Accelerated Mechanical Degradation of Anion Exchange Membranes via Hydration Cycling

Abstract

Ion exchange membranes are a promising class of materials for use in fuel cells, barrier layers, and water splitting devices. While the majority of research related to anion exchange membranes is on improving the chemical stability and increasing ionic conductivity, the mechanical performance under relevant operating conditions is vital to device lifetime. Using tensile testing and humidity cycling, we report the mechanical performance of Nafion 115, polyethylene-b-polyvinyl benzyl trimethylammonium diblock copolymer (PEb-PVBTMA), poly[t-butyl styrene-b-hydrogenated isoprene-b-vinyl benzyl trimethylammonium-b-hydrogenated isoprene-b-t-butyl styrene] (QP43), and aminated tetramethyl polyphenylene (ATMPP). Nafion 115 serves as a well-studied material to compare with the developmental anion exchange membranes. A home-built environmental chamber attached to a rheometer was utilized to perform all measurements under elevated temperature and relative humidity conditions. Standard mechanical properties are important to understanding membrane performance, but a simple metric comparing the mechanical elongation in a dry condition with the in-plane swelling percentage in liquid water can be used to predict how membranes will perform under hygral cycling. While chemical degradation remains a significant issue for membrane stability in anion exchange membranes (AEMs), mechanical stability is also an integral factor in fuel cell lifetime.1 The polymer needs to be processed into thin, durable membranes and withstand the stresses inside a fuel cell at operating conditions. Microcracks formed from repeated hydration and dehydration cycles in a fuel cell are the most common form of mechanical failure. Several attempts have been made to correlate mechanical failure with a common mechanical property, such as elongation at break 2,3 or Young's modulus and yield strength. 4 Cycling the humidity correlated well with a reduction in the strain at failure even though the yield stress and yield strain remained largely unchanged. 3Most of the available literature on mechanical properties of ion conducting polymer membranes is limited to stress-strain curves in various states of hydration. [5][6][7][8] Tensile strength and Young's modulus decrease as the polymer absorbs water. The elongation at break increases due to plasticizing effect of the absorbed water.7 Also noted by Marestin et al. 5 is the lack of a comprehensive study of mechanical properties including the combined effect of water content and temperature. Custom environmental chambers built around a mechanical tester are common for detailed exploration of properties at different humidities, 9-11 and the testing is usually performed at a constant temperature and relative humidity; time dependence is not considered. Bauer et al. investigated the dynamic mechanical response to Nafion 117 under different temperature and relative humidity conditions, concluding water acts as a plasticizer at lower temperatures, but stiffens the membrane at higher temperatures.11 Creep measurements are...

Water Uptake Study of Anion Exchange Membranes

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