Understanding reactant transport in gas diffusion electrodes for electrochemical CO2 reduction
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Date
2025
Publication Type
Doctoral Thesis
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Abstract
This Ph.D. thesis focuses on understanding reactant transport in gas diffusion electrodes (GDEs) during the electrochemical CO2 reduction (eCO2R). eCO2R has been identified as a key technology to close the carbon loop as it enables the conversion of the greenhouse gas CO2 into valuable platform chemicals, such as C2H4, from which many possible commodity chemicals and materials can be synthetized. GDEs address the challenge of the low solubility of CO2 in water by increasing the transport of CO2 to the catalyst through a porous microstructure, bypassing the mass transport limitations faced when the electrode is fully immersed in the electrolyte. This allows for higher reaction rates, improved efficiency, and better selectivity during eCO2R, making GDEs more suitable for industrial applications.
Despite numerous studies investigating eCO2R and GDEs, the transport of reactants (CO2 and H2O/H+) and products to/from GDEs has not been completely understood. This has led to a lack of design principles for GDE microstructures able to maximize catalytic performance. Additionally, the GDE performance decays over time but there is no full picture of the exact deactivation mechanism(s) at play. Finally, the fundamental question of whether eCO2R occurs at a double or at a triple phase boundary, i.e. whether CO2 reacts after dissolving in the electrolyte at the catalyst/electrolyte interface or from the gas phase at the gas/catalyst/electrolyte interface, remains open. These relevant questions are addressed in this Ph.D. thesis and the main takeaways are:
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The control over the microstructure of polymer-based GDEs combining electrospinning and capillary flow porometry enables establishing important structure-performance relationships between GDE substrate microstructure and catalytic performance.
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Designing polymer-based GDE substrates with small pores and high water entry pressure reduces electrolyte penetration within the GDE, which in turns facilitate CO2 transport to the catalyst and enhances selectivity towards carbon products. This leads to a record high Faradaic efficiency towards C2H4 of 55% in the case of Cu GDEs.
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The proton consumption associated with eCO2R at high current density is sufficient to create a local basic environment around the catalyst even when using strongly acidic electrolytes. This basic environment reacts with CO2 and leads to detrimental (bi)carbonate precipitation, as demonstrated by their detection via operando synchrotron X-ray diffraction, even in strongly acidic electrolytes.
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Periodic potential pulses used to mitigate GDE degradation enable 10s of hours of stable GDE operation, however they do not prevent (bi)carbonate precipitation as demonstrated by operando synchrotron X-ray diffraction. The co-existence of GDE stability and (bi)carbonate precipitation highlights the complex nature of GDE degradation, in which catalyst restructuring and change of its oxidation state also play an important role.
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Comparing the limiting current density for the reduction of CO2 and of CO shows that the GDEs microstructure dictates whether the reactant transport occurs across a double phase boundary (where CO2 is first dissolved in the electrolyte and then reaches the catalyst), as in the case of polymer-based GDEs with sharp hydrophobicity change , or across a triple phase boundary (where gaseous CO2 meets water on the catalyst), as in the case of carbon-based GDEs with distributed hydrophobicity.
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A novel state-of-the-art experimental setup equipped with temperature and pressure sensors, fast gas and liquid chromatography and accurate flow determination enables a precise monitoring of reaction parameters that are important for achieving high GDE performance. Additionally, eight parallel reactors and capability of automatic python-based data analysis allow for the standardization of the experimental work, paving the way for high-throughput experiments.
Overall, these results contribute to improve our understanding of reactant transport in GDEs for eCO2R bringing this technology a step closer to industrial deployment.
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Examiner: Niederberger, Markus
Examiner : Seger, Brian
Examiner: Battaglia, Corsin
Examiner : García de Arquer, F. Pelayo
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ETH Zurich
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Subject
Electrochemical CO2 Reduction; Gas diffusion electrodes microstructure; Transport phenomena; Deactivation mechanisms; Synchrotron radiation
Organisational unit
03763 - Niederberger, Markus / Niederberger, Markus
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