In the prototype device, sunlight passes through carbon dioxide dissolved in a solution before being absorbed by a semiconductor cathode, which converts photons into electrons. Aided by a catalyst, the electrons react with carbon dioxide to form carbon monoxide at the electrode. At the anode–a catalyst made of platinum–water is converted into oxygen.
To make a fuel, the carbon monoxide can be combined with hydrogen to create syngas in a well-known technology called the Fischer-Tropsch process, which has been widely used to make gasoline from coal. With the new process for creating syngas, however, fossil fuels could be unnecessary.
The system–which Kubiak began developing as a way of manufacturing oxygen for manned missions to Mars, which has a carbon-dioxide-rich atmosphere–is still a work in progress. In the first prototype, only about half of the energy needed for the reactions was supplied by the sun, with the rest coming from outside electricity. That’s because the researchers decided to prove the concept using silicon as the semiconductor. They are now working with a gallium-phosphide semiconductor, which has exactly the right electronic properties to drive the necessary reactions using sunlight alone.
At this early stage–Kubiak says that commercial systems could be 10 years away–the efficiency and economics of making fuels this way aren’t known. Kubiak says it’s likely that for large-scale applications, his group will need to use catalyst-coated nanoparticles to increase surface area, speeding up reactions.