PH dependence of catalytic activity for ORR of the non-PGM catalyst derived from heat-treated Fe-phenanthroline

The effect of pH on the ORR efficiency of catalysts derived from heat-treated Fe-phenanthroline is analyzed using a rotating ring-disc electrode (RRDE). The activity of pyrolyzed Fe-phenanthroline catalysts was tested in electrolyte solutions with pHs ranging from 1 to 13.7. Fe-phenanthroline has a half-wave potential that remains steady at 0.7 V from pH 1 to pH 7 and linearly increases from pH 7 to the maximum 0.9 V at pH 13.7. The percent hydrogen peroxide detected on the ring was below 5% over all pHs at steady state potentials. The RRDE results were analyzed using Koutecky-Levich and charge/mass balance methods. The kinetic current (i_k) linearly decreases from pH 1 to pH 7 where it reaches a minimum and linearly increases from pH 7 to 13.7 indicating that the ORR reaction with OH^- proceeds slower than with H⁺ as the substrate. The number of electrons transferred decreases from 3.77 ± 0.09 (pH 1) to 2.38 ± 0.11 (pH 13.7) electrons according to the Koutecky-Levich analysis. The charge/mass balance analysis yielded the number of electrons transferred to be just under 4 electrons from pH 1 to pH 12, with a sharp decrease from pH 12 to pH 13.7. The results from the E_1/2 and Koutecky-Levich analysis, when taken together, indicate a mechanism shift taking place at pH 7 resulting from changes in the double-layer structure and the reaction mechanisms.