The Importance of Substrate Pore Size and Hydrophobicity in Gas Diffusion Electrodes for CO2 Reduction

Abstract

Electrochemical CO2 reduction (CO2RR) is a promising way to convert detrimental CO2 emissions into sustainable fuels and chemicals, and move closer to a circular carbon economy. 1 To compete with traditional fuel/chemical production routes, electrocatalysts need to be highly active, selective towards a desired product, and stable. High activities can be achieved with gas diffusion electrodes (GDEs) that provide gaseous CO2 to the catalyst, however, product selectivity and stability still need to be improved before industrial implementation. In this work, we show that the selectivity and stability of Ag and Cu GDEs can be tuned by acting on the pore size and hydrophobicity of their polymeric substrates.2,3 We rationalize these findings with the extent to which electrolyte penetrates the polymer-based GDEs, which depends on both substrate pore size and hydrophobicity and is quantified by the water entry pressure. GDEs with larger pore size promote electrolyte penetration (low water entry pressure) and, in the Ag case, yield GDEs with poor CO selectivity and stability (Fig. 1). In contrast, GDEs with small pore size show a much higher resistance to electrolyte penetration (high water entry pressure), thus yielding a greatly improved Faradaic efficiency for CO (up to 95% at 100 mA/cm2 in neutral electrolyte) and remarkable long-term stability (97% of initial CO selectivity retained after > 40 h). In the Cu case, we observe a strong enhancement in C2H4 selectivity when employing GDE substrates with small pores and strong hydrophobicity (up to 55% Faradaic efficiency in neutral electrolyte at 200 mA/cm2 for > 3 h). Both CO2 and CO reduction reactions indicate that mass transport limitations are present for all GDEs, but can be mitigated by GDE's with small pore size. Our results emphasize the importance of substrate microstructure on GDE product selectivity and stability during CO2RR, and represent a new scalable strategy to improve GDE performance. References 1A. Senocrate, C. Battaglia, J. Energy Storage 2021, 36, 102373 2A. Senocrate, F. Bernasconi, D. Rentsch, K. Kraft, M. Trottmann, A. Wichser, D. Bleiner, C. Battaglia, ACS Appl. Energy Mater. 2022, 5, 14504 3F. Bernasconi, A. Senocrate, C. Battaglia et al., in preparation Figure 1