(326m) Continuum Modeling to Resolve Transport and Catalysis in Bipolar Membranes: From Fundamentals to Application

Bipolar membranes (BPMs) present substantial promise for implementation in a sustainable future due to their ability to convert applied electrical energy into gradients in pH. This conversion of electrostatic potential into changes in pH is accomplished by driving the dissociation water into protons (H⁺) and hydroxides (OH^–) within a catalyst layer (CL) using the large electric fields (> 10⁸ V m^–1) present at that catalytic interface. The generated H⁺ and OH^– can then transport out of the BPM through a cation exchange layer (CEL) and an anion exchange layer (AEL), respectively, on opposite sides of the CL, which develops a pH gradient across the BPM. The ability to sustainably generate changes in pH via breakdown of the solvent water, without the need of added acid and base, is critical to many processes in electrochemical energy storage and conversion (e.g., water electrolysis and CO₂ electrolysis), as well as environmental remediation (e.g., CO₂ capture and wastewater treatment). Unfortunately, the nature of electric field-enhanced water dissociation is poorly understood, and there is great need to manage the transport of competing co- and counter- ions (e.g., (bi)carbonates, sodium, chloride, etc.) for all applications.

In this talk, we develop a continuum model of a BPM immersed in electrolytes containing various ionic species. The continuum model resolves the local concentrations and fluxes of all ionic species (H⁺, OH^–, cations, and anions) and water using a modified Poisson-Nernst-Planck framework capable of resolving the effects the electric field on species activities and the equilibria of electric-field-enhanced dissociation reactions. Notably, the model can reproduce the electrochemical behavior of BPMs immersed in various electrolytes with a single set of fitting parameters, including buffering electrolytes like those containing (bi)carbonates or ammonium. The model enables a better understanding of the structure-property relationships that dictate co-ion crossover and water dissociation performance in BPMs. Lastly, we apply the developed modeling framework to simulate a BPM performing electrochemical carbon capture mediated via a pH swing, demonstrating that optimization of the CL and management of the mass transfer of bubbles are key to making pH-swing mediated carbon capture in a BPM energetically competitive with established thermal desorption processes. Ultimately, this work demonstrates the power of continuum modeling to rationalize and guide the performance and design, respectively, of materials and electrochemical devices across the nexus of energy and environmental technologies.

Figure 1: Schematic of BPM modeling across relevant length scales. (i.) Electric field enhanced water dissociation catalysis at the nanoscale. (ii.) Mesoscale ion transport within the BPM. (iii.) Device level modeling of a BPM-electrodialysis stack for carbon capture.