Carbon Nanofiber-Supported Fe₃O₄ nanoestructures As Efficient Electrocatalyst for the Oxygen Reduction Reaction

Abstract

Fe 3 O 4 nanostructures were supported on carbon nanofiber synthesized at two different temperatures (600 and 700 °C) and tested as cathode catalyts for oxygen reduction reaction (ORR) in alkaline medium. Briefly, an iron oxide precursor, Fe(NO 3 ) 3 .9H 2 O, is impregnated in carbon nanofiber (CNF 600 and CNF 700), followed by heat-treatment at 400 ° C for 4 h under Ar. The structure and morphology of the catalyst was evaluated by x-ray diffraction, raman spectroscopy scanning electron microscopy and nitrogen sorption isotherms. The electrocatalytic activity towards ORR was investigated by cyclic and linear voltammetries in 0.1 mol.L -1 KOH. Analysis of polarization curves showed the electrocatalytic activity of the materials towards oxygen reduction expressed by on-set potentials which Fe 3 O 4 /CNF 600 (+0.68 V vs RHE) presents more positive potential than Fe 3 O 4 /CNF 700 (+0.65 V vs RHE) and mass activity at +0.55 V vs RHE with values of 25.4 A.g-1 (Fe 3 O 4 /CNF 600) and 14.4 A.g-1 (Fe 3 O 4 /CNF 700). Nevertheless, the variation of number of exchage electrons ( n ) and peroxide ion percentage (%H 2 O - ) with applied voltage indicate a direct oxygen reduction starts (4 electrons pathway) at more positive potential in Fe 3 O 4 /CNF 700 (+0.21 V vs RHE) than Fe 3 O 4 /CNF 600 (+0.12 V vs RHE). These results were obtained by indirect calculation using the Koutecky-Levich equation and confirmed by direct determination of ion peroxide intermediate formation with rotating-ring disk electrode measurements. Figure 1