A promising avenue for the future of clean energy is to store it in the form of carbon-based fuels produced from renewable sources, effectively enabling the clean use of liquid fuels such as gasoline. A first step is the electrolysis of carbon dioxide into oxygen and carbon monoxide, which can be subsequently be transformed into liquid fuels. But current CO-forming catalysts are either not selective enough or too expensive to be industrially viable. EPFL scientists have now developed an Earth-abundant catalyst based on copper-oxide nanowires modified with tin oxide. A solar-driven system set up using this catalyst was able to split CO₂ with an efficiency of 13.4%. The work is published in Nature Energy, and is expected to help worldwide efforts to synthetically produce carbon-based fuels from CO₂ and water.

The research was carried out by the lab of Michael Grätzel at EPFL. Grätzel is known worldwide for the invention of dye-sensitized solar cells (“Grätzel cells”). The new catalyst, developed by Ph.D. student Marcel Schreier and several co-workers, is made by depositing atomic layers of tin oxide on copper oxide nanowires. Tin oxide suppresses the generation of side-products, which are commonly observed from copper oxide catalysts, leading to the sole production of CO in the electroreduction of CO₂.

The catalyst was integrated into a CO₂ electrolysis system and linked to a triple-junction solar cell (GaInP/GaInAs/Ge) to make a CO₂ photo-electrolyzer. Importantly, the system uses the same catalyst as both the cathode that reduces CO₂ to CO and the anode that oxidizes water to oxygen through what is known as the “oxygen evolution reaction.” Meanwhile the gases are separated with a bipolar membrane. Using only Earth-abundant materials to catalyze both reactions, this design keeps the cost of the system low.

The system was able to selectively convert CO₂ to CO with an efficiency of 13.4% using solar energy, thus setting a new benchmark for this reaction. The catalyst also reached a Faradaic efficiency of up to 90%, which describes how efficiently electrical charge is transferred to the desired product in an electrocatalysis system like the one developed here.

“This is the first time that such a bi-functional and low-cost catalyst is demonstrated,” adds Schreier. “Very few catalysts — except expensive ones, like gold and silver — can selectively transform CO₂ to CO in water, which is crucial for industrial applications.”

SOURCE: Ecole Polytechnique Fédérale de Lausanne