Elucidating Design Criteria for Enhancing Rates of Water Dissociation at Bipolar Membrane Interfaces Using Multiscale Modeling and Experiments
Abstract
Many electrochemical technologies rely on ion-exchange membranes for the critical role of maintaining separation of mobile chemical species while affording rapid conduction of specific mobile ions, typically H+ and/or OHÐ. While membranes can slow the crossover rate of these mobile species (i.e., redox-active molecules and/or inert counterions) they cannot completely prevent crossover. Unique to counterions is that they can be fixed to solid supports such that their crossover is prevented. This strategy is used in reversible H2/O2 fuel cells that employ membraneÐelectrode assemblies, where fixed counterions are present in the form of membrane electrolytes that when wetted with water dissociate forming H+ and/or OHÐ, which carry the ionic current. The pH of the membrane, in this case, dictates the local pH near the electro-catalysts and therefore, catalyst stability. When the membrane electrolyte is composed of an acidic cation-exchange membrane (CEM) directly interfaced to an alkaline anion-exchange membrane (AEM), the overall structure is referred to a bipolar membrane (BPM). BPMs wetted by water can theoretically support differences in pH indefinitely, and these pH values can be altered by varying the functionality of the membranes. Ideally the pH is adjusted to within a range where desired electro-catalysts are stable. The challenge in using BPMs in electrochemical technologies is that in order to facilitate an ionic current, BPMs must asymmetrically transport H+ and OHÐ. Asymmetric transport occurs via either formation of water via association of solvated H+ and OHÐat the CEMÐAEM interface (CEM|AEM), or generation of solvated H+ and OHÐat CEM|AEM through water dissociation. In order to determine the means to enhance water dissociation at CEM|AEM, we propose to perform parametric experimental studies as a function of the chemical functionality present at CEM|AEM, coupled with multiscale modeling on length scales from nanometers to hundreds of microns. We will utilize computational simulations (i.e. DFT calculations, MD and DPD simulations, microkinetic modeling, and continuum PoissonÐNernstÐPlanck simulations) and a suite of experimental tools (i.e., electrochemical impedance spectroscopy and other more standard techniques, Kelvin probe and other force microscopies, sum frequency generation, and X-ray beamline measurements) to better elucidate the mechanisms of water-related processes at CEM|AEM. Specifically we will seek answers to the following questions: (1) What are the physical limitations to the rate of water dissociation in BPMs?; and (2) How can we use this understanding to design BPMs that pass ionic current commensurate with currents observed in practical reversible fuel cells?
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 09, 2020
- Source ID
- W911NF2010241
Entities
People
- Stephen J Paddison
Organizations
- Army Contracting Command
- United States Army
- University of Tennessee