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 °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.