Relevance of Model Compounds to Electrode Materials

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffrac- 

Recent ECQM work and X-ray diffraction have confirmed the conversion of the a/y cycle to the / // cycle up on electrochemical cycling in concentrated alkali. Earlier ECQM studies of a- Ni(OH)2 films had shown a mass inversion in the microgravi metric curve after prolonged cycling [64] there is a mass decrease in charge instead of a mass increase. More recent work has confirmed that this mass 

The reaction scheme of Bode [11] was derived by comparison of the X-ray diffraction patterns of the active materials with those for the model compounds. How the 8-Ni(OH)2 in battery electrodes differs from the model compound is discussed in Section 5.3.I.3. In recent years, the arsenal of in situ techniques for electrode characterization has greatly increased. Most of the results confirm Bode s reaction scheme and essentially all the features of the proposed a/y cycle. For instance, recent atomic force microscopy (AFM) of o -Ni(OH)2 shows results consistent with a contraction of the interlayer distance fiom 8.05 to 7.2 A on charge [61-63]. These are the respective interlayer dimensions for the model a-Ni(OH)2 and y-NiOOH compounds. Electrochemical quartz crystal microbalance (ECQM) measurements also confirm the ingress of alkali metal cations into the lattice upon the conversion of a-Ni(OH)2 to y-NiOOH [45,64,65]. However, in situ Raman and surface-enhanced Raman spectroscopy (SERS) results on electro hemically prepared o -Ni(OH)2 in 1 mol NaOH show changes in the O-H stretching modes that are consistent with a weakening of the O-H bond when compared with results for the model a- and 8-Ni(OH)2 compounds [66]. This has been ascribed to the delocalization of protons by intercalated water and Na ions. Similar effects have been seen in passive films on nickel in borate buffer electrolytes [67]. 

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