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

Abstract

Economic operation of carbon dioxide (CO2) electrolyzers generating liquid products will likely require high reactant conversions and 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, alcohols) are introduced into electrolyte streams potentially leading to flooding. To understand the hydraulically stable operating envelopes in mixed aqueous-organic liquid domains, we connect intrinsic electrode wettability descriptors to operating parameters such as electrolyte flow rate and current. We first measure contact angles of water-organic dilutions on polytetrafluoroethylene (PTFE) and graphite surfaces as planar analogues for GDE components. We then use material balances around the reactive gas-liquid interface to calculate product mass fractions as functions of water sweep rate and current. Product composition maps visualize the extent to which changes in cell performance influence capillary pressure, a determinant of GDE saturation. Analyses suggest that formic acid mixtures pose little risk for GDE flooding across a wide range of conditions, but effluents enriched with less than 30% alcohol by mass may cause flooding. This study reveals opportunities to integrate microstructural features and oleophobic surface treatments into GDEs to repel aqueous-organic mixtures and expand the window of stable operating conditions.<br>

Concentrated Ethanol Electrosynthesis from CO2 via a Porous Hydrophobic Adlayer

“…The production of liquid products is known to induce flooding in the electrolyzer due to the low surface tension of these liquids. 28 Despite not perfectly simulating in situ operation due to the effect of electro-wetting, these ex situ measurements provide an indication of the wettability of the layer and suggest that the adlayer protects the catalyst layer from excessive hydration that would normally dilute the produced ethanol.…”

“…The hydro- phobicity of the GDL is important for the prevention of electrode flooding. 27,28 By analyzing the surface wettability using pure water, 0.1 M KHCO 3 (the anolyte used for performance testing), and 20 wt % ethanol in water solution (a low surface tension alcohol/water mixture), we found that the adlayer significantly changes the surface wettability of the Cu/ PTFE (Figure 4a and Table S3). It is evident from Figure 4a that the ratio of EC to CB has a strong effect on the surface contact angle, with a 1:2 EC/CB ratio having the most hydrophobic surface.…”

Controlling Product Distribution of CO2 Reduction on CuO‐Based Gas Diffusion Electrodes by Manipulating Back Pressure

“…Several studies suggest that flooding of GDEs is the key way in which electrolytes create CO 2 -depleted zones, leading to the dominance of HER. [60][61][62] Among other operational parameters, pressure has a unique role in CO 2 RR as it has a threefold effect: capillary effects on the electrode flooding, mechanical effects providing better contact between electrolysis cell components, and increase in CO 2 concentration in the gas phase and therefore the CO Ã 2 surface coverage on the catalyst. By means other than manipulating pressure, researchers have controlled CO Ã…”

“…As mentioned, catalysts producing more C 2þ relative to C 1 products may be more stable due to a possible graphitic intermediate in the C 1 route. [35,36,47,[59][60][61][62][63] In mechanistic studies conducted by Rahaman et al [43] and Akhade et al, [58] the poisoning C* species is postulated to be formed via reduction of COH*, which is an intermediate of CO*-to-CH 4 reduction. As discussed in the following subsections, abundant CO* coverage provided by back pressure suppresses C 1 hydrocarbon production (CH 4 ) and promotes C 2þ selectivity.…”

“…Another possibility is that back pressure keeps the capillary pressure (local pressure difference between wetting and nonwetting media) at a sufficiently high value that minimizes the flooding of the catalyst and GDLs, an otherwise highly likely cause of deactivation. [60][61][62] When flooding is minimized, the salts present in the aqueous electrolyte do not crystallize in the pores of the electrode and do not increase its hydrophilicity. We hypothesize that flooding occurs to a smaller degree or can be delayed under back pressure.…”