Chemical engineers have discovered a surprisingly simple way to improve the stability of electrochemical devices that convert carbon dioxide (CO2) into useful fuels and chemicals.
Their approach, which involves bubbling CO2 through acid rather than water, reduces the build-up of salts within a CO2 reduction system, extending its operational life more than 50-fold.
“This is a major finding for CO2 electrolysis,” says Ahmad Elgazzar, graduate student in chemical and biomolecular engineering at Rice University in Houston, Texas, and co-first author of a paper describing the research in the journal Science.
“Our method addresses a long-standing obstacle with a low-cost, easily implementable solution. It’s a step toward making carbon utilisation technologies more commercially viable and more sustainable.”
Electrochemical CO2 reduction powered by renewable energy could be used to convert CO2, which has been captured from the atmosphere or at a point source, into a range of useful products such as carbon monoxide, formic acid, methanol, methane, ethylene, ethanol, and propanol.
However problems stand in the way of implementing the technology at an industrial scale. One such issue is potassium bicarbonate salts.
“Salt precipitation blocks CO2 transport and floods the gas diffusion electrode, which leads to performance failure,” says Dr Haotian Wang, the corresponding author of the study and associate professor in the Department of Chemical and Biomolecular Engineering at Rice.
“This typically happens within a few hundred hours, which is far from commercial viability.”
To address this, the researchers used acid solutions, such as hydrochloric, formic, or acetic acid, rather than water, to humidify the CO2 gas input.
“Using the traditional method of water-humidified CO2 could lead to salt formation in the cathode gas flow channels,” says co-first author Dr Shaoyun Hao, postdoctoral research associate in chemical and biomolecular engineering at Rice.
“We hypothesised – and confirmed – that acid vapor could dissolve the salt and convert the low solubility potassium bicarbonate into salt with higher solubility.”
They found that any small salt deposits were eventually dissolved and carried out of the system.
In tests using a silver nanoparticle catalyst — a common benchmark for converting CO2 to carbon monoxide — the system operated stably for more than 2,000 hours in a lab-scale device and more than 4,500 hours in a 100cm2 scaled-up electrolyser.
In contrast, the team found that systems using standard water-humidified CO2 failed after about 80 hours due to salt buildup.
Their new approach was also effective when used with several different catalysts, including copper oxide and bismuth oxide, all of which are used to target different products of CO2 reduction. It can also be adopted without significant redesigns added costs.
“Our study presented an easy-to-engineer and robust strategy for enhancing the CO2 reduction reaction stability without compromising reaction selectivity or cell voltage, broadening the horizons for commercial applications of CO2 reduction reaction MEA electrolysers.”