Remarkably Stable Nickel Hydroxide Nanoparticles for Miniaturized Electrochemical Energy Storage

We report results on the synthesis of unsupported and carbon-supported nickel hydroxide nanoparticles (β-Ni(OH)2 NPs). Their characterization using X-ray diffraction (XRD) and transmission electron microscopy (TEM) reveals that the crystallite size is 2.2 nm and the particle size is 2.6 nm with a narrow size distribution. Thermogravimetric analysis (TGA) of the carbon-supported β-Ni(OH)2 NPs shows that the metal loading is ca. 27% wt. Cyclic voltammetry (CV) examination in 0.10 M aqueous NaOH demonstrates that the interconversions degree of β-Ni(OH)2 ⇆ β-NiO(OH) is ca. 61%, much more than that in the case of analogous bulk materials. The process is also monitored using confocal Raman spectroscopy, which reveals that the β-Ni(OH)2 ⇆ β-NiO(OH) interconversions occur in the interfacial region. Density functional theory (DFT) calculations point to the existence of NiO in the core of the nanoparticles, in agreement with the XRD data. CV, DFT, and Raman spectroscopy measurements demonstrate that the β-Ni(OH)2 ⇆ β-NiO(OH) interconversions occur only in the surface layer of the nanoparticles. Repetitive potential cycling of the β-Ni(OH)2 NPs in 0.10 M aqueous NaOH in the 0.05 V ≤ E ≤ 1.65 V range shows that they possess remarkable stability, thus making them suitable for application in miniaturized electrochemical energy storage.