Operando Synchrotron X-Ray Scattering Study of (Bi)Carbonate Precipitation in Gas Diffusion Electrodes for CO2 Reduction Using Neutral and Acidic Electrolytes
Authors: Senocrate Alessandro , Bernasconi Francesco , Plainpan Nukorn , Mirolo Marta , Battaglia Corsin 
Published: December 2024

Electrochemical CO2 reduction (eCO2R) offers a promising pathway to convert CO2 into sustainable fuels and chemicals, moving us closer to a net zero or net negative carbon economy.
1 To compete with established 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) fed by gaseous CO2, high activities (i.e. high current densities) can be achieved.
2 However, under these conditions rapid degradation typically takes place due to (bi)carbonate precipitation caused by excessive CO2 dissolution in the basic local environment around the catalyst. This phenomenon is so severe that is considered a potential showstopper for the eCO2R technology. A proposed strategy to overcome this limitation is to perform the eCO2R in acidic electrolytes, in which (bi)carbonate crystals are unstable.
In this work, we investigate in detail (bi)carbonate precipitation using operando synchrotron wide angle X-ray scattering experiments on polymer-based Cu GDEs, which have shown record high selectivity for C≥2 products and are impervious to flooding.3,4 We show that in neutral electrolytes (pH=4.5), (bi)carbonate precipitation takes place within minutes of switching on the eCO2R at technologically relevant current densities (200 mA/cm2). The extent of (bi)carbonate precipitation correlates with the decay of carbon product selectivity (particularly of C≥2 products) and with the increase of H2 production.5 Strikingly, using strongly acidic electrolytes (pH=1) under the same conditions does not lead to any delayed or reduced (bi)carbonate precipitation. Also here, the decay of carbon product selectivity correlates with the amount of precipitated (bi)carbonate and is even faster than in neutral electrolytes.5 Electrochemical studies with electrolytes of different pH corroborate these findings and show that, when lowering pH, an increasingly high current density is required for the eCO2R to be activated, with H2 being the sole product before reaching sufficiently higher currents. These results confirm that a high local pH close to the catalyst is necessary for effective eCO2R, thus requiring the higher proton consumption rates caused by higher current densities when using more acidic electrolytes. Our findings show that, while it is possible to operate Cu GDEs under sufficiently high current densities and achieve good selectivity for carbon products even in strongly acidic electrolytes (FEC2H4 > 45 % and FEC≥2 > 70 %), this takes place alongside substantial (bi)carbonate precipitation and thus at the expense of stability.

References

1Senocrate, A.; Battaglia, C.; J. Energy Storage
2021, 36, 102373.

2Verma, S.; Hamasaki, Y.; Kim, C.; Huang, W.; Lu, S.; Jhong, H. R. M.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P.; ACS Energy Lett.
2018, 3, 193–198.

3Senocrate, A.; Bernasconi, F.; Rentsch, D.; Kraft, K.; Trottmann, M.; Wichser, A.; Bleiner, D.; Battaglia, C.; ACS Appl.
Energy Mater.
2022, 5, 14504.

4Bernasconi, F.; Senocrate, A.; Kraus, P.; Battaglia, C.; EES Catalysis
2023, 1, 1009.

5Bernasconi, Plainpan, Mirolo, Battaglia, Senocrate, under review.