Electrochemical Impedance Spectroscopy of Metal Oxide Electrodes for Energy Applications

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.