The electrochemical CO2 reduction is a promising approach to convert this greenhouse gas to valuable products. The membrane electrode assembly (MEA) configuration integrates porous gas diffusion electrodes (GDEs) where the CO2 reduction reaction rapidly occurs thanks to the large surface area and the easy transport of CO2. The MEA electrolyzer also mitigates the ohmic potential loss in the device. Therefore, it becomes a potential device for industrial CO2 electrolysis. Nevertheless, the coupled multi-physical transport-reaction process in porous electrodes and the device is complex and experimentally not directly quantifiable. Additionally, there are various parameters in terms of operation, material, and design which require optimization. A comprehensive multi-scale and multi-physics study is therefore required to aid in the understanding.
In this thesis, the reaction environment was analysed by a pore-level model featuring a liquid-solid interface. Transport of species was resolved around catalyst surface including the electric double layer. The model allows for demonstration of the transport inside the Nernst diffusion layer and electric field distribution around the double layer, making it a key step forward on the path on understanding the local reaction environment in electrochemical CO2 reduction.
In terms of the device level study, a one-dimensional homogenized model for CO2 reduction to CO on an Ag-based GDE was firstly developed. The model was then extended to ethanol and ethylene joint production by a Cu-based catalyst with experimental validation. This model accounts for the gas phase for the delivery of reactant and product, the liquid phase for the transport of dissolved species and support the electrochemical reaction, and the solid phase for the transport of electrons. The dissolution of CO2 into electrolyte, CO2 reduction, H2 evolution and the reactions in aqueous solution are involved. The results demonstrated the influence of operation on CO2 concentration and local current density, which provided guidance on how to obtain fast production rate and high selectivity.
In addition, a two-dimensional model was developed for MEA on full device level for the performance prediction and formulation of design guidance. A combined computational and experimental study on the MEA CO2 electrolyzer with nickel-based single-site catalyst was performed. The continuum model was developed utilizing the kinetic parameters obtained from H-cell experiment, the geometrical characteristics and transport properties obtained by direct pore level simulations on catalyst meso-structure from Nano-tomography. This continuum model was compared with the experimental MEA measurements performed independently. The validated model was applied for extensive parameter studies, which aided in design guidelines, diagnosed the experimental operation, and provided insight on the heterogeneities. Lastly, a catalyst loading study was performed to analyse the mass transport limitation inside the catalyst layer and optimize the layer thickness.
The thesis will benefit the understanding of coupled multi-physics transport-reaction phenomenon in CO2 reduction devices on both pore scale and macroscopic scale, which is difficult to characterize and quantify via experiments. The optimization of operation parameters, materials, and geometry will contribute to the development of practical and scalable electrochemical CO2 reduction devices.