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Zinc oxide (ZnO) nanoparticles (NPs) anchored to carbon nanofiber (CNF) hybrids were synthesized using a facile coprecipitation method. This report demonstrates an effective strategy to intrinsically improve the conductivity and supercapacitive performance of the hybrids by inducing oxygen vacancies. Oxygen deficiency-related defect analyses were performed qualitatively as well as quantitatively using Fourier transform infrared spectroscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All of the analyses clearly indicate an increase in oxygen deficiencies in the hybrids with an increase in the vacuum-annealing temperature. The nonstoichiometric oxygen vacancy is mainly induced via the migration of the lattice oxygen into interstitial sites at elevated temperature (300 °C), followed by diffusion into the gaseous phase with further increase in the annealing temperature (600 °C) in an oxygen-deficient atmosphere. This induction of oxygen vacancy is corroborated by diffuse reflectance spectroscopy, which depicts the oxygen-vacancy-induced bandgap narrowing of the ZnO NPs within the hybrids. At a current density of 3 A g(-1), the hybrid electrode exhibited higher energy density (119.85 Wh kg(-1)) and power density (19.225 kW kg(-1)) compared to a control ZnO electrode (48.01 Wh kg(-1) and 17.687 kW kg(-1)). The enhanced supercapacitive performance is mainly ascribed to the good interfacial contact between CNF and ZnO, high oxygen deficiency, and fewer defects in the hybrid. Our results are expected to provide new insights into improving the electrochemical properties of various composites/hybrids.

Binder-free electrode materials offer high active material mass loading and usage rate, excellent connectivity between active materials and current collectors, and efficient electron and ion transport inside the electrodes. Herein, we demonstrate a binder-free in situ synthesis of microstructures of CuCo2O4/CuO composites grown on the Ni foam (CCO/NF) by wet chemical methods. Two different morphologies of microspheres (CCO/NF-IPA) and cross-linked microsheets (CCO/NF-DIW) result from solvents of isopropyl alcohol and deionized water, respectively. Using X-ray techniques, the nonstoichiometry of Cu, Co, and O in composites is measured. In the backdrop of the supercapacitor application, even though both electrodes have consistent electrochemical performance, the Co-excess of the CCO/NF-IPA composite has a higher specific capacity (369.6 C g–1 at 1 A g–1) and an extended cyclic performance (98% retention after 5000 cycles) compared to the other. The all-solid-state CCO/NF-IPA//activated carbon (AC) asymmetric supercapacitor (ASC) device with a full operating potential window of 0–1.5 V has exhibited a high specific capacity of 162.6 C g–1 at 1 A g–1. The ASC device retains its initial capacity of 97% over 5000 cycles and renders a notable energy density of 43.7 Wh kg–1 at 752.4 W kg–1 power density.

Hierarchical 3D flower-like ZnCo 2 O 4 (ZCO) microstructures assembled from petal-like nanosheets/flakes of non-uniform sizes were engineered by a polyvinylpyrrolidone (PVP)-assisted hydrothermal method. Four different samples/ morphologies of ZCO were obtained (PVP-L@6, PVP-H@6, PVP-L@12, and PVP-H@12) by altering the reaction parameters such as surfactant concentration (PVP) and reaction time, which can play a significant role in the formation of flower-/petal-/flake-like architectures. The alteration of the reaction parameters not only resulted in morphological changes but also affected the surface area, pore size/volume, crystalline nature, non-stoichiometry of Zn, Co, and O in ZCO, and their electrochemical performance. The metal (Zn/Co)/O deficiencies of ZCO samples were investigated via X-ray photoelectron spectroscopy and supported by the Rietveld refinement method. Furthermore, a plausible growth mechanism for these flower-like ZCO microstructures was projected based on the experimental results. The four dissimilar samples/morphologies of ZCO, which exhibit different electrochemical performances, were investigated. Our results show that PVP-H@12 exhibits higher specific capacitance (761/680 F g −1 at 0.35/1 A g −1) and good cycling constancy (90% capacitive retention after 2000 cycles at 5 Ag −1) among all the four samples.

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