Kirt A. Page, Joseph A. Dura, Sangcheol Kim, Brandon W. Rowe and Antonio Faraone

Abstract Polyelectrolyte membranes (PEMs) have been employed as solid elec­trolytes in fuel-cell technologies as early as the 1950s, when they were used in NASA’s Gemini program. However, PEM materials have only gained wide-spread attention in the last two decades due to advancements in membrane electrode­assembly (MEA) formation and the synthesis of new and interesting materials. Over the past several decades, various neutron techniques have played an instrumental role in measuring the structure and transport properties of PEMs in order to develop a deeper understanding of structure-property and performance relationships in PEM materials for fuel-cell applications.

10.1 Introduction

Proton-exchange (or polyelectrolyte) membrane fuel-cells (PEMFCs) have received increasingly more attention over the last two decades and is the principle subject of this chapter [1]. More specifically, this chapter presents an overview of how neutron techniques have been used to study polyelectrolyte membrane (PEM) materials. For an historical perspective on the use of polymers in fuel-cell technologies, the reader is encouraged to consulting the existing body of literature on the matter.

Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

K. A. Page (H) • S. Kim • B. W. Rowe

Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA e-mail: kirt. page@nist. gov

J. A. Dura • A. Faraone

Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg,

MD 20899, USA

© Springer International Publishing Switzerland 2015 273

G. J. Kearley and V. K. Peterson (eds.), Neutron Applications in Materials for Energy, Neutron Scattering Applications and Techniques,

DOI 10.1007/978-3-319-06656-1_10

It is generally understood that one of the key material properties influencing the conduction of protons through the PEM is the morphology of the material. The size — scale of the morphological features typically present in PEMs make small-angle neutron scattering (SANS) an ideal tool for probing the morphology. Additionally, researchers have recently shown an increasing interest in probing the structures that are present in PEM materials at interfaces, as these materials are used as binders in the catalyst layer and can be confined at interfaces with various types of material surfaces. For this, researchers have employed neutron reflectometry (NR). While the morphology of the material serves to provide a pathway for proton conduction, it is also understood that the polymer dynamics can play a role in charge transport. Moreover, the presence of water and water transport/dynamics is critical for optimal fuel-cell performance, and is therefore vital to elucidate the role and interdependent relationship that polymer and water dynamics have on charge transport in PEM fuel cells. Researchers have turned to neutron spectroscopic techniques such as quasi­elastic neutron scattering (QENS) and neutron spin-echo spectroscopy (NSE) to investigate the polymer and water dynamics in hydrated PEM materials.

The following chapter is an overview of the efforts to use neutron-scattering methods to study the structure and transport/dynamics in PEM materials and is divided into three sections. The first section gives a very brief overview of the characteristics of PEMs and highlights the material most studied using neutron techniques. The second section summarizes the structural studies on PEMs to date and demonstrates how techniques such as SANS and NR have aided in characterizing the bulk morphology and structures at interfaces, respectively. The third section focuses on the transport and dynamics in these materials, specifically describing how neutron spectroscopic techniques have been used to study the ion and water dynamics in hydrated PEMs. This chapter is not a comprehensive review of PEM materials and neutron techniques, but is intended to provide the reader with a demonstration of the many ways in which neutron measurements can aid in the understanding of structure — property and performance relationships in PEM materials.