Investigation of Zn-to-Co Chemometric Ratio in Zinc Cobaltite Spinel as Battery-Type Superior Redox-Active Electrodes for Hybrid Supercapacitors: Density Functional Theory Analysis

Engineering nonstoichiometric metal ions in the binary metal oxides is gaining importance in electrochemical energy storage and conversion devices. Zinc cobaltite (ZnCo₂O₄), a p-type spinel structure, has been stringently applied in these fields. In this work, the chemometric ratios of metal ions (Zn and Co) are varied, and the impact of this nonstoichiometry on the electronic structure-modified ZnCo₂O₄ and its effect on the electrochemical properties are studied in detail. The experiment uses the microwave-assisted hydrothermal method to vary the metal precursor to get the desired nonstoichiometry between Zn and Co in the ZnCo₂O₄ binary metal oxides. A set of four different samples is prepared by varying the Zn–Co precursor stoichiometric concentrations as 0.5–4, 1.0–4, 1.5–4, and 2.0–4, respectively. These four ZCO samples are first confirmed by X-ray diffraction analysis, followed by refining the structures to determine the nonstoichiometries of the elements and supported by X-ray photoelectron spectroscopy. Density functional theory is applied to determine the electronic structure of ZnCo₂O₄ with fully stoichiometric (bulk or no vacancy), three different, single-atom (Zn–, O−)- and dual atom (Zn– and O−)-deficient ZnCo₂O₄ and compared. Among four samples, ZCO-1.5–4 and ZCO-2.0–4 are constructed in a hybrid supercapacitor device for practical application. The optimum ZCO-1.5–4 with Zn- and O-deficiency electrodes has shown better specific capacity (312.5 C g^–1 at 1 A g^–1), energy density (49.4 W h kg^–1), and power density (10.168 kW kg^–1) than other electrodes. Therefore, the work will provide a database for constructing various chemometric ratios of binary metal oxides for achieving optimized nonstoichiometry for the enhanced electrochemical performance of the practical device.