Human World

Nate Lewis on artificial photosynthesis

Nate Lewis, the George L. Argyros Professor of Chemistry at the California Institute of Technology, works to develop new technologies to meet the immense energy needs of the future in a sustainable way. Lewis specializes in what’s called artificial photosynthesis. In nature, photosynthesis is the process plants use to make food from the sun’s energy. Dr. Lewis works to mimic that process. Using special materials, he builds tiny cells that – when hit by light, and surrounded by water – create hydrogen fuel. Hydrogen burns “clean.” That is, it doesn’t produce carbon dioxide (CO2) when it’s combusted. This podcast is part of the Thanks To Chemistry series, produced in cooperation with the Chemical Heritage Foundation. Generous sponsorship support was provided by the BASF Corporation. Additional production support was provided by the Camille and Henry Dreyfus Foundation, DuPont, and ExxonMobil. Nate Lewis spoke with EarthSky’s Beth Lebwohl.

Plants use sunlight to make food. That’s photosynthesis. But your lab is working on an artificial photosynthesis. What’s the goal?

Plants figured out that the best way to make and harness clean energy would be to take the biggest resource we have – the sun – and convert it into the thing that drives almost all energy and consumption on our planet today, which is chemical fuel. But plants don’t do it very efficiently, and they make a fuel that we can’t use, at least not directly, unless you want to eat the delicious vegetables that come out of it. But most of what plants make can’t directly be used as fuel by humans.

In the same way that birds have feathers, and we know that therefore it’s possible to fly, but we don’t build airplanes out of feathers, we know it’s possible to take the sunlight and make chemical fuel. We’re going to build our machines that are going to take sunlight and directly make fuel that anybody could use anywhere, anytime, for their energy.

Let’s talk about a specific product from your lab – a photoelectrochemical cell used in artificial photosynthesis with the goal of making hydrogen fuel – in the simplest possible terms. How will it work?

We know it’s possible with semiconducting materials like the ones used in solar panels, but a different set of materials like platinum and silicon, to actually take those materials, and instead of covering them with electrical wires, we immerse the material in water. And adding sunlight, one can split that water and produce hydrogen gas and oxygen gas directly. You would collect the hydrogen, and then could use it later in a fuel cell. Or you could convert it into a liquid fuel, or use it for other things. You would then get the oxygen back from the air at the point of combustion of the hydrogen or the other fuel you made. We know this already works.

You talked about splitting water. What exactly do you mean by that?

Water has the chemical formula of H2O. To split it, you re-juggle the bonds in the water, to make one molecule of H2, and one half of the O2 that makes the molecules of oxygen that are in our air.

The fuel that results from that is the hydrogen – the H2 – because that can be stored and then burned. Just like gasoline is burned with oxygen from the air, the hydrogen is burned with oxygen from the air. In this case, instead of making carbon dioxide, it would make water. So it is clean-burning, because the only byproduct is actually drinkable water from the combustion process.

What does this photoelectrochemical cell look like? What’s inside of it that’s making it do this work?

It’s just going to be a flexible material, kind of like the Slip ‘n Slide or bubble wrap, a multifunctional fabric that you’ll roll out, and there’ll be a top clear layer that will suck up water like a sponge from the air. Then the intermediate layer will absorb sunlight, and will decompose the water molecules into hydrogen and oxygen. We’re going to let the oxygen get vented just like through a rain jacket when you let it breathe. At the bottom we would wick out either the gaseous or the liquid fuel, collect it into a tank, and then we could use it to run our cars, to run fuel cells, to make liquid fuels out of, to provide the energy that we need even when he sun isn’t shining.

What is the timeline on this? When can we expect to see this on the market, in general use or in use in industry?

Our goal is to build prototypes that actually work in the first two years of this project, called the Joint Center for Artificial Photosynthesis, which is an energy innovation hub sponsored by the Department of Energy.

And so we are launching a very aggressive project, because no one has actually built a solar fuel generator that you can hold in your hand that is truly an artificial photosynthetic system. We know that the first prototypes we build are not going to work very well, or maybe not last very long, or maybe use too expensive pieces. And then we’re going to build a second one, and it’s going to work a little better. And then we’re going to build the third one, and it’s going to work better still. We’re going to learn from our mistakes until we build a fifth one that is really the one that is the one we’re try to think about moving into the commercial enterprise.

We think this is an evolving generation of technology development. But you can’t fly until you get off the ground, and our goal is to get off the ground, to build the thing that shows that we can create a technology that can really, directly do what plants do, but better, make fuel directly from the sun.

What are some of the big obstacles you’re facing now or have faced in the past with regard to artificial photosynthesis?

It’s chemically difficult to take the photons of light and the electrons that are produced willy-nilly all over the place in a material, and then to couple them together to make and break the chemical bonds that are needed to do real photosynthesis. We need to develop those catalysts that can do that, as well as the materials to absorb the light to deliver those electrons to those catalysts, so that all the pieces of the system work together in harmony all at the same time.

What’s an example of such a catalyst?

A catalyst right now that splits water into hydrogen and oxygen would be an expensive metal like platinum coupled with another expensive metal like ruthenium in the ruthenium dioxide form. We know they work extremely well. They just are way too expensive to think about using for covering very large areas needed to harness sunlight. We know that nature knows how to do this. It doesn’t use metal. In enzymes that bugs use to make hydrogen they use iron, a cheap metal that comes out of rust. They use nickel, the same stuff that we used to use to make our coin nickels. So they use really cheap stuff, and we need to figure out, as chemists, how to make the cheap metals work just as well as the expensive ones in order to really have an affordable technology.

What’s the most important thing you want people to know today?

The most important thing is to know that if we want to get to a clean energy system, we can get part of the way there with existing technology, with wind, with solar, with nuclear. But you can’t get all the way there with just making cheaper what we know. The two biggest challenges are how do you store massive quantities of electricity, and how do you make clean fuel for the 40 percent of transportation that cannot be electrified – our ships, our aircraft, our heavy-duty trucks? And other than a limited amount of biofuels, the only technical game in town that could solve both of those problems that we have to solve as a planet in order to make a sustainable, environmentally responsible secure future is to make fuel from the sun. And that’s why we are working so hard on that project.

Listen to the 8-minute and 90-second EarthSky interviews with Nate Lewis on artificial photosynthesis, at the top of the page.

November 8, 2011
Human World

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