Asymmetric Bipolar Membrane for High Current Density Electrodialysis Operation with Exceptional Stability

Abstract

Bipolar membranes (BPMs) enable isolated acidic/alkaline regions in electrochemical devices, facilitating optimized catalytic environments for water electrolysis, CO2 reduction, and electrodialysis. For economic feasibility, BPMs must achieve stable, high current density operation with low overpotentials. We report a graphene oxide (GrOx) catalyzed, asymmetric BPM capable of electrodialysis at 1 A cm-2 with overpotentials < 250 mV. Experiments and continuum modeling demonstrate that the low overpotentials for water dissociation are achieved by deprotonation at GrOx catalyst sites located within the high electric field BPM junction region. The asymmetric nature of the BPM allows it to overcome water transport limitations due to its thin anion exchange layer, while maintaining near unity Faradaic efficiency for acid and base generation. Additionally, the asymmetric BPM exhibits voltage stability exceeding 1100 hours at 80 mA cm-2 and 100 hours at 500 mA cm-2 and its freestanding architecture implemented in an electrodialysis cell stack demonstrates its real-world applicability.

Multi-scale physics of bipolar membranes in electrochemical processes

“…Typically, more hydrophobic ionomers like PFSAs possess phase-segregated domains, a hydrophobic backbone phase, and a development of advanced interfacial catalysts has significantly reduced BPM energy requirements [13][14][15] that have long been an important contributor to cost precluding their industrial application 16 , with recent demonstrations exhibiting performance close to the thermodynamic minimum even at high current densities approaching 1 A cm -2 (refs. 13,14,17). However, an untapped opportunity exists to leverage engineering approaches to optimize BPM systems.…”

“…These principles contextualize performance of three emerging BPM applications: (1) water electrolysis and CO 2 reduction (CO 2 R), (2) flow batteries and fuel cells, and (3) environmental remediation. Present challenges in BPM deployment at-scale are also discussed, ending with an outlook on the opportunities for chemical-engineering Nafion CEL, a PiperION AEL, and a nanoparticle catalyst layer 13,14 . Other polymer chemistries, such as hydrocarbon CELs, or junction morphologies, such as interwoven 138 and electrospun 59 , can appear in BPMs as well.…”

“…The emergent properties of BPMs render these materials susceptible to different failure modes from what is observed in monopolar membranes. Strong physicochemical interactions between AELs and CELs improve overall stability 14,24 ; however, unequal swelling (solvent uptake) and transport properties can cause BPM delamination 14 . Therefore, BPMs must be fabricated with chemically and mechanically compatible polymer layers.…”

Analysis of bipolar membranes for electrochemical CO2capture from air and oceanwater

“…To model the water dissociation catalyst, we treat the CL as a thin neutral region located between the AEL and CEL where the large field in between the two layers drives the dissociation of water by shifting the forward and reverse rate constants as per equations ( 12) and (13). While more detailed models of water dissociation catalysis have been presented that explicitly consider catalyst surface effects, 26,52 we choose a more simple model for water dissociation for computational efficiency. Furthermore, the interaction of carbon species with catalyst surfaces are still not well understood; because this work is more focused on modeling the impact of reactive carbon species on polarization behavior and mesoscale transport, the exact details of the catalyst surface are not crucial to capture.…”

Separation of Polymeric Charge Enables Efficient Bipolar Membrane Operation

“…Ionomers are often used as binders in catalyst ink mixtures to ensure good adhesion with an underlying substrate 29 and are necessary for stable catalyst operation in BPMs. 10,30 To understand how the inclusion of an ionomeric binder at the bipolar junction may affect BPM performance, we fabricated CEM | I | AEM composites, where I is either a cation exchange ionomer spraycoated directly on the AEM (Nafion D2020) or an anion exchange ionomer spray-coated onto the CEM (Sustainion XA-9). Employing the methodology established above, we used polyelectrolytes in place of H 2 SO 4 and KOH in order to disentangle the effect of the ionomer on the V mem value from the effect of co-ion crossover.…”