Interplay between Mesoscale Architecture and Catalytic Output in CO2 Gas-Diffusion Electrolyzers

Abstract

With the anticipated effects of global climate change upon us, induced by egregious CO2 emissions, it is imperative to implement technologies to close the carbon cycle. One potential technique to do this is storing the energy from intermittent sources (e.g. solar, wind, hydro) in the form of chemical bonds, such as the electrochemical reduction of CO2. By reducing CO2 to energetically and chemically useful products of CO2 (i.e. CO and C2H4) we can both reduce reliance on fossil fuels and mitigate atmospheric CO2 levels. However, controlling the selectivity towards which particular product is formed and preventing the parasitic hydrogen evolution reaction (HER) is a crucial challenge that must be overcome.1 Exploring fundamental mechanisms and new catalyst materials to overcome this challenge, can be skewed due to the low diffusivity (≈2x10-9 m2/s)2 of CO2 in the liquid phase causing inefficient utilization of catalyst active sites. Unlike the oxygen reduction reaction, use of a rotating disc electrode does not resolve the issue due to the competitive HER reaction. Therefore, it becomes imperative to study physical structure-function relations (e.g. catalyst layer thickness) in a cell that provides ample flux of solely CO2, such as a gas-diffusion electrolyzer (GDE) which greatly increases the diffusivity of CO2 and gas products by enabling transport through gas channels (≈1.6x10-5 m2/s). By filtering nanowires through a gas-diffusion electrode (Figure 1 a-b) we fabricated electrodes of varying thicknesses that were then applied as catalysts in a gas-diffusion electrolyzer (Figure 1c). Results show that even under high CO2 flux conditions there is an interplay between selectivity (Figure 1d), activity (Figure 1e) and catalyst layer thickness. Further exploration of this interplay in commercially relevant electrolyzers can expedite optimized implementation of catalysts in the industrial sector and hopefully aid in solving the global climate crisis at hand.(1) Verma, S.; Kim, B.; Jhong, H.-R. “Molly”; Ma, S.; Kenis, P. J. A. A Gross-Margin Model for Defining Technoeconomic Benchmarks in the Electroreduction of CO2. ChemSusChem 2016, 9 (15), 1972–1979. https://doi.org/10.1002/cssc.201600394.(2) Raciti, D.; Mao, M.; Ha Park, J.; Wang, C. Mass Transfer Effects in CO 2 Reduction on Cu Nanowire Electrocatalysts. Catalysis Science & Technology 2018, 8 (9), 2364–2369. https://doi.org/10.1039/C8CY00372F.Figure 1: Electron microscopy images of Ag nanowires (a) top-down and (b) cross-section, on a gas-diffusion electrode. This electrode is than applied as a CO2 reduction catalyst in a (c) gas-diffusion electrolyzer. The thickness of the Ag nanowire catalyst layer influences both the (d) selectivity and (e) mass activity of the reaction. Figure 1