De-protonation of Nickel Hydroxide by 200kV Electron Irradiation


Graham J.C. Carpenter* and Zbigniew S. Wronski Department of Natural Resources, Canada


*graham.carpenter@physics.org Abstract: Previous studies of nickel hydroxide (Ni(OH)2 ) powders have


shown that either heating or mechanical grinding can result in complete de-hydroxylation, leading to conversion to nickel oxide (NiO). In both cases, this process appears to occur in one stage, without evidence for any intermediate compounds being formed. During studies of Ni(OH)2


powders for applications in the positive electrodes of Ni metal


hydride (NiMH) rechargeable batteries, using transmission electron microscopy (TEM), we have observed significant changes caused by exposure to the highly energetic electron beam used for imaging and analysis. It is shown here, using electron energy-loss spectroscopy (EELS),


that de-hydroxylation under electron irradiation occurs in


two stages, with nickel oxy-hydroxide (NiOOH) being formed at the intermediate stage.


Keywords: nickel hydroxide, de-hydroxylation, electron irradiation, EELS


Introduction Since


the second half of the twentieth century, Ni


hydroxides have been used extensively in energy storage devices from rechargeable batteries to supercapacitors [1]. More recently, related compounds, in the form of layered metal hydroxides (LMH) and layered double (metal) hydroxides (LDH), have become important as novel inorganic functional materials. Nickel hydroxide (Ni(OH)2


structure with that of the well-known Mg(OH)2 compound, with hexagonal packing of large OH-


) shares its crystallographic brucite


negative ions


in alternate layers of octahedral sites filled by much smaller positive metal Ni ions, leading to the stacking of neutral- charged Ni-O layers. Bonding is anisotropic, with strong iono- covalent character within layers and more weak interaction between the layers. Te relatively weak bonding between layers is the reason behind the easy shearing or rotation of layers, the tendency for which can be further promoted by partial substitution of Ni+2


by Ni+3


Te latter has been applied to the development of high-energy, Co-doped batteries with Ni(OH)2


by transition metal atoms with distinct valences, such as Co+3 electrodes. Water molecules,


with Li+ ions from a KOH/LiOH electrolyte.


or even other larger molecular species, can be efficiently intercalated between layers, in particular, in LDH. However, battery storage applications benefit more from the doping of layered Ni(OH)2


interlayer space acquires a degree of mobility in the form of protons. In fact, Ni(OH)2


Te hydrogen of the hydroxyl ions that belong to the is a p-type semiconductor.


Importantly, LMH can be exfoliated into single-lamellar nanosheets which, in tandem with the semiconducting properties, makes them promising candidates for a variety of applications in electronics, electro-catalysis, CO2


reduction,


and hydrogen or oxygen evolution reactions on electrodes. Facilitating the de-protonation of an OH-


group to O is of great


importance for these applications. However, the processes 26


doi:10.1017/S155192952100136X


ions in the compound NiOOH, or .


occurring on the Ni layers during charge accumulation are not well understood. In an earlier study [2], it was shown that electrochemical behavior involving the de-protonation of Ni(OH)2 powder


could also be accomplished by either thermal or mechanical means, such as heating or mechanical grinding. Detailed work using a combination of scanning transmission electron microscopy (STEM) and differential scanning calorimetry (DSC) techniques showed that the transformation during heating occurred in one stage by means of the loss of H2


O


from the hydroxide layers. It was also shown that the same process could be induced in the hydroxide crystals by intensive mechanical milling [2,3] in a high-energy ball mill [4]. In addition, it was demonstrated that mechanical activation improves the mobility of hydrogen atoms (protons) in Ni(OH)2 crystals and hence improves the electrochemical performance of battery-grade hydroxide powders that are commonly used in NiMH rechargeable batteries [3]. During this study of Ni(OH)2


using TEM [2], it became


apparent that the nature of this compound changed in the microscope when exposed to the intense, high-energy electron beam. An in situ experiment was, therefore, conducted using electron energy-loss near edge structures (ELNES) in an effort to understand the decomposition in more detail. Coincidentally, a separate study using EELS [5] was being conducted to detect changes that occurred in Ni(OH)2


during charge/discharge


cycling of a positive NiMH electrode in which the active mass was nickel hydroxide Ni(OH)2


. Te results of that investigation


provided new information on the near-edge structure of the O-K core-loss edge, which proved to cast light on the process of de-protonation of Ni(OH)2


layers under electron irradiation.


Techniques Te Ni(OH)2


with diameters up to 10μm, that were made up of an agglomerate of


powder was in the form of porous spheres, thin platelets on the nanometer scale [2].


Specimens for TEM were prepared by embedding a sample of the powder in resin and cutting sections, ∼40nm in thickness, using an ultramicrotome [6]. Te specimens were examined in an FEI CM20FEG (field emission) TEM/STEM equipped with a Gatan Imaging Filter model 678, which permitted EELS spectroscopy at a resolution (FWHM) as low as 1eV.


Results Te progress of


the transformation under the electron


beam is shown in the ELNES spectra of Figure 1, using conditions of slow de-hydroxylation under a defocused electron beam, which also had the effect of averaging the crystallographic orientations of the nanocrystals. Te initial and final spectra were readily identified as Ni(OH)2


and NiO, www.microscopy-today.com • 2021 November


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