Central composite design as a tool to investigate the electrocatalytic activity of thermally treated nitrogen-doped graphene for the oxygen reduction reaction

Nitrogen-doped graphene (N-graphene) electrocatalysts prepared using a thermal treatment procedure were systematically investigated for the oxygen reduction reaction (ORR). A central composite experimental design (CCD) was adopted to evaluate the effect of the thermal treatment temperature (from 660 to 940 degrees C) and mass ratio of graphene and urea precursors (from 1:3 to 1:17) on the electron transfer number (n) of the ORR in alkaline medium. No interaction effect was observed between the two factors, and only temperature had a positive and statistically-significant effect on the n value. XPS results indicated that the improvement of the electrocatalytic activity of N-graphene with the increase of doping temperature cannot be associated with the N configuration or the overall N content, but is related to the formation of defects on graphene. This investigation represents an important step in the adoption of multivariate experimentation for the exploration of metal-free electrocatalysts for fuel cell cathodes.