Synthesis and characterization of sulfonated poly(ether ether ketone) for proton exchange membranes

https://doi.org/10.1016/j.memsci.2003.09.019Get rights and content

Abstract

Series of sulfonated poly(ether ether ketone)s (SPEEKs) were prepared by sulfonation of commercial Victrex® and Gatone® PEEK for a comparative study of proton exchange membranes (PEM) intended for fuel cell applications. The degree of sulfonation (DS) of the sulfonated PEEK was determined from deuterated dimethyl sulfoxide (DMSO-d6) solution of the purified polymers using 1H NMR methods. The second method using a solvent suppression technique, in which DS results were obtained directly from 1H NMR spectra of SPEEK dissolved in sulfuric acid (non-deuterated) reaction medium was evaluated. The variation between the two methods was determined. The room temperature sulfonation of PEEK, monitored directly by second 1H NMR method, proceeded rapidly initially, reaching DS∼0.8 within 1 week, but progressed slowly thereafter. A maximum DS of 1.0 was determined after 1 month at ambient temperature (∼22 °C). The thermal properties of SPEEK were characterized by means of DSC and TGA. The mass averaged molecular weights Mw of both Victrex® and Gatone® PEEK were estimated from intrinsic viscosities measured in sulfuric acid solutions. It was verified that higher temperature (55 °C) did not induce any apparent chain degradation of Victrex® (or Gatone®) PEEK by Mw tests. The water uptake and swelling properties of prepared films were studied and the proton conductivities at different temperatures were measured. The conductivities of the SPEEKs were found to increase with increasing DS and temperatures. The effect of film casting solvents on the conductivities is also discussed.

Introduction

Proton exchange membrane fuel cells (PEMFC) are among the most promising electrochemical devices for convenient and efficient power generation. The proton exchange membrane (PEM) is a key component in the system, which functions as an electrolyte for transferring protons from the anode to the cathode as well as providing a barrier to the passage of electrons and gas cross-leaks between the electrodes. Currently, the most commonly used PEM for both hydrogen (H2-PEMFC) and direct methanol fuel cells (DMFC) are perfluorinated copolymers such as Nafion®, which have high hydrolytic and oxidative stability and excellent proton conductivity. However, the perfluorinated polymers have three major drawbacks: very high cost; loss of conductivity at high temperature (>80 °C); and high methanol permeability, which hinder their further application [1], [2].

In view of this, there is widespread effort being spent to develop alternative more economical non-perfluorinated polymer PEMs for high temperature use. Many promising polymers are based on aromatic thermoplastics [1], [3], [4], [5], [6], such as poly(aryl ether ketone)s (PAEKs) (e.g. PEEK), poly(ether sulfone) (PES), polybenzimidazole (PBI), etc., which have excellent chemical resistance, high thermo-oxidative stability, good mechanical properties and low cost. By attachment of sulfonic groups to the polymers’ chains, these sulfonated PEEK [1], [3], [4], [5], [6], [7], [8], PES [9], [10], polyimides [11], [12] and PBI [13], [14] were applied as PEM materials. Sulfonation of polymers is typically conducted by either directly introducing the sulfonic acid group onto the polymer backbone by modification [15], [16], [17] or by polymerizing sulfonated monomers [9], [18]. Many polymers have been sulfonated to improve their membrane properties in terms of better wettability, higher water flux, higher antifouling capacity, better permselectivity, and increased solubility in solvents for processing. In particular, the solubility of certain sulfonated aromatic polymers such as PEEK allows casting from organic solution and offers a more convenient and less expensive process than fabricating perfluorosulfonic acid membranes. Hydrophilic SPEEKs have been used as membrane materials for the separation [19], [20] of liquids. However, in a number of studies, SPEEK was reported to be used as a PEM material [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [21], [22], [23], [24], [25]. In Ref. [24] for instance it was found that a SPEEK-based PEM could have a lifetime of more than 3000 h [24], which indicates that SPEEK may be durable enough under fuel cell operation conditions.

Recently, proton-conducting blend membranes [3], [26], [27], [28], [29], in which SPEEK was used as a major component, have been explored with the goal to obtain good mechanical properties, high proton conductivity, and optimized membrane properties. SPEEKs were used to prepare blend membranes with PES [3], [25], [26], and with polyetherimide [27] for PEMFC application and with polysulfone for direct methanol fuel cell (DMFC) [28].

Although several studies on SPEEK were previously conducted, the present paper is a comparative study of the sulfonation of commercial Victrex® and Gatone® PEEK and their membrane properties related to PEM. Although the sulfonation of Victrex PEEK has been studied and Victrex currently has a license agreement with a fuel cell company, there have apparently been no comparative studies of the sulfonation of Gatone PEEK, which is produced by a different polymerization method. In addition, a new technique for directly monitoring the sulfonation reaction of PEEK by NMR was evaluated.

Section snippets

Chemicals and materials

Victrex® PEEK grade 450G extruded pellets were kindly provided by Victrex PLC (England). Gatone® PEEK grade 5300P extruded pellets were kindly provided by Gharda Chemicals Ltd. (India). Sulfuric acid (95–98 wt.%), dimethylacetamide (DMAc), dimethylformamide (DMF) and methanol were obtained from Aldrich Chemical Corp. MEMBRA-CEL dialysis tubing was obtained from Helixx Technologies Inc. (Canada).

Sulfonation of PEEK

PEEK pellets (40 g) were gradually added during 20 min into vigorously mechanically stirred 1.4 l

Sulfonation and NMR study

Sulfonation of PEEK in sulfuric acid is an electrophilic substitution reaction, in which the sulfonic groups are introduced into the hydroquinone segment of the polymer chain activated for electrophilic substitution by the ether linkage. Table 1 shows the comparison of the DS of both Victrex® and Gatone® PEEK sulfonated at room temperature. Several studies on the sulfonation reaction kinetics and characterization of SPEEK were reported [7], [8], [14], [15], [16], [17], [29], [30], [31], [32],

Conclusion

A comparative study of sulfonation and product characterization of both commercial Victrex® and Gatone® PEEK was conducted. Both PEEKs show identical chain structure according to NMR spectroscopy and have similar molecular weights. It is verified that higher temperature sulfonation (55 °C) did not induce any apparent chain degradation of PEEK as shown by Mw tests. A new method for determining the DS of SPEEK directly in the sulfuric acid (non-deuterated) reaction solution was accomplished by 1H

Acknowledgements

Partial financial support was provided by National Science and Engineering Research Council (NSERC), Canada. The authors gratefully acknowledge the generous gift of PEEK pellets grade 450G from Victrex PLC and PEEK pellets grade 5300P from Gharda Chemicals Ltd.

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