Carbon dioxide can be turned into a valuable resource

One of the biggest challenges of our time is how to reduce the concentration of carbon dioxide (CO₂) and other greenhouse gases in the atmosphere. This is the only way to mitigate climate change. Rapid cuts in emissions produced by industry, transport and private households are not enough, however. To reach agreed climate targets on schedule, much of the research to date highlights the need to actively extract carbon dioxide from the atmosphere or capture it directly at its source, thus rendering it harmless.

An important question is what to do with this waste gas? One possibility is to physically lock it away. This already happens in carbon capture and storage (CSS) projects, where the CO₂ is pumped into underground storage reservoirs, such as old natural gas fields or saline formations. Another option is to use carbon capture and utilisation (CCU) to convert the CO₂ for use as a raw material, which can then serve directly as a refrigerant, fertiliser or fire-extinguishing agent. Its more profitable use, however, is as a raw material for other products.

Previous studies – including one published by PSI – have shown carbon monoxide (CO) and formic acid (HCOOH) to be particularly promising products, as they are very easy to manufacture from carbon dioxide using water and electrolysis. The resulting formic acid can be used as an antirheumatic in medicine, for example, or as a pickling and impregnating agent in the textile and leather industry. In a similar vein, carbon monoxide can serve as a reducing agent in iron ore smelting. Most importantly, however: it can be combined with hydrogen to produce synthetic fuels. Synfuels have a very promising future, as they are carbon neutral and can replace diesel, petrol and kerosene from fossil resources.

PSI is heavily engaged in research in this area: at the start of the year, it joined forces with the Swiss Federal Laboratories for Materials Science and Technology (Empa) to launch the SynFuels initiative, funded by the ETH board. PSI was also the lead institute in the government-sponsored Swiss Competence Center for Heat and Electricity Storage (SCCER). Both initiatives enabled the research of this current study.

The study investigated whether the electrolysis of CO₂ to produce CO or HCOOH is commercially viable and whether it consumes more carbon dioxide than it generates itself through its energy needs. In other words: Can the process earn money and at the same time act as a CO₂ sink to protect our climate?

Simulation for six different electrolysis plants

The research group around the study’s lead author Bernhard Pribyl-Kranewitter – a PhD student with PSI’s Electrochemistry Laboratory at the time – started by sifting through the scientific literature for data on the most efficient low-temperature electrolyser systems that produce CO and HCOOH. "We selected the four best designs for carbon monoxide production – including one developed in house and patented by PSI – as well as the two best designs for producing formic acid," Pribyl-Kranewitter explains. Next, for the associated cell architectures the researchers constructed two virtual models of large-scale electrochemical plants for the production of both chemicals. "We have used extremely sophisticated simulation software to some extent, allowing us to replicate as realistically as possible the cells’ performance on an industrial scale."

For their simulations, the researchers ran two different scenarios: the first was based on current technology and assumed a production volume of 75 tonnes of CO and HCOOH per day. The second optimistic scenario anticipated improvements in the technology over the coming years and advances in various key parameters and overall production. This simulation produced 100 tonnes per day. In both simulations, a lifespan of 25 years was assumed for the manufacturing plants.

The simulations showed that producing formic acid in microfluid cells on a profitable basis was possible in both scenarios. Nevertheless, the production process generates more CO₂ than it consumes. This is mainly because of the comparatively high energy consumption: formic acid occurs as a liquid in solution with water and must be separated using an energy-intensive process before it can be used. At the same time, less carbon dioxide is needed for production than for the same amount of carbon monoxide. The current electricity mix in the EU, with over half still coming from fossil fuels, produces 235 grammes of CO₂ per kilowatt hour. The production of formic acid could only serve as a CO₂ sink if this value is less than 137 g/kWh. Consequently, the percentage of renewables needs to be considerably higher before the electrolysis of CO₂ to produce formic acid has a positive climate effect.

Carbon monoxide already a potential CO₂ sink

When producing carbon monoxide in the alkaline cell systems typically used for this purpose, the value only has to be less than 346 g/kWh, because the product is in the form of a gas and can be easily separated. "So carbon monoxide production could already potentially serve as a CO₂ sink," says project leader Thomas Justus Schmidt, Head of the PSI Research Division Energy and Environment. "And this rises in line with the increasing proportion of renewables in the electricity mix." In the base-case scenario, however, it proved impossible to produce carbon monoxide on a profitable basis with any cell architecture, whereby the electrolysis cell developed at PSI showed the most potential here. In the optimistic scenario, on the other hand, all four architectures produced a positive result, with the PSI version even outperforming one of the cells for producing formic acid. "Even if these results are already encouraging, there is still a way to go before we can exploit the full commercial potential of our cell architecture," says Pribyl-Kranewitter. The researchers also hope they will be able to reduce the costs for catalysts and membranes.

"The results of the study are very promising," Professor Schmidt stresses. In the optimistic scenario, carbon monoxide production showed a 22 percent greater improvement on average compared with the production of formic acid. In other words, if the technology continues to develop and prices drop as expected, carbon monoxide definitely has the most potential for the profitable and eco-friendly use of carbon dioxide. "Production based on this method can make a significant contribution to the energy transition, as it is a negative emissions technology," Schmidt says. The market for carbon monoxide is also much bigger than for formic acid. Global carbon monoxide production in 2015 was 210 gigatons, compared with just 0.76 megatons of HCOOH in 2019 – a mere fraction. "So we should concentrate on the further development of CO₂ electrolysis for producing carbon monoxide," Schmidt recommends. "And then explore formic acid as an alternative."