Eliminating Flooding-related Issues in Electrochemical CO₂-to-CO Converters: Two Lines of Defense

Abstract

By using silver (Ag) in nanostructured (nanowire, nanosphere, etc.) or thin-layer form as a catalyst for electrochemical CO₂ reduction, very high CO-forming selectivity of almost 100% can be achieved. Supported by gas diffusion layers (GDLs),  the reactant CO₂ in the gas phase can approach and potentially access active Ag sites, which allows current densities in the range of a few hundred mA cm^–2 to be reached. Yet, the stability of gas diffusion electrode (GDE) based electrochemical CO₂-to-CO converters is far from perfect, and the activity of GDE cathodes, especially when operated at high current densities, often significantly decays during electrolyses after no more than a few hours. The primary reason of stability losses in GDE-based CO₂-to-CO electrolysers is flooding: that is, the excess wetting of the GDE that prevents CO₂ from reaching Ag catalytic sites. In the past years, the authors of this paper at Empa and at the University of Bern, cooperating with other partners of the National Competence Center for Research (NCCR) on Catalysis, took different approaches to overcome flooding. While opinions differ with regard to where the first line of defense in protecting GDEs from flooding should lie, a comparison of the recent results of the two groups gives unique insight into the nature of processes occurring in GDE cathodes used for CO₂ electrolysis.