Effect of reaction time and PVP contents on morphologies of hierarchical 3D flower‐like ZnCo ₂ O ₄ microstructures for energy storage devices

Abstract

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.

Construction of an Ultra‐High‐Energy Density Hybrid Supercapacitor with Self‐Assembled 3D Hydrangea Flower‐like Mo‐rich CoMoO4 Microstructures

“…Therefore, the O1 peak, connected to Co−O bonds, was centered at 529.4/529.8 eV, while the O2 (530.7/530.7 eV) is assigned to Mo−O. The O3 (531.9/532.3 eV) peak was related to water molecules adsorbed on the CMO‐DIW/CMO‐ETH samples′ surface, respectively [3,9] . Figure 3(b) compares Co 2 p core‐level spectra for CMO‐DIW and CMO‐ETH, which showed three distinct peaks: Co is +2 and +3 states, and the satellite peaks.…”

“…The ZCO‐ETH was lower crystalline than the ZCO‐DIW; this was shown by the fact that its diffraction peaks were broader and lower intensity peaks (Figure 1(a)). The Scherer equation was used to calculate the average crystallite size of CMO‐DIW and CMO‐ETH, which were determined to be around 28.6 and 24.4 nm, respectively [3] . Raman spectroscopy was used to further inspect the crystal structure and bonding in the two CMO samples.…”

“…Because of their considerable surface energy, the formed nanoparticles are thermodynamically unstable and would prefer to attach to flake/petal‐like CMO flakes to lower the surface energy, [3] as shown in Scheme 1, which can serve as the flower‐like structure‘s cores. The flakes were self‐assembled into irregular flower‐like architectures during the self‐assembly and Ostwald ripening processes [21,22] .…”

“…Furthermore, despite a 10‐fold increase in current density, the CMO‐ETH electrode maintained 74% of the initial specific capacity. In addition, the specific capacity of the CMO electrodes was decreased with the increase in current density; this is because of lower reaction kinetics at higher current densities, resulting in a corresponding capacity being decreased [3] …”

“…The demand for clean energy has led to substantial efforts to develop safe, sustainable, and economical energy storage and conversion technologies, owing to rising energy consumption and environmental contamination [1][2][3] Supercapacitors (SCs) are the most appealing energy storage systems because of their high power density, extended operating life cycle, quick charge-discharge ability, and environmental friendliness. [4,5] Due to their advantages, SCs are widely used in hybrid electric vehicles and portable electric devices.…”