Room-Temperature Chemical Transformation Route to CuO Nanowires toward High-Performance Electrode Materials

We demonstrated an efficient room-temperature chemical transformation route to CuO nanowires (NWs), from irregular particles to NWs coupled with a series of phase changes from CuCl, through Cu2(OH)3Cl, to Cu­(OH)2, and finally to CuO. The room-temperature chemical transformation of Cu­(OH)2 NW can reserve the initial NW morphology and made the synthesized CuO NW more active in electrochemical reactions. As the anode materials for lithium ion battery, these CuO NWs can exhibit a reversible capacity of 696.1 mAh g–1 after 40 cycles at the rate of 100 mA g–1. The high lithium-storage capacity can be ascribed to the unique structure of these CuO NWs with size of ∼10 nm and grain boundaries on the NWs surfaces, which show more active for the initial electrochemical reaction. CuO NWs and intermediate Cu­(OH)2 NWs can also be fabricated as pseudocapacitor electrodes; in KOH electrolyte, their specific capacitances are 118 and 114 F g–1 at the current density of 1 A g–1. The present results indicate that the current room-temperature chemical transformation route is promising to produce advanced electrode materials for both lithium ion batteries and supercapacitors

High Energy Density Hybrid Supercapacitor: In-Situ Functionalization of Vanadium-Based Colloidal Cathode

A novel and creative in-situ electrochemical activation method to transform vanadium ions to highly electroactive colloidal cathode in KOH solution under electric field has been designed. After undergoing electrochemical reaction, the in-situ-functionalized vanadium-based colloidal cathode can adapt their geometrical structure to the high pseudocapacitive activity. The vanadium-based colloids//activated carbon asymmetric supercapcitor displays a high energy density of 50.4 Wh/kg at a power density of 250 W/kg, which is higher than most reported vanadium-based supercapacitors. The main advantage of this system is that the materials synthesis and the device operation are performed in the same reactive environment. The obtained vanadium-based colloids can display high V3+ cation utilization ratios of about 100% for one-electron redox reactions. The present results highlight a new area of research on in-situ formation of reactive electrode materials under realistic environments, which can bring new chemistry and new structures of materials that are only present under the current in-situ reactive conditions

Phase Transformation of Ce³⁺-Doped MnO₂ for Pseudocapacitive Electrode Materials

Doping is one of the important methods to modify the physical and chemical properties of functional materials, which can be used to synthesize mixed ionic and electronic conducting metal oxides. Herein, the phase transformation of MnO₂ from β- to α-phase has been proven by doping Ce³⁺ ions. With the increase of the amount of Ce³⁺ ions, the sizes of MnO₂ nanorods were first decreased to 10–20 nm, then increased to 70 nm. The capacitive performance indicated that the specific capacitance of Ce-doped MnO₂ electrode materials increased 10-fold compared with undoped MnO₂, while the charge transfer resistance of Ce-doped MnO₂ decreased. The present results show that rare earth ions can be used as a promising dopant to modify the crystallization behavior and electrochemical performance of MnO₂ electrode materials

Microwave–Hydrothermal Crystallization of Polymorphic MnO₂ for Electrochemical Energy Storage

We report a coupled microwave–hydrothermal process to crystallize polymorphs of MnO₂ such as α-, β-, and γ-phase samples with plate-, rod-, and wirelike shapes, by a controllable redox reaction in MnCl₂–KMnO₄ aqueous solution system. MnCl₂–KMnO₄ redox reaction system was for the first time applied to MnO₂ samples under the coupled microwave–hydrothermal conditions, which shows clear advantages such as shorter reaction time, well-crystallized polymorphic MnO₂, and good electrochemical performances as electrode materials for lithium ion batteries. For comparison, we also did separate reactions with hydrothermal only and microwave only in our designed MnCl₂–KMnO₄ aqueous system. The present results indicate that MnCl₂–KMnO₄ reaction system can selectively lead to α-, β-, and γ-phase MnO₂, and the as-crystallized MnO₂ samples can show interesting electrochemical performances for both lithium-ion batteries and supercapacitors. Electrochemical measurements show that the as-crystallized MnO₂ supercapacitors have Faradaic reactivity sequence α- > γ- > β-MnO₂ upon their tunnel structures, the intercalation–deintercalation reactivity of these MnO₂ cathodes follows the order γ- > α- > β-phase, and the conversion reactivity of these MnO₂ anodes follows the order γ- > α- > β-phase. MnCl₂–KMnO₄ reaction system can also lead to the mixed-phase MnO₂ (β- and γ-MnO₂), which can provide better anode performances for lithium-ion batteries. The current work deepens the fundamental understanding of several aspects of physical chemistry, for example, the chemical reaction controllable synthesis, crystal structure selection, electrochemical property improvement, and electrochemical reactivity, as well as their correlations