Chemists have shown that it is possible to use solar energy, paired with the right catalyst, to convert carbon dioxide into a raw material for making a wide range of products, including plastics and gasoline.
Researchers at the University of California, San Diego (UCSD), recently demonstrated that light absorbed and converted into electricity by a silicon electrode can help drive a reaction that converts carbon dioxide into carbon monoxide and oxygen. Carbon monoxide is a valuable commodity chemical that is widely used to make plastics and other products, says Clifford Kubiak, professor of chemistry at UCSD. It is also a key ingredient in a process for making synthetic fuels, including syngas (a mixture largely of carbon monoxide and hydrogen), methanol, and gasoline.
The work is part of a growing effort to find practical uses for carbon dioxide, a leading greenhouse gas, says Philip Jessop, professor of chemistry at Queen’s University, in Ontario, Canada. Converting carbon dioxide into carbon monoxide is difficult to do, which Jessop says makes the UCSD work impressive and exciting.
At least at first, such a process will not make a significant impact on reducing greenhouse gases in the atmosphere–that would take quite large-scale operations, Kubiak says. But “any chemical process that you can develop that uses CO2 as a feedstock, rather than having it be an end product, is probably worth doing.” He adds that “if chemical manufacturers are going to make millions of pounds of plastics anyway, why not make them from greenhouse gases rather than making tons of greenhouse gases in the process?”
The system may also be part of a solution to a continuing problem with solar energy. For solar panels to be useful when the sun isn’t shining, the electricity they produce has to be stored. A potentially practical way of doing that is by converting the electrical energy into chemical energy. One popular approach is to use solar cells to produce hydrogen, which could then be used in fuel cells. But hydrogen gas is much more difficult to transport and store than are liquid fuels, such as gasoline, which contain far more energy by volume than hydrogen does. The UCSD system shows that it is possible to use solar energy to make carbon monoxide that then, together with hydrogen, can be converted into gasoline. Currently, carbon monoxide is made from natural gas and coal. But carbon dioxide is a more attractive raw material in part because it’s very cheap–indeed, it’s something industrial companies will pay to get off their hands, Jessop says. “There are very few chemicals which are cheaper than free, and carbon dioxide is one of them,” he says.
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.