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

Abstract

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...

Water Uptake Study of Anion Exchange Membranes

“…The stress–strain characteristics of HMT-PMBI, PPO-TMA, and PPO-AGO changed between drier and wetter conditions at 60 °C, which agrees with other AEMs and PEMs. ,− By using two different humidities, the range of mechanical properties of a film can be captured within the context of the water uptake study; a more exhaustive study was not completed as it is outside of the scope of this work. The mechanical properties of ETFE-TMA were studied previously and qualitatively agree with the changes with water content discussed here .…”

“…Solid lines are Park model fitting.Mechanical properties. The stress-strain characteristics of HMT-PMBI, PPO-TMA and PPO-AGO changed between drier and wetter conditions at 60 o C, which agrees with other AEMs and PEMs 76,[85][86][87][88][89]. By using two different humidities, the range of mechanical properties of a film can be captured within the context of the water uptake study; a more exhaustive study81 was not completed as it is outside of the scope of this work.…”

A Single-Step Monomeric Photo-Polymerization and Crosslinking via Thiol-Ene Reaction for Hydroxide Exchange Membrane Fabrication

“…Table II summarizes the properties of all photo-crosslinked HEMs for fuel cell applications presently found in the literature. The only other photo-crosslinked HEMs synthesized via thiol-ene chemistry have been reported by Stoica et al, 12 Sollogoub et al, 13 and Vandiver et al 14 The others were polymerized and crosslinked using traditional free-radical, chain-growth mechanisms. Thiol-ene photopolymerization reactions proceed through a rapid radical mechanism, but the molecular weight builds through a step-growth mechanism, which typically produces a homogeneous polymer network.…”

Novel Processing of a Poly(phenyleneoxide) −b–Poly(vinylbenzyltrimethylammonium) Copolymer Anion Exchange Membrane; The Effect On Mechanical And Transport Properties

“…Increased OHtransport by increasing the ion exchange capacity (IEC) of a membrane has been demonstrated; Ran et al have shown that the increase in IEC leads to larger water uptake that enhances ion transport giving a higher ionic conductivity [9]. However, the larger water uptake and dimensional swelling can compromise the mechanical strength of polymer membranes [10]. Increasing IEC does not guarantee higher conductivity, and Ertem et al have observed that the ionic conductivity increases with increasing IEC up to 1.54 mmol.g -1 and then is level at higher IECs for random copolymers of poly(isoprene) and poly(vinylbenzyl chlorine) AEMs [11].…”

“…Hence, an in-depth study of the solvent cast membranes was not pursued further. The literature has reported an increase in membrane strength by increasing cross-linking [10,11,30,31]. Alternatively, melt pressing of the copolymer at 240 °C and 30 MPa created an insoluble membrane with controlled swelling, reduced water uptake, and greater mechanical strength.…”