Finding productive uses for carbon dioxide remains a key challenge in reducing emissions of the greenhouse gas. Now, researchers at the University of Delaware, Newark, report that bismuth can serve as an electrocatalyst to transform CO2 into liquid fuels and chemical feedstocks. Moreover, the catalyst can be tailored or tuned to efficiently promote multiple types of reactions, notes Joel Rosenthal, a professor at the university’s Department of Chemistry and Biochemistry who led the research. He refers to bismuth’s ability to do this as “catalytic plasticity.”
By applying an electrical current to bismuth film placed in a salty liquid bath containing imidazolium and amidinium ions, the researchers were able to tune the chemical reaction to convert CO2 to either a liquid fuel, such as gasoline, or to formic acid. ACS Catalysis contains details on their work. The electricity could come from renewable sources such as the sun or wind instead of from conventional power plants, further reducing CO2 emissions, they add.
Bismuth doesn’t bind strongly to common contaminants found in CO2, so, poisoning doesn’t pose an issue, say the researchers: “We are not particularly concerned about the stability of the bismuth cathode in and of itself. It is quite robust, especially under the cathodic conditions needed for CO2 electrolysis,” comments Rosenthal. We have run our system as long as 24–48 hours without losses in activity. Going forward, we will be focusing much of our effort to understanding if and how the ionic-liquid containing electrolytes that we employ for the bismuth catalysts degrade during electrolysis. They seem to hold up well for day-long electrolyses but it will be important to show that this holds for longer electrolysis times.”
The researchers currently are preparing to test their bismuth CO2 electrolysis platform in a flow cell; the team is finalizing the design of the initial prototype. “Once we start collecting data on this new advanced cell, we will have a better idea of what next steps need to be taken to further boost the current densities and efficiencies for the CO2 electrolysis catalysts we have developed,” says Rosenthal.
“At the very least we expect to achieve a doubling of the rate of CO2 electrolysis when we use a more advanced cell,” he adds. “We are optimistic that we can realize improvements in kinetics of a full order of magnitude or greater, based on prior work using such cell designs with other CO2 reduction catalysts.”
With further fine-tuning, the researchers believe the system can be even more selective.
“At this point, the catalyst system can run and produce HCOO- as the only non-volative CO2 reduction product. … roughly 15–20% of the current goes toward CO production, which isn’t a major issue since this coproduct has value and is easily separated from the non-volatile formate. Having said that, we have already been able to show it is possible to further tune the selectivity of our catalyst system to be even more selective for HCOO- production by controlling the applied potential and electrolyte composition. We have already achieved selectivities for formate of ~90% and I think further gains are possible,” explains Rosenthal.
Once the researchers demonstrate the catalyst platforms can be integrated into an advanced flow-type electrolyzer, testing will begin on a pilot-plant scale. “If our research proceeds as planned, we could be ready to take this step within a year to 18 months,” Rosenthal reveals. “We would welcome help from an industrial partner to push this system forward, particularly with efforts to scale up the electrolyzer assemblies and to separate and recover the HCOO- from the post electrolysis stream,” he adds.
Before any of that takes place, the researchers need to verify the catalysts can be robustly integrated with gas diffusion electrodes that feature in the advanced cell designs. “We also need to take advantage of existing technologies to show that we can separate and recover the formic acid produced during electrolysis,” notes Rosenthal.
“This [catalytic plasticity] finding is important as we have shown that the reactivity of our bismuth catalysts with CO2 can be tuned to selectively produce either CO or HCOO- depending on the ionic-liquid additives that are introduced to the electrolyte solution. … the concept of catalytic plasticity is likely general and can be applied to other families of electrocatalysts as well. Moreover, the discovery of catalytic plasticity has real-world implications for the fields of catalysis and chemical production because it presages that one type of catalyst + electrolyzer may be able to be used for production of a variety of reduced carbon-containing products simply by tweaking the composition of the electrolyte solution. One can start to think about an electrolyzer assembly that can be used to convert CO2 to CO on one day, to HCOO- the next, and to MeOH the day after without having worry about any major changes in the design of the electrolyze or catalyst assembly,” concludes Rosenthal.