Stresses in Proton Exchange Membranes Due to Hydration-Dehydration Cycles

Abstract

Durability of the proton exchange membranes (PEM) is a major technical barrier to the economic viability of stationary and transportation applications of PEM fuel cells. In order to reach Department of Energy objectives for automotive PEM fuel cells, a design lifetime of 5,000 hours over a wide temperature range is required. Reaching these lifetimes is an extremely challenging technical problem. Though good progress has been made in recent years, there are still issues that need to be addressed to assure successful, economically viable, long-term operation of PEM fuel cells. The lifetime is limited due to gradual degradation of both the electro-chemical and hygro-thermo-mechanical properties of the membranes. Eventually the system fails due to a critical reduction of the voltage or mechanical damage. However, the hygro-thermo-mechanical loading of the membranes and how this effects the lifetime of the fuel cell is not understood. The long-term objective of the research is to establish a fundamental understanding of the mechanical processes in degradation and how they influence the lifetime of PEMs. In this paper, we discuss the finite element models developed to investigate the in-situ stresses in polymer membranes.

Mechanical behavior of fuel cell membranes under humidity cycles and effect of swelling anisotropy on the fatigue stresses

“…This is an extension of previous work [13,14], where only the elastic behavior of the membrane was considered. An uncoupled theory is assumed, for which the addi tional temperature changes brought by the strain are neglected.…”

“…The model used in the current investigation is an extension of a model developed in previous studies [13,14]. In the cur rent work, we incorporate temperature and humidity dependent properties of the PEM, as well as including plasticity and inves tigating the response to one simplified duty cycle.…”

Stresses in Proton Exchange Membranes Due to Hygro-Thermal Loading

“…The cases where the grooves are aligned will be referred to as "aligned" and the nonaligned cases as "alternating." Our previous study [21] showed that the magnitudes of the stresses in the membrane are much higher than that in GDEs or the plates. Also, the mismatches in the thermal and swelling coefficients lead to compression in the membrane and bending in the GDE.…”

“…From our ongoing research, the dimensional change of NAFION® from 35% to 100% relative humidity at 85° C is approximately 15%. The units of f we used in the model are mm/mm per percent relative humidity change, giving the swelling coefficient for NAFION® as 2300 (10 −6 mm/ mm per percent relative humidity change) [21].…”

A simple numerical method of cycle jumps for cyclically loaded structures

“…thermal barrier coatings [1 -3]), humidity (e.g. fuel cells [4], silica gtass [5]). and environ mental attacks (e.g.…”