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

High-Performance Sodium-Ion Pseudocapacitors Based on Hierarchically Porous Nanowire Composites

Electrical energy storage plays an increasingly important role in modern society. Current energy storage methods are highly dependent on lithium-ion energy storage devices, and the expanded use of these technologies is likely to affect existing lithium reserves. The abundance of sodium makes Na-ion-based devices very attractive as an alternative, sustainable energy storage system. However, electrodes based on transition-metal oxides often show slow kinetics and poor cycling stability, limiting their use as Na-ion-based energy storage devices. The present paper details a new direction for electrode architectures for Na-ion storage. Using a simple hydrothermal process, we synthesized interpenetrating porous networks consisting of layer-structured V₂O₅ nanowires and carbon nanotubes (CNTs). This type of architecture provides facile sodium insertion/extraction and fast electron transfer, enabling the fabrication of high-performance Na-ion pseudocapacitors with an organic electrolyte. Hybrid asymmetric capacitors incorporating the V₂O₅/CNT nanowire composites as the anode operated at a maximum voltage of 2.8 V and delivered a maximum energy of ∼40 Wh kg^–1, which is comparable to Li-ion-based asymmetric capacitors. The availability of capacitive storage based on Na-ion systems is an attractive, cost-effective alternative to Li-ion systems

Transition from Battery to Pseudocapacitor Behavior via Structural Water in Tungsten Oxide

The kinetics of energy storage in transition metal oxides are usually limited by solid-state diffusion, and the strategy most often utilized to improve their rate capability is to reduce ion diffusion distances by utilizing nanostructured materials. Here, another strategy for improving the kinetics of layered transition metal oxides by the presence of structural water is proposed. To investigate this strategy, the electrochemical energy storage behavior of a model hydrated layered oxide, WO₃·2H₂O, is compared with that of anhydrous WO₃ in an acidic electrolyte. It is found that the presence of structural water leads to a transition from battery-like behavior in the anhydrous WO₃ to ideally pseudocapacitive behavior in WO₃·2H₂O. As a result, WO₃·2H₂O exhibits significantly improved capacity retention and energy efficiency for proton storage over WO₃ at sweep rates as fast as 200 mV s^–1, corresponding to charge/discharge times of just a few seconds. Importantly, the energy storage of WO₃·2H₂O at such rates is nearly 100% efficient, unlike in the case of anhydrous WO₃. Pseudocapacitance in WO₃·2H₂O allows for high-mass loading electrodes (>3 mg cm^–2) and high areal capacitances (>0.25 F cm^–2 at 200 mV s^–1) with simple slurry-cast electrodes. These results demonstrate a new approach for developing pseudocapacitance in layered transition metal oxides for high-power energy storage, as well as the importance of energy efficiency as a metric of performance of pseudocapacitive materials

<i>In Situ</i> Transmission Electron Microscopy of Lead Dendrites and Lead Ions in Aqueous Solution

An ideal technique for observing nanoscale assembly would provide atomic-resolution images of both the products and the reactants in real time. Using a transmission electron microscope we image <i>in situ</i> the electrochemical deposition of lead from an aqueous solution of lead(II) nitrate. Both the lead deposits and the local Pb²⁺ concentration can be visualized. Depending on the rate of potential change and the potential history, lead deposits on the cathode in a structurally compact layer or in dendrites. In both cases the deposits can be removed and the process repeated. Asperities that persist through many plating and stripping cycles consistently nucleate larger dendrites. Quantitative digital image analysis reveals excellent correlation between changes in the Pb²⁺ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies

<i>In Situ</i> Transmission Electron Microscopy of Lead Dendrites and Lead Ions in Aqueous Solution

