Design of polyphosphazene-based graft copolystyrenes with alkylsulfonate branch chains for proton exchange membranes

Highlights

• Polyphosphazene graft copolystyrenes with alkylsulfonate chains were synthesized.

• They were made into the proton exchange membranes with the cross-linker.

• The proton conduction was comparable or superior to Nafion 117.

• The proton conduction may attribute to the nanophase-separated morphology.

• These membranes all could inhibit significantly the methanol diffusion.

Abstract

In order to develop efficient proton exchange membranes used for direct methanol fuel cells, we are focusing on the design and synthesis of polyphosphazene-based copolymers. In the present study, block polystyrenes were grafted onto polyphosphazenes by atom transfer radical polymerization (ATRP) of styrene and 4-acetoxystyrene, and then alkylated with 1,4-butanesultone to yield two series of graft copolymers having branch chains of alkylsulfonic acids. They were further made into the corresponding membranes with the assist of simultaneous cross-linking reaction with 2,6-bis(hydroxymethyl)-4-methylphenol (BHMP) as the cross-linker. High proton conductivities were found for the membrane series of M-PS_x-PSBOS_y in the range of 0.184–0.266 S/cm, and for F-PS_x-PSBOS_y in the range of 0.147–0.284 S/cm under fully hydrated conditions at 80 °C, respectively, and were supported by the nanophase-separated structures by the transmission electron microscopy (TEM) investigations. Also, the membranes showed good methanol resistance (1.60–10.4×10⁻⁷ cm²/s) and high oxidative stabilities. These results suggest a potential of the membranes for the DMFCs application.

Introduction

The proton exchange membranes (PEMs) in fuel cell perform a number of critically functions. It separates the fuel and oxidant, and provides the pathway for the proton transfer. DuPont׳s Nafion is widely studied as good fuel cell membranes because of its high proton conductivity combined with oxidative and chemical stability [1]. However, some drawbacks of Nafion, such as high methanol crossover and high cost, hinder its widespread commercial use in proton exchange membrane fuel cells (PEMFCs) [2], [3].

Many efforts have been made to develop the alternative materials to Nafion in past years. Most of the acid-functionalized aromatic hydrocarbon polymers including sulfonated poly(arylene ether ketone) [4], sulfonated polyimides[5], [6], and sulfonated poly(arylene ether sulfone) [7], have been considered as promising candidates for PEMs due to their thermal stabilities, low fuel permeabilities and low costs. However, the sulfonated aromatic polymer membranes have relatively low proton conductivity than that of Nafion because of the lower acidity of the aryl sulfonic acid and less distinct phase separation between hydrophilic and hydrophobic domains. One of approaches to improve the performance is to design polymer structure consisted of hydrophilic and hydrophobic segments which are expected to form ion transport channels for efficient proton conduction, and it has received increasing attention as enhanced properties in terms of PEM performance were found in the so-designed polymers [8], [9], [10], [11], [12], [13], [14]. Recent studies found that the attachment of pendant alkylsulfonated side chains to hydrocarbon-based polymers contributed to the well-developed phase separation and thus improved the proton conduction. Ueda and co-workers reported a series of cross-linked polystyrene membranes consisted of a hydrophobic main chain and flexible pendent aliphatic sulfonic acid side chains by the treatment with polyfunctional benzylic alcohol of 4,4′-methylene-bis[2,6-bis(hydroxyethyl)phenol] (MBHP) [9], [14], [15]. They found well-developed phase separation in the membrane structure, and a significant effect of alkylsulfonated side chains introduced to the polystyrene scaffold on membrane properties of water uptake, proton conductivity, and oxidative stability.

Polyphosphazenes are hybrid organic–inorganic polymers with a phosphorus–nitrogen chain backbone and have valuable qualities as structural and functional materials. A number of researches have focused on sulfonated polyphosphazenes as proton exchange membranes for fuel cell membranes over the past few years [16], [17], [18], [19], [20], [21]. Pintauro and co-works reported that sulfonated with SO₃ and crosslinked poly[bis(3-methylphenoxy)phosphazene] membranes showed much low methanol diffusivities (≤1.2×10⁻⁷ cm²/s) and also lower proton conductivity than that of Nafion 117 [17]. In our previous work, we have prepared sulfonated polyphosphazene-graft-polystyrene copolymers that were synthesized by atom transfer radical polymerization (ATRP) of styrene and then selectively sulfonated with acetyl sulfate, and exhibited good methanol-resistant ability but remained insufficient proton conduction [18]. Moreover, the sulfonated phosphosphazenes polymers were all almost obtained by postsulfonation with sulfonating agents such as sulfur trioxide and concentrated sulfuric acid, and this sulfonation method often led to an unstable product that later underwent degradation at a higher level of ion exchange capacity (IEC). In the present study, we designed and synthesized two series of the graft copolymer-based membranes M-PS_x-PSBOS_y and F-PS_x-PSBOS_y, which have polyphosphazene-graft-copolystyrene and branched chains of alkylsulfonate. The sulfonic acid groups were incorporated into the macromolecular chain at the polymer synthesis step, and this could give better control over the membranes properties even at higher level of IEC. The graft copolymer membranes showed excellent proton conduction even much higher than that of Nafion 117 in a range of room temperature to 80 °C. Herein, we will describe the preparations and properties of the membranes.