Fuel Cell Performance of Graft Type Poly (Ether Ether Ketone) Electrolyte Membranes

Introduction

              A pre-irradiation grafting method is a fascinating technique for direct introduction of a new functional polymer phase as a grafting chain into polymer films serving as a substrate which allows those films to retain their characteristics such as thermal stability, mechanical strength, and electronic properties. The radiation technique has been widely applied to the preparation of high performance fuel cell polymer electrolyte membranes (PEMs) for vehicles and domestic co-generation systems [1]. The PEMs of aromatic hydrocarbon polymers, so-called “super engineering plastics” including poly (ether ether ketone) (PEEK), have useful characteristics such as high mechanical strength, gas barrier property, and radiation resistance. However, PEEK films also have high chemical resistance; thus, it is difficult to introduce grafting monomers into the films. Recently, we succeeded in radiation-induced grafting of Ethyl 4-styrenesulfonate (E4S) into a PEEK film with grafting degrees around 100% [2]. We report herein the synthesis, characterization, and electrolyte properties of graft-type PEEK PEM with thin PEEK films less than 50 mm thickness.  Fuel cell performance of graft-type PEEK PEM was estimated at 80 °C under 30 and 100% RH conditions.

Experiments

              Graft-type PEEK PEM was synthesized by pre-irradiation grafting of E4S into PEEK films (Scheme 1) [3]. The PEEK films were immersed in a 10 wt% DVB solution of 1,4-dioxane at 50°C for 24h. The PEEK-g-DVB films were pre-irradiated with a ⁶⁰Co γ-ray source (JAEA Takasaki, Gunma, Japan) at room temperature in argon atmosphere to a absorbed dose of 160 kGy. Then, the samples were immersed in 15 ml of the E4S solution of 1,4-dioxane (1/1 v/v) at 80 °C. The obtained PEEK-g-DVB-g-E4S film was hydrolyzed in 95 °C hot water for 24h (PEEK-g-DVB-g-PSSA PEM). The ion exchange capacity (IEC (mmol/g)), proton conductivity (S/cm) in-plane direction at room temperature, and water uptake (WU) was measured according to the previous report [2]. Proton conductivity was estimated at 80 °C in 30%RH; mechanical strength was measured at 80 °C in 100% RH, which is severe conditions for the fuel cell operation, was measured as previously reported [4]. The membrane/electrodes assembly (MEA) (Pt loading 0.5 mg/cm²) prepared by hot-press set into a 5 cm². The fuel cell test was carried out at 80°C in a RH range of 30-100%.

 

Results and Discussion

              According to the previously reported methods, we could prepare the PEEK-based PEM with grafting degrees of 88-107% and ion exchange capacity (IEC) of 2.3 – 2.5 mmol/g using the original PEEK substrates with film thickness of 6, 12, 16, 25, and 50 mm (Table 1). The variation of film thickness does not affect the conductivity and water uptake of the PEM. This is one of the thinnest PEM possessing sufficient conductivity as well as high mechanical strength (60 MPa of tensile strength at break).

              Then, we compare the fuel cell performance of the single fuel cell devices consisting of the PEEK-based PEM with various thicknesses in the operation condition at 80 ºC and 100%RH. As expected from the similar ion conductivity around 0.045 S/cm, the thinner PEM showed higher cell voltages at the current density in the range from 0.2 to 1.0 A/cm². Especially, compared with a Nafion212 film (thickness: 50 μm), higher cell voltages (namely, power densities (W/cm²)) can be achieved in the PEM with film thickness less than 12 mm (Figure 1). The above results imply the advantages of mechanically tough PEEK as an original polymer substrate to be able to decrease the film thickness, resulting in higher power density owing to the less cell resistance. 19 μm PEEK PEM has good proton conductivity (0.0068 S/cm) at 80°C and 30%RH. The elastic modulus of the grafted film showed twice higher than that of Nafion 212 film at 80 degree and 100%RH.

Acknowledgments

              For support of this work, the authors acknowledge the financial support from a Grant-in-Aid for Scientific Research (KAKENHI, No. 24550263) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References

[1] T. Yamaki, M. Asano, Y. Maekawa, Y. Morita, T. Suwa, J. H. Chen, N. Tsubokawa, K. Kobayashi, H. Kubota and M. Yoshida, Radiation Physics and Chemistry 2003, 67, 403-407.

[2] S. Hasegawa, K. Sato, T. Narita, Y. Suzuki, S. Takahashi, N. Morishita and Y. Maekawa, Journal of Membrane Science 2009, 345, 74-80.

[3] J. Chen, M. Asano, Y. Maekawa and M. Yoshida, Journal of Membrane Science 2008, 319, 1-4.

[4] T. D.Tap, S. Sawada, S. Hasegawa, Y. Katsumura, and Y. Maekawa, Journal of Membrane Science 2013, 447, 19-25.