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 synthesis of nanocrystalline, p-type delafossite metal oxides (CuMO2) via hydrothermal methods has been explored for a variety of energy conversion and storage applications. However, isolation of the pure phase ternary product is challenging due to the facile growth of unwanted, binary byproducts (CuO, Cu2O, and M2O3) which could ultimately influence the optoelectronic properties of the resulting nanocrystals. Here, we report on the optimized hydrothermal synthesis of CuCrO2 nanocrystals to limit the production of such byproducts. This material possesses a wide band gap and high reported conductivity, making it attractive for applications as the hole transport layer in a variety of heterojunction solar cells. An important aspect of this work is the consideration of Cr3+ as the reductant used to reduce Cu2+ to Cu+. This was confirmed by detection and quantification of CrO4 2– as a product of hydrothermal synthesis in addition to the fact that CuCrO2 purity was maximized at a ratio of 4:3 Cr/Cu, consistent with the proposed stoichiometric reaction: 4Cr3+ + 3Cu2+ + 20 OH– → 3CuCrO2 + CrO4 2– + 10 H2O. Using a 4:3 ratio of Cr/Cu starting materials and allowing the synthesis to proceed for 60 h eliminates the presence of CuO beyond detection by powder X-ray diffraction (pXRD). Furthermore, washing the solid product in 0.5 M NH4OH removes Cu2O and Cr2O3 impurities, leaving behind the isolated CuCrO2 product as confirmed using pXRD and inductively coupled plasma mass spectrometry.

The reactions of simple oxides or halides of trivalent lanthanides and actinides or bismuth with boric acid in the ionic liquid 1-butyl-3-methylimidazolium chloride at 150 °C result in the formation and crystallization of a series of isomorphous tetranuclear borate clusters with the general formula M4B22O36(OH)6(H2O)13 (M = La, Ce, Pr, Nd, Sm, Eu, Gd, Pu, and Bi). These clusters do not assemble with trivalent cations smaller than Gd(3+), suggesting that the formation of the clusters is dictated by the size of the metal ion. The cations are found in cavities along the periphery of a cage assembled from the corner- and edge-sharing interactions of BO3 triangles and BO4 tetrahedra, yielding a complex chiral cluster. Both enantiomers cocrystallize. The metal ions are nonacoordinate, and their geometries are best described as distorted tridiminished icosahedra. This coordination environment is new for both Pu(3+) and Bi(3+). In addition to detailed structural information, UV/vis-NIR absorption and photoluminescence spectra are also provided.

CuCrO2 nanocrystals were synthesized and fabricated into mesoporous thin films to study their electrochemical properties, where a strong [Li+] dependence was observed. An anodic shift in the Cu2+/+ redox potential was observed with increased [Li+] in the electrolyte, in addition to the growth of a new redox feature at E1/2=−0.43 V vs Fc+/0. This new feature was attributed to Cu2+/+ redox chemistry accompanied by Li+ occupation in copper vacancy surface defects. The equilibrium constant and maximum charge for Li+ occupation were determined to be K=0.057 M−1 and 15.5 mC, respectively. The maximum charge was close to the expected value of 11 mC based on the measured concentration of copper vacancies. The pronounced Li+ dependent electrochemistry suggests that CuCrO2 behaves similarly to cathodes in Li‐ion batteries. Thus, chronopotentiometry experiments revealed a 7.8 mA h g−1 charge capacity at cycle 2 and increasing cycling efficiency from 83 to 91 % over 10 cycles. However, a pronounced decrease in charge capacity was observed with increased cycles, attributed to a loss in lithium‐coupled electrochemistry. These studies add to our understanding of surface defects in p‐type oxides and their effect on hole recombination in solar cell devices.

The Cover Feature illustrates lithium‐coupled redox reactions between Cu2+ and Cu+ at the surface of CuCrO2 delafossite nanocrystals. The lithium‐coupled redox activity is the result of copper vacancy defects at the nanocrystal surface where Li‐ions occupy vacant sites in the reduced state and return to solution in the oxidized state. CuCrO2 nanocrystals are used as hole transport materials in a wide array of solar cell technologies where the existence and reactivity of these surface defects can have wide ranging effects on solar cell performance. More information can be found in the Research Article by A. L. Chown et al.