Oxygen Reduction Electrocatalysis with Epitaxially Grown Spinel MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$
Nanocrystalline MnFe$_{2}$O$_{4}$ has shown promise as a catalyst for the oxygen reduction reaction (ORR) in alkaline solutions, but the material has been lightly studied as highly ordered thin film catalysts. To examine the role of surface termination and Mn and Fe site occupancy, epitaxial MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$ spinel oxide films were grown on (001) and (111) oriented Nb:SrTiO$_{3}$ perovskite substrates using molecular beam epitaxy and studied as electrocatalysts for the oxygen reduction reaction (ORR). HRXRD and XPS show synthesis of pure phase materials while STEM and 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 lattice mismatch between the spinel films and perovskite substrate. Cyclic voltammograms under an N$_{2}$ atmosphere revealed distinct redox features for Mn and Fe surface termination based on comparison of MnFe$_{2}$O$_{4}$ and Fe$_{3}$O$_{4}$. Under O$_{2}$ atmosphere, electrocatalytic reduction of oxygen was observed at both Mn and Fe redox features; however, diffusion limited current was only achieved at potentials consistent with Fe reduction. This result contrasts with that of nanocrystalline MnFe$_{2}$O$_{4}$ reported in the literature where 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 N$_{2}$, in addition to low conductivity through the MnFe$_{2}$O$_{4}$ 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.