Flooded by Success: On the Role of Electrode Wettability in CO2 Electrolyzers That Generate Liquid Products

The economic operation of carbon dioxide (CO2) electrolyzers generating liquid products will likely require high reactant conversions and high product concentrations, conditions anticipated to challenge existing gas diffusion electrodes (GDEs). Notably, electrode wettability will increase as lower surface tension products (e.g., formic acid, methanol, ethanol, and 1-propanol) are introduced into flowing electrolyte streams potentially leading to flooding. To better understand the hydraulically stable electrolyzer operating envelopes in mixed aqueous-organic liquid domains, we connect intrinsic porous electrode wettability descriptors to system operating parameters such as electrolyte flow rate and applied current. Specifically, we first measure contact angles of various water-organic dilutions on polytetrafluoroethylene (PTFE) and graphite surfaces as ex situ planar analogues for the major GDE components. We then use material balances around the reactive gas-liquid interface, to calculate product mass fractions as a function of liquid water sweep rate (water source and diluent) and the total current. Product composition maps enable visualization of the extent to which changes in cell performance can lead to changes in capillary pressure, a crucial determinant of GDE saturation. These analyses reveal that formic acid product mixtures pose little risk for GDE flooding across a wide range of flow rate and current combinations, but that effluents enriched with less than 30% alcohols content by mass may cause flooding. This study provides initial guidance into estimating flooding conditions for PTFE-based GDEs in contact with organic-enriched streams and indicates opportunities for oleophobic surface treatments that repel aqueous and organic liquids, expanding regions of stable operation