Solar fuels could soon replace fossil fuels - and we already have the technology to do it, providing that society grasps the opportunity, claims Daniel Nocera, professor of energy at Harvard University in Cambridge, Massachusetts, and the inventor of the ‘artificial leaf’.
“If the society was willing to use hydrogen as a fuel, we’re already there. We could start building an energy infrastructure based on hydrogen today,” said Nocera at the First International Solar Fuels conference in Uppsala, Sweden. “But people are slow to adopt new energy or new fuel systems.”
In fact, even with fossil fuels, the sun has been the real hero all along.
Fossil fuels are, in a way, solar fuels. They were created in organisms through photosynthesis, which then turned into the fossils that eventually turned into coal, crude oil and natural gas. "The problem is that this happened over millions of years and we are now burning them up much faster and nature can't keep up with making more," says Uwe Bergmann, a physicist at SLAC National Accelerator Laboratory in Menlo Park, California.
Instead, we can make a radical swap to a completely new energy system that plugs into the existing infrastructure, simply by using fuel cells that store energy generated by existing photovoltaics. We’re not there yet, says Nocera, but predicts that it’s “the future of the next five to 10 years”.
Nocera was the first to demonstrate that we can mimic what nature has been doing for billions of years: use sunlight to produce hydrogen and oxygen from water. And once you have hydrogen, you can make various types of fuels such as gasoline, alcohols or sugars.
However, not everybody is convinced that we are that close to a solution. Many researchers at the conference in Uppsala say they are still busy perfecting their concepts and trying to find ways that would make their technologies cheaper and more efficient so that – eventually – they can rival fossil fuels.
In 2011, Nocera introduced the world to the ‘artificial leaf’ – neither green nor growing, but performing some of the processes inside a real leaf. His device is a stamp-sized photovoltaic cell sandwiched between two thin metal oxide catalysts. Submerse it in a glass of water at room temperature and ambient atmospheric pressure, then hold it up to sunlight and it converts that into electricity, just like a real leaf or a solar panel. The current is then routed to the catalysts. One catalyst accelerates the formation of hydrogen and the other oxygen, which bubble on specific sides of the wafer.
His breakthrough inspired researchers around the world. “Several years ago, no one had the materials to split water and now everyone is doing it,” says Nocera, referring to the multitude of posters and discussions on water splitting at the conference. “This part is really under control. That’s been the big revolution of the last several years.”
Of course, a real leaf also makes carbohydrates and this is the other part of the solar fuel research – to produce liquid fuel from sunlight artificially. “It’s like turning water into wine – or in our case, into fuel,” laughs Bergmann.
Nocera’s team is one of many experimenting with the idea and recently published a paper on what he calls a ‘bionic leaf’ – a device that combines a biological system with the artificial leaf, converting solar energy into hydrogen fuel.
In it, the hydrogen gas is channelled to a metabolically engineered version of a bacterium called Ralstonia eutropha. The bacteria divide to make more cells and in the process combine the hydrogen with carbon dioxide, producing isopropanol (rubbing alcohol) that can be burned in an engine similar to the gasoline additive ethanol. Or simply put, “the bacteria breed in the hydrogen and in the carbon dioxide and make liquid fuel,” says Nocera.
It’s just the beginning, of course, but Nocera is convinced that science has already delivered enough to run our whole world on sunlight and water. “Our society has an energy infrastructure worth over $100 trillion and we have already paid it off - it’s a paid-off investment,” he says. “So the will has to be there on the part of the people who say: ‘Granted, we paid it off, but it’s a bad energy system and we’re going to have to spend money for a new energy system’.”
Not all solar fuels researchers share his enthusiasm, though. “You first need to unravel the water-splitting mechanism of the reaction. Nature developed it 2.6 billion years ago and it can work with natural, very abundant metals,” says biochemist Petra Fromme of Arizona State University.
She says that although water splitting has in principle been achieved, artificial catalysts developed so far have two disadvantages: either you have to put a lot of energy in to get them going, or they are very inefficient.
Another problem, she adds, is that many of these catalysts are either very fast but not stable and disintegrate quickly, or relatively stable but very slow. “In nature, there’s the same problem, but nature has solved it by having a small catalytic centre and big proteins surrounding it, so that you can make large changes to the catalytic process and the molecule doesn’t disassemble, because all the proteins keep the catalytic centre in place,” says Fromme. “The trick would be to unravel how natural photosynthesis works so that we can build artificial photosynthesis based on the same principle. And then also look at catalysts, for the same kind of reaction. At the moment, the optimisation of molecular catalysts is like fishing in the dark.”
One way to look inside a molecular reaction is by using ultra-fast X-ray pulses. Fromme and her team, as well as Bergmann and his colleagues, are doing just that with the help of powerful new X-ray free-electron lasers (XFELs).
Artificial photosynthesis is not the only way to get solar fuel and a hydrogen-powered car in your garage. Biologist Anastasios Melis at the University of California, Berkeley, is trying to achieve it using natural photosynthesis.
Somewhat similar to Nocera’s experiments with his ‘bionic leaf’, although bypassing the artificial leaf part, Melis’s team has been working with live microorganisms – cyanobacteria and photosynthetic bacteria – and unicellular green algae.
These organisms generate organic hydrocarbon molecules naturally and “we just want to make them give us more,” says Melis. “Our effort is not to design photosynthesis itself – it is to tweak the flow of photosynthesis, to reprogram it to make these microorganisms generate more hydrocarbons – terpenes – fuel-type molecules that could be helpful in our daily life.”
His group, and a few others, managed to show that it can be done – and now it’s the issue of yield and economics, says Melis.
To get the cells to produce more terpenes, scientists turn to metabolic engineering to raise the cells’ metabolism, increasing the activity of enzymes that synthesise terpenes and making them work harder.
Usually, these microorganisms direct four per cent of their metabolic activity towards terpenes, but to prove that the technology is commercially viable, the target is to engineer them in such a way that they give 40 per cent of their activity in the form of terpenes. “It’s like an apricot tree that normally makes very small fruit and we manipulate its ability to give us much bigger fruit by directing the metabolic activity of the tree,” says Melis. “So far, we’ve gone up to 10 per cent.”
Whichever concept turns out to be the better solution, and however long it may take, the solar fuels community is convinced that getting the sun on our side is the future.
Plants that use sunlight to grow then become food for others or turn into fossil fuels that we burn to make more food. When we eat, we are literally eating sunlight: as Nocera puts it, "biting the light of the sun." The question is now to start using our sun much more efficiently.