A Tetradentate Phosphonate Ligand-based Ni-MOF as a Support for Designing High-performance Proton-conducting Materials.

Developing a robust metal-organic framework (MOF) which facilitates proton hopping along the pore channels is very demanding in the context of fabricating an efficient proton-conducting membrane for fuel cells. Herein, we report the synthesis of a novel tetradentate aromatic phosphonate ligand H8 L (L=tetraphenylethylene tetraphosphonic acid) based Ni-MOF, whose crystal structure has been solved from single-crystal X-ray diffraction. Ni-MOF [Ni2 (H4 L)(H2 O)9 (C2 H7 SO)(C2 H7 NCO)] displays a monoclinic crystal structure with a space group of P 21 /c, a=11.887 Å, b=34.148 Å, c=11.131 Å, α=γ=90°, ß=103.374°, where a nickel-hexahydrate moiety located inside the void space of the framework through several H-bonding interactions. Upon treatment of the Ni-MOF in different pH media 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 proton-conducting medium after external proton doping. Due to the presence of a large number of H-bonding interactions and the 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 the SA-doped material. Enhancement of proton conductivity by proton doping under humid conditions suggested a very promising feature of this Ni-MOF.