Batteries, particularly those for portable electronic devices, can contain flammable liquids, such that when a battery is damaged a fire can ensue. To mitigate this safety risk, batteries include additional housing and safety features, and while this improves safety during operation, this strategy also increase the size and weight of the battery. An alternative strategy is to replace the flammable liquid with a plastic membrane that allows ions, such as lithium, to pass through without allowing electrons to pass. Prof. Winey's group has been studying single-ion conducting polymers that could be valuable for battery applications and for other membrane applications. In previous NSF-funded work they have uncovered a variety of new nanoscale structures that arise when the active chemical groups are evenly placed along a linear polymer molecule. One of these nanoscale structures, an alternating layered arrangement of ions and crystalline polymer, was recently found to have exceptional proton transport properties when hydrated. To capitalize on this finding, the PI has established new design rules for polymer membranes and built multiple collaborations with synthetic chemists who are incorporating these design concepts into new polymers. Winey's group will explore the nanoscale structures and conductivities of these newly designed polymers as a function of their polymer chemistry and processing to refine and extend their design rules for single-ion conducting polymer membranes. Given the current societal challenges related to clean water, energy storage and energy conversion, the fundamental understanding afforded by this project will have an important societal impact.
PART 2: TECHNICAL SUMMARY
A strong interest in ionomers and other polymers with acid, ionic and polar groups is fueled by their potential ability to selectively transport charged species, which is relevant to batteries, water purification technologies, and fuel cells. The prevailing research directions in the field of solid polymer electrolytes have consolidated around two general classes of homogeneous materials wherein the ions are uniformly distributed throughout the material: polymers mixed with salts and single-ion conductors. The ubiquitous design strategy in these materials systems is based on the understanding that ion conductivity is associated with chain dynamics and ions must be dissociated from their counterion. Unfortunately, these approaches have only limited success in developing suitable polymer-based electrolytes. Winey's group is exploring an alternative hypothesis, namely that efficient ion conductivity in polymers can be broadly achieved when the ions are sequestered into spatially-continuous nanoscale aggregates and the ions dissociate from their counterions. This project builds upon a promising result from the PI and collaborators wherein proton conductivity of a hydrated precise polyethylene with sulfonic acid groups on exactly every 21st carbon is somewhat higher than a commercial membrane. This precise polyethylene self-assembled into nanoscale layers lined with the acid groups and separated by a crystalline alkyl spacer. The high proton conductivity is evidence that the conducting protons are decoupled from the motion of the much slower polymer backbones. The proposed project will expand upon this singular finding to establish the merits of the proposed alternative hypothesis. The planned research combines conductivity measurements, structural characterization, and molecular dynamics simulations to rigorously interrogate this hypothesis using new nanostructured precise polymers. The proposed alkyl polyester sulfonates and telechelic oligomers are expected to have crystalline domains that direct the assembly of layered aggregates; these layered morphologies will be aligned in thin films on interdigitated electrodes to explore the fundamentals of conductivity. Random percolated structures in precise polyethylenes with short carbon spacers will also be investigated. The PI and her group will undertake this project with a set of unfunded collaborators: Prof. Stefan Mecking (Konstanz), Prof. Justin Kennemur (Florida State University), Prof. Paul Nealey (U Chicago), Dr. Amalie Frischknecht (Sandia), and Dr. Mark Stevens (Sandia). .
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
