A More Effective Carbon Dioxide Reduction Reaction

Simply bubbling CO2 gas through acid shows a 50-fold improvement in reaction duration, researchers find

The electrochemical carbon dioxide reduction reaction (CO2RR) is a promising means of carbon capture and use. The reaction not only removes carbon dioxide from the atmosphere but also transforms it into useful by-products such as stock chemicals and fuel. If the reaction were to be powered by renewable energy, the resultant products would be considered carbon neutral. But the technology is still emerging, constrained by numerous bottlenecks.

One particular architecture for the reaction, the zero-gap membrane electrode assembly (MEA) electrolyzer, is known for its high current density and impressive energy efficiency. But the formation of bicarbonate salts in the cathode chamber impedes carbon dioxide flow, which can result in flooding and system failure.

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In a recent paper, researchers report a “surprisingly simple” way to stabilize the MEA system—to bubble the carbon dioxide gas through an acid solution. This increases durability dramatically, enabling the system to operate for up to 4,500 h, compared with 80 h under more-typical conditions (Science 2005, DOI: 10.1126/science.adr3834)

Salt formation happens because of the presence of cations like potassium in the anode electrolyte or anolyte in the electrolyzer setup, says coauthor Ahmad Elgazzar, a graduate student at Rice University. MEA electrolyzers have an anion exchange membrane (AEM) to enable the crossover of ions, improving the efficiency of the reaction.

"Even though you have an anion exchange membrane, which is supposed to only let anions go through . . . cations [also] end up crossing over, especially at higher current densities," Elgazzar says. Under an electrical field, cations migrate across the AEM to the cathode chamber and combine with carbon dioxide to form carbonates or bicarbonates under high pH levels. These carbonates are first contained within droplets, but the water eventually evaporates, leaving salt crystals that deposit on the gas flow channels, blocking them.

Normally, carbon dioxide gas is bubbled through water to prevent membrane dehydration, but that also encourages salt formation, Elgazzar says. "So what we thought: if you have a salt and you want to dissolve that, the easiest chemical way to do it is to react it with a strong acid."

They replaced the water humidifier with an acid, such as hydrochloric acid, formic acid, or acetic acid, at very low concentration—from 0.01 to 6 mol. "We identified that around 0.05 mol is the sweet spot where you get really good removal of these salts, and you’re not affecting the reaction," Elgazzar says.

As a result, salt formation is substantially reduced, and the gas flow channels remain unblocked. In their experiments with MEA electrolyzers, the researchers used a silver catalyst, as is common in reactions converting carbon dioxide to carbon monoxide. With acid humidification, they achieved 2,000 h of stable operation in the laboratory and 4,500 h in a scaled-up reactor. This is a 50-fold improvement over standard water humidification. The researchers also found success with other catalysts, including zinc oxide, copper oxide and bismuth oxide, which result in CO2RR products other than carbon monoxide. The researchers report that the method can be scaled up, and the low concentration of acid causes minimal corrosive damage to the system.

The study’s first author, Shaoyun Hao, also at Rice University, adds, however, that factors beyond salt formation also affect the stability of CO2RR electrolyzers.

Mirza Galib, a chemistry researcher at Howard University who wasn’t involved in the study, says that this work offers a practical step forward in making CO2RR more viable. But he echoes Hao in that there is still work to be done. "To enable commercialization, ongoing research must also aim to improve further the conversion efficiency, product selectivity, and membrane stability," he says.