Nafion membranes annealed at high temperature and controlled humidity: structure, conductivity, and fuel cell performance

Abstract

The relationship between electrical and morphological properties of annealed Nafion samples is investigated by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), atomic force microscopy (AFM), and impedance spectroscopy. Experimental data reveal that the heat treatment at high temperature (T ∼130–140 °C) with low relative humidity (RH ∼0%) results in significant changes of Nafion such as increased crystallinity and decreased average distance of hydrophilic domains. Such effects were practically absent when the same heat treatment was carried out at high RH ∼100%. The effects of annealing with controlled RH were reflected in the polymer electrolyte fuel cell (PEFC) tests in which the measured performance was markedly reduced for Nafion samples annealed at low RH. Such a feature was related to decreased microstructural stability, water sorption and proton conductivity of the annealed membrane. The observed effects are relevant to evaluate degradation of Nafion during both fuel cell assembly and harsh PEFC operating conditions. Moreover, the experimental results contribute to advance the understanding of Nafion’s properties at high temperature for the development of high-performance ionomer membranes.

Introduction

Nafion is a leading polymer electrolyte from the family of perfluorosulfonate ionomer membranes (PIM) [1]. PIMs are known to exhibit high chemical/mechanical resistance and, more importantly, one of the highest proton conductivity at low temperatures (T ∼80 °C) among solid proton conductors [2], [3], [4]. Owing to such high proton conductivity (σ ∼10⁻¹ Scm⁻¹), PIMs have been extensively used in polymer electrolyte fuel cells (PEFC) to produce high power density output [3].

Optimized operating conditions for PEFC using Nafion have been widely studied. However, further exploring extreme conditions of temperature and/or RH is of relevance. One crucial issue that inhibits higher performance is the water management in PEFCs. For example, the excessive amount of water at the fuel cell cathode causes electrode flooding, which slower the diffusion of oxygen towards the catalyst layer [5]. In this scenario, fuel cell tests have been performed at various temperatures and humidity conditions in order to minimize the cathode flooding and maximize the fuel cell performance [5], [6], [7]. In fact, due to the excellent mechanical and electrical properties, Nafion has been considered as an electrolyte for PEFCs operating at high temperatures. Among these studies, the development of Nafion-based composite electrolytes for fuel cells at higher temperature and lower relative humidity (RH) have received a great deal of attention [5], [6], [12], [15]. The addition of hydrophilic ceramic fillers aims at improving the hydration of the Nafion matrix for PEFC operation at high T and low RH [4], [5]. However, relatively little attention has been given to changes of Nafion morphology at such conditions, which can decrease the membrane conductivity and, consequently, the fuel cell performance [4], [6], [15]. Several factors are involved in the reduction of the proton conductivity of Nafion at high T and low RH such as the loss of absorbed water, the increase of the membrane crystallinity, and the thermal transitions of the ionic phase [3], [13], [14].

Nafion properties have been widely studied and several models have been proposed to account for the relationship between the morphology, mechanical, and electrical properties [1], [9]. However, the available models considered experimental data collected in a limited temperature range (mostly, from room temperature to 80 °C). In this low temperatures, below the α‐transition of Nafion at Tα ∼110 °C, changes on the ionomer’s properties such as crystallinity and proton conductivity are negligible [1], [4]. Indeed, high temperature characterization of Nafion crystallinity has been performed mostly in dry conditions, hampering the application of models to describe the role of the water at T > Tα. Therefore, such models do not reflect the PEFC operating conditions, requiring new efforts to understand the relation between the morphology and electrical properties of PIMs at high temperature and different relative humidity [6], [7], [8].

Concerning the morphology of Nafion, it has been confirmed by several techniques that the sulfonic acid groups form ionic clusters and that the tetrafluorethylene (TFE) segments of the main chains crystallize during the fabrication process, such as extrusion (lamination) or casting. Both ionic and nonionic phases are nanometer sized and are ubiquitously distributed in the polymer matrix [1]. In this framework, two processes are likely to occur at high T: the crystallization of the TFE segments and the conformational changes of the polymeric aggregates, changing both ionic and nonionic domains of Nafion. The influence of ionic and nonionic phases on α-transition have been a subject of debate for several decades [9], [10]. For example, the endotherm peak at ∼130 °C, observed in differential scanning calorimetry runs of Nafion, was attributed to the melting of ill-formed crystallites allowing for the growth of a more crystalline phase [9]. In contrast, such a peak has also been assigned to the conformation changes of Nafion backbone due to electrostatic interactions [11]. Therefore, the understanding of the morphology changes of Nafion at low RH and high T is important for controlling the main properties that maximize the PEFC performance in such conditions. Moreover, the study of the microstructure of ionomer membranes at conditions that mimic the fuel cell operating conditions, using more suitable techniques for assessing changes of the morphology of PIMs, such as atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) are scarce.

In this study, the influence of both thermal and humidification history of Nafion membranes on the performance of PEFCs was investigated. The results indicated that at low RH and at T > Tα, crystallization of the TFE moieties of the polymer backbone and destabilization of the hydrophilic domains takes place, contributing for decreasing both the proton conductivity and the performance of the fuel cell. On the other hand, the crystallization of samples annealed at high RH was inhibited, a feature that resulted in a lower reduction of the proton conductivity, as compared to the sample annealed at low RH, and allowed reaching enhanced PEFC performance.