Understanding anion transport in an aminated trimethyl polyphenylene with high anionic conductivity

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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...

Section: Resultsmentioning

confidence: 74%

Section: Resultsmentioning

confidence: 99%

Section: H678mentioning

confidence: 99%

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

Vandiver

Caire

Carver

et al. 2014

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“…27 ATMPP is widely studied and shows promise as a platform for an anion exchange membrane in a working fuel cell. [28][29][30][31][32] Rheometry.-Rheological measurements were done on an ARES-G2 rheometer (TA Instruments) using a modified Sentmanat Extensional Rheometer (SER). 33 Details on the SER fixture are available elsewhere.…”

Section: Methodsmentioning

confidence: 99%

Accelerated Mechanical Degradation of Anion Exchange Membranes via Hydration Cycling

Caire

Vandiver

Pandey

et al. 2016

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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...

“…26,27 Our previous work was focused on understanding the transport properties of ATMPP. 28 Low water uptake, water content, and high ion conductivities up to 86 mS/cm at 90…”

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confidence: 99%

A Direct Methanol Alkaline Fuel Cell Based on Poly(phenylene) Anion Exchange Membranes

Janarthanan

Pilli

Horan

et al. 2014

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The Investigation of the performance of poly(phenylene) based anion exchange membranes (AEMs) in a direct methanol fuel cell is reported. The fuel cell performances were tested with different anode gas diffusion layers as a function of concentrations of fuel and electrolyte using two commercially available platinum catalysts supported on carbon. Normalized current density values demonstrated good performance for a poly(phenylene) AEM, with a cation consisting of six methylene spacers attached to a trimethyl ammonium group, with a maximum current density of 11.8 mA cm −2 mg −1 in a KOH free fuel. The platinum catalyst (46%) from Tanaka showed a better performance (10.4 mA cm −2 mg −1 ) compared to the platinum catalyst from E-TEK (5.1 mA cm −2 mg −1 ) in 1M methanol fuel feed under identical conditions. We observed that the MEA with a Zoltek gas diffusion layer on the anode showed the best performance (226 mA cm −2 and 53.8 mW cm −2 ) in the presence of KOH, suggesting that a hydrophilic gas diffusion layer plays a significant role in improving the fuel cell performance. A durability test for a MEA in 1M methanol and 1M KOH for 67 h showed an overall degradation rate of 400 μV h Direct methanol fuel cells based on proton exchange membranes (PEMs) have received considerable attention, as compared to hydrogen fuel cells, they have potential system simplicity, high system energy density, improved volumetric fuel storage and fuel supply. 1,2Direct methanol alkaline fuel cells (DMAFCs) which employ an anion exchange membrane (AEM) electrolyte are more attractive than devices based on PEMs as they should provide improved electrode kinetics, simple water management, lower methanol permeability, and use of non-platinum metal catalysts.3-6 Solid-state AEM electrolytes are more attractive than the conventional liquid KOH electrolytes as the carbonates and bicarbonates formed during operation do not precipitate as the cation is the AEM, and although adding an additional ionic resistance to the electrode, do diffuse through the cationic ionomer. 7,8 In spite of these advantages, it is important to note that except for a few reports, high performance in DMAFCs are shown only in the presence of the aqueous basic electrolyte, even for commercially available AEMs.9,10 In general, alkaline DMFC studies employ commercially available AEMs, such as Tokuyama A-006, A-010, Fumatech FAA-2, and Morgane ADP from Solvay. [9][10][11][12][13][14][15][16] Radiation grafted membranes have been demonstrated to be promising AEMs for DMAFCs with a KOH free methanol fuel.17,18 Still a great amount of research is being focused on improving the performance of available commercial AEMs as well as on development of new AEMs to achieve stable, highly conducting, and mechanically robust membranes for fuel cells with high performance and long durability.There are very few alkaline direct methanol fuel cell reports where neat methanol is used and aqueous methanol is still the fuel of choice, in fact high power densities have only been achieved for these ...