Enhanced proton conductivities of nanofibrous composite membranes enabled by acid–base pairs under hydrated and anhydrous conditions

• Nanofibrous composite membranes are prepared by SPEEK nanofiber mat and CS polymer.
• The bicontinuous phases donate two distinct continuous pathways for proton transfer.
• Interfacial interactions enrich and assemble acid–base pairs onto nanofiber surface.
• The membranes achieve much higher conductivity at hydrated and anhydrous conditions.
• The membranes display enhanced thermal and structural stabilities.

Herein, a series of nanofibrous composite membranes (NFCMs) are designed and prepared by incorporating chitosan (CS) matrix into electrospun sulfonated poly(ether ether ketone) (SPEEK) nanofiber mats with controllable structures. Fourier transform infrared results suggest that the −SO3H groups of SPEEK and the −NH2 groups of CS assemble into acid–base pairs along the nanofiber surface. The electrostatic attractions within the pairs inhibit the chain mobility of CS and SPEEK, endowing NFCMs with reinforced thermal and structural stabilities. Besides, the attractions drive the enrichment of acid/base groups near the nanofiber surface, thus providing more proton-hopping sites in the perpendicular direction of NFCMs. Together with the conducting groups in the amorphous CS phase, these sites donate significant enhancement in proton conduction via a low-energy-barrier manner. Particularly, a hydrated conductivity of 0.153 S cm−1 is achieved by the NFCM, much higher than those of CS (0.024 S cm−1) and SPEEK (0.037 S cm−1) casting membranes. Meanwhile, the formed acid–base pairs display unique anhydrous transfer ability, affording the NFCM a high anhydrous conductivity (59.6 mS cm−1) at 120 °C. Moreover, the influence of sulfonation degree and diameter of SPEEK nanofiber on proton conductivity is systematically investigated.

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