Creating fuel from thin air with artificial leaves
Artificial leaves could one day provide fuel
The sun produces more than enough energy for human activity, but we still cannot capture enough of it, says Erwin Reisner, professor of energy and sustainability at Cambridge University.
He's leading a team of researchers trying to get more of this free energy.
While solar panels have made great strides in recent years and have become cheaper and more efficient, they only provide electricity, not storable liquid fuels, which are still in high demand.
"If you look at the global energy portfolio and demand, electricity only covers 20-25%. So the question is when will we have that 25% covered. What do we do next?" asks Prof. Reisner.
His answer is: Looking at nature: "Plants are a great inspiration because over millions of years they have learned how to absorb sunlight and store the energy in energy carriers.
"I really believe artificial photosynthesis will be part of that energy portfolio over the next two decades."
When plants photosynthesize, they absorb water and carbon dioxide and use sunlight to convert these raw materials into the carbohydrates required for growth.
Prof. Reisner is optimistic that artificial photosynthesis will become an important energy supplier
"We want to repeat this, but we don't really want to make carbohydrates because they produce lousy fuel. Instead of making carbohydrates, we try to make something that is easier to use," says Prof. Reisner.
An additional problem is that plants are not very good at photosynthesis and only convert about a percent or two percent of the sun's energy into fuel. The US Department of Energy has concluded that for artificial photosynthesis to be economically viable, efficiency must increase to five to 10%.
Prof. Reisner's team has worked on a number of approaches, including a system that mimics natural photosynthesis and uses enzymes to split water and produce hydrogen as fuel.
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However, the efficiency is still low and hydrogen is difficult to store as a gas.
Perhaps more promising in the long term is the recent development by his team of a small device that converts sunlight, carbon dioxide and water into oxygen and formic acid, a liquid fuel with a high energy density.
The device contains a plate that sits in a bath of water and carbon dioxide. When exposed to sunlight, the panel releases electrons which combine with the carbon dioxide and protons in the water to form formic acid.
"These systems are like panels or sheets. It's a very thin device - you can almost imagine it like a sheet of paper," says Prof. Reisner.
Perhaps the biggest step forward with the device is the fact that it is stand-alone. There is no need for an external power source or charging additional catalytic converters.
The artificial leaf contains a plate that reacts with sunlight, carbon dioxide and water to make fuel
Despite the challenges, artificial photosynthesis is attracting heavyweight investments. In the US, the Department of Energy recently announced a $ 100 million (£ 76 million) funding over five years.
The money will go to two separate projects: the Center for Hybrid Approaches to Solar Energy for Liquid Fuels (Chase) and the Liquid Sunlight Alliance (Lisa).
Chase, headed by the University of North Carolina at Chapel Hill (UNC), is working on practical applications similar to the Cambridge device by developing systems that, like solar panels, use semiconductors to absorb light and then use various catalysts to do that Light convert carbon dioxide into fuel.
According to the Deputy Director of Chase, Prof. Jillian Dempsey, a particular research focus is the concept of cascade catalysts. To convert carbon dioxide into a usable fuel, more than one chemical conversion must be performed - and catalysts can only process one at a time.
"The first takes the first step and then passes its product on to the next catalyst," she says. "Everyone would carry out a very selective process and pass it on to the partner after this single step."
US researchers are studying the use of sunlight to produce storable liquid fuels
The Lisa project takes a more theoretical approach and focuses on improving all stages and components of artificial photosynthesis. Potential catalysts and processes are modeled by the computer before they are tried out.
"We have made great theoretical efforts, and theory and experiment go hand in hand," says project leader Prof. Harry Atwater from Caltech. "We now have the largest database in the world right now, period."
The bad news is that we probably won't see fields full of photosynthesis panels in the near future. According to Prof. Dempsey, there are still major stumbling blocks.
Bringing all of the technology together in one package is a problem.
"There's an incredible science out there about light collection, about the catalysis that fuel is made from, and about managing systems," she says.
"However, integrating these individual components into a system for artificial photosynthesis is a major challenge."
It is also difficult to ensure that the reactions produce a commercially viable fuel, and many of the catalysts that can achieve this are too expensive or too inefficient for large-scale use.
After all, according to Prof. Dempsey, durability is an issue: "When you deal with constant radiation [sunlight] it can lead to a reaction that can be very harmful and corrosive."
As a result, artificial photosynthesis still cannot produce liquid fuels cheaply enough to compete with fossil fuels.
"But the dynamics can change very quickly," says Prof. Reisner.
"The price of oil can change, taxation can change. And if things change, at some point in the future the price of artificial photosynthesis will go down and the price of fossil fuels will go up. The only question is when these lines cross." .
"If you go back 10 years, even the most optimistic predictions for the cost of photovoltaic electricity did not match. The cost has decreased by 85%, which is incredible. Once the economies of scale are in, a lot is possible so I'm very much optimistic. "
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