The electrochemical conversion of captured carbon dioxide (CO2) at low temperatures holds promise as a sustainable method for producing materials and fuel using renewable energy sources. However, technological hurdles such as mass transfer limitations and operational instability hinder its industrial application. This dissertation aims to address these challenges by exploring the use of gas-liquid Taylor flow (series of confined gaseous CO2 bubbles, which are separated from each other by liquid electrolyte and from the channel walls by a thin liquid film) in electrolysis, which can enhance mass transfer without requiring complex electrode designs, potentially improving long-term operational reliability. Additionally, a multi-scale modelling framework is introduced to evaluate electrolyser designs from an economic standpoint, aiding in the identification of bottlenecks and guiding technology development.
In Chapter 2, we propose a tubular electrolyser design operating under gas-liquid Taylor flow to overcome mass transfer limitations. By developing a numerical model, we investigate the relationship between process conditions, mass transfer, and reactor performance. Insights gained from this model allow us to derive an easy-to-use analytical relation to evaluate the impact of changes in inlet flow rates on Faradaic efficiency and current density. We find that long gaseous CO2 bubbles and low velocities enhance the current density towards CO, outperforming traditional H-cells. However, achieving performance comparable to flow-through electrolysers operated with a gas diffusion electrode (GDE) requires means to increase CO2 solubility in the liquid electrolyte, by for example increasing pressure.
Chapter 3 focuses on experimentally testing how Taylor flow influences the electrolyser performance within the established zero-gap water electrolyser concept adapted for CO2 reduction, by employing a silver gauze as the cathode. Our experimental findings reveal that Taylor flow enhances the Faradaic efficiency towards CO compared to single-phase flow, with minimal influence from gas holdup within the studied velocity range. Contrary to the tubular design, high velocities are desirable to increase the Faradaic efficiency towards CO in the rectangular flow channel. We find that further optimisation of
cathode design and fabrication is needed to fully exploit the potential of this electrolyser concept.
In Chapter 4, techno-economic aspects of electrochemical CO2 conversion are addressed, aiming to optimise operational parameters for industrial applications. A multiscale model capturing mass transfer effects over the channel length of a GDE electrolyser is integrated into an economic framework to analyse the interdependencies of key performance variables on the economic outlook. The analysis indicates that optimal current densities may differ significantly from previously reported benchmarks, emphasising the importance of multi–scale modelling for evaluating electrolyser designs under economic considerations.@en

The production of base chemicals by electrochemical conversion of captured CO₂ has the potential to close the carbon cycle, thereby contributing to a future energy transition. With the feasibility of low-temperature electrochemical CO₂ conversion demonstrated at lab scale, research is shifting toward optimizing electrolyser design and operation for industrial applications, with target values based on techno-economic analysis. However, current techno-economic analyses often neglect experimentally reported interdependencies of key performance variables such as the current density, the faradaic efficiency, and the conversion. Aiming to understand the impact of these interdependencies on the economic outlook, we develop a model capturing mass transfer effects over the channel length for an alkaline, membrane electrolyser. Coupling the channel scale with the higher level process scale and embedding this multiscale model in an economic framework allows us to analyze the economic trade-off between the performance variables. Our analysis shows that the derived target values for the performance variables strongly depend on the interdependencies described in the channel scale model. Our analysis also suggests that economically optimal current densities can be as low as half of the previously reported benchmarks. More generally, our work highlights the need to move toward multiscale models, especially in the field of CO₂ electrolysis, to effectively elucidate current bottlenecks in the quest toward economically compelling system designs.

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Electrochemical reduction of CO₂using renewable energy is a promising avenue for sustainable production of bulk chemicals. However, CO₂electrolysis in aqueous systems is severely limited by mass transfer, leading to low reactor performance insufficient for industrial application. This paper shows that structured reactors operated under gas-liquid Taylor flow can overcome these limitations and significantly improve the reactor performance. This is achieved by reducing the boundary layer for mass transfer to the thin liquid film between the CO₂bubbles and the electrode. This work aims to understand the relationship between process conditions, mass transfer, and reactor performance by developing an easy-to-use analytical model. We find that the film thickness and the volume ratio of CO₂/electrolyte fed to the reactor significantly affect the current density and the faradaic efficiency. Additionally, we find industrially relevant performance when operating the reactor at an elevated pressure beyond 5 bar. We compare our predictions with numerical simulations based on the unit cell approach, showing good agreement for a large window of operating parameters, illustrating when the easy-to-use predictive expressions for the current density and faradaic efficiency can be applied.

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