Energy Efficient Methods for Product Separation and Concentration in Scalable CO2 Electrolysis
Authors: Lee Noho , Hong Sungwon , Kim Minkyung , Cho Kang Hee , Kim Jaeeun 
Published: December 2024

Facing the critical challenges of climate change and the escalating need for sustainable carbon management, the electrochemical reduction of CO2 emerges as a pivotal strategy for carbon recycling and the production of renewable energy. This approach addresses the imperative to mitigate CO2 emissions while harnessing the potential of electrochemical processes to transform CO2 into ethanol, a valuable renewable fuel. Unlike conventional methods reliant on biomass, which are limited by significant land use and potential environmental repercussions, electrochemical CO2 reduction offers a sustainable and efficient alternative. It enables the direct conversion of CO2 from its most oxidized state into more energetically favorable products through a process that integrates water splitting and CO2 hydrogenation into a single electrochemical step. This not only simplifies the conversion process but also facilitates the synthesis of products difficult to achieve through thermal processes, operating at or near ambient conditions to minimize energy consumption and environmental impact.

Central to our study is the experimental approach using flow cells and MEA cells, which are primarily utilized for the large-scale electrochemical conversion of CO2. Both systems enable high-current density reactions (large-scale processing) by continuously flowing electrolyte, thus achieving mass processing capabilities. The dilution of reaction products within the electrolyte presents a significant challenge. Additionally, in the case of an MEA cell, there's the issue of the produced products undergoing oxidation at the anode, causing problems in stability during operation. Therefore, an economical method is needed to separate the products within the electrolyte.

To overcome this, our research innovatively integrates a pervaporation system employing zeolite-based membranes within a flow cell configuration. This setup allows for the effective concentration of ethanol produced in the electrochemical reduction process, addressing the significant challenge of product dilution in the electrolyte.

In our efforts to generate ethanol electrochemically, we utilized a Cu-based catalyst, conducting experiments in both neutral and alkaline electrolyte environments. It was observed that ethanol forms in real-time through the electrochemical reactions at concentrations lower than 1wt%. Furthermore, we successfully implemented a pervaporation process using a hydrophobic all-silica zeolite membrane. The pervaporation process using a hydrophobic all-silica zeolite membrane can selectively separate low concentrations of ethanol from catholyte due to the high selectivity of the zeolite membrane. In this study, we fabricated an MFI type silicalite-1 membrane on a tubular support and investigated the selective removal of ethanol in a water/ethanol mixture. The silicalite-1 membrane has a high Si/Al ratio and exhibits high molecular-level ethanol selectivity. In addition, the membrane has high chemical stability, making it applicable to long-term operation under harsh conditions.

As a results, we achieved an ethanol concentration in the permeate of over 50% under conditions where < 1 wt% ethanol was present in the feed. This not only addresses the challenge of separating ethanol from highly diluted streams but also significantly improves the sustainability and scalability of the overall process.

Our research underscores the potential of integrating advanced separation technologies in carbon recycling, offering a promising route for the environmentally friendly utilization of CO2 and contributing to the advancement of renewable energy solutions

Ref)

CO2 Electrolyzers. Chemical Reviews, 2024.

Suppressing the liquid product crossover in electrochemical CO2 reduction. SmartMat, 2021, 2.1: 12-16.

Energy-Efficient Ethanol Concentration Method for Scalable CO2 Electrolysis. ACS Energy Letters, 2023, 8.7: 3214-3220.

Pure-water-fed, electrocatalytic CO2 reduction to ethylene beyond 1,000 h stability at 10 A. Nature Energy, 2024, 1-11.

Identifying and alleviating the durability challenges in membrane-electrode-assembly devices for high-rate CO electrolysis. Nature Catalysis, 2023, 6.11: 1042-1051.