Raman spectroscopic evidence supporting the existence of Ni4(OH)44+ in aqueous, Ni(NO3)2 solutions

Nickel-based rechargeable batteries are used daily in satellites, cell phones and computers, and they may be the choice for future electric or hybrid vehicles due to their long life. The electrodes in nickel-based batteries are typically prepared by deposition of Ni(OH) 2 via the reduction of free NO3- in Ni(NO3)2 solutions. The loading efficiency of Ni(OH)2 on such electrodes is dependent on the Ni2+ concentration and is known to decrease with increasing Ni2+ concentration. Mathematical models have been developed to describe this decrease by considering the formation of an intermediate species prior to Ni(OH)2 deposition. Several independent theoretical models indicate the intermediate species to be hydroxonickel(II) tetramer, Ni4(OH)44+. Spectroscopic evidence of Ni4(OH)44+ has been reported using UV-Vis spectrometry, however, identification of such high-energy UV bands is ambiguous. We report here direct Raman spectroscopic evidence supporting the existence of Ni4(OH)44+ in aqueous Ni(NO3)2 solutions. In this study, a low frequency, not, vert, similar490 cm^-1, polarized vibrational mode attributed to Ni4(OH)44+ has been identified in 1 M Ni(NO3)2 solutions in the pH range of 6.1–6.31. The intensity increase of ~490 cm^-1 Raman band from pH 6.1 to 6.31 is consistent with theoretical equilibrium calculations for Ni4(OH)44+.