Wettability of Gas Diffusion Electrode Materials for CO2 Reduction

Abstract

Rapidly decreasing cost of wind and solar electricity coupled with the increasingly urgent need to reduce carbon emissions motivate the decarbonization of the electric power sector and offer exciting opportunities for the electrification of the chemical industry. The development of efficient carbon dioxide (CO2) electroreduction processes, which leverage renewable electrons, would simultaneously curb anthropogenic CO2 emissions and provide sustainable pathways to a range of fuels, chemicals, and plastics. While progress has been made in CO2 electrocatalysis, most materials are evaluated in electrochemical cells that operate at relatively low current density (ca. 1 – 10 mA·cm-2). In contrast, practical electrolyzers must achieve current densities in the range of 0.5 – 1 A·cm-2, typical of existing industrial electrochemical processes (e.g., chlor-akali electrolyzers), which are needed to enable cost-effective scaling, without sacrificing efficiency1. Aqueous-phase CO2 delivery at ambient conditions is hampered by low solubility and diffusivity which, in turn, lead to mass transport limitations that set an upper limit on current density of 10 – 20 mA·cm-2. Gas-phase delivery offers an alternative configuration whereby gaseous CO2 is fed into an electrolysis cell and interfaces with a liquid electrolyte or ion-selective membrane via catalyst-coated gas diffusion electrodes (GDEs), which were originally developed to manage gas, liquid, electron, and heat transport at electrochemical interfaces within polymer electrolyte fuel cells (PEFCs). In this configuration, the high diffusive rates of gaseous CO2 and shorter diffusion lengths enable increased current densities, typically an order of magnitude or larger, over atmospheric liquid-phase cell designs. As such, most emerging gas-fed CO2 electrolyzers employ GDEs based on repurposed PEFC materials, which demonstrate high geometric-area-specific electrochemical activity for a variety of both CO- and hydrocarbon-selective metal catalysts although longevity remains a challenge2. While differential conditions are typical of laboratory scale flow cells, industrial electrolyzers will likely operate near maximum single-pass conversion to reduce the necessity for material recycling and to avoid high separation costs associated with dilute product streams. Achieving high conversion requires either high current densities and/or long residence times which, for gas-to-liquid electrolysis, lead to organic product enrichment (e.g., formic acid, ethanol) and the formation of mixed aqueous-organic electrolytes. Such mixtures are anticipated to interact with the GDEs differently than water, which is the benchmark fluid for the wettability of gas diffusion layer (GDL) materials in the context of PEFC cathodes3. Conventionally, GDL engineering has focused on hydrophobicity/-philicity in order to manage cathode flooding in PEFC operating at high power. However, the addition of organic species to the electrolyte environment for CO2 electrolysis complicates the existing GDL design space because common wet-proofing materials (i.e., PTFE) may not repel aqueous-organic mixtures. In this talk, we will discuss the fundamental interactions of CO2-reduction-associated liquids (e.g., water, formic acid, ethanol)/electrolytes (e.g., carbonates, hydroxides) with a variety of solid materials (e.g., carbon, PTFE, metals). Sessile drop contact angle measurements on flat substrates provide simple, yet clear insights into surface wettability. However, they can also provide misleading information for porous materials where roughness and entrapped gases obscure the apparent contact angle4. For porous materials, we conduct studies involving capillary pressure phenomena (e.g., imbibition, Washburn method) to obtain information about the internal wettability in a format more aligned with GDE operation. While the propensity of pure liquid components to either resist wetting (solid-liquid contact angles > 90°) or readily wick (solid-liquid contact angles < 90°) into porous media may be known, the behavior of mixed organic-aqueous electrolytes is less obvious. Adding organic CO2-reduction-product species to a solution is anticipated to reduce the surface tension and density, while increasing the electrolyte salt concentration is expected to increase the surface tension and the density. The nuanced interplay between these two trends is understood neither in the absence nor in the presence of solid surfaces. To our knowledge, a focused study of such mixed electrolyte-solid interactions in the CO2 electrolysis field has not yet been compiled and may provide useful design principles for future GDE engineering. Funding Acknowledgement We gratefully acknowledge funding support from the US Department of Energy SBIR Program Grant # DE-SC0015173. References (1) Pletcher, D. Electrochemistry Communications 2015, 61, 97–101. https://doi.org/10.1016/j.elecom.2015.10.006. (2) Chen, C.; Khosrowabadi Kotyk, J. F.; Sheehan, S. W. Chem 2018. https://doi.org/10.1016/j.chempr.2018.08.019. (3) Gostick, J. T.; Ioannidis, M. A.; Fowler, M. W.; Pritzker, M. D. Journal of Power Sources 2009, 194 (1), 433–444. https://doi.org/10.1016/j.jpowsour.2009.04.052. (4) Cassie, A. B. D.; Baxter, S. Transactions of the Faraday Society 1944, 40, 546. https://doi.org/10.1039/tf9444000546.