Jing Li

Although sulfonic acid (SA)-based proton-exchange membranes (PEMs) dominate fuel cell applications at low temperature, while sulfonation on polymers would strongly decay the mechanical stability limit the applicable at elevated temperatures due to the strong dependence of proton conduction of SA on water. For the purpose of bifunctionally improving mechanical property and high-temperature performance, Nafion membrane, which is a commercial SA-based PEM, is composited with fabricated silica nanofibers with a three-dimensional network structure via electrospinning by considering the excellent water retention capacity of silica. The proton conductivity of the silica nanofiber–Nafion composite membrane at 110 °C is therefore almost doubled compared with that of a pristine Nafion membrane, while the mechanical stability of the composite Nafion membrane is enhanced by 44%. As a result, the fuel cell performance of the silica nanofiber-Nafion composite membrane measured at high temperature and low humidity is improved by 38%.

Herein, a modified in situ sol–gel method was applied to prepare the Nafion/silica composite membrane with targeted filling of silica into ionic clusters. The low humidity (20–60% RH) proton conductivity of Nafion was therefore doubled at elevated temperatures (110–120 °C), and the high-temperature fuel cell performance was significantly improved by 45%.

A hyper-branched sulfonated polyimide (s-PI) was synthesized successfully and composited with polyvinylidene fluoride (PVDF) to achieve ultra-high methanol-permeation resistive for direct methanol fuel cell application. The optimized s-PI-PVDF composite membrane exhibited methanol resistivity low to 1.80 × 10⁻⁸ cm²/s, two orders of magnitude lower than the value of the commercial Nafion 117 membrane (60 × 10⁻⁷ cm²/s). At the same time, the tensile strength of the composite membrane is 22 MPa, which is comparable to the value of the Nafion 117 membrane. Therefore, the composite membrane is promising for application in direct methanol fuel cell.

Based on a previously developed polyamide proton conductive macromolecule, the nano-scale structure of the self-assembled proton conductive channels (PCCs) is adjusted via enlarging the nano-scale pore size within the macromolecules. Hyperbranched polyamide macromolecules with different size are synthesized from different monomers to tune the nano-scale pore size within the macromolecules, and a series of hybrid membranes are prepared from these two micromoles to optimize the PCC structure in the proton exchange membrane. The optimized membrane exhibits methanol permeability low to 2.2 × 10⁻⁷ cm²/s, while the proton conductivity of the hybrid membrane can reach 0.25 S/cm at 80 °C, which was much higher than the value of the Nafion 117 membrane (0.192 S/cm). By considering the mechanical, dimensional, and the thermal properties, the hybrid hyperbranched polyamide proton exchange membrane (PEM) exhibits promising application potential in direct methanol fuel cells (DMFC).

High H⁺/V selectivity was achieved for the ion exchange membrane due to specially designed hyperbranching structure and intermolecular force.