Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe₂O₄ and Fe₃O₄

Abstract

Nanocrystalline MnFe2O4 has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been sparingly studied as highly ordered thin-film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe2O4 and Fe3O4 spinel oxide films were grown on (001)- and (111)-oriented Nb:SrTiO3 perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). High-resolution X-ray diffraction (HRXRD) and X-ray photoelectron spectroscopy (XPS) show the synthesis of pure phase materials, while scanning transmission electron microscopy (STEM) and reflection high-energy electron diffraction (RHEED) analysis demonstrate island-like growth of (111) surface-terminated pyramids on both (001)- and (111)-oriented substrates, consistent with the literature and attributed to the lattice mismatch between the spinel films and the perovskite substrate. Cyclic voltammograms under a N2 atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe2O4 and Fe3O4. Under an O2 atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, a diffusion-limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe2O4 reported in the literature where the diffusion-limited current is achieved with Mn-based catalysis. This difference is attributed to a low density of Mn surface termination, as determined by the integration of current from CVs collected under N2, in addition to low conductivity through the MnFe2O4 film due to the degree of inversion. Such low densities are attributed to the synthetic method and island-like growth pattern and highlight challenges in studying ORR catalysis with single-crystal spinel materials.

Nitrogen doped carbonaceous materials as platinum free cathode electrocatalysts for oxygen reduction reaction (ORR)

“…The two-electron route often produces a lower voltage than the four-electron pathway. Because the 4-electron pathway ORR mechanism converts oxygen more effectively and avoids the corrosive side products created by the 2-electron pathway, which can harm other fuel cell components, it is more advantageous in both acidic and alkaline situations [14,15]. Because of superior kinetics and lower overpotentials, the ORR in alkaline medium at ambient temperature is preferable to that in acidic conditions [16].…”

Synthesis and advantages of spinel-type composites

“…13d. 165 The distinct redox features for Mn and Fe surface termination were researched by comparing MnFe 2 O 4 and Fe 3 O 4 . The constructed electrodes were used as ORR catalysts in alkaline solutions, in which perovskite substrates were used due to their high conductivity.…”

Supercritical-Assisted Ball-Milling Synthesis of Multicomponent Si/Fe3O4/C Composites for Outstanding Lithium-Storage Capability

“…The hierarchical conductive network composed of folded graphite and CNTs not only promotes electron transfer but also relieves the volume expansion stress of Si. , However, there exist several problems such as mechanical fracture, poor interfacial combination, and low initial Coulombic efficiency in mostly reported Si/C composites. Adding another component is considered to be an effective method to improve the overall performance of Si/C composites. − It is revealed that iron oxide (Fe 3 O 4 , Fe 2 O 3 ) can buffer volume expansion and stabilize the solid electrolyte interface (SEI) film. , For example, Fe 2 O 3 coating on the Si surface promotes the formation of a uniform and dense SEI film and significantly improves the rate capability and storage capability. , …”

“…23−26 It is revealed that iron oxide (Fe 3 O 4 , Fe 2 O 3 ) can buffer volume expansion and stabilize the solid electrolyte interface (SEI) film. 27,28 For example, Fe 2 O 3 coating on the Si surface promotes the formation of a uniform and dense SEI film and significantly improves the rate capability and storage capability. 29,30 In this work, a new strategy to synthesize a silicon/iron oxide/carbon (Si/Fe 3 O 4 /C) composite is developed by ballmilling micron-Si powder, MCMB, and Prussian blue as an iron source in the scCO 2 fluid.…”