Durable perfluorosulfonic polymer electrolyte membranes prepared from alkaline-ion-assisted heat treatment

Abstract

Perfluorosulfonic acid (PFSA) electrolyte heat-treated at elevated glass transition temperature (T_g, 140–290 °C) with alkaline metal ions as the sulfonic group protector have been used to prepare stable PFSA/polytetrafluoroethylene (ePTFE) polymer electrolyte membrane for fuel cells. Thermal mechanical analysis (TMA) and X-ray diffraction results revealed that the PFSA resin with alkaline metal ions (Li, Na, and K) exhibits a significant increase of T_g, as well as an improved crystalline structure after heat treatment at T_g. Resin solubility measurement indicated the heat-treated PFSA show a better solvent resistance which favors membrane stability. The effect of ionic modification on PFSA/ePTFE composite PEMs performances, such as gas crossover, shrinkage stress, proton conductivity, and mechanical properties has been investigated in details. In general, the heat-treated PFSA/ePTFE composite PEMs showed lower dissolved fraction in water–alcohol solvents and better dimensional stability. The increased heat-treated temperature also enhanced the membrane performance because of the improved interface compatibility between PFSA resin and PTFE. The PFSA/ePTFE membrane prepared from heat-treated PFSA in Na-form at 290 °C (T_g of Na-form PFSA) increased the performance up to 0.744 V@600 mA cm⁻², significantly higher than those of other PEMs because of the T_g of Na-PFSA were close to the T_g of PTFE.

Introduction

Polymer electrolyte membrane (PEM) fuel cells have been considered as one of the most promising environment-friendly power generations and energy conversion devices for their high energy efficiency, high power density and low greenhouse gas emissions [1], [2]. PFSA membranes, such as Nafion^® membranes, are widely used for the high mechanical properties, high conductivity and outstanding chemical stability [3]. However, recent researches revealed that the internal stress due to the water-uptake change is one of the fatal factors to degrade the PEM besides the attack of peroxide/radical [4], [5]. Meanwhile, Stucki et al. reported that membrane thinning and local dissolution exist after a long time operation at 80 °C and this process will be enhanced by the local stress [6], [7]. And the mechanical creep would be accelerated in hot and wet conditions because the glass transition temperature of PFSA decreases as the polymer hydration, which leads to the membranes failure. Thus, development of durable polymer electrolyte with highly dimensional stability and low solubility are most critical requirements for durable membrane for PEMFCs.

PFSA/PTFE composite membrane with high strength porous PTFE as support is favorable to develop durable PEM because of higher dimensional stability and lower hydrothermal stresses [8], [9], [10]. It is difficult to impregnate the hydrophilic PFSA solution into ePTFE micro pores due to difference of the interface property [11], [12]. In our former work, we developed durable composite PEMs with high PFSA loading by converting the H-form PFSA ionomer into Na-form as well as the chemical modification of the ePTFE matrix [13], [14], [15]. Through heat treatment at elevated temperature, the PFSA monomers were firmly fixed in the PTFE microspores, which improved the mechanical stability of the composite membrane. However, the effect of alkaline-ion modification and heat-treatment on properties of PFSA and PFSA based composite membranes have not been reported till now.

The aim of this research is to prepare durable perfluorosulfonic polymer electrolyte membranes through heat treating the polymer electrolyte at elevated glass transition temperature with the assistance of alkaline metal ions. The alkaline-ion-assisted heat treatment can improve the crystalline and physical stability of the perfluorosulfonic acid electrolyte, resulting in a decrease of polymer solubility and reduce in humidity-generated stress of the electrolyte membrane. Additionally, the elevation of the heat-treated temperature is also favorable to the composite membrane performance because of the improved interfaces compatibility between PFSA resin and the PTFE support.