Breakthrough in CO2 Conversion: Cost-Effective Methods Discovered

Researchers found that using ionic liquids as electrolytes with metal hydroxides enhances the electrochemical transformation of CO2 into hydrocarbons.

Electrochemical conversion of CO₂ into fuel and chemicals, powered by electricity, offers a viable method for reducing emissions. This technique enables the transformation of carbon captured from industrial outputs and the atmosphere into valuable resources traditionally derived from fossil fuels.

To advance ongoing research on efficient electrochemical conversion, scientists from Doshisha University have introduced a cost-effective method to produce valuable hydrocarbons from CO₂. The study was made available online on 17 May 2024 and will be formally published in the journal Electrochimica Acta on 20 July 2024. The research team, led by Professor Takuya Goto and including Ms. Saya Nozaki from the Graduate School of Science and Engineering and Dr. Yuta Suzuki from the Harris Science Research Institute, produced ethylene and propane on a basic silver (Ag) electrode by utilizing an ionic liquid containing metal hydroxides as the electrolyte.

“Most studies on CO₂ electrolysis with room-temperature liquid electrolyte have focused on the electrode’s catalytic properties. In this groundbreaking study, we focused on the electrolyte and succeeded in producing valuable hydrocarbon gas even on a simple metal electrode,” says Prof. Goto.

Ionic liquids offer unique advantages for the electrochemical reduction of CO₂. They operate over a wide range of voltages without decomposing, are non-flammable, and have high boiling points. This stability enables the electrolyte to withstand the high temperatures generated during exothermic CO₂ reduction.

Advantages of DEME-BF4 Electrolyte

In their study, researchers investigated the electrochemical conversion of CO₂ and water with N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate (DEME-BF₄) as the electrolyte. The DEME-BF₄ electrolyte provides optimal conditions for maximizing CO₂ reduction. DEME⁺ ions enhance the solubility of CO₂, allowing a greater number of CO₂ molecules to participate in the reaction. Moreover, due to its hydrophilic nature, the hydrogen ions required for reducing CO₂ to hydrocarbons can be easily supplied by mixing the electrolyte with water.

The production of hydrocarbons occurs through two intermediates formed on the surface of the silver electrode to produce useful hydrocarbons like ethylene, ethane, propylene, and propane. Credit: Takuya Goto from Doshisha University

The researchers determined that the electrochemical conversion of CO₂ to hydrocarbons could be increased with the addition of aqueous solutions containing metal hydroxides like calcium hydroxide (Ca(OH)₂), sodium hydroxide (NaOH), and cesium hydroxide (CsOH) to the electrolyte. The hydroxides in the ionic liquid can react with CO₂ to form bicarbonates (HCO₃⁻) and carbonates (CO₃²⁻), further enhancing the availability of CO₂ to participate in electrochemical reactions.

Achieving High Conversion Efficiencies

Under room temperature electrolysis (298 K or 25°C) in a CO₂ atmosphere, the researchers successfully reduced CO₂ to ethylene (C₂H₄), ethane (C₂H₆), propylene (C₃H₆), and propane (C₃H₈). They achieved the highest current efficiencies for each product using DEME-BF₄ electrolyte mixed with water and containing Ca(OH)₂, with efficiencies reaching up to 11.3% for propane and 6.49% for ethylene. This efficiency surpassed those obtained with other metal hydroxides by over 1000 times.

The reason for this high efficiency was explained using Raman spectroscopy and density functional theory (DFT) calculations. These analyses revealed that bicarbonate ions, formed when CO₂ interacts with OH^– ions in the electrolyte, interact with DEME⁺ and BF₄^– ions of the electrolyte to form a stable structure [DEME⁺-BF₄⁻-HCO₃⁻-Ca²⁺].

CO₂ and HCO₃^– DOI: 10.1016/j.electacta.2024.144431

The study was funded by the Japan Society for the Promotion of Science and the Iron and Steel Institute of Japan.