Pseudocapacitive oxide materials for high-rate electrochemical energy storage

Electrochemical energy storage technology is based on devices capable of exhibiting high energy density (batteries) or high power density (electrochemical capacitors). There is a growing need, for current and near-future applications, where both high energy and high power densities are required in the same material. Pseudocapacitance, a faradaic process involving surface or near surface redox reactions, offers a means of achieving high energy density at high charge–discharge rates. Here, we focus on the pseudocapacitive properties of transition metal oxides. First, we introduce pseudocapacitance and describe its electrochemical features. Then, we review the most relevant pseudocapacitive materials in aqueous and non-aqueous electrolytes. The major challenges for pseudocapacitive materials along with a future outlook are detailed at the end

Mechanistic understanding of microstructure formation during synthesis of metal oxide/carbon nanocomposites

Nanocomposite materials consisting of metal oxide and carbon are of interest as electrode materials for both high rate intercalation-type and high capacity conversion-type charge storage processes. Facile synthesis processes like the pyrolysis of an organic carbon-source can yield a well-dispersed carbon phase within the metal oxide structure. Detailed understanding of the carbon formation process is required to tailor the resulting material microstructure. Herein, both the formation and the final microstructure of a molybdenum oxide/carbon nanocomposite are studied in detail. Octylamine assembled in the interlayer space of layered MoO3 serves as a carbon source. The structural changes during pyrolysis are characterized using a combination of in situ heating X-ray diffraction with simultaneous FTIR- and mass spectroscopy-coupled thermogravimetric analysis experiments. These reveal mobility and partial desorption of octylamine and interlayer water at low temperatures, octylamine decomposition and loss of long-range order at intermediate temperatures, and carbothermic reduction of molybdenum oxide at high temperatures during pyrolysis. The resulting nanocomposite mainly contains nanocrystalline MoO2 domains surrounded by a well-dispersed carbon phase, as observed with scanning transmission electron microscopy of focus-ion beam prepared cross-sectional lamellae. The electrochemical behavior is evaluated in organic, lithium-containing electrolyte for both intercalation and conversion-type reactions, showing good intercalation kinetics and a high first cycle efficiency for the conversion-type reaction

Electrochemical Kinetics of Nanostructured Nb2O5 Electrodes

Pseudocapacitive charge storage is based on faradaic charge-transfer reactions occurring at the surface or near-surface of redox-active materials. This property is of great interest for electrochemical capacitors because of the substantially higher capacitance obtainable as compared to traditional double-layer electrode processes. While high levels of pseudocapacitance have been obtained with nanoscale materials, the development of practical electrode structures that exhibit pseudocapacitive properties has been challenging. The present paper shows that electrodes of Nb2O5 successfully retain the pseudocapacitive properties of the corresponding nanoscale materials. For charging times as fast as one minute, there is no indication of semi-infinite diffusion limitations and specific capacitances of 380 F g−1 and 0.46 F cm−2 are obtained in 40-μm thick electrodes at a mean discharge potential of 1.5 V vs Li+/Li. In-situ X-ray diffraction shows that the high specific capacitance and power capabilities of Nb2O5 electrodes can be attributed to fast Li+ intercalation within specific planes in the orthorhombic structure. This intercalation pseudocapacitance charge-storage mechanism is characterized as being an intrinsic property of Nb2O5 that facilitates the design of electrodes for capacitive storage devices. We demonstrate the efficacy of these electrodes in a hybrid electrochemical cell whose energy density and power density surpass that of commercial carbon-based devices

Characterization of nanostructured materials for lithium-ion batteries and electrochemical capacitors

In this dissertation, nanostructured materials are examined for electrochemical energy storage devices with high energy and power densities. While previous research on nanostructured materials for energy storage has mostly focused on the effects of reduced dimensionality on diffusion distances, the research presented here demonstrates how nanostructuring can lead to new charge storage mechanisms. The first part of the dissertation describes the low-potential reactivity of V2O5 aerogels and how nanostructuring leads to significantly improved reversibility of the charge storage process. The second part details the rapid kinetic response of T-Nb2O5 and in addition, how the combination of nanostructure and appropriate crystalline structure leads to a mechanism called intercalation pseudocapacitance. The third part examines how a 2D nanosheet morphology changes both the redox potentials and kinetics of lithium ion storage in TiO2. These investigations underscore how reducing a material's dimensions and morphology leads to unique electrochemical behavior beyond simple decreasing of diffusion distances, and how such structures could lead to ultimately higher energy and power density electrochemical energy storage devices

How to Store Energy Fast

Representing the Molecularly Engineered Energy Materials (MEEM), this document is one of the entries in the Ten Hundred and One Word Challenge. As part of the challenge, the 46 Energy Frontier Research Centers were invited to represent their science in images, cartoons, photos, words and original paintings, but any descriptions or words could only use the 1000 most commonly used words in the English language, with the addition of one word important to each of the EFRCs and the mission of DOE energy. The mission of MEEM, using inexpensive custom-designed molecular building blocks, aims to create revolutionary new materials with self-assembled multi-scale architectures that will enable high performing energy generation and storage applications

Effects of Chemical versus Electrochemical Delithiation on the Oxygen Evolution Reaction Activity of Nickel-Rich Layered Li<i>M</i>O₂

Nickel-rich layered Li<i>M</i>O₂ (<i>M</i> = transition metal) oxides doped with iron exhibit high oxygen evolution reaction (OER) activity in alkaline electrolytes. The Li<i>M</i>O₂ oxides offer the possibility of investigating the influence of the number of d electrons on OER by tuning the oxidation state of <i>M</i> via chemical or electrochemical delithiation. Accordingly, we investigate here the electrocatalytic behavior of LiNi_0.7Co_0.3O₂ and LiNi_0.7Co_0.2Fe_0.1O₂ before and after chemical delithiation. In addition to varying the oxidation state of the transition-metal ions, we find that chemical delithiation also affects the local chemical environment and morphology. The electrochemical response differs depending on whether the delithiation occurred ex situ chemically or in situ during the electrocatalysis. The results point to the important role of in situ transformation in Li<i>M</i>O₂ in alkaline electrolytes during electrocatalytic cycling

Evidence of Localized Lithium Removal in Layered and Lithiated Spinel Li_1–<i>x</i>CoO₂ (0 ≤ <i>x</i> ≤ 0.9) under Oxygen Evolution Reaction Conditions

The electrocatalytic oxygen evolution reaction performance of various forms of lithium cobalt oxide has been studied to systematically establish the surface-level catalytic mechanism. The low-temperature lithiated spinel form of LiCoO₂ (designated as LT-LiCoO₂) exhibits lower overpotentials than the high-temperature layered form of LiCoO₂ (designated as HT-LiCoO₂), but this is shown to be a result of the increased surface area afforded by lower-temperature synthesis conditions. Raman spectroscopy, along with the presence of an irreversible peak during the first cycle of the oxygen evolution reaction (OER), demonstrates that the mechanism for OER is the same for both the forms of LiCoO₂. At the surface level, lithium is removed during the first cycle of the OER, forming Co₃O₄ on the surface, which is likely the active site during the OER. This work highlights the importance of determining the nature of the catalyst surface when investigating the electrocatalytic properties of bulk materials