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.

Sulfonic acid functionalized graphene (S-graphene) is explored as a potential inorganic filler as well as a solid acid proton conducting medium to realize a hybrid membrane with Nafion for a direct methanol fuel cell (DMFC). The simple, but effective, functionalization of graphene is performed by sulfonic acid containing aryl radicals to increase the number of sulfonate groups per unit volume of graphene domain. Nafion−S-graphene hybrid membranes increase compactness of ionic domains and enhanced proton conductivity while restricting the methanol crossover across the membrane. DMFCs with a Nafion−S-graphene (1 wt %) hybrid membrane deliver a peak power density of 118 mW cm −2 at a load current density of 450 mA cm −2 while operating at 70 °C under an ambient pressure. By contrast, operating under identical conditions, a peak power density of 54 mW cm −2 at a load current density of 241 mA cm −2 is obtained with the pristine recast Nafion membrane. The Nafion−S-graphene hybrid membranes are extremely beneficial and useful for DMFCs in addressing many critical problems associated with commercial Nafion membranes.

Nafion-silica composite membranes are fabricated by embedding silica particles as inorganic fillers in perfluorosulfonic acid ionomer by a novel water hydrolysis process. The process precludes the use of an added acid but exploits the acidic characteristic of Nafion facilitating an in situ polymerization reaction through a sol-gel route. The use of Nafion as acid helps in forming silica/siloxane polymer within the membrane. The inorganic filler materials have high affinity to water and assist proton transport across the electrolyte membrane of the polymer electrolyte fuel cell ͑PEFC͒ even under low relative humidity ͑RH͒ conditions. In the present study, composite membranes have been tested in hydrogen/oxygen PEFCs at varying RH between 100 and 18% at elevated temperatures. Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy studies suggest an evenly distributed siloxane polymer with Si-OH and Si-O-Si network structures in the composite membrane. At the operational cell voltage of 0.4 V, the PEFC with an optimized silica-Nafion composite membrane delivers a peak power density value five times higher than that achievable with a PEFC with conventional Nafion-1135 membrane electrolyte while operating at a RH of 18% at atmospheric pressures. The polymer electrolyte fuel cell ͑PEFC͒ is an attractive power source for a variety of applications 1 due to its high efficiency and environment-friendly characteristics. The current PEFC technology utilizes perfluorosulfonic acid ͑PFSA͒ polymer membranes, e.g., Nafion, as electrolyte and hence is limited to low-temperature applications. In order to realize the optimum PEFC performance, the Nafion membrane needs to be fully wet as the proton conduction in Nafion relies on the dissociation of protons from the constituent SO 3 H groups in the presence of water.2 However, the performance of PEFCs is enhanced at elevated temperatures by improved kinetics of the cathode and anode reactions and the reduction in adsorption of poisoned species such as CO. [3][4][5][6] To this end, Nafion-composite membranes suitably modified with ceramic/inorganic fillers, namely SiO 2 , TiO 2 , ZrO 2 , etc., are widely used 7-13 to facilitate proton conductivity in the membranes at elevated temperatures even under low relative humidity ͑RH͒ conditions. Watanabe et al. 14 have employed modified Nafion membrane fabricated by incorporating nanosized particles of SiO 2 , TiO 2 , Pt, Pt-SiO 2 , and Pt-TiO 2 to alleviate the humidification requirements of PEFCs. When operated under low humidification, PEFCs with an alternative membrane reportedly exhibited lower ohmic drops in relation to Nafion. In situ platinum particulates with sorption of the water produced on the incorporated oxide fillers attribute such an improvement accompanied with suppression of hydrogen crossover. The benefits of these composite membranes appear to be in the steady operation of PEFCs at about 130°C due to the higher rigidity of the membranes in relation to commercial Nafion membra...

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