A novel tetradentate phosphonate ligand based Ni-MOF as a support for designing high performance proton conducting material.

Developing a robust metal-organic framework (MOF), which facilitates proton hoping along the pore channels is very demanding in the context of fabricating an efficient proton conducting membrane for the fuel cells. Herein, we first report the synthesis of a novel tetradentate aromatic phosphonate ligand H8L (L= tetraphenylethylene tetraphosphonic acid) based Ni-MOF, whose crystal structure has been solved from single crystal X-ray diffraction. Ni-MOF [Ni2(H4L)(H2O)9(C2H7SO)(C2H7NCO)] displayed monoclinic crystal structure with space group of P 21/c, a=11.887 A, b=34.148 A, c=11.131 A, α=γ=90o, β=103.374o, where a nickel-hexahydrate moiety located inside the void space of the framework through several H-bonding interactions. Post treatments of the Ni-MOF at different pH mediums as well as solvents the framework remained unaltered, suggesting the presence of strong H-bonding interactions in the framework. High framework stability of Ni-MOF bearing H-bonding interactions motivated us to explore this metal-organic framework material as the proton conducting media after external proton doping. Due to the presence of large number of H-bonding interactions and presence of water molecules in the framework we have carried out the doping of organic p-toluenesulfonic acid (PTSA) and inorganic sulphuric acid (SA) in this Ni-MOF and observed high proton conductivity of 5.28 × 10-2 S cm-1 at 90 °C and 98% relative humidity for SA doped material. Enhancement of proton conductivity by proton doping under humid conditions suggested a very promising feature of this Ni-MOF.