Ionic liquid electrolyte enables efficient CO₂ conversion to fuels and chemicals

Converting CO₂ into fuel and chemicals using electricity, also known as electrochemical conversion of CO₂, is a promising way to reduce emissions. This process allows us to use carbon captured from industries and the atmosphere and turn it into resources that we usually get 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 is published in the journal Electrochimica Acta.

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.

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 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.

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 1,000 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^- species then adsorb onto the electrode surface forming adsorbed species CO⁻ _ads. The adsorbed CO^- ions then strongly interact with Ca²⁺ ions present in the electrolyte, forming two distinct intermediate structures: One structure A, consisting of a Ca²⁺ ion coordinated with two CO⁻ ions adsorbed on three Ag atoms, and the other Structure B, where the Ca²⁺ ion is coordinated with two CO⁻ ions adsorbed on two Ag atoms.

This interaction with Ca²⁺ ions is crucial as it increases the stability of the adsorbed species, making the subsequent electrochemical reactions possible.

Among these structures, researchers suggest that structure B is more stable and is the preferred pathway for ethylene, while structure A leads to the production of propane.

"We showed that tailoring the electrolyte can lead to molecular-level changes in the phase transformation of CO₂ in bulk solution and at the electrode/ionic liquid electrolyte interface and proposed a process that enables the synthesis of unique hydrocarbons such as C₃," says Prof. Goto.

These findings shed light on the processes involved in the conversion of CO₂ at the interface between ionic liquid-based electrolytes and metal electrodes, such as the role of calcium ions. Such insights can help in the development of electrolytes for the efficient production of useful hydrocarbons from CO₂.

"The physicochemical knowledge of this new route from CO₂ decomposition to synthesizing useful hydrocarbons, as revealed in this study, will be instrumental in advancing CO₂ utilization technology and contributing to academic progress in materials science," concludes Prof. Goto.