Block-type proton exchange membranes prepared by a combination of radiation-induced grafting and atom-transfer radical polymerization
Introduction
Polymer electrolyte fuel cell (PEFC) is expected as a power source for portable electronic devices and automobiles because of high energy conversion efficiency and low environmental load [1], [2]. The most critical component of a PEFC is the proton exchange membrane (PEM) that conducts protons from the anode to the cathode. Currently, the most widely used PEM for PEFC applications is Nafion (DuPont Co.), which consists of polytetrafluoroethylene backbone chains and perfluorinated ether side chains terminated with sulfonic acid groups [3]. Nafion has sufficient proton conductivity, mechanical strength, and chemical stability, all of which are requirements for high-performance PEFC materials. However, there is a drawback that the production of Nafion is expensive because of the complicated fluorine chemistry synthesis process. Accordingly, much effort is being worldwide devoted to the development of alternatives to Nafion for use in low-cost PEMs [1], [4], [5].
In recent years, a radiation-induced graft polymerization (RIGP) has gained interest as a promising technique to prepare PEFC PEMs [6], [7], [8]. The RIGP method commonly involves the following three steps: irradiation of fluoropolymer films to generate initiation radicals, grafting of styrene (St) monomers starting from the radicals, and sulfonation of aromatic rings of the grafted St units. The resulting graft-type PEMs have hydrophobic fluoropolymer backbones and hydrophilic poly(styrene sulfonic acid) (PSSA) grafts for mechanical strength and proton conduction, respectively. The grafting degree of St can be widely controlled by changing the experimental conditions such as the irradiation dose and grafting time, thereby varying the ion exchange capacity (IEC) of the resultant PEMs.
Recently, the operation of PEFCs at low relative humidity (RH) and high temperature has become of interest because of some advantages. Low RH diminishes the flooding problem caused by water transportation from the anode to the cathode [9], and high temperature enhances the electrode reactions to generate greater electric power. We have investigated the proton conduction properties of poly(ethylene-co-tetrafluoroethylene) (ETFE)-based graft-type PEMs at 30–98% RH and 80 ºC [10], [11]. The proton conductivity of these PEMs significantly decreased as the RH decreased. At the lowest RH (30%), only the PEMs with IECs of more than 2.7 mmol/g had proton conductivity higher than that of Nafion (IEC =0.91 mmol/g). However, the PEMs with these high IEC values absorb large quantities of water and are mechanically fragile under both wet and dry conditions. Accordingly, it is very important to achieve high proton conductivity under low RH and high temperature conditions for graft-type PEMs while maintaining low IEC values.
The proton conduction properties of PEMs are influenced by their morphologies. Dissipative particle dynamics simulation of RIGP-PEMs have revealed that the PSSA graft chains were mixed with absorbed water to form large hydrophilic regions [12], [13]. These hydrophilic regions are connected with each other and form proton-transport ion channels. However, the PSSA graft chains are somewhat entangled with the hydrophobic base fluoropolymer chains because these two types of the chains are covalently bonded. It has been speculated that the fluoropolymer matrix hinders connections between the hydrophilic regions, resulting in the insufficient proton conductivity under low RH conditions. Therefore, the spatial separation of PSSA grafts from the fluoropolymer matrix is a prospective strategy for the design of PEMs with higher proton conductivity.
Considering the factors mentioned above, in this study, we have designed novel ETFE-based graft-type PEMs consisting of proton-conducting graft polymer attached to a hydrophobic spacer graft polymer. These PEMs were prepared by a combination of RIGP and atom-transfer radical polymerization (ATRP), which is a widely used living radical polymerization technique for the production of polymers with well-defined compositions and architectures [14]. As shown in Scheme 1, our preparation procedure involves three steps: (1) RIGP of St with a small amount (one tenth) of chloromethyl styrene (CMS) into the base ETFE film, (2) ATRP of ethyl styrene sulfonate (EtSS) initiated by reaction of methyl chloride on the grafted CMS units, and (3) hydrolysis, converting poly(EtSS) to PSSA. In this PEM, the hydrophilic PSSA grafts are separated from the hydrophobic ETFE chains by the copoly(St/CMS) grafts, which should act as flexible spacers. The designed polymer architecture should enhance the molecular mobility of the proton-conducting PSSA grafts to aggregate with each other, thereby forming a well-connected ion-channel network that facilitates proton conduction.
To date, only a few studies on the preparation of PEMs via RIGP and ATRP have been reported. Holmberg et al. prepared a PEM by RIGP of CMS into a poly(vinylidene fluoride) base film, ATRP of St starting from the chloromethyl groups in poly(CMS), and sulfonation of the St units using chlorosulfonic acid [15]. The sulfonation of the poly(St) grafts accompanied the sulfonation of aromatic rings in poly(CMS), leading to change of both poly(St) and poly(CMS) grafts into hydrophilic chains. This molecular structure is quite different from that of our PEMs, which have hydrophilic and hydrophobic block-type copolymer grafts (Scheme 1). In our study, during the ATRP reaction in the St/CMS-grafted films, we have tried to suppress undesirable thermal polymerization, which deteriorates the molecular architecture shown in Scheme 1. The proton conductivity and water uptake of the prepared RIGP/ATRP-PEMs were measured at 30% RH at 80 ºC, and compared with those of the conventional RIGP-PEMs. The morphology of the PEMs was investigated by small-angle X-ray (SAXS) scattering measurement. Finally, the proton conduction properties of the PEMs were examined on the basis of water content and the structures of ion channels consisting of graft polymers and absorbed water.
Section snippets
Materials
ETFE films (Asahi Glass Co., Japan) with thickness of 50 µm were used as the base fluoropolymer. St (Wako Junyaku Kogyo Co., Japan) and EtSS (TOSOH Co., Japan) were used without further purification. CMS (AGC Seimi Chemical Co., Japan) was purified by the extraction of a polymerization inhibitor in a 1 mol/L aqueous NaOH solution before use. Dioxane (Wako Junyaku Kogyo Co., Japan) and acetone (Wako Junyaku Kogyo Co., Japan) were used as received. CuIBr (Sigma-Aldrich Co., USA) was purified by
RIGP of St/CMS into ETFE films
As the first step of PEM preparation shown in Scheme 1, the base ETFE film was irradiated with 15 kGy γ-rays, and then St and CMS monomers with 90/10 vol% were co-grafted into the ETFE film at 60 ºC for 30–12 h. Fig. 1 shows the GD1 values, which correspond to the amount of copolymer grafts calculated by Eq. (1), as a function of the RIGP time. When the monomer concentration was 50%, the GD1 increased from 23% to 68%, depending on the RIGP time. At a monomer concentration of 25%, as expected, the
Conclusion
We prepared novel block-type PEMs by a combination of RIGP and ATRP methods. In the RIGP, the base ETFE film was irradiated with γ-rays, and St and CMS were co-grafted into the film. ATRP of EtSS was initiated from the grafted CMS units in the St/CMS-grafted film with GD1 values of 60–65%. ATRP was performed at 50 ºC, because undesirable thermal polymerization of EtSS was restricted at this temperature. The GD2 for EtSS increased with increasing ATRP reaction time, and reached more than 240%
Acknowledgement
This work was supported by Grant-in-Aid for Young Scientists (B) from Japan Society for the Promotion of Science (JSPS) (KAKENHI, Grant Number: 25810130). We would like to thank Editage (http://www.editage.com) for editing and reviewing this manuscript for English language.
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