Q&A: MacArthur ‘genius’ explains why artificial leaves need to work better than real ones
It took nature millions of years to figure out how to turn sunlight into chemical energy that plants can store for a cloudy day. It took UC Berkeley chemist Peidong Yang about 10 years to accomplish a similar feat with the help of semiconducting nanowires and bacteria.
That's one of the reasons Yang was among the 24 people who received a “genius” grant from the MacArthur Foundation this week. The $625,000 award can be spent however he sees fit; there are no strings attached.
Yang and his collaborators have created a synthetic leaf that uses the ingredients plants require for photosynthesis — water, sunlight and carbon dioxide — to make liquid fuels like methane, butane and acetate. And just like nature's version, the synthetic leaf releases oxygen into the air.
The technology is still several years from being commercially viable, but it represents an important step on the road to creating a truly carbon-neutral and sustainable fuel system. “Yang’s advances in the science of nanomaterials are opening new horizons for tackling the global challenge of clean, renewable energy sources,” the MacArthur Foundation said in its award announcement.
Yang spoke with The Times about his work with synthetic leaves, the future of artificial photosynthesis, and how soon we can hope to be powering our cars with fuel made from the sun.
So, how does a person learn they’ve been dubbed a “genius”?
The foundation called me on my cellphone. It was a total surprise. It is such a huge honor to get this — it is great recognition for the research we have been doing.
How did you get interested in artificial photosynthesis?
This work started in 2002 as a continuation of our semiconductor nanowire research. These nanowires are a unique type of one-dimensional nano-structure made of a semiconductive material like silicon that are about 100 to 1,000 times thinner than a human hair. We spent a lot of time looking at their special optical properties, and that led to our current research.
When you want to convert solar energy to chemical energy you need something to capture the light, and semiconductor nanowires are very good at capturing solar energy.
How is your artificial leaf different from a rooftop solar panel?
In a solar panel you have a semiconductor absorbing solar energy, and that energy is converted to electricity. But in our artificial photosynthesis, the energy captured by the semiconductor is stored in the carbon-carbon bond or the carbon-hydrogen bond of liquid fuels like methane or butane.
How similar is artificial photosynthesis to the photosynthesis the plants around us do every day?
Green leaves absorb solar energy, take in CO2 and water and then release oxygen into the environment. In the meantime, they convert the CO2 into fuel they can use later —sugar, for example.
Our artificial photosynthesis does exactly the same thing. We start with water and CO2, and we put in solar energy. We convert that to oxygen and also acetate, methane or butanol, just like in a green leaf.
Your system includes bacteria too. What role do they play?
The bacteria take electrons from the nanowires and use them do the CO2-reduction chemistry for us. We feed them a solar-generated electron, and they spit out the product.
But bacteria are just one type of catalyst. We are also using synthetic catalysts like nanoparticles and semiporous organic metals. At the moment bacteria have the best activity and selectivity, but they live and die, and that’s a problem.
Can you collect actual liquid fuel from the system you have set up?
Yes. In the lab, we culture the bacteria in a one-inch-square forest of nanowires. We dip that into water, pump in CO2 and shine solar light on it. Then, after several hours or several days, you collect the chemical product.
How would those chemical products be used?
Now we are photosynthesizing chemicals like methane and butanol. In the future, we hope to make even more long-chain carbons. They can be considered the fuel of the future, not just for our cars but for anything that requires energy.
How close are you to being able to use artificial photosynthesis on a large scale?
This year, we finally came up with a first-generation, fully-functional system — and that’s after 10 years of research. We demonstrated its feasibility, but in terms of robustness and cost and efficiency, it is not close to being commercially viable.
To do basic research, we have to be patient. I’m a big believer that discovery cannot be planned. It requires support from the government and industry. It will take the work of one or two generations of talented people to solve this problem.
Do you think artificial photosynthesis can ever compete with natural photosynthesis?
We want to learn from nature, but we have to be better than nature.
It took evolution millions of years to get green plants and leaves to their current stage, but their solar-to-chemical-energy efficiency is not that high. All they need to do is make enough energy to survive. To come up with a commercially viable technology, we have to do better than that.
Is that possible?
Theoretically, it is certainly possible. In solar panels the energy conversion efficiency is above 20%, much higher than what is happening in leaves. So in terms of design, we have the advantage — nature doesn't have silicon to use. We do.
This interview has been edited for length and clarity.