Mechanical testing of small, thin samples in a humidity-controlled oven

Pandey

et al. 2016

J. Electrochem. Soc.

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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...

Section: Methodsmentioning

confidence: 99%

Accelerated Mechanical Degradation of Anion Exchange Membranes via Hydration Cycling

Pandey

et al. 2016

J. Electrochem. Soc.

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“…The SER fixture is typically used to measure extensional viscosity of polymer melts, but has also been shown to accurately measure tensile properties of solid polymers [25,28]. The SER drums were modified to have screw down pins that would hold thin samples, a small amount of silicon rubber on the inside of the pin prevented film slip and generation of pinch points that would cause premature failure [29]. Double sided tape was also placed on the drum surface to aid sample loading and further prevent film slip.…”

Section: Extensional Testsmentioning

confidence: 99%

Effect of hydration on the mechanical properties and ion conduction in a polyethylene-b-poly(vinylbenzyl trimethylammonium) anion exchange membrane

Journal of Membrane Science

Pandey

et al. 2016

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a b s t r a c tAnion exchange membranes (AEM) are promising solid polymer electrolytes utilized in alkali fuel cells and electrochemical energy conversion devices. AEMs must efficiently conduct ions while maintaining chemical and mechanical stability under a range of operating conditions. The ionic nature of AEMs leads to stiff and brittle membranes under dry conditions while at higher hydrations, water sorption causes significant softening and weakening of the membrane. In this work, a new polyethylene-b-poly(vinylbenzyl trimethylammonium) polymer (70 kg/mol) was cast into large (300 cm 2 ), thin (12 7 3 μm) membranes. These membranes exhibited improved elasticity over previously tested AEMs, minimal dimensional swelling, and moderate ionic conductivity (5 7 2 mS/cm at 50°C, 95% RH in the bromide form). Extensional testing indicated a 95% reduction in Young's modulus between dry and hydrated states. Further investigation of the complex modulus as a function of hydration, by dynamic mechanical analysis, revealed a sharp decrease in modulus between dry and hydrated states. Mechanical softening was reversible, but the location of the transition displayed hysteresis between humidification and dehumidification. Conductivity increased after membrane softening; suggesting bulk mechanical properties can identify the hydration level required for improved ion transport. Understanding the relationship between ion conduction and mechanical properties will help guide AEM development and identify operating conditions for sustained performance.

“…The SER fixture typically measures elongation viscosity of polymers in their melt state, but has also been shown to measure tensile properties of solid polymer films. [31][32][33] The SER fixture has two counter-rotating drums that suspend the film. The drums rotate, stretching the film to its failure point, while the stress applied to the film is measured by the rheometer.…”

Section: H208mentioning

confidence: 99%

Mechanical Performance of Polyiosoprene Copolymer Anion Exchange Membranes by Varying Crosslinking Methods

Ertem

et al. 2015

J. Electrochem. Soc.

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Anion exchange membranes (AEM) are polymer electrolytes that facilitate ion transport in alkaline fuel cells and electrochemical devices. Fabrication of mechanically durable AEMs with high ionic conductivity is a challenge. Here, a copolymer of isoprene and vinylbenzyl trimethylammonium and a terpolymer of isoprene, vinylbenzyl trimethylammonium and styrene were crosslinked by various methods, and properties, including conductivity and mechanical strength, were investigated at dry and saturated conditions. Polymer chemistry and degree of crosslinking significantly influenced conductivity, swelling, and mechanical properties. The terpolymer had a higher proportion of vinylbenzyl trimethylammonium units increasing the ion exchange capacity (IEC), but membranes could still be rendered insoluble by crosslinking. The higher IEC of the terpolymer resulted in higher chloride conductivity, 20-75 mS/cm at 50 • C and 95%RH, compared to 4-17 mS/cm for the copolymer at the same conditions. At dry conditions films were stiff, having Young's moduli between 100-740 MPa, but hydration caused severe softening, reducing moduli by 1-2 orders of magnitude. The severe softening effect of hydration was confirmed by dynamic mechanical analysis. The AEMs studied did not have adequate mechanical durability at hydrated conditions, additional work is needed to determine polymer chemistries and crosslinking methods that will produce robust AEMs for long-term use in fuel cells and electrochemical devices. Fuel cells are attractive energy conversion devices that utilize hydrogen to produce electrical energy for stationary and transportation applications, with the by-product being water.1,2 Electrolyzers utilize a similar principle to produce hydrogen from water using electrical energy. Fuel cells and electrolyzers both utilize a polymer electrolyte membrane to separate the catalyst layers and efficiently transport ions, while limiting crossover of other species.1,3 Proton exchange membranes (PEM), specifically perfluorosulfonic acid polymers such as Nafion, have been utilized almost exclusively for current fuel cells, owing to their high proton conductivity and suitable chemical and mechanical stability. 4,5 While PEM fuel cells have been widely researched and developed over the last few decades, anion exchange membrane (AEM) fuel cells offer potential advantages over PEMs. [6][7][8][9][10] The alkali nature of an AEM fuel cell allows redox reactions with nonplatinum catalysts and improves oxygen reduction kinetics. 6,9,10 On going research seeks to develop robust, well performing AEMs for use in fuel cells, electrolyzers, and other electrochemical devices.Anion exchange membranes must have a high ionic conductivity and be chemically and mechanically stable over the life-time of the fuel cell.6,9,10 Chemical stability is a concern in AEMs as hydroxide ions present in the membrane have the potential to degrade the polymer backbone and cation functionalities. 6,11,12 Hydroxide transport is inherently slower than proton transport, to compensate...