| Abstract
| - The redox processes occurring at the nickel oxyhydroxide electrodes were followed by insitu neutron diffraction. The aim was to get a deeper insight into the existing phases andthe reactivity mechanisms involved in the reduction process, paying special attention to theso-called “second plateau” phenomenon, occasionally appearing during electrochemicalreduction at a potential of 0.8 V vs Hg/HgO. Chemically prepared protonated or deuteratednickel hydroxides, having different phase compositions, oxidation state, and particle sizewere studied to serve as reference samples. The electrochemically driven structural evolutionof four samples upon discharge and charge was followed by in situ neutron powder diffractionusing a specially designed cell. For both γ and β-NiOOH phases, the neutron diffractionresults evidenced a direct and continuous structural transformation into the β-Ni(OH)2 phaseupon reduction, on both the first and the second plateau, with no discontinuity whenencountering the second plateau. This confirms that the second plateau phenomenon is notdue to any intrinsic structural properties of the active material but is related to its surfaceproperties being prone to be strongly dependent upon the electrode preparation.
- The electrochemically driven structural evolution of nickel hydroxide phases, usually used as positive materials in Ni−Cd batteries, was followd by in situ neutron diffraction upon charge and discharge. The results unambiguously show that the redox processes taking place in the first and second plateau (appearing respectively at 1.25 V and 0.8 V vs Cd/Cd(OH)2) are the same and consist of a direct reduction of β-III- or γ-III-NiOOH phases into β-II-Ni(OH)2.
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