Developing Fluorine-Free Coatings to Repel Low Surface Tension Liquids in Gas Diffusion Electrodes for CO2 Electrolyzers
Authors: Erer Mert Can , Forner-Cuenca Antoni 
Published: February 2024

Decarbonization of the chemical industry is critical to combat climate change and ensure resource availability for key sectors. In this regard, the electrochemical reduction of carbon dioxide (CO2) has emerged as a promising negative-carbon technology to produce valuable chemicals. Recently, gas diffusion electrodes have been incorporated in CO2 electrolyzers to reach industrially relevant current densities (>250 mA cm-2) by introducing a porous medium to improve CO2 mass transport rates, enable shorter diffusion lengths, and circumvent the low CO2 solubility in water persistent in the catholyte flow.1 State-of-the-art gas diffusion electrodes are made of a gas diffusion media substrate, typically composed of two layers of distinct porosity, coated with a layer of electrocatalyst nanoparticles.2 To facilitate a resistance to flooding by the electrolyte, the gas diffusion media are coated with hydrophobic polytetrafluoroethylene (PTFE).2 Although the PTFE-coated diffusion media is reluctant towards water intrusion, they fail to prevent the infiltration of low surface tension (<30 mN/m) liquid products, such as ethanol, limiting the reactor operation to several hours.3 Furthermore, the fluorinated structure of the polymer poses a threat to the environment and human health, and could result in a broad ban of per- and polyfluoroalkyl substances.4 To this end, several efforts have been focused on understanding the effect of catalyst layer wettability on the reactor performance and stability.5 However, the impact of the gas diffusion media wettability and prospective influence of new non-fluorinated coating chemistries remain largely unexplored.
In this study, we investigate the application of non-fluorinated, alcohol-repellent coatings with low surface energy on the gas diffusion medium. First, we apply the coatings onto model, flat carbon electrodes, as the wettability of planar surfaces can be characterized more reliably and efficiently than for complex porous media. Utilizing a variety of probe solvents including the liquid products of CO2 electrolysis, contact angle measurements are performed to assess the surface repellencies and extract the surface energies of the coated substrates. The most promising coatings are translated to carbon fiber substrates, where the internal surface energies are determined. The chemical and morphological properties of the developed surfaces are analyzed via spectroscopy and microscopy. We hypothesize that we can leverage the fundamentals of coating science and surface chemistry to boost the stability of the gas diffusion media by preventing electrode flooding and thereby enhancing the reactor performance for CO2 electrolysis.

Acknowledgements

The authors gratefully acknowledge the Dutch Research Council (NWO) for financial support through the project “Designing gas diffusion electrodes with tailored wettability for CO2 reduction electrolyzers” (10030962) of the research program “Open Competition ENW”.

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