Low-cost renewable electricity has raised the prospect of using electrochemistry to synthesise chemicals presently sourced from fossil fuels. Now, researchers at Stanford University in the US claim to have developed electrochemical cells that efficiently convert carbon monoxide derived from carbon dioxide into commercially viable compounds.
Such a system may also be useful for space exploration, the team believes. The group is working with researchers at the NASA Ames Research Center to combine electrochemical synthesis with microbial biosynthesis to recycle the CO2 breathed out by astronauts into food and nutrients.
In order to be viable, electrolysis must convert CO into products at a high rate with a low overall energy demand, says Matthew Kanan, associate professor of chemistry at Stanford University. Previous studies have shown that copper cathodes reduce CO to valuable C2 products such as ethylene, ethanol and acetate, but only at low CO conversions and typically low synthesis rates.
Previous electrochemical cells required a large excess of CO, which results in dilute products that must be concentrated and purified – a process that requires more energy, and more money. However, Kanan’s team gets around these problems with a modified electrochemical cell design that produces a concentrated stream of ethylene gas and a sodium acetate solution 1000 times more concentrated than product obtained with previous cells (Joule, doi: https://doi.org/10.1016/j.joule.2018.10.007). The cell uses a gas diffusion electrode (GDE) combined with a flow field that improves both the delivery of CO to the electrode surface and the removal of products.
‘Prior to this work, the combination of a high electrolysis rate, high CO conversion, and concentrated product streams had not been achieved,’ Kanan states. ‘Our results suggest that CO GDE electrolysis is viable for C2 synthesis and highlight a substantial advantage of CO over CO2 GDE electrolysis, where high rates have required low single-pass conversions.’
Now the team is scaling up their prototype to investigate if it needs modifications to work on an industrial scale. Kanan hopes they can eventually combine their CO electrolysis cells with existing technologies for converting CO2 into CO.