Microstructural and Mechanical Characterization of Catalyst Coated Membranes Subjected to In Situ Hygrothermal Fatigue

Abstract

Catalyst coated membranes (CCMs) in polymer electrolyte fuel cells are subjected to mechanical stresses in the form of fatigue and creep that deteriorate the durability and lifetime of the cells. The present article aims to determine the effect of in-situ hygrothermal fatigue on the microstructure and mechanical properties of the CCM. The fatigue process is systematically explored by the application of two custom-developed accelerated mechanical stress test (AMST) experiments with periodic extraction of partially degraded CCMs. Cross sectional and top surface scanning electron microscope (SEM) images of the end-of-test CCMs reveal the formation of mechanically induced cracks and delamination due to cyclic tensile and compressive fatigue stress. Tensile and expansion tests are conducted at different stages of degradation to evaluate the evolution in the mechanical and hygrothermal properties of the CCM. The tensile test results indicate gradual reductions in final strain, ultimate tensile strength, and fracture toughness with increasing number of fatigue cycles. The decay in tensile properties is attributed to the microstructural damage and micro-cracks formed during the AMST. Moreover, it is shown that the hygrothermal expansion of the CCM is more sensitive to conditioning than mechanical degradation. Polymer electrolyte fuel cells (PEFCs) are a prime candidate to replace gasoline and diesel internal combustion engines for transportation applications due to their environmental benefits combined with rapid start-up, high efficiency, and high power density at relatively low operating temperature.1 The commercialization of PEFCs is dependent on the development of membrane electrode assemblies (MEA) capable of meeting the automotive industry durability targets.2 However, the current PEFC technology is facing insufficient longevity, mainly because of the deterioration of the proton exchange membrane (PEM) component.1 Hence, an essential step to accomplish the commercialization requirements for PEFCs is to enhance the membrane durability and lifetime. Among various types of membranes utilized in PEFCs, perfluorosulfonic acid (PFSA) ionomer membranes (e.g., Nafion from DuPont) are the most widely used materials due to the superior chemical stability attributed to the chemically inert C-F bonds of the polytetrafluoroethylene (PTFE) base structure. 3Chemical and mechanical degradation mechanisms are recognized as the principal root causes for lifetime limiting failures of PFSA ionomer membranes in fuel cells. Understanding of the degradation mechanisms, their interactions, and the corresponding failure modes could provide valuable insight toward decelerating the rate of the membrane degradation and thereby extend the lifetime.2 Chemical degradation is caused by the attack of radical species in the form of hydroxyl (•OH) and hydroperoxyl (•OOH) radicals generated through decomposition of hydrogen peroxide (H 2 O 2 ) by metal contaminants.2,4,5 Hydroxyl radicals also form as a by-product of the electrochemical reaction bet...

Effect of catalyst layer defects on local membrane degradation in polymer electrolyte fuel cells

“…For a fresh MEA, these features could be formed during manufacturing or be a consequence of poor handling, bending, or stretching of the MEA. Membrane expansion and contraction during fuel cell operation may also result in catalyst layer cracks [45,51,53]. Delamination, which is observed when the catalyst layer is separated from the membrane, may also occur during the manufacturing processes or during operation [45,51].…”

“…Membrane expansion and contraction during fuel cell operation may also result in catalyst layer cracks [45,51,53]. Delamination, which is observed when the catalyst layer is separated from the membrane, may also occur during the manufacturing processes or during operation [45,51]. As discussed by Kundu et al [53], the high temperature applied in the catalyst layer drying stage can cause vapor to form at the catalyst layer/membrane interface, and create areas of poor adhesion.…”

Effect of catalyst layer on fatigue life and fracture mechanisms of fuel cell membrane

“…As the core component of PEFC, the performance and structural integrity of proton exchange membrane (PEM) are critical to the life of fuel cells. Perfluorosulfonic acid (PFSA) ionomer membranes (e.g., Nafion from DuPont) have been widely used and studied due to their outstanding performance 3 . During practical operation of fuel cells, PEMs will periodically swell and shrink due to water absorption and dehydration, resulting in mechanical fatigue 4 .…”

“…So it seems that there is no consensus on the effect of catalyst layer on fatigue life of fuel cell membranes. To further explore the cause of fatigue failure of CCM, Kjeang et al studied the mechanical degradation of membranes in fuel cells through accelerated stress tests (ASTs) and 4D visualization technology 3,36–38 . It was found that membrane–catalyst layer delamination and catalyst layer cracks led to the failure of the membrane.…”

Development and testing of a novel catalyst-coated membrane with platinum-free catalysts for alkaline water electrolysis

“…Whereas the CCM-MEA has been intensively studied for PEM fuel cells (PEMFC) [32][33][34][35][36][37][38][39][40][41], direct methanol fuel cells (DMFC) [42][43][44] and PEM electrolysers [44][45][46][47][48][49] to advantage, there is a lack of similar investigations for an alkaline environment due to the absence of an appropriate anion-selective polymer binder. To our knowledge, the only study evaluating the performance CCM-MEAs is that of Leng et al, who coated the surfaces of a commercial A201 membrane (Tokuyama Corp., Japan) with IrO2 catalytic ink (loading: 2.9 mg/cm 2 ) and Pt catalytic ink (loading: 3.2 mg/cm 2 ), using AS-4 ionomer (Tokuyama Corp., Japan) as a binder on the anode and cathode sides, respectively [50].…”