A simple and unexpected catalyst could help make hydrocarbons from carbon dioxide, rather than refining oil. The nickel-based system was developed by an international team of chemists from the National University of Singapore, ETH Zurich and the Chemical Research Institute of Catalonia, Tarragona, Spain.
Electrocatalysts made from abundant, non-toxic metals capable of converting carbon dioxide into hydrocarbons are of considerable interest in moving away from a fossil fuel-based economy. Copper has been the preferred metal so far, but it’s not without its problems. Making molecules longer than three carbon atoms has proven difficult, and copper ions are extremely mobile and electrocatalytic cells can often end up with copper ions seeping into the electrolyte, degrading the cell .
So finding another metal would be a big problem. “For several years, we wanted to develop a catalyst capable of producing molecules containing more carbon atoms. Having worked on copper ourselves and knowing its strengths and limitations, we had a feeling it wouldn’t be able to do the job,” says Javier Perez-Ramirez from ETH Zurich. The team knew that nickel was supposed to be inert, but they recalled an almost 30-year-old paper that described how metallic nickel could reduce carbon dioxide to trace amounts of C.3-4 hydrocarbons under high carbon dioxide pressure. “It made us wonder if there might be a missed opportunity to use nickel electrocatalysts,” says Pérez-Ramirez.
The team combined theoretical studies and experiments to test their intuition. Their first test, a metallic nickel disc, produced very small amounts of hydrocarbons. When the disk was modified by an electrochemical passivation technique, to make NiO, the production efficiency of the hydrocarbon chain more than doubled, although it was still only 0.8%.
The next step was to take a closer look at inorganic nickel oxygenates (INO) and see how good their catalytic power was. They tested INO catalysts made by electrochemically reducing nickel phosphate, nickel carbonate, nickel borate, nickel bicarbonate, nickel hydroxide and nickel oxide. All catalysts produced hydrocarbon chains up to six carbon atoms long, and the nickel-phosphate-derived INO worked best with yields up to 6.5%.
The reason for the catalytic activity seems to be that the nickel atoms become positively charged in the INOs, and not just because of their counterions. Theoretical and practical experiments have shown that it was nickel bound to oxygen that caused this polarization. And one consequence was that the catalysts were not poisoned by carbon monoxide – which is another danger of metallic nickel.
“Our results suggest a reaction mechanism profoundly different from that observed on copper catalysts with remarkable similarities to that which drives the classical Fischer-Tropsch process,” says the co-author of the study. Boon Siang Yeoat the National University of Singapore.
“This article shows that there is hope beyond copper,” says F Pelayo Garcia De Arquer at the ICFO research center in Barcelona. “One of the remarkable things with this paper is that not only can you make hydrocarbons, but you can also make more complex hydrocarbons than with a copper catalyst.” The next step, Arquer says, will be to test whether biased nickel systems can operate under more industry-relevant conditions, and to see if the crucial nickel bias can be maintained at high voltage. “It’s very exciting actually,” Arquer says.