Tweaking Nature: Harnessing Photosynthesis to Secure our Energy Future

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Fossil fuels are not a sustainable source of energy, but the use of renewable energy is hampered by their intermittency: the sun goes down every day, and there are times when the wind doesn't blow. To overcome this challenge, one team at CalTech are taking inspiration from nature to turn renewable energy into a storable fuel.

The team, lead by Nathan Lewis from the Division of Chemistry and Chemical Engineering, want to generate energy by mimicking the process of photosynthesis and then store that energy in the same form as in fossil fuels. Their idea involves an artificial photosynthetic system that uses energy from the sun to split water into hydrogen and oxygen, which is the same process used by plants to create energy. The trick is finding a way to safely and efficiently store the energy so that it could be used to power cars, houses, and industry; all of the things we use fossil fuels for today.

Lewis explains that the most energy dense form of storing energy, other than in atomic form, is within chemical bonds.

It's clear that the best way to take an intermittent resource like solar energy and to store it so it can be supplied on demand is to store it in chemical fuel, just like nature did to make all the fossil energy from photosynthesis over millennia to many, many hundreds of thousands of years. We just want to do it more efficiently and over a short enough time scale that it can be sustained instead of only used once, as we're doing with fossil fuels.

Nathan Lewis, Professor of Chemistry, CalTech

The system involves two light absorbers: one to absorb red light and one to absorb blue. Lewis explains that this is one of their tweaks to nature. Chlorophyll has two photosystems that both absorb green light, but this means that photons end up competing with each other. Separating out into a blue and red photosystem removes this problem and enhances efficiency, allowing the photosystems to absorb more photons.

The system also contains two catalysts: one to evolve oxygen from water and the other to reduce water and/or carbon dioxide to make a fuel. A membrane is also included to hold it together, and to avoid a dangerous mixture of hydrogen and oxygen over a catalyst.

That's the blueprint. Two absorbers, two catalysts and a membrane. Then we have to put all the pieces together and make sure they work under the same conditions, and make sure they're efficient, and make sure that they function hand in hand with all the parts of the sunlight spectrum that we can harvest in order to get the energy we need to make and break chemical bonds.

Nathan Lewis, Professor of Chemistry, CalTech

There are some remaining challenges when it comes to getting the system up and running. Lewis and his group are working to find a set up that is stable for both photoanodes and photocathodes. The silicon photocathodes that have been used to demonstrate their system are not stable in alkaline conditions, but there is not yet a photoanode that is stable in acid.

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In terms of implementing the technology, the researchers will be looking at ways to reduce costs while maintaining efficiency. Lewis envisions that the technology being incorporated into a multi-layer fabric that could be simply unrolled, and would wick water vapor from the air and absorb sunlight.

Much more like installing AstroTurf than like installing solar panels to electrolysers.

Nathan Lewis, Professor of Chemistry, CalTech

Lewis compares the work of his team to that of the Wright brothers trying to get the first plane into the air.

Our design principles are mapped out of the inspiration of natural photosynthesis, but they're adopted in the same way that, although birds fly with feathers, airplanes aren't built out of feathers to fly faster and further.

Nathan Lewis, Professor of Chemistry, CalTech

Lewis and his team's vision is clear, but ambitious;

We want to make an artificial photosynthetic system that in a scalable, sustainable way, with an efficiency ten times greater than the fastest growing crop, makes a fuel that we can directly use in our infrastructure from the sun. If we do that, then history shows that humans are very good at innovating and developing next generations of technology, making it faster, better, cheaper.

Nathan Lewis, Professor of Chemistry, CalTech