Electrocapacitive removal of malachite green and methylene blue using green-synthesized NiBi2O4 nanospinels

Bismuth based spinels are new spinels which possess special features for advanced material applications. In this study, NiBi2O4 was synthesized through a green sol-gel method using plant extract reductant for capacitive removal of malachite green dye. Parameters like effect of concentration, applied voltage, electrode spacing, and cycle were investigated. The NiBi2O4 spinel showed 91 % malachite green removal and followed the Elovich model at R2 of 0.99413. This confirms that the capacitive removal is chemisorption and depends on surface chemistry of the green NiBi2O4. This work confirms that high temperature destabilizes the surface chemistry of green bismuth-based spinels. XRD confirms the NiBi2O4 crystallized as a cubic spinel, the FTIR confirms the presence of Bi2O3 octahedron and NiO tetrahedron. XPS confirms Bi 2p1/2 and 2p3/2 at 798 and 776 eV respectively. Over the first five cycles, a charge capacitance and charge recovery kept at 214 to 212 and 99 to 97 % respectively confirms its capacitive stability over the spinels that were prepared at higher temperatures. The green NiBi2O4 spinels also showed 83 % removal efficiency for malachite green and methylene blue binary mixtures in the presence of an acidified iodine ion carrier. This work shows that green NiBi2O4 maintains good structural stability and shows good capacitive behavior at room temperature than spinels synthesized at higher temperatures.