Fe-based catalysts for oxygen reduction in proton exchange membrane fuel cells with cyanamide as nitrogen precursor and/or pore-filler

Fe/N/C catalysts for oxygen reduction reaction were synthesized via impregnation or ballmilling. The role of cyanamide (CM) as nitrogen precursor and/or pore-filler for a highly microporous carbon (Black Pearls 2000) was investigated. The use of CM in this work resulted in two main differences compared with phenanthroline from our previous work; (i) ballmilling the precursors did not result in improved activity of the resulting catalysts, and (ii) the activity after the first pyrolysis in argon was relatively high, but did not increase after a second pyrolysis in NH₃. These differences may be explained by TGA measurements of both pore-fillers, where complete gasification of CM is observed at temperatures above 750 °C in Ar, while pyrolysis of phenanthroline in Ar results in 20 wt% residual carbon-based material. Consequently, when using CM as pore-filler with a highly microporous carbon support, the maximum microporous surface area and nitrogen content is reached after only a single pyrolysis in Ar. The most active catalyst prepared with CM was obtained by pyrolysing in Ar at 950 °C a catalyst precursor containing 1 wt% Fe, 80 wt% CM and Black Pearls 2000. This catalyst possessed about 1/6th the catalytic activity of best reported using phenanthroline as a pore-filler. Changing the carbon support had effects on the activity and stability of the catalysts. The catalysts made with a non-porous furnace black (N330) or carbon nanotubes as a carbon support were more stable but less performing than those using carbon supports having high microporous surface area like Black Pearls 2000 or Ketjenblack. The desirable properties for a pore-filler molecule used in the synthesis of Fe/N/C-catalysts by the pore-filling method are discussed.