Study of hybrid Zn-Ni-Co/graphene nanoplatelet composite for supercapacitor applications

The evolution of the electrode materials for supercapacitor has become a pivotal role for advancing energy storage technologies. Among promising candidates, ternary mixed transition metal oxides (MTMOs) consisting of zinc (Zn), nickel (Ni), and cobalt (Co) offer multiple redox states, high theoretical capacitance, and tuneable electrochemical properties. However, the influence of Zn: Ni: Co stoichiometry and their interaction with conductive additives such as graphene nanoplatelets (GNPs) remains underexplored. This study aims to bridge these gaps by systematically synthesizing Zn Ni Co MTMOs with varying ratios (1:1:1, 1:2:1, 1:1:2, and 2:1:1) via the sol gel method and evaluating their electrochemical enhancement upon GNP incorporation (0.1, 0.2, and 0.3 wt.%). The synthesized materials were characterized using XRD, FESEM, EDX, FTIR, BET, and Raman spectroscopy, and evaluated in a three-electrode system using cyclic voltammetry (CV), galvanostatic charge discharge (GCD), electrochemical impedance spectroscopy (EIS), and Dunn’s analysis in 2 M KOH electrolyte. Among the samples, ZNC-GNP 0.2 exhibited the best performance with a specific capacitance of 478 Fg⁻¹ at 5 mV s⁻¹, owing to the synergistic contributions of electric double layer capacitance (EDLC) from GNP and the pseudocapacitive behaviour of the MTMOs. Dunn’s analysis confirmed a predominantly diffusion-controlled charge storage mechanism (85% contribution at 5 mV s⁻¹), gradually transitioning to increased capacitive contributions (up to 44%) at higher scan rates, indicating hybrid storage behaviour. The ZNC GNP 0.2 electrode also demonstrated excellent cyclic stability, retaining 97.05% of its initial capacitance after 1000 cycles, comparable to the performance of similar MTMOs-carbon systems. GCD confirmed hybrid supercapacitor behavior with favorable energy and power density of 24.06 Wh/kg and 1375 W/kg, respectively shown by ZNC-GNP 0.2 sample. EIS results revealed the lowest solution resistance (Rs = 5.43 Ω) and charge transfer resistance among all compositions, further confirming enhanced ion diffusion and electronic conductivity. In contrast, excessive GNP content (0.3 wt.%) resulted in increased resistance due to agglomeration, while insufficient GNP (0.1 wt.%) limited conductive network formation. Overall, the findings underscore the critical role of compositional optimization and carbon integration in achieving high performance, stable, and conductive supercapacitor electrodes. The ZNC GNP system, particularly at 0.2 wt.% GNP, offers a balanced architecture combining high energy density, long term stability, and efficient charge transport, positioning it as a strong candidate for advanced energy storage applications in portable electronics, electric vehicles, and grid systems.