Progress in neutron techniques: towards improved polymer electrolyte membranes for energy devices

Abstract

Design and implementation of advanced membrane formulations for selective transport of ions and molecular species are critical for creating the next generations of fuel cells and separation devices. It is necessary to understand the detailed transport mechanisms over time-and length-scales relevant to the device operation, both in laboratory models and in working systems under realistic operational conditions. Neutron scattering techniques including quasi-elastic neutron scattering, reflectivity and imaging are implemented at beamline stations at reactor and spallation source facilities worldwide. With the advent of new and improved instrument design, detector methodology, source characteristics and data analysis protocols, these neutron scattering techniques are emerging as a primary tool for research to design, evaluate and implement advanced membrane technologies for fuel cell and separation devices. Here we describe these techniques and their development and implementation at the ILL reactor source (Institut Laue-Langevin, Grenoble, France) and ISIS Neutron and Muon Spallation source (Harwell Science and Technology Campus, UK) as examples. We also mention similar developments under way at other facilities worldwide, and describe approaches such as combining optical with neutron Raman scattering and x-ray absorption with neutron imaging and tomography, and carrying out such experiments in specialised fuel cells designed to mimic as closely possible actual operando conditions. These experiments and research projects will play a key role in enabling and testing new membrane formulations for efficient and sustainable energy production/conversion and separations technologies.

Differences in Water Dynamics between the Hydrated Chitin and Hydrated Chitosan Determined by Quasi-Elastic Neutron Scattering

“…In a hydrated polymer electrolyte, the QENS results support that the water dynamics mechanism in Nafion is a diffusion mechanism in which H 3 O + moves rather than a concerted jump motion in which H 3 O + continuously builds up and disappears [28,29]. In addition, it is considered that the hydrated polymer electrolyte undergoes formation of H 3 O + via a slow jump mode with a characteristic time from 500 ps to 150 ps and long-range diffusion of water molecules within the confinement region via a fast jump mode with a characteristic time from 8.0 ps to 2.5 ps [28,29]. The residence times of mobile hydrogen atoms in chitin ranged from 0.26 ps to 3.1 ps is much faster than these motions observed in the hydrated polymer electrolyte.…”

Hydrogen Dynamics in Hydrated Chitosan by Quasi-Elastic Neutron Scattering

“…This is interesting considering that both phenomena involve the protonation of water from the viewpoint of a proton receptor. Such protonation to form a hydronium ion is considered to take place via a slow jump mode with a typical characteristic time of 150 to 500 ps in a hydrated polymer electrolyte [ 28 , 29 ]. Furthermore, the residence time of our slow Lorentzian component was found to range from 5 to 60 ps, which is significantly faster than the slow jump mode found in the polymer electrolyte, and also faster than the intermediate situation between the slow and fast jump modes in a polymer electrolyte.…”

“…Recently, microscopic observations of the dynamics of hydrated polymer membranes have been reported based on the use of neutron scattering measurements [ 28 ]. For example, in the case of Nafion, a polymeric membrane bearing hydrophilic sulfonic acid groups, the slow dynamics were related to the motion of hydronium ions, while the fast dynamics were related to the motion of hydration water [ 28 , 29 ].…”

“…Recently, microscopic observations of the dynamics of hydrated polymer membranes have been reported based on the use of neutron scattering measurements [ 28 ]. For example, in the case of Nafion, a polymeric membrane bearing hydrophilic sulfonic acid groups, the slow dynamics were related to the motion of hydronium ions, while the fast dynamics were related to the motion of hydration water [ 28 , 29 ]. Subsequently, the dynamics of the polymers and the water molecules present in polymeric membranes were revealed over different time scales using the quasi-elastic neutron scattering (QENS) technique.…”