Preparation And Performances Of Novel Proton Conducting Membranes Based On Polyphosphazenes And Polyetheretherketone

Proton exchange membrane fuel cells?PEMFCs?are clean and high-efficiency devices.As the central component in PEMFCs,proton exchange membranes?PEMs?separate the reactant gases and conduct protons from the anode to the cathode.Sulfonated polyphosphazene materials have been extensively investigated as promising candidates for alternative PEMs materials owing to their low cost and low fuel permeability.In the past decades,the PEMs based on polyetheretherketone?PEEK?and derivatives have displayed the considerable promises for PEMFC applications due to its excellent mechanical properties,low cost and thermal properties as well as superior processability.In this article,three novel proton exchange membranes based on polyphosphazene and polyetheretherketone were designed and prepared.Firstly,a series of polyphosphazene-based membranes and corresponding composite membranes were prepared.Poly?dichlorophosphazene??PDCP?was prepared by ring opening polymerization of hexachlorocyclotriphosphazene?NPCl₂?₃ at 250oC.The macroinitiator was then prepared from PDCP by substitutions with 4-trifluoromethylphenol and 4-methylphenol and subsequent bromination with NBS.Poly[?4-trifluoromethylphenoxy??4-methylphenoxy?phosphazene]-g-poly{?styrene?_x-r-[4-?4-sulfobutyloxy?styrene]_y}?CF₃-PS_x-PSBOS_y?were synthesized via atom transfer radical polymerization?ATRP?,and corresponding membranes were then prepared from these polyphosphazene graft copolymers.The prepared membranes showed good thermal stability and oxidative stability.Among them,CF₃-PS₁₁-PSBOS₃₃3 and CF₃-PSBOS₄₅5 showed higher proton conductivity than that of Nafion 117.Further,another series of composite membranes CF₃-PS_x-PSBOS_y-SCNT were obtained by the incorporation of sulfonated single-walled carbon nanotubes?S-SWCNTs?into CF₃-PS_x-PSBOS_y.Compared with native membranes,the composite membranes exhibit ed higher proton conductivity but also significantly reduced methanol permeabili ty,suggesting a great enhancement effect of S-SWCNTs on the proton conduction and methanol resistance.Specifically,membranes CF₃-PS₁₁-PSBOS₃₃-SCNT and CF₃-PSBOS₄₅-SCNT showed proton conductivity at 0.46 S/cm and 0.55S/cm under fully hydrated conditions at 100oC,respectively,which were 2.2 to2.6 times as much as that of Nafion 117.Morphological observations by the means of transmission electron microscopy?TEM?and atomic force microscopy?AFM?showed clear phase-separated structures in the composite membranes,indicating the formation of effective ionic pathways for the proton transfer.Then a series of membranes based on sulfonated polyetheretherketone and sulfonated polyphosphazene-graft copolymers were prepared by cross-linking reaction because the former material had good enough mechanical property while the latter was excellent in the proton transfer.The cross-linked membranes combined the advantages of the two kinds of polymers.Among them,the membrane CF₃-PS₁₁-PSBOS₃₃-SPEEK75 showed a proton conductivity at 0.143 S/cm under fully hydrated conditions at 80°C and performed tensile strength about 5 times as much as did the sulfonated polyphosphazene membrane CF₃-PS₁₁-PSBOS₃₃.The maximum power density of the fuel cell with the membrane CF₃-PS₁₁-PSBOS₃₃-SPEEK75 was 294mW/cm² at 80°C.Further doping of S-SWCNTs into the cross-linked membrane on the screening of additives gave composite membrane CF₃-PS₁₁-PSBOS₃₃-SPEEK75-SCNT possessing proton conductivity of 0.196S/cm even higher than that of Nafion 117 and a tensile strength comparable to that of Nafion 117.Lastly,a novel series of hydrocarbon-based copolymers containing flexible alkylsulfonated groups and hydroxylated polyetheretherketone backbones were designed and prepared as proton conducting membranes.The copolymer of polystyrenes was synthesized via a radical polymerization of4-methylstyrene and 4-acetoxystyrene using 2,2-azobisisobutyronitrile?AIBN?as a radical initiator.Then,hydrolysis of acetoxyl moieties in the copolymer using hydrazine and subsequent sulfonation on the sites with1,4-butanesultone led to the sulfonated polystyrenes.The polymer hydroxylated polyetheretherketone?SPO?was prepared by reducing sulfonated polyetheretherketone with reducing agent of sodium borohydride.Sulfonatedpoly?etheretherketone?_x-poly{?4-methylstyrene?-r-[4-?4-sulfobutyloxy?styrene]}_y[SPO_x-?PMS-PSBOS?_y]were synthesized via the substitution reaction between the SPO and brominated polystyrenes.Corresponding membranes were then prepared.Among the membranes,the membrane SPO₃-?PMS-PSBOS?₂ showed good proton conductivity at 0.137S/cm at 80°C,which was 2 times as much as that of the control membrane SPO.Further,incorporating the sulfonated graphene oxide?s-GO?into SPO₃-?PMS-PSBOS?₂ led to the composite membrane SPO₃-?PMS-PSBOS?₂-SGO,which exhibited higher proton conductivity compared to Nation 117 and the native membrane SPO₃-?PMS-PSBOS?₂.In addition,the composite membrane SPO₃-?PMS-PSBOS?₂-SGO showed lower methanol permeability and higher selectivity compared with Nafion 117.These results suggested that these membranes are promising materials for PEMFC applications.