The only way to produce ethanol from carbon dioxide at the moment is from green plants but this process is limited by the efficiency of photosynthesis, explains team leader Matthew Kanan. “We now envisage a two-step electrochemical synthesis route to produce liquid fuel from CO2 that could be powered by any renewable electricity source, such as photovoltaic cells or wind turbines. The first step would involve an electrochemical cell that converts CO2 to CO and the second would involve a cell that converts CO to ethanol using oxide-derived nanocrystalline copper as the cathode.”

Although there are many catalysts to convert CO2 into CO, there are not so many for producing ethanol from CO when using water as a hydrogen source. Copper is the only known material that can drive this second step, but prior to the work by Kanan’s team, there was no copper-based material that could electrochemically transform CO into ethanol with reasonable efficiency.

Very high overall light-ethanol efficiency

“While we do still need to improve our catalyst further, our study shows that the two-step synthesis we have proposed to produce liquid ethanol from CO2 is feasible,” Kanan told nanotechweb.org. In a solar energy-based device, for instance, the overall sunlight-to-ethanol conversion efficiency is defined as the efficiency of the photovoltaic cell employed (typically 20%) multiplied by the efficiency of the electrochemical part of the system, he explains. And, although it is far from optimized, an electrochemical cell utilizing our catalyst could already provide a very high overall light-to-ethanol efficiency.”

The new nanocrystalline copper catalyst is made up of a continuous network of nanometre-sized crystals linked by disordered regions called grain boundaries. Until now, most research on nanocatalysts focused on metallic nanoparticles, but the Stanford team has found that nanocrystalline materials have better catalytic properties than just simple nanoparticles.

“Our work is an important step forward for elaborating a system that uses renewable energy to convert CO2 and water into liquid fuel,” said Kanan. “This energy, which is often produced intermittently from photovoltaic cells and wind turbines, could thereby be efficiently stored for later use.”

Catalyst variants

The researchers, who have published their work in Nature, say that they are now busy studying the structural properties of oxide-derived nanocrystalline copper in more detail. “We are also preparing variants of this catalyst with different grain boundary structures and dopant atoms to enhance its activity even further,” revealed Kanan. “We are also working on incorporating the catalysts in electrochemical cells that have been specially engineered to produce more fuel.”

Aaron Appel in the Catalysis Science Group at Pacific Northwest National Lab in the US, who was not involved in this research, says that the new results “lay the groundwork for further advances. Because of the challenges of predicting fossil-fuel prices, renewable-energy production costs and incentives (such as carbon taxes) to use renewables in place of fossil fuels, the timetable for widespread adoption of renewable fuels is not clear,” he writes in a related News & Views article. “But the production of fuels from non-fossil sources will certainly require effective catalysts. Kanan and colleagues’ work is an excellent step in that direction.”