Aromatic ionic monomer bearing perfluorosulfonate moiety and its polycondensation toward high performance superacid ionomers

Highlights

• Synthesis of a new monomer having a superacid and two meta-polymerizable groups is studied.

• Purification of the monomer is optimized in time and technological realization.

• Polycondensation results in mixture of high- and low-molecular weight products.

• Cyclization of the chain end-groups may occur during polycondensation.

• The ionomers show good film-forming, thermo-mechanical and H⁺-conductive properties.

Abstract

Bottom-up synthesis of perfluorosulfonic acid ionomers based on the high performance backbone is rarely performed, due to the difficulty of obtaining ionic monomers of high purity. Such an approach, however, results in ionic polymers of well-defined chemical structure and ionic group distribution. Here copper-mediated synthesis of an ionic monomer bearing a perfluorosulfonate group and two polymerizable fluorine-sites was studied. Its synthesis and purification were investigated with accuracy to find the least time-consuming and the most technologically advantageous protocol (the overall yield of the monomer consisted 76%). The monomer was subjected to polycondensation with commercial non-ionic monomers, and a series of ionomers of different ionic concentration (and thus ion exchange capacity, IEC) was obtained. The resultant ionomers demonstrated excellent film-forming properties. The obtained membranes were characterized by high glass transition temperature (>160 °C) and high proton conductivity (10⁻²–10⁻¹ S/cm at 60–90 °C and 95% relative humidity). Such results make these materials promising for use in fuel cells.

Introduction

Due to their superacidity and highly organized structure with well-connected ionic domains [1], [2], [3], [4], ionomers bearing short or long perfluorosulfonic side chains exhibit high proton conductivity and excellent performance in a proton exchange membrane fuel cell (PEMFC). Therefore, Nafion, the commercial Teflon-based ionomer of DuPont, became the benchmark membrane for PEMFC. However, it is used in high humidification conditions and at temperature below 90 °C in order to keep high its conductivity and sufficient its mechanical strength. In addition, the material is expensive and its synthesis is far from being green. To overcome the Nafion drawbacks, recent studies were focused on the development of polyaryl-based ionomers. However, the related membranes show lower dissociation, worse organization and percolation of ionic domains and, thus, lower proton conductivity [5]. Such a drop is generally ascribed to high rigidity of the ionomer backbones together with lower acidity of the ionic groups, which are, in most cases, aryl sulfonic groups. To overcome the latter drawback polyaryl-based ionomers bearing perfluorosulfonic acid side groups were proposed in the last 6 years (a more detailed review may be found in [6]). These new materials, especially the multiblock-copolymers [7], [8], show excellent separation between hydrophilic and hydrophobic phases, thus, high ionomer structuration, and improved connectivity of the ionic domains. These properties led to membranes showing high proton conductivity in a wide range of temperatures and of relative humidity.

As reported in [6], polyaryl-based ionomers bearing perfluorosulfonic acid side groups are usually prepared by post-modification of polyphenylene backbones. This approach results in a poor control of ionomers’ structure, and more specifically, of the distribution and position of the ionic function along the main chain. The bottom-up approach, i.e. polycondensation of an ionic monomer with non-ionic ones, overcomes the main drawback of post-modification and leads to an ionomer of a predetermined structure. The earlier reported bottom-up syntheses of the ionomers dealt with ionomers consisting of relatively short superacid pendant groups: a perfluoromethylsulfonic group [9], [10], or a perfluoroethoxy sulfonic acid O(CF₂)₂SO₃H [11], [12]. While the former did not show satisfactory conductivity due to rigidity of the lateral group; the latter was found more conductive. It was until our recent report on highly nanostructured polyarylether ionomers bearing (CF₂)₂O(CF₂)₂SO₃H pendant groups (obtained by bottom-up synthesis of 1,3-dihydroxy-5-(1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoroethoxy-2-sulfonate)ethylsulfanyl)benzene (DHSPFS) with commercial non-ionic monomers), when the materials’ higher conductivity at low relative humidity, compared to Nafion, was observed [13].

In this work we report on the synthesis of a new ionic monomer bearing a dangling perfluorosulfonic function (CF₂)₂O(CF₂)₂SO₃K and two polymerizable fluorine-sites – potassium salt of 1,3-difluoro-5-(1,1,2,2-tetrafluoro-2-(1,1,2,2-tetrafluoroethoxy-2-sulfonate)ethyl)benzene (DFPFS). We discuss, in particular, about the reaction conditions allowing the monomer to be obtained with high purity and the side-product amounts to be decreased. The bottom-up synthesis of ionomers of different ion exchange capacity (IEC) is also presented here. Membranes, obtained by casting from solution of the ionomers, were characterized in terms of their thermal, mechanical and proton-conducting properties.