant in the earth’s crust as sand. There’s plenty of silicon. We might see issues with the indium used to make the film contacts, and we of course have big demands on indium for displays as well. We haven’t run out yet and it’s an issue that should be considered, but it’s not thought to be as stringent a concern as for instance iridium in electrolysis, which is obviously a big concern.
Q: Throughout your webinar talk, you mentioned being inspired by nature but not copying it. In what ways does nature/biomimicry inspire your research?
A: Artificial photosynthesis is completely, in our implementation, inspired by natural photosynthesis in that the blueprint of natural photosynthesis is two light absorbers, two chlorophylls hooked together like batteries in a series to give the voltage needed to make fuel. Plus, two catalysts: one to oxidize water and the other to reduce something to make the fuel that powers the cell. Or in our case, fuel that we can directly use in our infrastructure. Then, you need a membrane to separate the products and pass protons to neutralize the charge that passed across that membrane from the photo-generated event.
Those five components are a robust, we think, blueprint for how to build a photosynthetic system of any kind, natural or otherwise. So as our analogy said, birds fly but you don’t build aircraft to fly with feathers. You design them differently to be optimal for different functions and specificity. In our case, we want to use two absorbers that are different colors. They don’t fight for photons. We want to make catalysts that don’t have to be rebuilt every thirty minutes like the active site - the oxygen-evolving complex - of photosynthesis. We want to make a fuel not ATP and not use RuBisCo, a pretty inactive enzyme and thus one of most abundantly expressed proteins on earth. Instead we want to use a robust, inexpensive, scale-able catalyst to make the fuel. So, it’s stuff like in the analogy. Even if you have the Wright Brothers that doesn’t mean you have a 787 Dreamliner. We want to get to the ultimate cost-effective, scalable, functional system, not just to any system.
Q: Do you have advice for people who are starting up businesses that are facing the “valley of death” but want to sustainably scale their ideas?
A: This issue is related to setting a long term strategic vision of what needs to be done for scalability while also having the short-term tactical vision of building something that can address the marketplace. They’re both perfectly fine, but you need to do them both in parallel.
If we want to right now, we could use existing platinum and iridium-oxide based catalysts and PEM-based electrolysis and make hydrogen at some cost with solar or nuclear or other no-carbon or low-carbon electricity sources, and you could make a lot of hydrogen that way. Dow Chemical Corp. makes a lot of hydrogen that way through the chlor-alkali electrolysis process, where they also get value out of the chlorine they make as the primary product of commercial interest. They make a LOT of chlorine this way with a scarce element – ruthenium or sometimes even iridium. That doesn’t mean there’s not a good business to be had there. To the contrary, it does mean that if that’s the only end goal then along the way we won’t get to a clean fuels economy if we don’t consider the bigger picture of sustainability at the same time.
I like to say we don’t want to get to the Grand Canyon and not be able to cross it. We need to start with technology that can build a bridge over those key barriers by the time we get there so we’re not surprised. In the meantime, there are lots of other things we could and should be doing but we would be remiss if we didn’t recognize the key major scalability barriers and worked on them while we still had the chance to do so.
A short-term fix is not a substitute for the long term fix. But because you don’t have the long term fix doesn’t mean there aren’t things you shouldn’t also be doing in the short term, otherwise you never get to where you’re going.
Q: What would your advice be to researchers who are using critical materials (like platinum or rhodium) but don’t know how to get away from their use?
A: I think it’s been shown that you can get away from using these materials. We’ve gotten a lot of functionality out of earth abundant metal phosphide acid stable hydrogen evolution catalysts. We did that in collaboration with Raymond Schaak’s group at Penn State. It was inspired by an analogy with hydrodesulphurization catalysts and reactions. It was also inspired by – in addition to some chemical intuition – a theoretical prediction by Jose Rodriguez at Brookhaven National Laboratory . And so we combine the ways that catalysts are discovered. You have to want to discover them, you get some guidance from theory and you also use some chemical intuition to tell you which spaces you might be looking in are more promising than others. You can’t build a better mouse trap unless you agree that mice are a threat. So we have to do discovery research if we’re going to discover new things.
Q: What do you see as the biggest challenges in the way chemistry is currently taught, practiced and implemented?
A: Well I don’t know that I know enough to talk about that because I don’t have a broad, necessarily hands-on exposure to the total breadth of how chemistry is taught and experimented. I do think that there’s a lot more visibility on sustainability considerations, on clean energy considerations. I hear these calls for doubling the budgets in these areas, and that certainly couldn’t hurt anything. It would probably be a much needed step toward helping research in this field, whether we’ll get that is another story and depends on, I’m sure, to some extent what happens in the upcoming elections. It’s almost a shame that it needs elections in order to just do good sound science policy planning for our energy future. But clearly it’s going to be a factor, which is fine, but I think we should get that behind us and get on with it.
Q: Where do you think we’ll be 20 years from now in terms of incorporating green chemistry and engineering into academia and industry?
A: I think we’re going to be a lot further down the road. Again, it depends on unfortunately what the global emphasis is and what the funding picture is and how much the public and funding and emphasis continues to pay attention. We’ve got a good start, but we lost two decades. We didn’t really pay attention to this in the 1980’s and 90’s because even though we had the Kyoto Protocol going, we had cheap oil. And we still have cheap oil now. When you have cheap, abundant fossil energy it’s pretty difficult to see the forest because of the trees and say that sustainability is going to be an important consideration in the long term that we have to take care of doing the R&D now. That’s going to be a challenge. In fact, cheap shale gas is arguably a challenge for renewable electricity because it’s got now such a highly competitive, now inexpensive, fossil based source to compete with in the market place. That doesn’t mean we shouldn’t do it, and it doesn’t mean we shouldn’t be doing the R&D to make an affordable, scalable, clean, cheap energy system. It does mean it’s a little harder from the short-term, pure economic viewpoint to see the justification of doing it right now, although I think all the scientists could make a compelling case that we should be doing it.
Q: Do you have any particular thoughts or things you’re excited about for coming to the green chemistry and engineering conference?
A: We’re making a lot of progress. Not just me and my group, but the general community of researchers not only in solar fuels but in sustainable energy, and there’s no indication that we’re going to stop. I think this is all a very positive thing. We train students that are thinking the same way and we motivate people to think about the problem from many different perspectives. I’m excited to see engagement in the conversation at the national and international level and how it’s moving forward on both climate and energy policy, and observe that numerous very positive things that are happening. It’s our job just to keep the ball rolling and the momentum going and not screw it up. If we do that, then we’re in pretty good shape.
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