Advancement of Asymmetric Bipolar Membranes for Tailoring Chemical Environments in Electrochemical Systems

Citation

Lucas, Éowyn (2023) Advancement of Asymmetric Bipolar Membranes for Tailoring Chemical Environments in Electrochemical Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/jken-ty64. https://resolver.caltech.edu/CaltechTHESIS:02162023-233344819

Abstract

Under reverse bias, bipolar membranes (BPMs) enhance water dissociation (WD) at the junction between a cation exchange layer (CEL) and an anion exchange layer (AEL), often with additional improvement from an integrated WD catalyst. Recent research has shown promise for developing and implementing BPMs in renewable energy systems, such as carbon removal, water and CO₂ electrolysis, and energy storage. The economic feasibility of these carbon capture and conversion systems with incorporated BPMs, however, relies on BPMs to maintain stable operation at high current densities (>100 mA cm⁻²) and low overpotentials. Existing commercial BPMs are limited to current densities of ≤100 mA cm⁻² as water transport through the CEL and AEL cannot keep up with the increased rate of WD at the junction at higher current densities. In this work, we present a freestanding, high current density BPM (HCD-BPM) with a thin AEL (15 μm, PiperION 15R), a graphene oxide (GrOx) catalyst layer, and a mechanically supportive CEL (50 μm, Nafion 212) specifically designed to overcome water transport limitations. When tested under reverse bias in a custom electrodialysis cell with Luggin capillaries, this HCD-BPM demonstrates the lowest published overpotentials up to 1 A cm⁻². Furthermore, the HCD-BPM exhibits stabilities of >1000 hour at 80 mA cm⁻², >100 hours at 500 mA cm⁻², and >60 hours at 1 A cm⁻², Faradaic efficiencies for H⁺ and OH⁻ of >95%, and successful implementation into a multi-cell electrodialysis stack designed for integration into a DOC system. Additional characterization, such as SEM, Confocal microscopy, and titration, was performed to understand the structure and performance of the HCD-BPM. Additionally, the BPM was tested in forward bias to investigate its use for acid/base flow batteries. Overall, this thesis presents a novel BPM with record performance in multiple electrochemical systems that mitigate anthropogenic CO₂ emissions.

Thesis Files