Electrolysis powered by renewable electricity is a promising approach to converting carbon dioxide (CO₂) into valuable fuel and feedstocks. A major challenge associated with this technology is the limited supply of CO₂ to the catalyst where the electrochemical CO₂ reduction (CO₂RR) occurs. This limits the CO₂RR reactivity and selectivity. By tuning the structure and composition of the catalyst layer (CL), the supply of CO₂ to the catalyst can be enhanced, and CO₂RR performance and stability can be improved. Ionomers are a common component in the CL and are typically dispersed in a catalyst ink solvent during CL fabrication. While this solvent quickly evaporates, the solvent can still impact the final ionomer conformation in the CL.[1, 2] Here, we demonstrate the role of ionomer-solvent interaction in diluted and concentrated catalyst ink and its impact on CO₂RR performance.

In this study, we chose Aquivion® (a type of perfluorinated sulfonic acid, PFSA) as the ionomer model and compared acetone and methanol as the solvents. We show that acetone exhibits stronger interaction with the ionomer backbone than methanol, resulting in a more continuous catalyst layer with greater film hydrophobicity than the CL formed using methanol dispersion. This is attributed to acetone's lower solubility parameter, which induces greater Aquivion® backbone mobility than methanol. Consequently, the CL made with acetone yields greater selectivity to C₂₊ products at high current density, up to 30% higher than CL derived with methanol at 200 mA cm^-2. Predominantly, the C₂₊ primary product is ethylene, with ethylene faradaic efficiency reaching up to 39.8 ± 2.4 % for the CL prepared from acetone compared to 27.8 ± 8.0 % for CL prepared with methanol at 200 mA cm^-2. With a more continuous and hydrophobic ionomer film due to acetone, CL is modulated to show a high local pH and improved gas transport, synergistically promoting the formation of C₂₊ products. However, the acetone derived CL displays a higher overpotential at all current densities due to the high surface coverage of Aquivion's non-conductive backbone on CL, limiting the electron transfer. Our findings demonstrate the potential of an alternative strategy for optimising the local reaction environment, which will benefit all electrochemical designs.

References

[1] B. A. W. Mowbray, D. J. Dvorak, N. Taherimakhsousi, and C. P. Berlinguette, "How Catalyst Dispersion Solvents Affect CO2 Electrolyzer Gas Diffusion Electrodes," Energy & Fuels, vol. 35, no. 23, pp. 19178-19184, 2021/12/02 2021, doi: 10.1021/acs.energyfuels.1c01731.

[2] C. M. Johnston, K.-S. Lee, T. Rockward, A. Labouriau, N. Mack, and Y. S. Kim, "Impact of Solvent on Ionomer Structure and Fuel Cell Durability," ECS Transactions, vol. 25, no. 1, pp. 1617-1622, 2019/12/17 2019, doi: 10.1149/1.3210717.