Investigation of Nafion®/HPA composite membranes for high temperature/low relative humidity PEMFC operation

Nafion®-based organic/inorganic composite membranes with different inorganic heteropolyacid (HPA) additives such as phosphotungstic acid, silicotungstic acid phosphomolybdic acid and silicomolybdic acid were investigated for high temperature (120 °C) low relative humidity (35%) operation. A mechanism is proposed to explain possible pathways for conductivity enhancement in the composite membrane. The decomposition temperature of the composite membrane was extended by close to 150 °C by a suitable ion exchange treatment, permitting more stringent heat treatment. The additive particle size in the composite membrane was on the order of 1–10 μm. The conductivity of the composite membranes at 120 °C and 35% relative humidity were on the order of 0.015 S/cm. All membranes were evaluated at high temperatures and low relative humidities in an operating fuel cell. The membranes demonstrated low crossover currents of approximately 1 mA/cm2. The membrane electrode assemblies (MEAs) made using membranes containing molybdenum-based HPA additives were electrochemically unstable due to redox transitions of the additive in the fuel cell operating range. These MEAs also showed poorer performance at all conditions as evidenced by polarization results.