Transport Phenomena in Polymers for Energy Applications: Molecular Design Cues from Quasielastic Neutron Scattering

Polymers are nearly ubiquitous in technologies for clean energy and/or sustainability. In electrochemical energy storage, a polymer electrolyte balances the flow of electrons through the external circuit against the flow of ions through the storage device. In reverse osmosis (RO) desalination, a polymer membrane separates the flow of ions from the clean water through the membrane. And in direct air capture (DAC), a polymer sorbent both captures carbon dioxide from the atmosphere and then releases it on demand for capture. In all these instances, the transport of a small molecule through the polymer is directly linked to the energy efficiency of the device. In electrochemical storage, the diffusion of the ions through the polymer electrolyte is the rate limiting factor that determines the electrical performance. In an RO membrane, permeance of the clean water through the membrane must maximized to make the pressure driven separation energetically affordable. And in DAC, the carbon dioxide must readily diffuse into the polymer sorbent be captured through strong ionic interactions, but then also be released in a manner that is not energy intensive. In all these situations, the transport through the polymer adds impedance to the system and directly affects energy efficiency.



In this presentation, I will illustrate how quasielastic neutron scattering (QENS) is a powerful tool to understand the fundamental transport mechanism for a series of small molecule penetrants through a polymer membrane or sorbent. With its sub-nanosecond and nanometer scale resolution, QENS has the appropriate time and length scale to probe the local, microscopic interactions between the penetrant and the polymer. I will provide examples of how QENS can be used to develop design cues to improve the transport of water through polyamide membranes for RO desalination, water through ion containing fuel cell membranes, and combinations of water and carbon dioxide into polyethylenimine sorbents for DAC. These QENS measurements wilI be integrated with complimentary structural and chemical characterization techniqes and I will full connect the local dynamic interactions between the penetrant and the polymer to the transport properties and ultimate performance of the device.