A Quasielastic Neutron Scattering Study of Water Diffusion in Model Anion Exchange Membranes over Localized and Extended Volume Increments

Abstract

Anion exchange membranes provide the basis for potentially more cost-effective fuel cells, electrochemical energy conversion, and energy storage. The membranes consist of a polymer domain with tethered cationic side groups and a pervading aqueous domain that conducts water and anions. Water diffusion is investigated through temperature-dependent quasielastic neutron scattering (QENS) at application relevant water contents, employing the BASIS spectrometer at ORNL. The spectrometer resolves two distinct quasielastic components corresponding to localized water diffusion (on length scales lower than the water domain thickness) and to extended water diffusion (on length scales approaching the water domain thickness). Water diffusion coefficients in membranes are time and length scale dependent and water exchanges between diffusion on a localized and extended volume increment. An exchange model for QENS is developed to describe the exchange of water between the two volume increments. The populations and lifetimes of water in each volume are discussed in terms of their connection to long-range water transport and membrane nanomorphology.

Mechanism of Facilitation of Ion Mobility in Low-Water-Content Fuel Cell Membranes

“…15 The high value of ν for mobility over tens of nanoseconds is consistent with experimental results of D as a function of time scales in commercial AEM with comparable water content. 41,43 Most significantly, we find that subdiffusivity coefficients ν for the mobility of anions are independent of the length N of the cation tether, consistent with the insensitivity 15 of the structure of hydrophilic channels to tether length. Figure 3 presents D of anions as a function of tether length for their displacements over times t 30−60 ns computed using eq 2.…”

“…The HRCG model , has shown to reproduce the experimental position and width of the ionomer peak that characterizes the dimensions of nanophase segregation of PPO-TMA membranes , and their electro-osmotic water drag coefficient, as well as to produce a distribution of pore widths in excellent agreement in simulations using polarizable all-atom models of the same membranes and experiments of related poly-aryl TMA membranes . The chloride form of the PPO-TMA membrane with the TMA cation directly bound to the PPO backbone and ion-exchange capacity 3.22 has a conductivity ∼1 mS cm –1 in simulations with our model at water content λ = 5, intermediate between the experimental conductivity of the same membrane in bromide and fluoride forms and IEC 3.7 .…”

“…The mobility of water and hydrogen-containing ions in fuel cell membranes has been investigated across a range of time scales using quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR). ,− These studies evince that the diffusion coefficient of water and ions in these membranes depends on the time scale of observationan evidence of subdiffusive behavior that results from the mobility of these species in tortuous narrow hydrophilic channels. , However, the gulf between the larger time scales accessible with QENS (∼1 ns) and the shorter ones accessible through NMR (∼0.01 s) hampers the study of the dynamics of tether chains bound to the glassy polymer matrix and whether they couple to the dynamics of ions as they diffuse through the spatially heterogeneous membrane.…”

Dynamics of Water in a Catalyst Layer of a Fuel Cell by Quasielastic Neutron Scattering

“…Similar results have been reported in bulk anion-exchange membranes . As mentioned in the Introduction Section, Nafion in the wet CL is in a thin-film state.…”

“…Similar results have been reported in bulk anion-exchange membranes. 23 As mentioned in the Introduction Section, Nafion in the wet CL is in a thin-film state. The confinement effects must cause the difference in the fast motion between the wet CL and the wet Nafion.…”