The electrochemical reduction of CO2 for producing carbon-neutral fuels and chemicals is a pivotal technology for the future decarbonization of the chemical and energy sectors. Among the various electrolyzer architectures, forward bias bipolar membrane (BPM) systems have emerged as a promising solution, combining high selectivity and CO2 utilization with a pure-water feed, salt-free design. However, performance limitations arising from water transport imbalance, membrane degradation, and kinetics overpotentials at the membrane interface hinder their industrial deployment.

This thesis addresses these challenges through systematic investigations across four key axes. The first part focuses on water transport, using synchrotron-based, operando X-ray tomography and diffusion measurements to analyze water management and hydration dynamics in a commercial BPM-based cell. It reveals that cathode flooding is not a limiting factor at the investigated current densities (up to about 200 mA cmâ 2), as the GDL saturation stays below 10%. Instead, insufficient water supply to the cathode and membrane over-swelling are identified as the dominant challenges for high current density operation.

The second part focuses on degradation mechanisms within the membrane-electrode assembly. Pre- and post-mortem analysis, again by X-ray tomography, reveals the evolution of structural damage. We show that membrane delamination occurs at current densities above 100 mA cmâ 2. To resolve this, a series of semi-porous BPMs with engineered gas-permeable anion exchange layers is designed and investigated. These structures enable enhanced back-transport of recombined CO2 toward the cathode, thereby relieving interfacial pressure buildup at the membrane junction. Among the tested designs, a microporous ionomer-nanoparticle composite layer proves particularly effective in suppressing membrane delamination under up to 200 mA cmâ 2 and reducing anode catalyst layer damage area by up to 90% compared to commercial BPMs.

The third part addresses the kinetic limitations at the BPM junction by integrating metal-oxide catalyst layers (e.g., TiO2, SiO2, IrO2) directly at the membrane interface. Complete catalyst coverage at 20â 30 ÎŒg/cm2 enables up to 100% higher current density at similar iR-corrected voltages, providing a scalable strategy to overcome interfacial limitations.

Finally, the fourth part investigates the role of the catalyst-layer microenvironment and its influence on reaction kinetics in cation-free systems. By designing a fully gas-fed setup with pretreated membranes, the study isolates and evaluates the impact of trace alkali cations migrating across the membrane. Furthermore, the use of anion exchange ionomers with high ion-exchange capacity (IEC) is found to promote beneficial double-layer capacitance and stabilize the catalytic interface over multi-hour operation. The results support a mechanism where both mobile and fixed cations shape the local environment on the silver catalyst surface, thereby enabling stable CO production under pure-water-fed, salt-free conditions.

Altogether, this work presents a comprehensive framework for enhancing the performance of a pure-water fed CO2 electrolyzer with a BPM in forward bias. By coupling advanced diagnostics with targeted material strategies, the thesis contributes to both fundamental understanding and practical design principles to guide the next generation of scalable and durable CO2 electrolysis systems.