  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.
