A degradation study of Nafion proton exchange membrane of PEM fuel cells

Abstract

The durability and degradation behavior of the Nafion NR111 proton exchange membranes (PEMs) is investigated in detail under various mechanical, chemical and polarization conditions. It was found that the fatigue strength or the safety limit of the cyclic stress for Nafion NR111 membrane is ∼1.5 MPa that is 1/10 of the tensile strength of the membrane. The cyclic stress and dimensional change (or strain) induced by the water uptake can be substantial and are the main causes for the mechanical degradation and failure of the membrane. For example, in the case of RH cycling of water soaked state to 25% RH state, the cyclic stress of the Nafion membrane was as high as 2.23 MPa and the dimensional change was ∼11%. Both FTIR and NMR analysis indicate that the decomposition of the Nafion polymer in the H₂O₂ solution in the presence of trace Fe, Cr and Ni ions started from the ends of the main chain, resulting in the loss of the repeat units and the formation of voids and pinholes in the membrane. The high degradation rate of the membrane at the open circuit voltage most likely results from the attack of H₂O₂ formed between O₂ and H₂ crossovered through the membrane.

Introduction

Fuel cells utilizing perfluorosulfonate acid proton exchange membranes (PFSA PEMs) have received much attention because they provide high power density at relatively low operating temperatures. These fuel cells are promising candidates for portable and stationary power sources and for electric vehicle applications. DuPont's Nafion membranes are the state-of-the-art PEMs because of their high proton conductivity and excellent chemical stability. However, enhancement of the stability and durability of the proton exchange membranes are critical to the lifetime and commercial viability of the polymer electrolyte membrane fuel cells (PEMFCs) [1], [2]. In order to meet requirements for commercial applications, PEM fuel cells are required to demonstrate durability of about 6000 h under normal operating conditions (e.g. automotive conditions) [3], [4]. The stability and integrity of the proton exchange membranes (PEMs) is one of the most crucial factors affect the lifetime of the fuel cells since the PEMs function both as electrolyte and as a separator of the reactant gases.

Durability issue of the proton exchange membrane has attracted extensive attention in recent years [5]. It has been found that the degradation of the fuel cell performance is primarily due to the decay of the membrane-electrode-assembly (MEA) [6], [7], [8]. The early failure of the PEMs (<1000 h) is usually contributed to the structural failure of the membranes resulting from the cracking, tearing, puncture, mechanical stresses, inadequate humidification and reactant pressure. The decomposition of the polymer proton exchange membrane associated with the loss of fluoride ion and the decrease of conductivity is also a problem facing the long-term stability of the fuel cell [9], [10]. It was shown that the formation of H₂O₂ in the cathodic reaction region can cause the chemical degradation of the membrane [11], [12]. The presence of the H₂O₂ had been detected in situ during the fuel cell operating conditions [13]. Pozio and co-workers suggested that the metal ions released from the bipolar plates would accelerate the decay of the proton exchange membrane [14]. The degradation of PEMs may also depend on the operation voltages. It was reported that the degradation rate could be as high as 5.8 mV h⁻¹ for the cell operating at open circuit [15].

The long-term operation data obtained from a fuel cell under real operating conditions is very useful for the performance evaluation of a fuel cell in terms of the durability. However, testing of fuel cells for lengthy periods of time is time-consuming and also costly. Thus, ex situ tests were commonly used to study the degradation behaviors [16], [17], [18]. For example, using nuclear magnetic resonance (NMR) spectroscopy and mass spectroscopy, common degradation products of Nafion were shown to be produced by in situ (fuel cell operation) and ex situ (Fenton test) testing of membrane [19]. However, the degradation mechanism of the proton exchange membrane is still not clear and not completely understood. In this paper, we investigate various mechanical, chemical and polarization factors affecting the durability and stability of the Nafion polymer electrolyte membrane in detail. The relationship between the membrane breach and the mechanical and chemical effects is discussed.