An ideal technique for observing nanoscale assembly would provide atomic-resolution images of both the products and the reactants in real time. Using a transmission electron microscope we image <i>in situ</i> the electrochemical deposition of lead from an aqueous solution of lead(II) nitrate. Both the lead deposits and the local Pb²⁺ concentration can be visualized. Depending on the rate of potential change and the potential history, lead deposits on the cathode in a structurally compact layer or in dendrites. In both cases the deposits can be removed and the process repeated. Asperities that persist through many plating and stripping cycles consistently nucleate larger dendrites. Quantitative digital image analysis reveals excellent correlation between changes in the Pb²⁺ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies

<i>In Situ</i> Transmission Electron Microscopy of Lead Dendrites and Lead Ions in Aqueous Solution

An ideal technique for observing nanoscale assembly would provide atomic-resolution images of both the products and the reactants in real time. Using a transmission electron microscope we image <i>in situ</i> the electrochemical deposition of lead from an aqueous solution of lead(II) nitrate. Both the lead deposits and the local Pb²⁺ concentration can be visualized. Depending on the rate of potential change and the potential history, lead deposits on the cathode in a structurally compact layer or in dendrites. In both cases the deposits can be removed and the process repeated. Asperities that persist through many plating and stripping cycles consistently nucleate larger dendrites. Quantitative digital image analysis reveals excellent correlation between changes in the Pb²⁺ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies

<i>In Situ</i> Transmission Electron Microscopy of Lead Dendrites and Lead Ions in Aqueous Solution

An ideal technique for observing nanoscale assembly would provide atomic-resolution images of both the products and the reactants in real time. Using a transmission electron microscope we image <i>in situ</i> the electrochemical deposition of lead from an aqueous solution of lead(II) nitrate. Both the lead deposits and the local Pb²⁺ concentration can be visualized. Depending on the rate of potential change and the potential history, lead deposits on the cathode in a structurally compact layer or in dendrites. In both cases the deposits can be removed and the process repeated. Asperities that persist through many plating and stripping cycles consistently nucleate larger dendrites. Quantitative digital image analysis reveals excellent correlation between changes in the Pb²⁺ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies

<i>In Situ</i> Transmission Electron Microscopy of Lead Dendrites and Lead Ions in Aqueous Solution

An ideal technique for observing nanoscale assembly would provide atomic-resolution images of both the products and the reactants in real time. Using a transmission electron microscope we image <i>in situ</i> the electrochemical deposition of lead from an aqueous solution of lead(II) nitrate. Both the lead deposits and the local Pb²⁺ concentration can be visualized. Depending on the rate of potential change and the potential history, lead deposits on the cathode in a structurally compact layer or in dendrites. In both cases the deposits can be removed and the process repeated. Asperities that persist through many plating and stripping cycles consistently nucleate larger dendrites. Quantitative digital image analysis reveals excellent correlation between changes in the Pb²⁺ concentration, the rate of lead deposition, and the current passed by the electrochemical cell. Real-time electron microscopy of dendritic growth dynamics and the associated local ionic concentrations can provide new insight into the functional electrochemistry of batteries and related energy storage technologies

<i>Operando</i> Atomic Force Microscopy Reveals Mechanics of Structural Water Driven Battery-to-Pseudocapacitor Transition

The presence of structural water in tungsten oxides leads to a transition in the energy storage mechanism from battery-type intercalation (limited by solid state diffusion) to pseudocapacitance (limited by surface kinetics). Here, we demonstrate that these electrochemical mechanisms are linked to the mechanical response of the materials during intercalation of protons and present a pathway to utilize the mechanical coupling for local studies of electrochemistry. <i>Operando</i> atomic force microscopy dilatometry is used to measure the deformation of redox-active energy storage materials and to link the local nanoscale deformation to the electrochemical redox process. This technique reveals that the local mechanical deformation of the hydrated tungsten oxide is smaller and more gradual than the anhydrous oxide and occurs without hysteresis during the intercalation and deintercalation processes. The ability of layered materials with confined structural water to minimize mechanical deformation likely contributes to their fast energy storage kinetics