Nate Lewis
Chemistry Professor, CalTech

The Key to Solving the Energy Crisis: A Leaf?

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A process that mimics photosynthesis could create a fuel that provides energy in a convenient form.

Nate Lewis

Dr. Nathan Lewis, George L. Argyros Professor of Chemistry, has been on the faculty at the California Institute of Technology since 1988 and has served as Professor since 1991. He has also served as the Principal Investigator of the Beckman Institute Molecular Materials Resource Center at Caltech since 1992. From 1981 to 1986, he was on the faculty at Stanford, as an assistant professor from 1981 to 1985 and as a tenured Associate Professor from 1986 to 1988. Dr. Lewis received his Ph.D in Chemistry from the Massachusetts Institute of Technology.
Question: How do we close the gap in terms of finding a cost-effective solution for storing energy? 

Nate Lewis: I think that, personally, ultimately although we can think about storing electricity in batteries, or think about storing it in compressed air, those suffer from energy density. The amount of energy you can actually pack into a given volume. Now, we know that this is intuitively true because even though you can compress air and store energy, everybody knows you can’t go very far in your car by storing the energy and deflating your tires. Everybody knows that you can’t really go very far on batter energy for powering your house. You can’t even light up that little dome light in your car for very long before you drain the battery; imagine trying to run your big screen TV or your toaster, or some other electrical load. You’d need banks and banks of batteries. 

The reason for this is the energy density in the best battery, so far, is 200 watt hours in a kilogram of the whole mass of the battery. The energy density in a gallon of gasoline is 13,000 watt hours per kilogram. Chemical fuels and chemical bonds are by far the best way to store energy we know of in the universe other than in the nucleus getting it from the atom. 

And so I personally believe that we’re going to have to find a way to do what nature figured out. We’re going to have to be able to store energy in chemical bonds, just like a plant does, storing it in the bonds through photosynthesis. Now, plants make fuels that humans can’t use. We have to work hard to put an energy and boil it and cook it and process it to make lignocelluloses junk into biofuels like ethanol, or butenol, or something else that we could use. But we do have to find a way to directly take the sun and make a fuel that humans can use, like hydrogen, or gasoline, or diesel fuel, or biodiesel, equivalent so that we can get energy to where ever people need it whenever they want it. 

Question: What are the steps involved in capturing solar energy in an artificial leaf? 

Nate Lewis: I personally do research in the area of artificial photosynthesis, of trying actually to mimic by inspiration the functions of what a leaf does in photosynthesis which in its essence is taking the energy from sunlight and converting it directly into stored chemical fuel. 

Now, plants don’t do this very well. In fact, they stop working at a light intensity only one-tenth of that in the bright sun. And the reason is, the plants are meant to be working in a canopy of shade. And if you get too bright sunlight on them, they actually have radical damage from their free radicals and they have to expend a lot of energy to rebuild their photosynthesis machinery every 30 minutes, even under this dim sunlight. And if they get more bright sunlight, they have to spend a lot more energy just to stay alive. 

So, we have to find other components that do what the photosynthetic chlorophylls in plants do, but that don’t saturate at a tenth of the light intensity of sun. That don’t involve trading off land that we need to grow for food, instead for fuel and then don’t have the drawbacks of having to have these living systems that we need precious green matter on our planet for other things. We think we can do this. We have pieces that work much better than a plant’s pieces do. In the same way that although birds have feathers airplanes don’t, and they still fly pretty fast. So, we’re taking little pieces of semi-conductors on the nano and micro scale and using them instead of the chlorophyll that nature uses to absorb sunlight, they capture that light and then they separate that electricity but we don’t run it through wires. Instead, we hook it up to catalysts that just like a leaf; store that energy, not as electricity, but in making the chemical fuels hydrogen and oxygen from only sunlight and water. This is a true artificial photosynthesis because we are doing the function of a plant of taking sunlight and cheap chemicals and as the output we’re making a fuel that can provide energy to people to use in a convenient form. 

Question: How do we make this process affordable? 

Nate Lewis: So, right now in this field, there are three things that we would all like to have. We would like it to last a long time, we would like it to be very efficient so we don’t have to cover very large land areas, and we would like it to be really cheap. And right now we can have two out of those three, but not all three be at the same time. The pieces that we have that last a long time and that work really well are too expensive. The pieces that we have that are cheap, and that last a long time, don’t work very well. And this is the issue. So, we have a demonstration system that does tell us there is a light at the end of the tunnel. But we have to find ways to replace expensive metals, like platinum, with cheap metals like iron. We have to find ways to replace expensive, pure semi-conductors with cheap light absorbers. 

Now we know that nature has figured this out because nature doesn’t use platinum in algae that makes hydrogen; it uses a cheap metal, iron. Nature’s catalysts aren’t poisoned by carbon monoxide and sulfur; they actually use them in the molecule to keep it function. So, people like my colleagues in chemistry are fishing these active sites out of bacteria and trying to build models of them as molecules to do just what nature did what the same cheap chemicals that it used as opposed to expensive ones. 

Question: Sounds like alchemy. 

Nate Lewis: It’s actually not alchemy– it’s bioinspired. Once you knew that a bird could fly, you didn’t build an airplane out of feathers? We built it with other materials. And so, really what it is, it’s like building a B-2 Bomber in the sense that we have a system that we think could be a new design of flight. It’s going to be stealthy and it’s going to have a different range, and it’s going to have these wings that are different than normal airplane wings, and it’s going to fly. But to do it, we have to put all the pieces together. We have to invent some of these pieces. And it’s not enough to just invent the engine. You’ve got to have the wings. You’ve got to have the new materials. You’ve got to have the plane in the air. It’s got to fly for a certain distance, and it better come back on one piece, and you’ve got to be able to make a lot of them, not just one. 

And so we have these criteria that a successful system isn’t just one piece, but is the whole thing flying on it’s mission and returning safely and being able to make a lot of these energy conversion devices so that anybody can put them up anywhere. 

Recorded on February 3, 2010