Facile and cost-effective NiO/MgO-SiO₂ composites for efficient oxygen evolution reaction and asymmetric supercapacitor systems

Biomass waste from grapefruit peel extract was used for the preparation of MgO-SiO₂ composites in situ in order to develop effective electrocatalytic composites based on NiO/MgO-SiO₂. The MgO-SiO₂ composites were subsequently deposited with NiO using a modified hydrothermal method. The synthesized materials were analyzed to investigate their morphology, crystal structure, chemical composition, functional group, and optical band gap. The structural analysis allowed us to determine the orientation of the nanoparticles, the cubic phase of NiO and MgO, the significant loss of optical band gap, and the enriched functional groups on the surface of NiO/MgO-SiO₂ composites. The electrochemical properties were investigated in the presence of an alkaline solution of KOH. To study the oxygen evolution reaction (OER) in 1 M KOH aqueous solution, different NiO/MgO-SiO₂ composites were investigated. It was found that the NiO/MgO-SiO₂ composite that contained the highest amount of MgO-SiO₂ (sample 3) had a lower overpotential than the NiO/MgO-SiO₂ composite with the lowest amount of MgO-SiO₂. Sample 3 exhibited an overpotential of 230 mV at 10 mA cm⁻² over a period of 40 hours with excellent stability. The superior electrochemical activity of the NiO/MgO-SiO₂ composite (sample 3) was demonstrated in an energy storage device using 3 M KOH aqueous solution, and asymmetric supercapacitor devices were fabricated in 3 M KOH solution. According to the ASC's specifications, a specific capacitance of 344.12 F g⁻¹ and an energy density of 7.31 W h kg⁻¹ were found for the device at a fixed current density of 1.5 A g⁻¹. After over 40 000 galvanic charge-discharge repeatable cycles at 1.5 A g⁻¹, sample 3 of the NiO/MgO-SiO₂ composite exhibited excellent cycling stability with 88.9% percent capacitance retention. During the performance evaluation of the NiO/MgO-SiO₂ composites, grapefruit peel extract was confirmed as a potential biomass waste for the fabrication of high-performance energy conversion and storage devices.