Electron Irradiation
Figure 1: EELS spectra showing an intermediate stage in the transition from Ni(OH)2
to NiO during high-energy electron irradiation in the electron microscope.
respectively, by comparing them with the near-edge structure of those collected previously as reference spectra from pure reactant-grade powders [7]. It should be noted that compared to the reference spectra, additional fine structure is visible in the spectra of Figure 1 because they were collected under conditions of enhanced energy resolution. It is clear from Figure 1 that a distinct intermediate stage
exists, for which no comparable reference spectra could be found in the published literature. However, during a TEM investigation of the changes in a positive nickel electrode caused by charge/discharge cycling, where the active mass was Ni(OH)2
, an ELNES spectrum was obtained for an electrode
in the charged condition [5] (Figure 2). As discussed in more detail in the Discussion section, the effect of charging this form of electrode causes the Ni(OH)2
to be electrochemically
oxidized to the well-established nickel oxy-hydroxide phase, NiOOH. A comparison of
the peak positions in the fine
structure of the EELS spectrum, particularly that at 527eV, shows clearly that the spectrum from the intermediate stage of de-protonation must indeed correspond to that from NiOOH. Further evidence for this designation was obtained from a high-resolution X-ray photoemission spectroscopy (XPS) study, which revealed an extra peak at a comparable energy (∼529eV) in the oxygen O1s edge for a NiOOH layer forming on Ni metal exposed to oxidation by water vapor [8]. A similar peak was observed in a de-convoluted profile for solution processed NiOx for photovoltaic applications [9]. To eliminate any doubt that the intermediate spectrum
could have been caused by the overlap of the hydroxide and oxide phases, the two spectra were summed for comparison purposes. Tis was not possible in a direct manner because the
original digital data could no longer be accessed. Te printed output for the initial hydroxide phase and the final oxide was therefore re-digitized using the application WebPlotDigitizer© (Automeris) and the data summed and re-plotted using Excel. Te results, displayed in Figure 3, show quite clearly that
28
Figure 3: The data of Figure 1, re-plotted to allow comparison of the interme- diate spectrum with the sum of the spectra for Ni(OH)2
and NiO.
www.microscopy-today.com • 2021 November
Figure 2: EELS spectrum from the active mass of a NiMH electrode in the charged condition after charge/discharge cycling. Reproduced with kind permission from Microscopy Today.
the spectrum from the intermediate phase is quite different from the sum of the spectra from the start and end of the de-protonation experiment.
Discussion Tis is the first time the process of de-protonation of dry has been observed to occur via an intermediate stage, with the formation of NiOOH, apart from by electro-chemical
Ni(OH)2
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