PA-doped nanofiber composite proton exchange membranes with ultrahigh proton conductivity and methanol barrier performance for direct methanol fuel cells

Development of proton exchange membranes (PEMs) with low methanol permeability and high ionic conductivity is an effective way to improve the lifespan and power density of direct methanol fuel cells (DMFCs). Here, we report a novel phosphoric acid (PA)-doped quaternary ammonium ion-modified poly (vinylidene fluoride) nanofiber composite PEM (QA@PVDF/SPEEK-PA) for DMFCs. Benefiting from the rich acid-base ion-pair interface (–NH 2 /-N(CH 3 ) 3 + and −SO 3 H/H 2 PO 4 - ) constructed within the membrane, QA@PVDF/SPEEK-PA exhibited an ultrahigh conductivity of up to 202.31 mS cm −1 at 80 °C, corresponding to 2.6 and 1.31 times the conductivity of SPEEK (77.8 mS cm −1 ) and commercial Nafion 211 (154.03 mS cm −1 ), respectively. More importantly, the introduction of 3D nanofibers within the membrane together with the formation of acid-base interactions led to the excellent methanol barrier effect over a wide methanol range. Even with a 10 M CH 3 OH fuel, the cross-current density of methanol remained at a mere 161.21 mA cm −2 , while the open-circuit voltage (OCV) of the fuel cell reached an impressive 0.73 V. The DMFC assembled with the membrane exhibited a peak power density (PPD) of 121.38 mW cm −2 , representing a 1.36-fold relative to that of Nafion 211. After 260 h of in-situ durability test, the OCV was reduced by only 7.49 %. These excellent results suggest that the PA-doped nanofiber composite PEMs are promising candidates for future DMFCs applications.