Heat-Treated Charge-Transfer Polymer Blend Membrane for Application in Polymer Electrolyte Fuel Cells
Authors: Nishihara Masamichi , Feng Shiyan , Christiani Liana , Hwang Byungchan , Kondo Shoichi , Kikuchi Takamasa , Sasaki Kazunari 
Published: December 2020

Introduction
We developed polymer blend type electrolyte membranes (PEMs) using charge-transfer (CT) interactions for polymer electrolyte fuel cells (PEFCs) and evaluate effect of heat treatment to characteristics and performance of the CT polymer blend membranes.
CT interaction is one of molecular interaction, and CT complex can be formed by electron accepting units and donating units which have optimal molecular structures. CT complex formation was applied to control the position of proton conductive groups in PEMs. By applying this characteristic, we have developed CT complex polymer hybrid membranes. Especially, CT polymer blend consisting of two polymers, electron-accepting sulfonated polyimide (SPI) and -donating polyether (Poly-DAN), showed high mechanical strength and gas barrier property.[1] During this study, it was confirmed that CT complex concentration increased by heat treatment. In this study, to further understand the effect of CT complex formation on PEM performance, heat treatment was performed to enhance the extent of CT complex formation in the membrane.
Experimental
In this work, we used two model polymers with simple chemical structure. Sulfonated polyimide homopolymer (SPI) was used as the electron-accepting polymer, while polyether-containing electron-rich dialkoxynaphthalene (Poly-DAN) was used as the electron-donating polymer.[2]
SPI/Poly-DAN blend membranes were prepared with or without a heat treatment step.
SPI and Poly-DAN can form a CT complex in the blend membrane during solvent evaporation at 60 °C in vacuo, and CT blend membranes without heat treatment were thus obtained. Heat-treated CT blend membranes were obtained by heating at 150 °C in a N2 atmosphere for several hours. The heat-treated membranes were then immersed in water to remove thermally decomposed sulfuric acid moieties.
Visible spectroscopy was used to confirm CT complex formation and the amount. Thermal property, mechanical strength and structural analysis of the CT blend membranes were evaluated by TGA, tensile test, 1H NMR etc. IEC, proton conductivity and fuel cell performance at 80˚C and 110 ˚C were also evaluated.
Results and discussion
A characteristic absorbance is observed in the visible spectra of the untreated and the heat-treated membranes at 530 nm. After heat treatment at 150 °C for 50 h, the concentration of CT complex in the membrane was significantly enhanced by about 13 times. The 1H NMR and FT-IR results suggest that the main chain structure of SPI does not significantly change during heat treatment. From DSC and XRD results, we suggested that Poly-DAN unit with low glass transition temperature inserted between two SPI main chains due to CT complex formation by heat treatment.
After heat treatment, SPI/Poly-DAN blend membranes did not completely dissolve in DMSO, and the weight loss after water immersion of the heat-treated SPI/Poly-DAN blend membranes was smaller than that of the untreated membrane. These results indicate that a cross-linked structure was not only formed by CT complex formation but also due to chemical cross-linking during heat treatment. Because of chemical cross-linking of sulfonic acid by heat treatment, experimental IEC of SPI/Poly-DAN (1:0.05 mol) was smaller (2.51) than theoretical value (3.41).
Heat-treated SPI/Poly-DAN blend membranes showed higher mechanical strength (50.8 MPa) than Nafion 212 (15.5 MPa) and highly chemical durability compared to the untreated membrane by the synergetic effect of enhanced CT complex formation and chemical cross-linking. Heat-treated SPI/Poly-DAN membranes showed comparable proton conductivity (32.3 mS cm−1, 80 °C, and 90% RH), although some cross-linking occurred between sulfonic acid units due to the heat treatment process.
In fuel cell tests, heat-treated SPI/Poly-DAN membranes had maximum power densities of 255 mW cm−2 at 80 °C and 95% RH and 59.0 mW cm−2 at 110 °C and 31% RH, indicating that these heat-treated CT complex membranes could be used for fuel cell applications.
References

S. Feng et al., Journal of Membrane Science, 548 (2018).

S. Feng et al., ACS Applied Energy Materials, 2, 8715–8723 (2019).

Figure 1