The Effects of Water and Ion Management for Modulate CO2 Reduction Selectivity Via Anode Control
Authors: Kim Dogyeong , Yu Seung-Ho , Oh Hyung-Suk , Lee Woong Hee 
Published: December 2024

The process of conversion ethylene through carbon dioxide reduction reaction (CO2RR) shows potential in catalysis by encouraging the formation of C-C bonds while reducing the occurrence of competing reactions(1). Despite their potential, a deeper understanding of membrane electrode assembly (MEA) systems is needed due to their low selectivity. The MEA configuration, which eliminates the need for a catholyte, delivers humidified CO2 to the cathode, enhancing CO2 solubility and minimizing overall ohmic resistance by maintaining a narrow electrode gap, thus boosting energy efficiency(2). However, in MEA, since protons and ions from the anolyte for reaction are supplied to the catalyst through a membrane, additional understanding of the electrolyte supply effect is needed to optimize the overall system(3). Ions and water of electrolytes are essential for CO2RR. Water serves as a proton source for CO2RR and facilitates ion transport. However, the delicate balance of water is crucial in CO2RR systems. While water is necessary for proton supply, an excess of water at the cathode can lead to complications, particularly in promoting the competing hydrogen evolution reaction (HER)(4). HER can occur alongside CO2RR and consume protons and electrons, thereby reducing the selectivity of CO2 conversion and diminishing overall efficiency. Balancing the concentrations of ions and water is crucial for optimizing cell voltage and overall performance, highlighting the necessity of effective water and ion management approaches. While much research on water management has focused on cathodes, electrolytes, and membranes, the properties of the anodes have been relatively neglected. This study proposes the hypothesis that modulation of the anode electrode can have a significant impact on CO2RR performance. When comparing Ti-mesh, which facilitates the passage of electrolyte, with carbon paper, which restricts electrolyte movement, localized pH changes and optimal ethylene production efficiency were observed. In particular, in situ/operando analysis shows that under conditions of insufficient water and ion supply, local pH effects are promoted and Cu valence increases during CO2RR, resulting in optimal ethylene production efficiency at low current densities. However, at high current densities, the reaction was not evenly distributed and catalytic activity decreased. On the other hand, when water and ions are supplied smoothly, the ethylene production efficiency increases as the current density increases, showing that the material is sufficiently transferred. In addition, it was confirmed through the backside image that water crossed over to the cathode at the same time. This shows that the anode can affect the catalytic activity of the cathode side.

Therefore, this study highlights the importance of effective ion and water management in improving CO2RR efficiency in MEA and highlights the central role of the anode in this process.

References

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Figure 1