Insights from operating under gas-liquid Taylor flow and techno-economic optimisation

Abstract

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.