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Abstract

Renewable electricity-driven oxygen and carbon dioxide reduction reactions (2e-ORR and CO₂RR) to electro-synthesize hydrogen peroxide and formate could replace conventional fossil-fuel-operated thermochemical processes. However, continuous improvement in the electrochemical system, including catalyst development, electrode design, two-phase flow dynamics, and reactor operation, is needed for industrial realization. This thesis systematically investigates the challenges associated with each of these components through a rigorous experimental approach to developing a durable and efficient gas diffusion electrode (GDE)-based electrolyzer system for 2e-ORR and CO₂RR processes. In the first part of the study, a simple electrode system comprised of a hydrophobic microporous carbon layer (MPL) coated on one side of the gas diffusion layer (GDL) was discovered for efficient 2e-ORR to alkaline peroxide electrosynthesis. The effects of gas and liquid flow rates with corresponding pressures and the mode of electrolyzer operation on the electrode stability as a function of current density were further investigated. The study shows that high gas and low liquid flow rates enable efficient peroxide generation (> 90% Faradaic efficiency) at a high current density of up to 500 mA cm-2 by keeping the differential pressure window below the threshold liquid breakthrough pressure. While the high activity was achieved under gas-fed flow-by mode with optimized flow dynamic conditions, an operational electrode stability (> 20 h) was only established with the flow-through (or quasi-flow-through) approach. The second part of the thesis demonstrates the promotional effects of CeO₂ and a novel step-pulse-reverse polarity strategy in enhancing the activity and stability of SnO₂ and In₂O₃ electrodes for CO₂RR to formate process on a 20 cm² size electrode system. At a high current density of 500 mA cm⁻², SnO₂ and In₂O₃ suffer from degradation owing to their reduction to zero valence states and subsequent dissolution in an alkaline electrolyte. CeO₂, due to its strong redox properties, prevents the transformation of these metal oxides to the metallic state, thereby retaining the activity for a longer period. The findings from the 2e-ORR work, with regards to the flow properties, GDE composition, and reactor operation, were successfully implemented in the CO₂RR process to prevent liquid flooding-induced malfunctioning of the GDE.