Controlling CO2 and H2o Mass Transport in Gas Diffusion Electrodes for CO2 Reduction by Acting on the Substrate
Authors: Senocrate Alessandro , Bernasconi Francesco , Plainpan Nukorn , Battaglia Corsin 
Published: December 2024

Electrochemical CO2 reduction (eCO2R) is a very promising way to convert detrimental CO2 emissions into sustainable fuels and chemicals, and move us closer to a net zero or net negative carbon economy.
1 To compete with traditional fuel/chemical production routes, electrocatalysts needs to be highly active, selective towards a desired product, and stable. Thanks to the use of gas diffusion electrodes (GDEs) that provide gaseous CO2 close to the electrocatalyst, high activities can be achieved.
2 However, product selectivity as well as stability still need to be improved before applications. An emerging strategy to drive product selectivity and improve stability is to control the local environment surrounding an electrocatalyst, i.e., the concentration of the products and reagents therein.3

In this work, we achieve such control by tuning the mass transport of the eCO2R reactants, namely CO2 and H2O, to the reaction sites by varying pore size and hydrophobicity of polymeric GDE substrates coated with Ag or Cu electrocatalysts.4,5

We show that for GDEs with large pore size and weak hydrophobicity, a deeper penetration of water/ aqueous electrolyte takes place, yielding, in the Ag case, poor selectivity towards CO and a significant production of H2. In contrast, GDEs with small pore size prevent this local flooding of the electrocatalyst, thus yielding a greatly improved Faradaic efficiency towards CO (up to 95% at 100 mA/cm2 in neutral electrolyte), minimal H2 evolution, and remarkable long-term stability (97% of initial CO selectivity retained after > 40 h).4 In the Cu case, we observe a strong enhancement in C2H4 selectivity when employing GDE substrates with small pores and strong hydrophobicity (up to 50% Faradaic efficiency in neutral electrolyte at 200 mA/cm2 for > 3 h).5 Both CO2 and CO reduction experiments on Cu GDEs indicate the presence of mass transport limitations for the gaseous reactants in all GDEs, however this limitation is less severe for GDEs with small pore sizes. We rationalize these findings with the presence of an electrolyte layer covering the electrocatalyst, forcing the electrocatalytic reaction to take place at double phase boundaries. Under these conditions, CO2 mass transport limitations are governed by the extent of electrolyte penetration within the GDE and can thus be controlled by acting on the substrate.4,5 In addition to fundamental understanding, our results also provide a scalable strategy to improve product selectivity and stability of GDEs for eCO2R.

References

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2021, 36, 102373.

2García de Arquer, F. P.; Dinh, C.-T.; Ozden, A.; Wicks, J.; McCallum, C.; Kirmani, A. R.; Nam, D.-H.; Gabardo, C.; Seifitokaldani, A.; Wang, X.; Li, Y. C.; Li, F.; Edwards, J.; Richter, L. J.; Thorpe, S. J.; Sinton, D.; Sargent, E. H. Science
2020, 367, 661–666.

3Veenstra, F.; Ackerl, N.; Martín, A.; Pérez-Ramírez, J.; Chem
2020, 1–16.

4Senocrate, A.; Bernasconi, F.; Rentsch, D.; Kraft, K.; Trottmann, M.; Wichser, A.; Bleiner, D.; Battaglia, C.; ACS Appl.
Energy Mater.
2022, 5, 14504.

5Bernasconi, F.; Senocrate, A.; Kraus, P.; Battaglia, C.; EES Catalysis
2023, 1, 1009.