Metal oxides have been of great importance to the development of energy conversion and storage technologies including heterojunction solar cells, Li-ion batteries, and electrocatalysts/photocatalysts for water splitting and CO2 reduction. The role of metal oxides in these devices has been diverse, from charge transport layers to catalytic surfaces to protective blocking layers. Understanding the fundamental structural and electronic properties of these materials will continue to allow for advancement in the field of renewable energy. Electrochemical impedance spectroscopy (EIS) is one of the most utilized methods to characterize these electrodes in the context of energy applications. The utility of EIS stems from its ability to differentiate multiple interfaces (i.e., solid/electrolyte, solid/solid) within devices on the basis of their frequency response to a modulated potential and the subsequent decoupling of resistive and capacitive circuit components. In this review, the fundamental theory of EIS is first described with a physical and mathematical basis, followed by a discussion of equivalent circuit modeling. The review then covers examples from the literature where EIS has been particularly important in the understanding of electronic properties related to metal oxide electrodes within energy conversion and storage devices. A specific focus is placed on metal oxides that are used as heterojunction solar cells, ion batteries, and photocatalysts/electrocatalysts. Common themes are discussed within each application such as the study of electron and hole diffusion in solar cells, the dependence of recombination reactions and catalysis on surface defect/trap states for solar cells and photocatalysts, and the formation of passivation layers at the solid electrolyte interface in Li-ion batteries.

The unique redox cycle of NiII(dtc)2, where dtc– is N,N-diethyldithiocarbamate, in acetonitrile displays 2e– redox chemistry upon oxidation from NiII(dtc)2 → [NiIV(dtc)3]+ but 1e– redox chemistry upon reduction from [NiIV(dtc)3]+ → NiIII(dtc)3 → NiII(dtc)2. The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate NiII(dtc)2 and six-coordinate [NiIV(dtc)3]+. Cyclic voltammetry (CV) experiments show that these 1e– and 2e– pathways can be controlled by the addition of pyridine-based ligands (L) to the electrolyte solution. Specifically, the addition of these ligands resulted in a 1e– ligand-coupled electron transfer (LCET) redox wave, which produced a mixture of pyridine-bound Ni­(III) complexes, [NiIII(dtc)2(L)]+, and [NiIII(dtc)2(L)2]+. Although the complexes could not be isolated, electron paramagnetic resonance (EPR) measurements using a chemical oxidant in the presence of 4-methoxypyridine confirmed the formation of trans-[NiIII(dtc)2(L)2]+. Density functional theory calculations were also used to support the formation of pyridine coordinated Ni­(III) complexes through structural optimization and calculation of EPR parameters. The reversibility of the LCET process was found to be dependent on both the basicity of the pyridine ligand and the scan rate of the CV experiment. For strongly basic pyridines (e.g., 4-methoxypyridine) and/or fast scan rates, high reversibility was achieved, allowing [NiIII(dtc)2(L) x ]+ to be reduced directly back to NiII(dtc)2 + xL. For weakly basic pyridines (e.g., 3-bromopyridine) and/or slow scan rates, [NiIII(dtc)2(L) x ]+ decayed irreversibly to form [NiIV(dtc)3]+. Detailed kinetics studies using CV reveal that [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ decay by parallel pathways due to a small equilibrium between the two species. The rate constants for ligand dissociation ([NiIII(dtc)2(L)2]+ → [NiIII(dtc)2(L)]+ + L) along with decomposition of [NiIII(dtc)2(L)]+ and [NiIII(dtc)2(L)2]+ species were found to increase with the electron-withdrawing character of the pyridine ligand, indicating pyridine dissociation is likely the rate-limiting step for decomposition of these complexes. These studies establish a general trend for kinetically trapping 1e– intermediates along a 2e– oxidation path.

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.

Transition metal oxides have long been an area of interest for water electrocatalysis through the oxygen evolution and oxygen reduction reactions. Iron oxides, such as LaFeO3, are particularly promising due...

This manuscript describes electrochemical experiments in which a Nickel(III) intermediate is kinetically trapped by the addition of derivatized pyridine ligands to the electrolyte solution. EPR and DFT studies support the conclusion that pyridine coordinates in a trans- configuration and contains a small equilibrium between mono- and bis-pyridine structures. Electrochemical kinetic data provides evidences for decomposition pathways which ultimately result in Ni(IV) complexes.