Modification of ultrathin polyetheretherketone film for application in direct methanol fuel cells
Introduction
Direct methanol fuel cells (DMFCs) are very promising as power sources for mobile applications. To date, Nafion® is typically used as the polymer electrolyte membrane (PEM) to transport protons from the anode to the cathode. However, it still has some critical problems such as high methanol permeability. The crossover of methanol from the anode to the cathode lowers the DMFC efficiency and affects the total fuel economy. Finding solution for eliminating or reducing methanol crossover through the PEM is a current subject of research.
The high methanol crossover of Nafion® may be due to its perfluorinated structure and the large phase separation between its hydrophobic and hydrophilic layers, through which methanol have a high permeability [1], [2], [3]. Although thick Nafion® up to 175 μm was used to prevent the methanol crossover, the cell performance was still not acceptable despite the increase in PEM cost. Partially fluorinated PEMs such as sulfonated styrene-grafted poly(ethylene-co-tetrafluoroethylene) (ETFE-g-SS) membranes are less methanol-permeable than Nafion® [4], [5]. Furthermore, the non-fluorinated hydrocarbon PEMs often exhibit several tens of times lower methanol permeability than Nafion® [6], [7].
These hydrocarbon PEMs are usually prepared by introducing sulfonic acid groups to thermally stable aromatic polymers. Among PEMs prepared in these ways, sulfonated polyimide (sPI) and sulfonated polyetheretherketone (sPEEK) have received particular attention [8], [9], [10]. These hydrocarbon PEMs exhibit certain advantages, in especial low methanol permeability. The possibility for use of these PEMs in DMFCs needs to be further investigated.
For hydrocarbon PEMs, the sulfonic acid groups are generally directly attached to the backbones. However, the intrinsic properties of the backbones may thus be lost to a great extent. Specifically, the PEMs become brittle in a dry state and weak in a wet state. For instance, the sPEEK with an ion exchange capacity (IEC) of 1.65 mmol g−1 highly swells in water at 80 °C, resulting in low mechanical and dimensional stability [10]. Recently, some new hydrocarbon PEMs with pendent sulfonic acid groups on the backbones were reported to be useful in high temperature fuel cells [11], [12]. On the other hand, blending of unsulfonated polymers, such as poly(vinylidene fluoride) (PVDF), polyaniline and polybenzimidazole (PBI), with sPEEK is an effective method to restrain the water-swelling [13], [14], [15]. Crosslinking of the sPEEK chains by irradiation or chemical treatment is another effective method to improve the PEM performance [16], [17], [18].
Radiation grafting technique offers a convenient method to modify a preformed film to have proton conductibility [23], [24], [25], [26], [27], [28], [29], [30], [31]. The preformed films and monomers for grafting can be selected from a wide range of commercially available products, and thus PEMs with high mechanical strength and high proton conductivity can be designed. On aromatic-free films such as ETFE and PVDF it is easy to radiation graft vinyl monomers, such as styrene, acryl acid and their derivates [4], [19], [20], [21], [22], [23]. However, it is difficult to graft onto aromatic films because of their high radiation-resistance [17], [25]. In addition, when styrene is grafted and the post-sulfonation is executed, the aromatic rings on the backbones are sulfonated as well, which decreases the PEM strength drastically. For example, although the styrene can be radiation grafted onto the PEEK film in a n-propanol solution at 80 °C after a long grafting time, a PEM with a smooth and uniform surface suitable for a fuel cell is difficult to obtain after the sulfonation [24].
Therefore, to avoid the sulfonation, direct grafting of a monomer containing functional groups that can be converted to sulfonic acid groups, such as ethyl styrenesulfonate (ETSS) and sodium styrenesulfonate (NaSS), was proposed [25], [26], [27]. For the ETSS-grafting, an active divinylbenzene (DVB) monomer was first grafted to the PEEK films to form active regions. Thus, the subsequent ETSS-grafting was enhanced and a PEEK-g-DVB-g-ETSS film with high grafting yield could be prepared. The ETSS units of the grafted film were readily hydrolyzed in hot water into styrenesulfonic acid (SS) units, resulting in a PEEK-g-DVB-g-SS PEM. In our previous works [26], [27], the 25 and 50 μm thick PEEK was used as the base films, and the resulting PEMs with about 43 and 96 μm in thickness, respectively, were tested in a hydrogen-fed polymer electrolyte membrane fuel cell (PEMFC). The long-term durability and degradation of the PEMFC using these membranes at 95 °C were reported.
For the hydrogen-fed PEMFC, the long-term durability of more than ten years is highly desired. Therefore, much thick base films were used to prepare the PEMs with high robustness. On the other hand, for DMFC used in mobile systems, high power density comparable to Li-ion battery has to be met. A thin PEM in a DMFC can further decrease the cell resistance, improve the water management efficiency, and thus increase the power density of the DMFC.
Because the mechanical property of the PEEK-g-DVB-g-SS PEM is very excellent [26], we tried to prepare a PEM as thin as possible for use in DMFC. For this purpose, a commercially available 6 μm thick PEEK film was used as the starting material to prepare the ultrathin PEM. The grafting of DVB and ETSS onto the PEEK film was studied in detail, including the effect of temperature on the DVB- and ETSS-grafting and the electrochemical, thermal and physical properties of the resulting PEM. Finally, the resulting PEM was tested in a DMFC.
Section snippets
DVB-grafting
To graft DVB (80% mixture of m-, p-isomers, and 20% of ethylvinylbenzene, Aldrich, Japan) onto the low crystallinity PEEK film (14.6% crystallinity, Vitrex PLC, Japan), a PEEK film (70 mm × 70 mm) with a thickness of 6 μm was immersed in 100 ml of DVB/dioxane solution (20 wt%) for 24 h to induce the DVB-grafting. After the grafting, the PEEK-g-DVB film was washed in dioxane to remove the absorbed monomer and the ungrafted homopolymer, and then dried in vacuum at 40 °C for 24 h.
The amount of the grafted
Two-step grafting of DVB and ETSS
In our previous study, we found that the low crystallinity PEEK can be thermally grafted with DVB even without irradiation or chemical initiators [25], [26]. On the other hand, the PEEK with a high crystallinity of about 30% and the other wholly aromatic polymers, such as polyimide and polyarylenethersulfone, cannot be grafted under the same conditions.
Scheme 1 shows the process for the preparation of the PEM. The DVB was first grafted onto the PEEK film by thermal polymerization. Then, the
Conclusions
A PEEK-g-DVB-g-SS PEM was successfully developed by thermal grafting of DVB, followed by radiation grafting of ETSS and hydrolysis of the ETSS units into SS units in succession. Both the DVB- and ETSS-graftings could be enhanced by elevating the grafting temperature. In addition, the second-step ETSS-grafting was greatly enhanced by the first-step DVB-grafting.
The properties of these PEEK-g-DVB-g-SS PEMs, such as water uptake and proton conductivity, could be controlled freely by changing the
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