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

Abstract

The catalyst layer (CL) in polymer electrolyte fuel cells (PEFCs) plays a critical role in the performance of a PEFC. In this study, we investigate the water dynamics in the CL using quasielastic neutron scattering (QENS) and small-angle neutron scattering. The temperature dependence of the mean square displacement ⟨u 2⟩ shows that the freezing of water does not occur at a melting temperature in the Nafion thin film of the CL, suggesting that the water is confined in a smaller region than ∼15 Å. The QENS measurements established three kinds of water in the CL: immobile water tightly connected to a sulfonic group, water in a fast mode assigned to free diffusion restricted in a sphere, and water in a slow mode described by a jump diffusion model. Assuming that these three modes were independent, the number of water molecules in each mode was estimated. On discussing the structure and dynamics elucidated in the study, we finally conclude that the coupled model of the fast and slow modes is plausible for describing the diffusion of water confined in the thin Nafion film of the CL.

Revealing the Ion Dynamics in Li10GeP2S12 by Quasi-Elastic Neutron Scattering Measurements

“…Such a dependence is characteristic for a model in which the mobile species diffuse in a restricted space of a characteristic radius r QENS . Previously, the model of diffusion inside a sphere has been applied to analyze the hydrogen dynamics in MoS 2 and the water dynamics in the catalyst layer of a fuel cell, in which the EISF is connected to the spherical Bessel function of the first kind ( j 1 ), the immobile fraction ( f ), and the radius of the sphere ( r QENS ): …”

Molecular Dynamics Study of Reaction Conditions at Active Catalyst-Ionomer Interfaces in Polymer Electrolyte Fuel Cells

“…40 This water was mostly confined to a relatively small region at the interface, of roughly 15 Å thickness. 41 The flooding of the pore in the present simulation cell aims to reproduce this saturation effect during operation.…”

Challenges and opportunities for characterisation of high-temperature polymer electrolyte membrane fuel cells: a review

“…417 QENS studies have been used to probe water dynamics in LT-PEMFCs, including investigations into water distribution in the catalyst layers. 418 QENS data allowed for the quantication of water molecules per sulphonic acid group of different types (immobile, slow and fast), as well as identifying the effect of temperature on the type of water molecules observed. Although there is minimal liquid water in operating HT-PEMFCs, QENS could still be a useful technique for understanding water dynamics, particularly during start-up/shutdown.…”