The continuing release of carbon dioxide into the atmosphere is a major driver of global warming and climate change with increased extreme weather events. Researchers at Johannes Gutenberg University Mainz (JGU) have now presented a method for effectively converting carbon dioxide into ethanol, which is then available as a sustainable raw material for chemical applications. "We can remove the greenhouse gas CO2 from the environment and reintroduce it into a sustainable carbon cycle," explained Professor Carsten Streb from the JGU Department of Chemistry. His research group has shown how carbon dioxide can be converted to ethanol by means of electrocatalysis. Assuming green electricity is used for this process, it would also be sustainable –and food crops currently used to produce ethanol for fuels would be available for food again. According to Carsten Streb, the conversion technique, which has so far been carried out on a laboratory scale, could also be realized on a larger scale. The research results have been published in ACS Catalysis.
Efficient tandem system achieves selective electrocatalytic conversion
The electrochemical conversion of CO2 to multicarbon products, such as ethanol, would be an ideal way to obtain high energy density fuels and valuable chemical raw materials, while at the same time using CO2 as a precursor and thus removing it from the atmosphere to a certain extent. "To achieve this, we require suitable catalysts capable of this conversion with high selectivity so that we obtain a high yield of the desired product, which – in our case – is ethanol," said Streb.
To this end, his research team has designed a special electrode where the chemical reactions take place. It is coated with a black powder containing cobalt and copper in precisely dosed quantities. The two metals also have to sit in very specific positions on the electrode. "The initial challenge is to get carbon dioxide to react," said Streb. "The bonds between the atoms of the molecule are very strong, but cobalt can break them." This initially produces carbon monoxide, which is not an ideal feedstock for the chemical industry. Therefore, in a second step, copper is used to carry out the reaction to ethanol. "However, this only works if cobalt and copper are close to each other on the electrode," said Streb, outlining the trick that led to success.
Level of selectivity to be improved in future
Currently, the selectivity of the process is equivalent to 80 percent, i.e., 80 percent of the starting material is converted to ethanol – the best result achieved in research to date. Dr. Soressa Abera Chala played a key role in optimizing the results. He is lead author of the paper and came as a postdoc with a Humboldt Research Fellowship to Mainz from Ethiopia. Two of the co-authors, Dr. Rongji Liu and Dr. Ekemena Oseghe, are also working in Streb's group as fellows of the Alexander von Humboldt Foundation. The team is currently working on improving the yield of the process to 90 to 95 percent. A catalyst that achieves 100 percent selectivity would be desirable so that no other substances, apart from ethanol alone, would remain on completion of the process.
Cooperation within the Collaborative Research Center / Transregio "CataLight"
Success depends on process control and particularly on the loading of the electrode with cobalt and copper. "We need to see the individual atoms, which is possible using a special kind of electron microscope," said Streb. To achieve this, the Mainz-based chemists have joined forces with colleagues at Ulm University as part of the Collaborative Research Center / Transregio "CataLight" (CRC/TRR 234). Their goal is to develop a catalyst which is not only efficient, but functions well for as long as possible. The system itself is perfectly stable without any loss of performance even after several months.
Finally, the sheer abundance of cobalt and copper present on earth is a key factor in the choice of these two metals. The entire process could also be set up with precious metals like platinum or palladium, but at high costs without commercial prospects.
Sustainable production of ethanol conserves food resources and provides a new source of energy
"By using globally available raw materials as catalysts, we are following an approach in current research to increasingly focus on non-precious metals," emphasized Professor Carsten Streb. In future, this process could be used to produce ethanol sustainably from green electricity and carbon dioxide coming from power plants, for instance. Large quantities of ethanol are at present produced from sugar cane and maize in Brazil, meaning that these food crops are not available as sources of nutrition for the local population. The process presented here would open up an innovative and sustainable method of producing ethanol that could be stored and used for decentralized power generation as required.
