Hybrid Composite Ni(OH)₂@NiCo₂O₄ Grown on Carbon Fiber Paper for High-Performance Supercapacitors

We have successfully fabricated and tested the electrochemical performance of supercapacitor electrodes consisting of Ni­(OH)₂ nanosheets coated on NiCo₂O₄ nanosheets grown on carbon fiber paper (CFP) current collectors. When the NiCo₂O₄ nanosheets are replaced by Co₃O₄ nanosheets, however, the energy and power density as well as the rate capability of the electrodes are significantly reduced, most likely due to the lower conductivity of Co₃O₄ than that of NiCo₂O_4. The 3D hybrid composite Ni­(OH)₂/NiCo₂O₄/CFP electrodes demonstrate a high areal capacitance of 5.2 F/cm² at a cycling current density of 2 mA/cm², with a capacitance retention of 79% as the cycling current density was increased from 2 to 50 mA/cm². The remarkable performance of these hybrid composite electrodes implies that supercapacitors based on them have potential for many practical applications

Unraveling the Nature of Anomalously Fast Energy Storage in T‑Nb₂O₅

While T-Nb₂O₅ has been frequently reported to display an exceptionally fast rate of Li-ion storage (similar to a capacitor), the detailed mechanism of the energy storage process is yet to be unraveled. Here we report our findings in probing the nature of the ultrafast Li-ion storage in T-Nb₂O₅ using both experimental and computational approaches. Experimentally, we used <i>in operando</i> Raman spectroscopy performed on a well-designed model cell to systematically characterize the dynamic evolution of vibrational band groups of T-Nb₂O₅ upon insertion and extraction of Li ions during repeated cycling. Theoretically, our model shows that Li ions are located at the loosely packed 4g atomic layers and prefer to form bridging coordination with the oxygens in the densely packed 4h atomic layers. The atomic arrangement of T-Nb₂O₅ determines the unique Li-ion diffusion path topologies, which allow direct Li-ion transport between bridging sites with very low steric hindrance. The proposed model was validated by computational and experimental vibrational analyses. A comprehensive comparison between T-Nb₂O₅ and other important intercalation-type Li-ion battery materials reveals the key structural features that lead to the exceptionally fast kinetics of T-Nb₂O₅ and the cruciality of atomic arrangements for designing a new generation of Li-ion conduction and storage materials

Probing the Charge Storage Mechanism of a Pseudocapacitive MnO₂ Electrode Using <i>in Operando</i> Raman Spectroscopy

While manganese oxide (MnO₂) has been extensively studied as an electrode material for pseudocapacitors, a clear understanding of its charge storage mechanism is still lacking. Here we report our findings in probing the structural changes of a thin-film model MnO₂ electrode during cycling using <i>in operando</i> Raman spectroscopy. The spectral features (e.g., band position, intensity, and width) are correlated quantitatively with the size (Li⁺, Na⁺, and K⁺) of cations in different electrolytes and with the degree of discharge to gain better understanding of the cation-incorporation mechanism into the interlayers of pseudocapacitive MnO₂. Also, theoretical calculations of phonon energy associated with the models of interlayer cation-incorporated MnO₂ agree with the experimental observations of cation-size effect on the positions of Raman bands. Furthermore, the cation-size effects on spectral features at different potentials of MnO₂ electrode are correlated quantitatively with the amount of charge stored in the MnO₂ electrode. The understanding of the structural changes associated with charge storage gained from Raman spectroscopy provides valuable insights into the cation-size effects on the electrochemical performances of the MnO₂ electrode

Functionalized Bimetallic Hydroxides Derived from Metal–Organic Frameworks for High-Performance Hybrid Supercapacitor with Exceptional Cycling Stability

A hybrid supercapacitor consisting of a battery-type electrode and a capacitive electrode could exhibit dramatically enhanced energy density compared with a conventional electrical double-layer capacitor (EDLCs). However, advantages for EDLCs such as stable cycling performance will also be impaired with the introduction of transition metal-based species. Here, we introduce a facile hydrothermal procedure to prepare highly porous MOF-74-derived double hydroxide (denoted as MDH). The obtained 65%Ni-35%Co MDH (denoted as 65Ni-MDH) exhibited a high specific surface area of up to 299 m² g^–1. When tested in a three-electrode configuration, the 65Ni-MDH (875 C g^–1 at 1 A g^–1) exhibited excellent cycling stability (90.1% capacity retention after 5000 cycles at 20 A g^–1). After being fabricated as a hybrid supercapacitor with N-doped carbon as the negative electrode, the device could exhibit not only 81 W h kg^–1 at a power density of 1.9 kW kg^–1 and 42 W h kg^–1 even at elevated working power of 11.5 kW kg^–1, but also encouraging cycling stability with 95.5% capacitance retention after 5000 cycles and 91.3% after 10 000 cycles at 13.5 A g^–1. This enhanced cycling stability for MDH should be associated with the synergistic effect of hierarchical porous nature as well as the existence of interlayer functional groups in MDH (proved by Fourier transform infrared spectroscopy (FTIR) and in situ Raman spectroscopy). This work also provides a new MOF-as-sacrificial template strategy to synthesize transition metal-based hydroxides for practical energy storage applications