Crosslinking and grafting of polyetheretherketone film by radiation techniques for application in fuel cells

Abstract

An aromatic hydrocarbon polymer electrolyte membrane (PEM) was prepared by direct modification of polyetheretherketone (PEEK) film using the radiation-induced crosslinking and grafting techniques. The crosslinking structure induced by the electron-beam irradiation enhanced the insolubility of the PEEK base film. The film shape of the crosslinked PEEK (cPEEK) was well maintained under sulfonation. A small amount of divinylbenzene (DVB) was introduced into the cPEEK film to enhance the radiation-induced styrene-grafting. The styrene-grafted film was sulfonated under a mild sulfonation condition to obtain the PEM. The PEM with different ion exchange capacity (IEC) was prepared by changing the grafting yield of styrene. The new PEM exhibited lower methanol permeability and higher mechanical properties, and was proven to be durable in a direct methanol fuel cell (DMFC) at high temperature, reaching a high maximum power density of 106 mW cm⁻² at 95 °C.

Introduction

The polymer electrolyte membrane (PEM) is one of the key components in the polymer electrolyte membrane fuel cell (PEMFC), conducting protons from the anode to the cathode, as well as separating the two electrodes to avoid the fuel and oxygen mixing. When the methanol aqueous solution was used as a fuel to feed the anode, the PEMFC is also called the direct methanol fuel cell (DMFC). To date, an ideal PEM with high proton conductivity and low methanol permeability, as well as high durability, is still under developing [1], [2], [3], [4], [5], [6], [7], [8]. The traditional perfluorosulfonic membrane, such as Nafion^®, has better chemical durability, but the methanol permeability is too high to effectively separate the anode and cathode [1]. On the other hand, the hydrocarbon PEMs often show more than one order of magnitude lower methanol permeability than Nafion^®, but the chemical durability of these PEMs have to be largely improved for practical use in fuel cells [9], [10], [11].

We are trying to develop a PEM by modification of the existent commercial polymer film with proton conductivity using the radiation-induced grafting method [12], [13], [14], [15], [16], [17]. For the non-aromatic hydrocarbon polymer films, such as polyethylene (PE), polypropylene (PP), and the fluoropolymer films, such as polytetrafluoroethylene (PTFE), poly(ethylene-co-tetrafluoroethylene) (ETFE), poly(vinylidene fluoride) (PVDF) and poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP), the high grafting yield of styrene can be reached due to the easy generation of the radicals under the irradiation [12], [13], [14], [18], and thus the designed functional membranes can be prepared.

On the other hand, we are also trying to develop an aromatic hydrocarbon PEM by radiation-induced grafting method [19], [20], [21]. Such a PEM is expected to have excellent mechanical and gas barrier properties because of the robust aromatic hydrocarbon base film. In addition, the resultant PEM is fluorine-free and environmentally friendly. However, the aromatic hydrocarbon polymer films, such as polyimide, polysulfone, and polyetheretherketone (PEEK), are radiation-resistant materials [22], and the radicals generated in the films under irradiation are not sufficient for the graft polymerization to reach a satisfactory grafting yield and grafting rate. Additionally, most of the aromatic hydrocarbon polymer films are dissoluble either in the monomer solution or in the sulfonation solution, and were difficult to be used for developing the PEM.

In general, the PEEK-based PEM can be prepared by direct sulfonation of commercial PEEK polymer with subsequent membrane casting. However, although the sulfonated PEEK (sPEEK) membranes with a higher sulfonation level exhibit higher proton conductivity, they also exhibit much higher water uptake [23], [24], [25], [26]. For instance, the sPEEK membrane with a sulfonation level of 61% can be dissolved in water at 80 °C [23]. To solve this problem, different strategies such as introducing crosslinking and organic/inorganic hybrid structures into the sPEEK membranes have been explored [27], [28], [29].

Recently, we successfully grafted styrene onto PEEK film by a radiation-induced grafting method [19]. However, although the grafting yield of styrene was low (16%), the resulted PEM was still weak due to the high water uptake of more than 200%. This was because that the PEEK backbone was also quite sulfonated and partially dissolved, resulting in the serious deterioration of the intrinsic properties of the PEEK backbone. To avoid the sulfonation and dissolution of the PEEK backbone, more recently, we grafted ethyl styrenesulfonate (ETSS) onto the PEEK base film [16], [17], [21]. The PEM was obtained by hydrolysis of the ETSS-grafted film without sulfonation. In that study, a small amount of divinylbenzene (DVB) was first thermally grafted onto the PEEK film, where more radicals were generated under the irradiation, and thus the ETSS-grafting was enhanced. The resulted PEM was robust and was confirmed to be durable in a fuel cell for more than 1000 h at a high temperature of 95 °C [17].

However, the ETSS is yet not a commercial monomer. Therefore, we still want to graft the styrene onto the PEEK film for the PEM development. We found that the PEEK film can be crosslinked by electron-beam irradiation. The crosslinked PEEK (cPEEK) film was not dissolvable in any solution and can be directly sulfonated to prepare the PEM, but the water uptake of the crosslinked sPEEK PEM was still too high even with a high crosslinking irradiation dose of 100 MGy [27].

In this study, we use the cPEEK as the base film, and the styrene as the monomer to develop the PEM for fuel cells. The cPEEK was crosslinked with a lower irradiation dose of 40 MGy. To enhance the radiation-induced grafting of styrene, the DVB was first thermally grafted onto the cPEEK film. The styrene-grafted film was sulfonated under a mild condition, where the polystyrene graft chains have a preference for sulfonation over the PEEK backbone chains. The resultant PEM was characterized in terms of ion exchange capacity (IEC), water uptake, proton conductivity, methanol permeability, mechanical properties as well as the DMFC performance at different cell temperatures.