Effect of Precursor-Solution pH on the Phase Composition and Microstructure of Precipitated NiO, and By-Product Retention in Hydrothermally Derived NiO–MnO2 Powders

Nickel oxide (NiO)/manganese dioxide (MnO2) composites are widely studied pseudocapacitive electrode materials, but the role of the precursor-solution pH in their synthesis is poorly established. NiO was precipitated at precursor pH 8, 9, 10, and 11, calcined, and subjected to a hydrothermal MnO2-growth step, and the products were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDS). Cubic NiO was the only Ni-bearing oxide, accompanied by a large halite (NaCl) by-product fraction (32–41 wt%). Scherrer, Williamson–Hall, and size–strain-plot analyses gave a NiO crystallite size of 20–30 nm and a microstrain near 1×10−3, and Nelson–Riley refinement gave a lattice parameter of 4.178 Å, equal to the bulk value; neither these quantities nor the NaCl content varied systematically with pH. The hydrothermal products were dominated by a phase indexed on a primitive cubic cell (a = 10.1175 Å) consistent with langbeinite-type K2Mn2(SO4)3; the reflections attributable to tetragonal MnO2 coincide with those of this phase, so MnO2 formation is not established. FE-SEM showed sheet- and flake-like features after the hydrothermal step, and EDS detected Ni, Mn, and O, but neither technique identifies the Mn-bearing phase. Over pH 8–11 the precursor pH is therefore not an effective processing variable: the retained by-products dominate the material, and purification, rather than pH, controls this route.