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Phosphoric acid (PA)-doped polybenzimidazole (PBI) proton exchange membranes have received attention because of their good mechanical properties, moderate gas permeability, and superior proton conductivity under high temperature operation. Among PBI-based film membranes, nanofibrous membranes withstand to higher strain because of strongly oriented polymer chains while exhibiting higher specific surface area with increased number of proton-conducting sites.In this study, PBI electrospun nanofibers were produced and doped with PA to operate as high temperature proton exchange membrane, while changes in proton conductivity and morphologies were monitored. Proton conductive PBI nanofiber membranes by using the process parameters of 15 kV and 100 μL/h at 15 wt% PBI/dimethylacetamide polymer concentration were prepared by varying PA doping time as 24, 48, 72, and 96 hours. The morphological changes associated with PA doping addressed that acid doping significantly caused swelling and 2-fold increase in mean fiber diameter. Tensile strength of the membranes is found to be increased by doping level, whereas the strain at break (15%) decreased because of the brittle nature of H-bond network. 72 hour doped PBI membranes demonstrated highest proton conductivity whereas the decrease on conductivity for 96-hour doped PBI membranes, which could be attributed to the morphological changes due to H-bond network and acid leaking, was noted. Overall, the results suggested that of 72-hour doped PBI membranes with proton conductivity of 123 mS/cm could be a potential candidate for proton exchange membrane fuel cell. Figure 1) is capable of absorbing acids (pKa~5.5), which is essential to be used in fuel cell membranes and other protonconducting applications. 2 To overcome the drawbacks of the low-temperature proton exchange membrane fuel cell (PEMFC) such as CO catalyst poisoning, necessity of humidification, heat management, and low diffusion rates of protons, PBI-based membranes were preferred because of their superior proton conductivity particularly both at high temperatures 3-5 and at 0% relative humidity. 6 Phosphoric acid-doped polybenzimidazole membranes were first successfully prepared by Wainright et al. 7,8 These membranes were recommended as electrolyte for high-temperature proton exchange membrane fuel cell (HT-PEMFC) operating at temperatures of up to 200°C. 8,7 They also exhibited good mechanical properties and low gas permeability [9][10][11] compared to water-containing membranes including Nafion whose proton conductivity decreases with increased temperature because of the evaporation of H 2 O molecules. 12 In addition, the results showed that an increase in doping level resulted in better proton conductivity and so more efficient HT-PEMFC performance. 7,13 After blended with PA, PBI films might suffer from deterioration because of the slow elution of water-soluble PA, when the vapor was produced. 14 Moreover, it was reported that these film membranes also sacrificed the

The present study uses the radiation‐induced grafting method and applies it onto poly(ethylene‐alt‐tetrafluoroethylene) (ETFE) for the synthesis of proton‐exchange membranes by using monomers 4‐vinyl pyridine (4VP), 2‐vinyl pyridine (2VP), N‐vinyl‐2‐pyrrolidone (NVP) followed by phosphoric acid doping. Phosphoric acid that provides Grotthuss mechanism in proton mobilization is used to transform the graft copolymers to a high temperature membrane state. Resultant proton‐exchange membranes are verified with their proton conductivity, water uptake, mechanical and thermal properties, and phosphorous distribution as ex situ characterization. Our most important finding as a novelty in literature is that ETFE‐g‐P4VP phosphoric acid doped proton‐exchange membranes exhibit proton conductivities as 66 mS cm–1 at 130 °C, 53 mS cm–1 at 120 °C, 45 mS cm–1 at 80 °C at RH 100% and 55 mS cm–1 at 130 °C, 40 mS cm–1 at 120 °C, 35 mS cm–1 at 80 °C at dry conditions. Moreover, ETFE‐g‐P4VP membranes still conserves the mechanical properties, i.e., tensile strength up to 48 MPa. ETFE‐g‐P4VP membranes were tested in PEMFC at 80, 100, and 120 °C and RH <2% and exhibit promising performance as an alternative to commercial Nafion® membranes. The single cell testing performance of ETFE‐g‐P4VP membranes is presented for the first time in literature in our study.