(52b) Direct Seawater Electrolysis Enabled By Robust Ion Transport Control in Membrane Electrolyzers with Asymmetric Electrolyte Conditions

Enabling renewable electricity-driven seawater electrolysis is a modern imperative that stands to unlock vastly abundant resources for integration in contemporary chemical fuel and feedstock production. However, significant technological barriers impede the translation of design principles for state-of-the-art proton-exchange membrane (PEM) electrolyzers to systems that integrate contaminated water streams as feedstocks. While PEM electrolyzers are highly efficient in generating H₂ and O₂ from ultra-pure water feeds, the ionic composition of naturally occurring ocean waters significantly complicate their operation with seawater owing both to significant Nernstian voltage losses as well as anodic microenvironments that facilitate the oxidation of Cl^- to corrosive OCl^-, HOCl, and even dissolved Cl₂. In this work we present strategies for controlling the intrinsic ion transport phenomena of permselective ionomer membrane electrolyzers that underpin premature device failure. Specifically, we highlight the stability afforded by bipolar membrane (BPM) electrolysis in comparison to PEM electrolysis, quantify the effects of controlling Cl^- transport through permselective membranes, and present the longest sustained operation of direct seawater electrolysis at a significantly large current density (250 mA/cm²) that has been reported to date.

Herein we evaluate the strategy for generating H₂ directly via seawater electrolysis of preventing the transport of anionic constituents—particularly Cl^- owing to the corrosive nature of its oxides—across the membrane electrode assembly from the catholyte to the anolyte. We also quantify the shift in pH conditions for hydrogen evolution and oxygen evolution catalysis away from ideality when this transport is not mitigated, as well generation of particularly corrosive anolyte conditions that destroy overall cell performance. To quantify the degree to which permselective ionomer architectures mitigate the aforementioned failure modes, we 1) performed electrolysis under asymmetric electrolyte conditions that included HER in simulated or real seawater with OER in deionized water, and 2) coupled electrochemical with quantitative compositional analyses of device components and electrolyte conditions over time. Our methodology for screening the performance and stability of seawater electrolyzers highlights the promise of bipolar membrane electrolyzers as incredibly robust against feed contamination, with >19 hours of continuous operation with real seawater. We further implicate the use of BPM electrolyzers in an array of energy-conversion applications involving un-treated water feedstocks.