The ORR kinetics of ZIF-derived FeNC electrocatalysts

Abstract

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.