Gap Attack Platform B

Problem Statement

Relative to biological membranes, today’s synthetic membranes have very poor selectivity for removing solutes of interest from aqueous streams. While biological K+ channels have extraordinary K+/Na+ specificity, synthetic membranes largely lack the ability to separate solutes of the same valency. We will develop new functionalities, based on inclusion of highly selective metal organic frameworks (MOFs) into UMCP polymers to improve ion specific selectivity (Bates, Humphrey, Page, Henkelman). Ion transport is central in both hydrated polymers for water filtration and dry polymers for electrochemical applications such as batteries. While remarkable progress has been made in the development of polymeric battery electrolytes, and hydrated membranes for both desalination and energy applications (e.g., fuel cells, electrolyzers, and solar fuel generators), further progress hinges on developing fundamental insights into mechanisms of ion solvation/solubility and diffusion that are the foundations of ion permeability and ionic conductivity. Developing this insight requires bridging understanding among both dry and hydrated membranes. Based on our success in developing ion selective membranes, we will explore new design paradigms for ionic separations and develop an improved understanding of the role of hydration in the physics of ion transport (Freeman, Segalman, Ganesan, Clément). Furthermore, many membranes (e.g., RO membranes) bear charged groups, such as carboxylic acids and amines, in low water content, confined environments, where much less fundamental information is known, relative to aqueous solutions. The dissociation of these charged functional groups determines charge density in the RO membranes and, in turn, Donnan exclusion and, therefore, ion rejection (i.e., selectivity) of such membranes. The impact of ionic strength, background solution complexity (e.g., mixtures of solutes and ions) on protonation/deprotonation, complexation and ion pairing have been well studied in aqueous solution. In contrast, our knowledge of such interactions among solutes and solute/polymer functional groups within polymers is still lacking. The impact of continuum level parameters (e.g., ionic strength and dielectric constant) on these interactions has been identified in numerous studies, but robust models for predicting the influence of these parameters have not been fully developed for highly complex source waters. We will construct model UMCP materials bearing relevant functional groups to identify the underlying molecular phenomena reflected in the observed influence of continuum properties (e.g., water content, dielectric constant, ionic strength) on functional group-solute interactions (e.g., pKa and binding constants) in both confined and unconfined geometries (Christopher, Katz, Henkelman). These macroscopic studies will synergize with molecular/atomic-scale studies in GAP A. That is, GAP A’s studies on how surface chemistry impacts ion solvation at surfaces will provide molecular insights for the macroscopic studies performed in GAP B.

Molecular-scale permeation of water and solutes through dense, nonporous polymers controls separation properties of reverse osmosis (RO) and electrodialysis (ED) water purification membranes. Such membranes are invariably hydrated, with hydration varying over a wide range, from as low as 5 vol.% in RO membranes to 50 vol.% or more in some ion exchange membranes (IEMs) used for ED94. Water and solute permeability vary by orders of magnitude with polymer water uptake. Polymer membranes for energy applications (e.g., batteries, fuel cells, electrolyzers, and solar-fuel cells) are often chemically similar to IEMs used in water purification membranes. However, these membranes are either rigorously dry (in the case of solid polymer electrolytes (SPE’s) for batteries) or much less hydrated (membranes for fuel cells, electrolyzers, and solar-fuel cells) than those used as water purification membranes. Solute transport (e.g., ion diffusion) is critical in both classes of membranes. While significant progress has been made in the development of the membranes listed above, further progress hinges on a deep understanding of the fundamental origins of permeability and selectivity and how the presence of water modulates these properties. Indeed, a curation and analysis of a database of published polymer Li+- electrolyte conductivity performance suggests that, with the exception of activation energy, individual features commonly explored by this community, are poor predictors of performance. Further, the performance of conventional lithium-ion batteries is limited by low selectivity of the metal cation relative to its counterion (where selectivity is parameterized by the transference number, t+). Indeed, SPE performance appears to be subject to an upper bound in permeability/selectivity similar to that which has stymied the development of seemingly unrelated gas and water separation membranes. GAP B will assemble a team with diverse expertise to bridge our understanding of wet and dry water-energy membranes with the goal of presenting design paradigms for future membranes. These design rules will be rooted in a fundamental understanding of the role of hydration on solute (e.g., water, ions, neutral molecules) transport in such membranes. We seek to cross-pollinate theory, characterization tools and methods, and materials between the energy and water membrane communities to accelerate translation of discoveries in one field (e.g., dry membranes) to the other (e.g., hydrated membranes) and vice versa. This knowledge will elucidate the impact of water on ion sorption, diffusion, permeation, ionic conductivity, as well as membrane structure and dynamics.

