(482c) Understanding the Thermodynamics and Kinetics for Water-Splitting in Micropatterned Bipolar Membranes

The palette of applications for bipolar membranes (BPMs) has expanded in recent years to numerous electrochemical energy and conversion applications including fuel cells, electrolyzers, and photoelectrochemical cells. BPMs have historically been deployed in electrodialysis setups for mineral acid and base production. The need to have disparate pH environments in electrochemical cells, and prevent species crossover, have motivated researchers to examine BPMs as an electrolyte separator. BPMs have the unique capability to split water into protons and hydroxide ions charge carriers in addition to conducting those ions in opposite directions to maintain current flow in the electrochemical setup. A BPM consists of a cation exchange membrane (CEM) appended to an anion exchange membrane (AEM) that have intimate contact and feature a water dissociation catalyst between the individual membrane layers.

Most materials related research for BPMs has focused on water-dissociation catalysts. There are few reports that investigate the importance of high-quality bipolar junction interfaces for improving water-splitting in BPMs. Although it has been shown that intimate contact between the oppositely charged layers at the interface improves BPM performance, no model or scaling relationships exists that relate bipolar junction interfacial area to the on-set potential for water-splitting and the current density to water-splitting. Our research has aimed to address this knowledge gap by preparing BPMs with systematically varied interfacial area values using soft lithography. Polarization experiments with the new, micropatterned interface BPMs reveal a 250 mV reduction in the on-set potential when increasing the interfacial area by 2.2x. With respect to current density, the same increase in interfacial area resulted in a 15% increase in current density at 2 V (i.e., marginal improvement). This talk explains these observations by examining the change in local electric field using Poisson-Boltzmann equation and limitations in enhancing reaction kinetics due to mass transfer. Finally, the soft-lithography approach devised was successful for making BPMs with different chemistries ranging from perfluorinated AEMs and CEMs to alkaline stable, ether-free poly(arylene) hydrocarbon AEMs. These polymer chemistries are more robust for fuel cell and electrolysis applications.

This work was supported from NSF Award #: 1703307.