Electrospun multi-scale nanofiber network: hierarchical proton-conducting channels in Nafion composite proton exchange membranes

In this study, a unique multi-scale nanofiber membrane prepared by electrospinning with adding the tetrabutylammonium chloride (TBAC)  was applied to proton exchange membrane for direct methanol fuel cell. Three types of multi-scale nanofiber membranes of cellulose acetate (CA), nylon 6 (PA6) and poly-m-phenyleneisophthalamide (PMIA) were carefully selected as effective conductive fillers to be incorporated into Nafion as composite membranes (T-CA-Nafion, T-PA6-Nafion and T-PMIA-Nafion). At 80 °C, the proton conductivity of the multi-scale nanofiber composite membranes could reach 0.192 S cm⁻¹ (T-CA-Nafion), 0.287 S cm⁻¹ (T-PA6-Nafion) and 0.225 S cm⁻¹ (T-PMIA-Nafion), which were higher than that of the ordinary nanofiber composite membrane. At the same time, the methanol permeability was also significantly reduced. The above superiorities could be attributed to the following aspects: Firstly, the unique multi-scale nanofiber structure could provide hierarchically consecutive long-range channels for proton conducting. Meanwhile, the hydrophilicity of TBAC additives made the membrane with high water-absorbing capacity, which could be beneficial to provide more water molecule carriers for proton conduction via the Vehicle mechanism. Moreover, the cross-linked nanofiber network can be acted as barriers to further hinder methanol penetration. Specifically, the –NH (amido bonds in the PA6 and PMIA) groups could be interconnected with –SO₃H groups in Nafion matrix via electrostatic attractions, leading to the formation of effective –NH^…–SO₃H pairs in the composite membrane. The effective acid–base pairs can facilitate the proton hopping through Grotthuss mechanism, which also well illustrated the better proton conducting behavior of the T-PA6-Nafion and T-PMIA-Nafion membranes.