Non-Precious Metal Catalysts for Oxygen Reduction in PEM Fuel Cells

Metal-free carbon-based catalysts for oxygen reduction is synthesized with high activity selectivity and durability. The catalysts were synthesized by introducing oxygen and nitrogen groups from various oxygen and nitrogen precursors. The nature of nitrogen surface groups and the effect of pyrolysis temperature on the activity of the catalyst have been evaluated. XPS indicates that higher concentration of pyridinic type nitrogen groups doped on nanostructures graphitic carbon increase the catalysts activity. The metal-free catalysts showed an onset potential close to 0.9 V (NHE) in 0.5 M H2SO4 solution, similar to that of Pt. The amount of hydrogen peroxide (H2O2) generated was less than 1 % indicating a complete four-electron reduction of oxygen to water. The fuel cell performance indicated a current density of 0.49 A/cm2 at 0.5 V (catalyst loading of 2mg/cm2) and 2.0 A/cm2 at 0.2 V with a maximum power density of 0.49 W/cm2. The nature of surface nitrogen groups has been examined. The XPS characterization studies indicated that on the carbon surface, only pyridinic and graphitic nitrogens act as catalytic sites for oxygen reduction: particularly, pyridinic nitrogen, which possesses one lone pair of electrons in addition to the one electron donated to the conjugated bond, facilitates the reductive oxygen adsorption and eliminates H2O2 formation. Currently, our goal is to increase the catalyst activity by increasing the concentration of pyridinic and quaternary nitrogen on nanostructured carbon surface. More studies should be performed to decrease the hydrophilic character of the catalyst and to increase its durability.