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We developed polymer electrolyte membranes (PEMs) utilizing charge-transfer (CT) interactions for polymer electrolyte fuel cells (PEFCs). CT complex formation was applied to control the position of proton conductive groups in the membranes. To 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. In this work, sulfonated polyimide (SPI) was used as the electron-accepting polymer, while polyether-containing electron-rich dialkoxy­naphthalene (Poly-DAN) was used as the electron-donating polymer. After heat treatment at 150 °C for 50 h, the concentration of CT complex in the membrane was significantly enhanced by about 13 times. Heat-treated SPI/Poly-DAN 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 also showed reasonable 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 single 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.

Sulfonated poly(ether sulfone)s (SPESs) are developed which have different sulfonation level as a polymer electrolyte membrane for high‐temperature operation. The sulfonation level of SPESs is calculated by 1H NMR, and the molecular structure and crystalline structure of SPESs are evaluated by Fourier transform infrared and X‐ray diffraction. SPES membranes are thermally stable up to 250 °C. SPES membranes can keep their shapes at 120 °C and 23%RH. Water uptake at 120 °C and 23%RH is 5.7–6.4 wt%, while Nafion 212 shows 2.4 wt%. Proton conductivity measurements of SPESs are carried out from 30 to 120 °C at different relative humidity. With increasing sulfonation level of SPES, proton conductivity increases in all humidity. The proton conductivity obtained from all SPESs is more than 100 mS cm–1 at 120 °C in high humidity (>90%RH), and high‐sulfonation SPES shows higher conductivities than Nafion 212 at 120 °C, 20%RH.