Mechanical Characterization of Anion Exchange Membranes by Extensional Rheology under Controlled Hydration

Abstract

Alkali anion exchange membrane (AEM) based devices have the potential for electrochemical energy conversion using inexpensive catalysts and a variety of fuel types. Membrane stability and anion transport must be improved in AEMs before these devices can be fully realized. Mechanical failure of the membrane can contribute to failure of the device, thus membrane durability is critical to overall system design. Here, a study of the mechanical properties of three well-established AEMs uses a modified extensional rheometer platform to simulate tensile testing using small membrane samples. Mechanical properties were tested at 30 and 60 • C under dry or water saturated gas conditions. Water in the membrane has a plasticizing effect, softening the membrane and reducing strength. PEEK membrane reinforcement limits swelling producing negligible softening and only a 9% decrease in strength from dry to hydrated conditions at 30 • C. Higher cation concentration increases water uptake resulting in significant softening, a 57% reduction in Young's modulus, and a 67% reduction in strength when hydrated at 30 • C. In a working electrochemical device, AEMs must maintain integrity over a range of temperatures and hydrations, making it critical to considering mechanical properties when designing new membranes. Polymer electrolyte membrane fuel cells and electrolyzers are potentially disruptive technologies that will replace traditional heat engines such as internal combustion engines for transportation applications, portable electronics, and are scalable to larger energy storage facilities. Polymer electrolyte membrane fuel cells are suitable for transportation applications due to their low temperature start-up and operation, high power density, and quick refueling.1-3 Proton exchange membranes (PEMs) have dominated polymer electrolyte membrane fuel cell development in the last several decades, resulting in the development of relatively stable, well performing membranes.1-4 Current PEM fuel cells remain cost prohibitive due to high catalysts costs, as well as long-term durability issues. 3,5,6 Anion exchange membranes (AEMs) can also be utilized in polymer electrolyte membrane devices and have several potential benefits over PEMs. AEM fuel cells benefit from increased kinetics in an alkali media allowing more complex fuels then hydrogen and have the potential to utilize non-platinum catalysts to reduce costs. 7-11However, a number of challenges must be overcome before AEMs reach the performance and durability necessary for fuel cells and other electrochemical energy conversion devices. Hydroxide present in the AEM degrades many of the proposed cationic groups and some polymer backbones, making development of chemically stable AEMs difficult. 9,11,12 Additionally, transport of hydroxide in AEMs is inherently slower than protons in PEMs, 13 to compensate, the concentration of ionic groups is often increased in AEMs.8 Increasing ion concentration in AEMs increases water sorption in the polymer and can result in significant dimensional...

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

“…• C. 37 While the moduli values of the films were fairly high at dry conditions, the stress to failure was low to moderate compared to other AEMs (5-70 MPa), 12,32,38 crosslinked AEMs (2-27 MPa) 20,21,27 and PEMs 4,37 ( Figure 7). At dry conditions the thermally crosslinked copolymer films had relatively low stress to break, between 3 to 6 MPa, while the thermally crosslinked terpolymer films had slightly higher stress to break, 7 to 15 MPa.…”

“…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.…”

“…Films were extremely stiff at dry conditions and underwent severe softening when hydrated ( conditions, all films are stiff having moduli between 100 and 740 MPa; this is comparable to moduli reported for other AEMs. 32,[34][35][36] Generally, the terpolymer was slightly stiffer than the copolymer due to the addition of glassy polystyrene to the polymer matrix. A lower degree of crosslinking (3 hr thermal) resulted in softer films than those with a higher degree of crosslinking (24-hr thermal/UV).…”

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

“…The SER has two counter-rotating drums; films are suspended between drums and stretched to failure. 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].…”

“…Functionalization of a polymer with cations reduces elasticity and elongation [13], making it difficult to produce AEMs that maintain adequate mechanical durability. Additionally, water sorption in AEMs can severely soften and weaken AEMs when they are hydrated [25]. Producing AEMs that have sufficient elasticity and elongation at dry conditions and maintain adequate strength at hydrated conditions is a continued challenge.…”