A new strategy for the optimal electroreduction of Co2 to high-value products

A KAIST research team presented three novel approaches for modulating local carbon dioxide (CO₂) concentration in gas-diffusion electrode (GDE)-based flow electrolyzers. Their study also empirically demonstrated that providing a moderate local CO₂ concentration is effective in promoting Carbon-Carbon (C-C) coupling reactions toward the production of multi-carbon molecules. This work, featured in the May 20th issue of Joule, serves as a rational guide to tune CO₂ mass transport for the optimal production of valuable multi-carbon products.

Amid global efforts to reduce and recycle anthropogenic CO₂ emissions, CO₂ electrolysis holds great promise for converting CO₂ into useful chemicals that were traditionally derived from fossil fuels. Many researches have been attempting to improve the selectivity of CO₂ for commercially and industrially high-value multi-carbon products such as ethylene, ethanol, and 1-propanol, due to their high energy density and large market size.

In order to achieve the highly-selective conversion of CO₂ into valuable multi-carbon products, past studies have focused on the design of catalysts and the tuning of local environment related to pH, cations, and molecular additives.

Conventional CO₂ electrolytic systems relied heavily on an alkaline electrolyte that is often consumed in large quantities when reacting with CO₂, and thus led to an increase in the operational costs. Moreover, the life span of a catalyst electrode was short, due to its inherent chemical reactivity.

In their recent study, a group of KAIST researchers led by Professor Jihun Oh from the Department of Materials Science and Engineering reported that the local CO₂ concentration has been an overlooked factor that largely affects the selectivity toward multi-carbon products.

Professor Oh and his researchers Dr. Ying Chuan Tan, Hakhyeon Song, and Kelvin Berm Lee proposed that there is an intimate relation between local CO₂ and multi-carbon product selectivity during electrochemical CO₂ reduction reactions. The team employed the mass-transport modeling of a GDE-based flow electrolyzer that utilizes copper oxide (Cu2O) nanoparticles as model catalysts. They then identified and applied three approaches to modulate the local CO₂ concentration within a GDE-based electrolytic system, including 1) controlling the catalyst layer structure, 2) CO₂ feed concentration, and 3) feed flow rate.

Contrary to common intuition, the study showed that providing a maximum CO₂ transport leads to suboptimal multi-carbon product faradaic efficiency. Instead, by restricting and providing a moderate local CO₂ concentration, C-C coupling can be significantly enhanced.

The researchers demonstrated experimentally that the selectivity rate increased from 25.4% to 61.9%, and from 5.9% to 22.6% for the CO₂ conversion rate. When a cheap milder near-neutral electrolyte was used, the stability of the CO₂ electrolytic system improved to a great extent, allowing over 10 hours of steady selective production of multi-carbon products.

Dr. Tan, the lead author of the paper, said, "Our research clearly revealed that the optimization of the local CO₂ concentration is the key to maximizing the efficiency of converting CO₂ into high-value multi-carbon products."

Professor Oh added, "This finding is expected to deliver new insights to the research community that variables affecting local CO₂ concentration are also influential factors in the electrochemical CO₂ reduction reaction performance. My colleagues and I hope that our study becomes a cornerstone for related technologies and their industrial applications."

This work was supported by the Korean Ministry of Science and ICT (MSIT) Creative Materials Discovery Program.