Solute size, polarity and solvation radii are critically important for determining solute diffusion coefficients in aqueous solutions and membranes. However, for ions, their extent of hydration depends on the water content and dielectric properties of their environment, which can change enormously as membrane water content varies from dry (e.g., dielectric constant of 5-10) to highly hydrated environments having dielectric constants similar to water(~80). Additionally, ion association (i.e., ion pairing) is favored in dehydrated, low dielectric media and disfavored in highly hydrated media. Ion association impacts effective ion size and charge, which can markedly affect ion transport rates in response to concentration gradients (e.g., RO) and electric fields (e.g., ED). Determination of individual ion diffusion coefficients from ion permeability and ionic conductivity measurements relies on the validity of the Nernst-Planck relation and on knowledge of ion association, both of which are poorly understood in hydrated membranes for water purification. Similarly, recent rigorous calculations of transference numbers for dry SPEs are substantially lower than those approximated from ideal solutions (Nernst-Einstein), presumably due to formation of neutral ion clusters and charged ion triplets. Indeed, such phenomena have led to reports of seemingly non-physical negative transference numbers largely driven by the presence of negatively charged [anion-cation-anion]- ion triplets.

Research Questions

1. Can molecular insights of ion transport processes and mechanisms in wet and dry polymers be unified to design new, highly selective, water and energy membranes?
2. What is the role of water in ion/solute transport and functional group (e.g., carboxylate, amine) speciation in systems of varying levels of hydration?
3. Can high selectivity MOFs be designed that are hydrolytically stable and can be readily incorporated into the UMCP without compromising membrane mechanical strength?

Research Approach

In polymer electrolytes, ion conductivity is achieved through salt dissociation into unpaired cation/anion charge carriers and subsequent transport/diffusion of these ions through the polymer. These two steps strongly depend on factors including, cation solvation thermodynamics, polymer backbone dynamics that control solvation site connectivity, and the rearrangement of covalently tethered coordination sites (i.e., ligands) with metal cations. These phenomena and the relationships between ligand bond chemistry and the physics of polymer chains have been widely studied in dry polymer electrolytes towards the design of highly conductive and selective polymers for energy storage devices. In the development of these materials, combined Pulsed Field Gradient (PFG)-NMR Spectroscopy and electrochemical measurements have demonstrated that ion aggregation and pairing profoundly affect these key metrics for electrolyte performance. The performance of hydrated ion exchange membranes, frequently implemented for water purification (e.g., crosslinked poly(styrene sulfonate) and quaternary ammonium functionalized poly(styrene-co-divinyl benzene)) and fuel cells (e.g., Nafion), can also be strongly affected by ion dissociation (i.e., formation of ion pairs). However, molecular scale understanding and design rules similar to the dry polyelectrolyte context have not yet emerged. Additionally, variations in water content of hydrated membranes strongly influence ion-ligand interactions, causing ion solubility and diffusivity to vary significantly in poorly predicted ways. M-WET’s earlier studies of selective ion transport 12-crown- 4 (12C4)-functionalized membranes demonstrated that Li+ ions, which bind selectively to 12C4 groups in nonaqueous media, exhibited little affinity for 12C4 groups in hydrated membranes. Instead, Na+ ions binded strongly with the 12C4 groups, which markedly impeded their diffusion, resulting in the highest Li+/Na+ selectivity reported to date for dense, hydrated polymer membranes. In hydrated polymer systems, there are two limiting regimes for ion diffusion: at low water content, ion diffusion is controlled by local polymer segmental dynamics, and in highly hydrated systems, ion diffusion proceeds through highly hydrated regions of the membrane, with the polymer chains serving as essentially immobile, fixed obstacles that increase tortuosity and reduce available area for transport. In both cases, the concept of free volume is often used to rationalize observed trends of ion diffusion coefficients with respect to ion size and membrane water content. In dry polyelectrolyte systems, the concept of free volume is often implicitly invoked via, for example, interpretation of ionic conductivity via Vogel-Tammann-Fulcher (VTF) scaling.

We propose to bridge the detailed polymer physics understanding of dry polymer electrolyte systems with the important applications of hydrated membranes by exploring several fundamental questions: If one starts with a dry polymer electrolyte/salt system exposed to humid air, where does the first water go (i.e., to a hydration shell around the ion or to plasticizing the polymer?) How do ion size, density, and chemistry influence this initial hydration? How does polymer chemistry (local polarity and chemical environment around ion vs. continuum scale dielectric constant) influence hydration? What is the role of hydration on ion-membrane interactions and ion pairing/aggregation? While ion pairing/aggregation is discussed extensively in dry polymer electrolytes, this topic is rarely considered in highly hydrated membranes. Lack of fundamental understanding regarding this phenomena in water purification membranes stymies progress in designing highly permeable and selective membranes for treatment of highly complex water sources in RO or ED or in new applications, such as resource (e.g., Li+)recovery from highly saline brines. Similarly, our lack of knowledge of the impact of water contenton pKa values of carboxylic acids in RO membranes limits our ability to predictively design high performance membranes based on these ubiquitous functional groups.