Preparation And Characterization Of Polyphosphazenes For Proton Exchange Membrane

The proton exchange membrane(PEM) in fuel cells performs a number of critical functions. It separates the fuel and oxidant, and provides the pathway for the protons transfer. The general requirements of the ion exchange membrane in a proton exchange membrane fuel cell include high proton conductivity under cell operating conditions, long-term chemical and mechanical stability, low fuel crossover, and low cost. DuPont’s Nafion or other poly(perfluorosulfonic acid) membranes are widely studied as good fuel cell membranes, because of their high proton conductivity combined with oxidative and chemical stability. However, some drawbacks of Nafion hinder its widespread commercial use in proton exchange membrane fuel cells(PEMFCs), such as low proton conductivity at high temperature, high methanol crossover and high cost. Thus, the study of the new proton exchange membranes materials becomes a hot research.Polyphosphazenes are inorganic polymers, with a backbone consisting of alternating phosphorus and nitrogen atoms, with two side groups attached to each phosphorus, having high thermal stability and chemical stability. In addition, a very broad range of groups can be easily incorporated into the polyphosphazene chain to address the requirements of the polymer electrolyte membranes. Based on the development of polyphosphazenes, three series of proton exchang membranes were prepared based on polyphosphazene materials.Two series of graft copolymers with flexible alkylsulfonated side chains, poly[(4-methoxyphenoxy)(4-methylphenoxy) phosphazene]-g-poly{(styrene)x-r-[4-(4-sulfobutyloxy)styrene]y}(M-PSx-PSBOSy) and poly[(4-fluorophenoxy)(4-methylphenoxy)phosphazene]-g-poly{(styrene)x-r-[4-(4-sulfobut yloxy)styrene]y}(F-PSx-PSBOSy) were designed and synthesized. The proton exchange membranes were prepared using 2,6-Bis(hydroxymethyl)-4-methylphenol(BHMP) as the crosslinker in the presence of methanesulfonic acid. The obtained graft copolymer membranes had high proton conductivities of 0.284 S/cm at 80 oC, higher than Nafion 117(0.191 S/cm). The methanol permeability of the membranes were in the range of 1.60 to 10.4×10-7 cm2/s, which were lower than Nafion(15.8×10-7cm2/s). In addition, The obtained graft copolymer membranes had a high oxidative stability against Fenton’ s reagent, and good thermal stability.A series of polyphosphazene-based copolymers containing alkylsulfonated side chains were prepared and made into the corresponding membranes PFMPPx-r-PFSPPy by the reaction with 2,6-bis(hydroxymethyl)-4-methylphenol(BHMP) as the cross-linking reagent. All the PFMPPx-r-PFSPPy membranes showed good proton conductivity, lower water uptake and dimensional swelling. They displayed significant methanol resistance ranging from 1.35 to 7.18×10-7 cm2/s, which were lower than Nafion(15.8×10-7cm2/s). In addition the PFMPPx-r-PFSPPy membranes had high oxidative stability and good thermal stability.Poly(bis(phenoxy)phosphazene)(SPBPP)/phosphotungstic acid(PWA)/silica composite membranes for fuel cells were prepared. Incorporation of PWA particles and silica particles into the SPBPP polymer matrix had a specific interaction between the PWA and SPBPP. The composite membranes had high thermal stability. The doped membranes showed proton conductivity(0.123 S/cm) at room temperarure.