Mechanically Tough Polymer Electrolyte Membranes Prepared By Radiation-Induced Graft Polymerization of Poly (ether ether ketone) Film
Authors: Hasegawa Shin , Yoshimura Kimio , Hiroki Akihiro , Zhao Yue , Maekawa Yasunari 
Published: December 2024

The development of state-of-the-art polymer electrolyte membranes is indispensable for solid polymer fuel cells. To make a desired membrane, we have reported the radiation-induced graft polymerization (RGP) method in our previous studies, which is a fascinating technique with great advantages of low-cost fabrication and imparting new functionality via graft polymers to commercially available substrate polymer films, whose excellent mechanical/thermal properties are maintained. Among various substrate polymers, poly (ether ether ketone) (PEEK) is regarded as the most promising aromatic hydrocarbon material for the long life-time operation requirement of fuel cells due to its excellent mechanical stability and high gas barrier property at high temperature. However, graft polymerization on PEEK substrate is rather difficult. To successfully obtain PEEK-based polymer electrolyte membranes with suitably-balanced properties, we have extensively investigated the graft polymerization and irradiation conditions. In this work, we will report the synthesis of both PEEK-based proton- and anion-exchange membranes (PEEK-PEMs and PEEK-AEMs, respectively), and the fuel cell performance using these two types of membranes.
1)RGP for making PEEK-PEMs and PEEK-AEMs.
The PEEK base films used for RGP was previously immersed in (1, 4-dioxane) DOX at 50°C for 18 hours as a solvent annealing treatment. Ethyl styrene sulfonate (ETSS) and chloromethyl styrene (CMS) are used as precursors for PEMs and AEMs, respectively (Scheme 1).
For the synthesis of PEEK-PEMs, the solvent-annealed PEEK films were pre-irradiated with 60Co ϒ-ray at room temperature in argon atmosphere with a total dose 160 kGy. Then the samples were immersed in ETSS/DOX solution (1/1 v/v) at 80°C under the argon atmosphere. The resultant grafted-PEEK membranes has a grafting degree (GD) of 150%. After hydrolysis treatment, the final PEEK-PEM shows an ion-exchange capacity (IEC) of 2.5 mmol/g and a proton conductivity of 0.1 S/cm at 25 °C.
For the synthesis of PEEK-AEMs, the same irradiation and polymerization procedures as in the synthesis of PEEK-PEMs were performed, by using CMS as monomers instead of ETSS. The CMS-grafted PEEK with high GD of 66% was obtained in tetraethylenglycol as a solvent. The grafted PEEK was converted to AEM by treating in trimethylamine with a quaternization degree of 76%. The obtained AEM has an IEC of 1.7 mmol/g and an anion conductivity of 0.128 S/cm at 60°C.
The mechanical strength and elongation of the resulting PEEK-PEM at 80°C and 100%RH were measured to be 22 MPa and 50%, respectively, which are superior to those of Nafion (10 MPa and 200%, respectively). PEEK-AEM also shows good mechanical strength and elongation of 94 MPa and 18%, respectively, at room temperature and 70% RH.
2)Fuel cell performance evaluation.
Fuel cell test using PEEK-PEMs and AEMs was carried out according to previous reports [1]. The membrane was sandwiched by anode and cathode electrodes (Pt or Pt/Ru loading 0.5 mg/cm2) and hot-pressed, and then obtained membrane/electrodes assembly (MEA) was then set into a 5 cm2 fuel cell for testing. The cell was fed with pure hydrogen and oxygen gases at a flow rate of 0.05 L/min each to the anode and cathode, respectively, under atmosphere pressure. The cell condition was 80°C and 100%RH.
Fuel cell test with an MEA incorporating 16 μm PEEK-PEM shows a good performance of an open circuit of 0.9 V and a power density of 860 mW/cm2 at a current density of 2240 mA/cm2 (Figure 1(a)). Using the PEEK-g-AEM membrane, it shows an open circuit voltage value of more than 0.9 V and a power density of 673 mW/cm2 at a current density of 1199 mA/cm2 (Figure 1(b)).
As described above, utilizing radiation-induced graft polymerization, we have for the first time successfully developed two types of polymer electrolyte membranes, PEEK-PEM and PEEK-AEM, using the mechanically tough and chemically stable aromatic hydrocarbon polymer PEEK as the substrate.
[1] J. Chen, et al. J.Membr. Sci. 2008, 319, 1-4.

Figure 1