A new polyfunctional acid material for solid state proton conductivity in dry environment: Nafion doped with difluoromethandiphosphonic acid

A major application of polyfunctional acid materials is the fabrication of solid electrolytes exhibiting high proton conductivity in the solid state for operation in fuel cells at medium temperature (< 160 \ub0C) and low humidity (RH). A most recent trend of research in this field is the search of low vapour pressure doping agents designed to provide liquid-like proton transfer in perflurosulfonic acid membranes. A new polyfunctional acid material made of Nafion doped with difluoromethandiphosphonic (DFMDP) acid is reported in the present work. The DFMDP doping agent, melting at 146 \ub0C and stable up to 160 \ub0C, has been found to enhance by two orders of magnitude the conductivity of neat Nafion in dry environment at 120 \ub0C, and to be stable at this temperature even in the presence of 30% RH. The material chemistry responsible of the enhanced conductivity has been studied by several techniques: i.e., potentiometric titration (PT), scanning electron microscopy, wide (WAXS)- and small (SAXS)-angle X-ray scattering, and X-ray photoelectron spectroscopy (XPS). The results allow a number of conclusions. The PT data prove that a multilevel proton acidity wide gradient is present in the DFMDP doped Nafion samples constituting the driving force for proton mobility through the membrane phase. The WAXS and SAXS data prove that the DFMDP molecules are incorporated into the Nafion SO3H ionic domains. The XPS data provide direct evidence of the interaction between the DFMDP PO3H2 and Nafion SO3H groups which allows the proposition of the establishment of H-bonding network between the proton acceptor P = O and P\u2013OH sites and the proton donor SO3H group. The conductivity data for the DFMDP membranes, compared to other data obtained for membranes made by other sulfonated polymers doped with H3PO4, offer intriguing perspectives for the design of solid electrolytes exhibiting enhanced conductivity in dry or low humidity environment