Building bridges: Crosslinking of sulfonated aromatic polymers—A review

Abstract

Sulfonated aromatic polymers (SAP) are promising inexpensive polymer electrolyte membranes for fuel cells, but there still exist some challenges about this kind of materials including excessive swelling in water, poor mechanical strength, low dimensional stability, especially for highly sulfonated SAP. Crosslinking is widely proposed as an efficient strategy to deal with these challenges. Here, the state-of-the-art of crosslinked SAP is reviewed and some prospects for further development are outlined. The following points are addressed: (i) Crosslinked SAP via covalent bonds, including esterification, addition, Friedel–Crafts reactions and formation of –SO₂– bridges; (ii) crosslinked SAP by ionic bonds; (iii) combined covalent and ionic crosslinking.

Introduction

The harsh working conditions encountered in electrochemical energy technologies, such as proton exchange membrane fuel cells (PEMFC), redox flow batteries and water electrolysers, are a great challenge for the cell components, especially for the key one—the proton exchange membrane [1], [2]. Up to now, there is no completely satisfactory polymer membrane for these applications. As a standard PEM, Nafion, a perfluorinated sulfonated polymer, possesses some excellent properties such as a high proton conductivity, chemical inertia, a high thermal stability and good nanophase separation, and thus has been widely investigated. However, its high cost, high fuel permeability and the loss of proton conductivity above 80 °C are significant disadvantages for its application in fuel cells [3], [4]. Therefore, inexpensive, easily available and low permeable sulfonated aromatic polymers (SAP) have become promising alternative PEM materials. Common SAP are shown in Fig. 1, including sulfonated poly–ether–ether–ketone (SPEEK) [5], [6], sulfonated poly–ether–sulfone (SPES) [7], [8], [9], [10], sulfonated poly–ether–sulfone–ketone (SPESK) [11], sulfonated poly–phenyl–sulfone (SPPSU) [12], [13], sulfonated polyimide (SPI) [14], sulfonated benzyl polybenzimidazole (N-benzylsulfonated PBI) [15], [16], sulfonated phenylene oxide (SPPO) [17].

Unfortunately, an excessive swelling in water and poor mechanical properties also reduce the applicability of SAP, especially those with an ion exchange capacity (IEC) above 2.0 meq/g. Nevertheless, both high IEC and high water uptake are useful, because they are closely related to the proton conductivity, an essential property for PEM. As shown in Fig. 2 [18], a high IEC offers enough acidic groups and a high water uptake facilitates the proton dissociation from –SO₃H groups and the transport inside hydrated channels. How to simultaneously maintain a high proton conductivity and good mechanical properties is the central challenge for SAP development.

This problem is particularly marked when the operation temperature is higher than 80–90 °C. In fact, the utilization of SAP is nowadays limited to direct methanol fuel cells (DMFC) operating at around 60–70 °C [19]. However, an increase in the operation temperature, to around 110 °C, is desired especially for stationary H₂-fueled PEMFC [20].

A crosslinking reaction can be defined as a process where a polymer becomes a dense three-dimensional network by varied interactions such as hydrogen bonds, ionic or covalent bonds (Fig. 3) [21].

Calculations on SAP show that the preferred macromolecular conformation is approximately spherical, because the polymer chains are generally hold together and entangled by long- and short-range interactions [22], [23]. It is known that the spherical shape is the most stable, which is attributed to the low surface energy state. The interactions become stronger and stronger with the increase of the crosslinking (XL) degree. Without reticulation, interactions among the polymers are weak, resulting in a parallel gliding of the polymer chains, leading to poor mechanical strength and excessive swelling behaviour in the macroscopic scale [23].

The presence of XL, especially by covalent bonds, makes the polymers more resistant to harsh environment because of XL-induced properties of the polymers such as good chemical resistance, low solubility in solvents, low fuel permeability, a high dimensional stability and an excellent mechanical strength [24], [25], [26]. On the other hand, the free volume and the proton conductivity usually also decrease [27], [28], [29], accompanied by an enhancement of the glass transition temperature due to changes in the macromolecular assembly [30], [31]. The reticulation effect on the physicochemical properties of the polymers depends on the XL degree and the uniformity of the network.

Crosslinking reactions can occur via various pathways, initiated by heat, a change in pH or radiation. XL formation can be macroscopically observed by a decrease of the polymer solubility in various solvents. In addition, the colour usually becomes deeper with increasing XL degree [32], [33]. In order to modify SAP, besides introducing inorganically or organically reinforced materials into the polymer matrix [34], [35], [36], [37], [38], reticulation is a facile, reliable and powerful strategy for combining high proton conductivity with good mechanical properties, suppressed swelling in water and low fuel permeability. Kerres et al. did many experiments about XL SAP [39], [40], [41], [42], [43], [44], [45], [46], [47], and Kerres also reviewed the research progress of this topic in [48]. Since then, many new articles have been published and investigations on crosslinking reaction mechanisms and degradation tests (ex situ or in situ) of XL SAP have also been reported [49], [50], [51], [52], [53], [54]. Recently, Lee et al. reviewed the advances of sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs), including polymer sulfonation and physico-chemical properties. Crosslinking is described as one of the physico-chemical strategies for polymer optimization [55].

In this work, the recent advances exclusively about various crosslinked SAP are organized in terms of the strength of crosslinking interactions as shown in Fig. 4. Conventional and unexpected effects of XL on SAP properties are discussed and future prospects are indicated.