The ORR kinetics of ZIF-derived Fe N C electrocatalysts

FeNC oxygen reduction reaction (ORR) catalysts, based on metal organic frameworks (MOFs) with highly-dispersed active sites embedded in porous carbon matrix, are presently regarded as the most promising noble metal-free ORR catalysts in acidic electrolyte. However, the detailed and specific ORR kinetics of the FeNC catalysts in several successive steps of oxygen reduction, still remains unclear. In this study, a series of zeolitic imidazolate frameworks (ZIF)-derived and Fe-coordinated catalysts (Fe-ZIF-8) were prepared and investigated in much details of half-wave potentials, ring currents, H2O2 yields and apparent coverage ratios on catalyst surfaces, hydrogen peroxide reduction reaction (HPRR) activities and diffusion-limited current plateaus, in comparison to benchmark Pt/C catalyst. Specially, through intentionally introducing varied concentrations of H2O2 as the probe molecule into the acidic electrolyte during ORR tests, we verified an interesting phenomenon that relatively lower ORR catalytic activity of Fe-ZIF-8 is not associated with the over-production of intermediate H2O2 as compared to Pt, and in fact, Fe-ZIF-8 produced significantly lower H2O2 amount than that by Pt in the ORR. Based on the above studies and findings, this work attempts to clarify the possible underlying kinetics in ORR by using Fe-ZIF-8 as the ORR electro-catalysts. It is concluded that, the oxygen-involved kinetics, such as oxygen molecule adsorption, activation and reduction into H2O2 in the first 2e step, rather than the proton-related reactions such as superoxide anion protonation in the first 2e step, nor the HPRR in the second 2e step, is kinetically sluggish on Fe-ZIF-8, in comparison with Pt/C.