(128b) Investigation of the Effects of Alkali Metal Cations and Cathode Wettability on the Selectivity and Stability of CO Electroreduction through Membrane Modifications

Carbon dioxide (CO₂) electroreduction is a promising pathway to achieve net-zero emissions by 2050, enabling the conversion of waste CO₂ from flue gas into valuable fuels like ethylene and ethanol. However, direct CO₂ electroreduction in alkaline electrolytes faces challenges such as carbonate formation, which causes significant CO₂ loss and reduces system stability by forming salts. A two-step approach—initial electroreduction of CO₂ to carbon monoxide (CO) using solid oxide electrolyzers, followed by CO electroreduction—offers a solution. This method minimizes carbonate formation and enhances the production of multi-carbon products, as CO is an intermediate in CO₂ electroreduction. This research investigates critical factors influencing CO electroreduction performance, including the roles of alkali metal cations near the cathode and cathode wettability, with a focus on membrane design and modification.

Although CO electroreduction avoids carbonate salt formation, it introduces unique challenges concerning selectivity and stability. Using a cation exchange membrane results in excessive K⁺ flux to the cathode, which promotes the undesired hydrogen evolution reaction. Alternatively, an anion exchange membrane improves selectivity but suffers from low stability. Achieving an optimal balance of selectivity and stability requires operating at very low anolyte concentrations to limit alkali metal cation diffusion to the cathode. However, this approach creates issues such as rapid pH fluctuations and oxidation of liquid products at the anode. Furthermore, the most effective anode material, Ni foam, becomes unstable when the pH drops below 12. Liquid product formation near the cathode further exacerbates performance by lowering surface tension, accelerating electrode flooding. These interconnected challenges necessitate innovative membrane solutions.

To address these issues, a thin, sprayed Nafion membrane was implemented to restrict K⁺ transport, allowing experiments at higher KOH concentrations. In this setup, charge balance was maintained primarily through OH^- diffusion, reducing K⁺ electro-migration. This strategy stabilized the anode by mitigating pH fluctuations and preventing liquid product oxidation. The hydrophobic properties of the Nafion membrane also prevented cathode flooding, enabling prolonged operation. However, the thin Nafion membrane exhibited durability issues due to oxygen crossover, leading to the formation of membrane-degrading radicals through oxygen reduction reactions. This degradation limited the system's long-term stability.

To overcome these limitations, a hybrid membrane combining sprayed Nafion and a commercial anion exchange membrane was developed. This dual-layer design retained the advantages of the Nafion membrane, resistance to K⁺ transport and cathode protection—while addressing durability concerns. The commercial membrane added structural robustness and enhanced resistance to oxygen crossover, effectively mitigating degradation. This configuration functioned like a bipolar membrane, where charge balance was achieved mainly through OH^- transport from cathode to anode, significantly limiting K⁺ flux to the cathode. The hybrid design improved operational stability tenfold compared to the initial setup while significantly enhancing selectivity.

This research underscores the importance of membrane modification in addressing key challenges in CO electroreduction. By optimizing alkali metal cation effects, cathode wettability, and membrane durability, this study provides critical insights into developing robust, scalable CO electroreduction systems, advancing carbon utilization technologies toward achieving net-zero emissions.