Researchers at the University of Toronto's Faculty of Applied Science & Engineering have designed a catalyst that can efficiently convert captured carbon into valuable products - even in the presence of contaminants that degrade the performance of current versions.
The discovery, described in a paper published in Nature Energy, is an important step toward more economically viable techniques for carbon capture and storage that could be added to existing industrial processes.
"Today, we have more and better options for low-carbon electricity generation than ever before," says David Sinton, professor in the department of mechanical and industrial engineering and senior author on the paper. "But there are other sectors of the economy that will be harder to decarbonize: for example, steel and cement manufacturing. To help those industries, we need to invent cost-effective ways to capture and upgrade the carbon in their waste streams."
Sinton and his team use devices known as electrolyzers to convert CO2 and electricity into products such as ethylene and ethanol. These carbon-based molecules can be sold as fuels or used as chemical feedstocks for making everyday items such as plastic.
Inside the electrolyzer, the conversion reaction happens when three elements - CO2 gas, electrons and a water-based liquid electrolyte - come together on the surface of a solid catalyst.
The catalyst is often made of copper but may also contain other metals or organic compounds that can further improve the system. Its function is to speed up the reaction and minimize the creation of undesirable byproducts like hydrogen gas, which reduce the efficiency of the overall process.
While several high-performing catalysts have been developed around the world, nearly all of them are designed to operate with a pure CO2 feed. But if the carbon in question comes from smokestacks, the feed is likely to be anything but pure.
"Catalyst designers generally don't like dealing with impurities, and for good reason," says Panos Papangelakis, a PhD student in mechanical engineering and a co-lead author on the paper.
"Sulphur oxides such as SO2 poison the catalyst by binding to the surface. This leaves fewer sites for CO2 to react, and it also causes the formation of chemicals you don't want.
"It happens really fast: whereas some catalysts can last hundreds of hours on a pure feed, if you introduce these impurities, within minutes they can be down to five per cent efficiency."
Although there are well-established methods to remove impurities from CO2-rich exhaust gases before feeding them into the electrolyzer, these require substantial time, energy and expense. Furthermore, in the case of SO2, even a little bit can be a big problem.
"Even if you bring your exhaust gas down to less than 10 parts per million, or 0.001 per cent of the feed, the catalyst can still be poisoned in under two hours," says Papangelakis.
In the paper, the team describes how two key changes to a typical copper-based catalyst can make it more resilient to SO2.
On one side, they added a thin layer of polyteterafluoroethylene, also known as Teflon. This non-stick material changes the chemistry at the catalyst surface, impeding the reactions that enable SO2 poisoning to take place.
On the other side, they added a layer of Nafion, an electrically conductive polymer often used in fuel cells. This complex, porous material contains some areas that are hydrophilic, meaning they attract water, as well as other areas that are hydrophobic, meaning they repel water. This structure makes it difficult for SO2 to reach the catalyst surface.
The team then fed this catalyst with a mix of CO2 and SO2, with the latter at a concentration of about 400 parts per million, typical of an industrial waste stream. Even under these tough conditions, the new catalyst performed well.
"In the paper, we report a Faraday efficiency - a measure of how many of the electrons ended up in the desired products - of 50 per cent, which we were able to maintain for 150 hours," says Papangelakis.
"There are some catalysts out there that might start at a higher efficiency, maybe 75 per cent or 80 per cent. But again, if you expose them to SO2, within minutes or at most a couple of hours, that drops down to almost nothing. We were able to resist that."
Papangelakis says that because his team's approach doesn't affect the composition of the catalyst itself, it should be widely applicable. In other words, teams that have already perfected high-performing catalysts should be able to use similar coatings to confer resistance to sulphur oxide poisoning.
Although sulphur oxides are the most challenging impurity in typical waste streams, they are not the only ones, and it's the full set of chemical contaminants that the team is turning to next.
"There are lots of other impurities to consider, such as nitrogen oxides, oxygen, etc.," says Papangelakis.
"But the fact that this approach works so well for sulphur oxides is very promising. Before this work, it was just taken for granted that you'd have to remove the impurities before upgrading CO2.
"What we've shown is that there might be a different way to deal with them, which opens up a lot of new possibilities."
