Advancement of PVDF and its copolymer-based proton exchange membranes for direct methanol fuel cells: A review

Direct methanol is a promising portable electronic device that converts chemical energy into electrical energy by using easily available and environmentally friendly methanol fuel. Among its different components, the proton exchange membrane (PEM) is responsible for continuing the flow of hydration ions from anode to cathode, protecting from fuel crossover and keeping the electrons pushing towards the external circuit, which produces power. The Nafion-based proton exchange membrane is widely studied in direct methanol fuel cells because of its easy wettability, high ion exchange capacity, minimum methanol crossover, and mechanical strength due to its nono-porous structure. However, the methanol crossover increases in the Nafion membrane due to easy swelling when temperature raise due to reactions in the direct methanol fuel cells (DMFC) and dramatically reduces overall cell performance. Several types of polymers have been applied as PEMs for the replacement of costly Nafion membranes. Among these polymers, polyvinylidene fluoride (PVDF) and its copolymers such as polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) and poly (vinylidene fluoride-chlorotrifluoroethylene) (PVDF-CTFE) are considered effective for the production of high-class proton exchange membranes because of their less swollen nature, easy film forming ability, high mechanical strength, thermal stability and chemical inertness. Their easy modification with other hydrophilic polymers and nanoparticles as well as surface functionalization convert their hydrophobic nature that boosts proton conductivity. Moreover, the nano-porous structure with tortuous paths create with PVDF and copolymers efficiently reduces the methanol crossover. From high-crystalline PVDF to reduced-crystalline copolymers, effects on mechanical strength and water uptake of the proton exchange membrane. This review article presents the overall application of all PVDF family polymers and their modified forms as proton exchange membranes in direct methanol fuel cells.