ETFE-g-pentafluorostyrene: Functionalization and proton conductivity

•New PEM based on pentafluorostyrene grafting from ETFE is developed.•Grafting is optimized to achieve control on degree of grafting.•Grafting is optimized to decrease the activation energy and termination reactions.•Efficient post-functionalization results in PEMs with IEC = 1–1.5 mmol g−1.•The conductivity (170 mS cm−1 at 120 °C, 90% RH) exceeds this of Nafion 117.

In this study we propose a relatively simple and versatile method for the preparation of a polyelectrolyte membrane based on poly(pentafluorostyrene) (PFS) owing proton exchange functionalities for proton conducting applications. The method includes two steps: grafting of the PFS from an irradiated ETFE film, followed by post-functionalization. The advantage of our approach lies in grafting a polymer with highly reactive functional groups suitable for efficient post-sulfonation/phosphonation. The grafting step is optimized both in terms of the system (bulk, non-/solvent and gas-phase grafting) and the conditions (radiation doses, time and temperature) used. The kinetics of the grafting shows a linear increase of grafting degree with both time and temperature suggesting a lower diffusion control and a reduced termination reaction rate for the optimized grafting process. Both phosphonation and sulfonation of the PFS grafted ETFE result in a corresponding high functionalization degree of 50% and 70% with IEC values of 1 and 1.5 mequiv g−1 and conductivities of 170 and 90 mS cm−1 at 120 °C, 90% RH for the sulfonated and phosphonated ETFE-g-PFS respectively. The high conductivity is attributed to the homogeneous distribution of the PFS within the ETFE matrix (EDX-SEM micrographs) and relatively high acidities of the sulfonic and the phosphonic acids.

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