Fuel cells turn chemicals into electricity. Now, engineers have adapted technology from fuel cells to do the reverse: harness electricity to make valuable chemicals from waste carbon (CO2).
In a hydrogen fuel cell, hydrogen and oxygen come together on the surface of a catalyst. The chemical reaction releases electrons that are captured by specialized materials within the fuel cell and pumped into a circuit. The opposite of a fuel cell is an electolyzer, which uses electricity to drive a chemical reaction. The electrolyzer converts CO2 into other carbon-based molecules such as ethylene — one of the most widely produced chemicals in the world. It is used to make everything from antifreeze to lawn furniture. Today, it is derived from fossil fuels but if it could be made by upgrading waste, it would provide a new economic incentive for capturing carbon.
Today’s electrolyzers do not yet produce ethylene on a scale large enough to compete with what is derived from fossil fuels. Part of the challenge lies in the unique nature of the chemical reaction that transforms CO2 into ethylene and other carbon-based molecules. The reaction requires three things: CO2, a gas; hydrogen ions, which come from liquid water; and electrons, which are transmitted through a metal catalyst. The challenge is bringing those three different phases — especially the CO2 — together quickly.
In the new electrolyzer design, the team used a unique arrangement of materials to overcome the challenges of bringing the reactants together. Electrons are delivered using a copper-based catalyst that the team had previously developed. But instead of a flat sheet of metal, the catalyst in the new electrolyzer is in the form of small particles embedded within a layer of a material known as Nafion.
Nafion is an ionomer — a polymer that can conduct charged particles known as ions. Today, it is commonly used in fuel cells, where its role is to transport positively charged hydrogen (H+) ions around within the reactor. The design enables gas reactants to reach the catalyst surface fast enough and in a sufficiently distributed manner to significantly increase the rate of reaction.
With the reaction no longer limited by how quickly the three reactants can come together, the team was able to transform CO2 into ethylene and other products ten times faster than before. They accomplished this without reducing the overall efficiency of the reactor, meaning more product for roughly the same capital cost.
Despite the advance, the device remains a long way from commercial viability. One of the major remaining challenges concerns the stability of the catalyst under the new higher-current densities. Also, electrons can be pumped in ten times faster but the system can only operate for about ten hours before the catalyst layer breaks down. This is still far from the target of thousands of hours that would be needed for industrial application.