Characterization of the viscoelastic properties of gore® 5510 proton exchange membrane

Abstract

Fuel cells have been a consistent focus of research over the past decade, offering potential for cleaner and more efficient energy produced from a range of sources. Though many types of fuel cells exist, of particular interest to portable power and transportation needs is the Polymer Electrolyte Fuel Cell (PEFC) consisting of Proton Exchange Membranes (PEM). When a fuel cell is put to use, mechanical stresses develop within the PEM and vary widely with internal operating environment. Accurately predicting the mechanical durability of these stressed membranes is still one of the barriers for commercializing the PEM fuel cells. It is known that higher moisture content increases the conductivity of the membrane. The hygrothermal stresses associated with hygral contraction and expansion at the operating conditions is believed to be critical in performance of the membrane. The viscoelastic constitutive characterization of PEM is essential for making hygrothermal stress predictions in an environment of transient temperature and humidity. The tensile relaxation modulii of a commercially available PEM were obtained from stress relaxation, and creep tests conducted at various humidity levels and temperatures. These tests were performed using dynamic mechanical analyzers (DMA) (TA Instruments 2980 and Q800). The machine is capable of applying specified tensile loading conditions to a small piece of membrane at a given temperature. A special humidity chamber was built in the shape of a cup that would enclose the tension clamps of DMA. The chamber would then be inserted in the heating furnace of DMA and would be connected to a gas humidification unit through a slot on the chamber. Additionally the chamber would seal off humid air from entering DMA furnace. Heating pads were applied on the base of the chamber in order to minimize condensation in the chamber. Thermal and hygral master curves were constructed and were used to form a hygrothermal master curve using the principle of time temperature moisture superposition. Horizontal shifts of the stress relaxation modulus in the hygral master curve implied the applicability of time moisture superposition. A simple approximate interrelation was used to validate the relaxation modulus data. The hygrothermal master curve was fitted with a 10-term Prony series for subsequent use in numerical models.