Supplying the World's Energy Needs with Light and Water
A leading chemist says that a better understanding of photosynthesis could lead to cheap ways to store solar energy as chemical fuel.
While researchers and technologists around the world scramble to find cleaner sources of energy, some chemists are turning to nature’s own elegant solution: photosynthesis. In photosynthesis, green plants use the energy in sunlight to break down water and carbon dioxide. By manipulating electrons and hydrogen, oxygen, and carbon atoms in a series of complex chemical reactions, the process ultimately produces the cellulose and lignin that form the structure of the plant, as well as stored energy in the form of sugar. Understanding how this process works, thinks Daniel Nocera, professor of chemistry at MIT, could lead to ways to produce and store solar energy in forms that are practical for powering cars and providing electricity even when the sun isn’t shining.
What’s needed are breakthroughs in our understanding of the fundamental chemical processes that make photosynthesis possible, according to Nocera, a recognized photosynthesis expert. He is studying the principles behind photosynthesis and applying what he learns to making catalysts that use solar energy to create hydrogen gas for fuel cells. Nocera’s goal: a world powered by light and water.
Technology Review: What’s the biggest challenge related to energy right now?
Daniel Nocera: The real challenge with energy is the scaling problem. We’re going to have this huge energy need, and when you start looking at all the numbers, there’s only one supply that has scale, and it’s the sun. But it’s still a research problem. Technologies all follow lines; then there’s a discovery and a new line that’s better. We’re on a very predictable line now in solar. Most things you hear about are incremental advances.
TR: You’re studying photosynthesis to get ideas for how to convert sunlight into a chemical fuel–hydrogen–for use when the sun isn’t shining or in powering fuel-cell vehicles.
DN: You can use the electricity directly when the sun is out, in places that have sun. [But] you need storage. There’s absolutely no way around it. I am distilling the essence of photosynthesis down to be able to use it.
TR: Why is photosynthesis attractive in finding a source of clean energy?
DN: [Photosynthesis] does three things. It captures sunlight, and [second,] it converts it into a wireless current–leaves are buzzing with electricity. And third, it does storage. It stores the converted light energy in chemical energy. And it uses that chemical energy for its life process, and then it stores a little.
It turns out [that] photosynthesis is one of the most efficient machines in the world for energy conversion. But it’s not great for storing energy because that’s not what [a plant] was built to do. It was built to live and grow and reproduce.
And so that’s the approach we take. Can we now do what the leaf is doing artificially, which is the capture, conversion, and storage in chemical bonds? But my device doesn’t have to live: it can take a lot more of that energy and put it into chemical bonds.
TR: And you’ve had some success putting what you’ve learned to use.
DN: We did make a compound that makes hydrogen using light. We have something that you can dissolve in solution, shine light on it, and hydrogen comes bubbling up. It didn’t do it that efficiently. But it was a big advance because it had a lot of new concepts in there to show how you can use sunlight to make hydrogen.
TR: What are some of the research problems you’re addressing that you hope can lead to a big step forward in solar?
DN: Something we’ve really been working hard at is [understanding] the design principles that photosynthesis operates off.
One is that when [photosynthesis] splits water into hydrogen and oxygen, it uses more than one electron. This current that’s running is going one electron at a time. But then [the plant] stores them and uses four electrons at once. We don’t know how to do multi-electron reactions very well. We don’t even have theories to describe them.
And then you have to manage protons–and that’s what biology does really well. It takes electrical current and then it converts it to a chemical current, and the thing conducting the chemical current is protons. And then it sends atoms. What a photovoltaic does is send electrons to a point. Photosynthesis actually sends not an electron but an atom. And that’s even a tougher thing to do because atoms are so much heavier than electrons. So we’ve gotten down deep into understanding, how do you move atoms [such as hydrogen atoms] around from point A to B so that they can join up with each other? How do you assemble them so they can unite?
TR: You’ve written that chemistry “will likely play the most central role of all the sciences” in addressing energy problems. How would you summarize the role of chemistry?
DN: For game changers, it’s really easy. There’s three.
Make photovoltaics cheap, which is a lot of chemistry. It’s inventing new materials to make PV cheap.
Replace noble metals–things like platinum–with abundant metals. Because there’s not enough stuff. When you’re talking about this much scale, you better be using things like iron and manganese. You better look at your book that says what are the most abundant elements on the face of the earth.
TR: And this is for fuel cells, and also for photovoltaics.
DN: Photovoltaics–everything. That’s the real technology issue that you have to keep in your mind. Not something that’s so great, it’s 100 percent efficient–and oh, by the way, I’m using ruthenium. I can use ruthenium now to teach me a principle, but ruthenium’s below iron [on the periodic table]. So I better figure out, how can I take everything I’m learning with ruthenium and apply it to iron?
TR: And the third game changer?
DN: Split water with light. You do those three things, and you have a full new energy economy. It’s hard for me to say exactly what that technology will look like, because the science is missing. But at the beginning of the 1900s, we built an entire society based on a new energy system. And I believe, once solar is in place, with help from biofuel, with a little help from wind, we will invent our society again from a new energy source.
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