Sulfonated Graphene-Nafion Composite Membranes for Polymer Electrolyte Fuel Cells Operating under Reduced Relative Humidity

Sulfonic acid-functionalized graphene (S-graphene) is employed as a promising inorganic filler as well as a solid acid proton conducting medium to realize a composite membrane with Nafion for polymer electrolyte fuel cell (PEFC) applications under reduced relative humidity (RH). The functionalization of graphene is performed by sulfonic acid-containing aryl radicals to increase the number of sulfonate groups per unit volume of a domain. A Nafion-S-graphene composite membrane is obtained by embedding S-graphene in Nafion, which provides high absorption of water and fast proton-transport across the electrolyte membrane under low RH values. The proton conductivity of the Nafion-S-graphene (1%) composite membrane at 20% RH is 17 mS cm^-1, which is five times higher than that of a pristine recast Nafion membrane. PEFCs incorporating the Nafion-S-graphene composite membrane deliver a peak power density of 300 mW cm^-2 at a load current density of 760 mA cm^-2 while operating at optimum temperature of 70 °C under 20% RH and ambient pressure. By contrast, operating under identical conditions, a peak power density of 220 mW cm^-2 is achieved with the pristine recast Nafion membrane. The Nafion-S-graphene composite membrane could be used to address many critical problems associated with commercial Nafion membranes in fuel cell applications.