Novel anion-exchange membranes with accelerated hydroxide ion conduction through a quaternized covalent organic framework-doped electrospinning binary polymer†

Abstract

The mutual restriction between hydroxide ions' conductivity and alkaline stability is the main obstacle for the practical application of anion-exchange membranes (AEMs) in anion-exchange membrane fuel cells. In this research, we designed a binary polymer nanofiber of polyvinylidene fluoride (PVDF) and polystyrene-block-poly (ethylene-ran-butylene)-block-polystyrene (SEBS) through an electrospinning technique. A quaternized covalent organic framework (QACOF) was then synthesized to accelerate the conduction of hydroxide ions consisting of successive and hydrophilic hydroxide ion conduction channels based on the quaternary ammonium groups. Additionally, the ordered microchannel structures of QACOF could further accelerate the hydroxide ion conduction process. The novel AEMs were thus constructed via the re-stacking of PVDF–SEBS binary polymer nanofibers with the designed QACOF. The QACOF could closely adhere to the PVDF–SEBS binary polymer nanofibers even when the PVDF–SEBS/1% QACOF membrane was immersed in 2 M KOH solution for 480 h. As a result, a fabricated single fuel cell equipped with the PVDF–SEBS/1% QACOF membrane exhibited the maximum power densities of 89.8 mW cm⁻² at 30 °C and 264.2 mW cm⁻² at 60 °C. In particular, reinforced hydroxide ion conduction and remarkable conductivity stability at subzero temperature were realized owing to the confinement of hydroxide ion conduction by the chemically inert PVDF–SEBS binary polymer nanofibers. For instance, the hydroxide conductivity of the PVDF–SEBS/1% QACOF membrane was 2.58 mS cm⁻¹ at −25 °C and 32.4 mS cm⁻¹ at 80 °C in a 480 h test.