In Situ Neutron Powder Diffraction of a Nickel Hydroxide Electrode

The redox processes occurring at the nickel oxyhydroxide electrodes were followed by in situ neutron diffraction. The aim was to get a deeper insight into the existing phases and the reactivity mechanisms involved in the reduction process, paying special attention to the so-called "second plateau" phenomenon, occasionally appearing during electrochemical reduction at a potential of 0.8 V vs Hg/HgO. Chemically prepared protonated or deuterated nickel hydroxides, having different phase compositions, oxidation state, and particle size were studied to serve as reference samples. The electrochemically driven structural evolution of four samples upon discharge and charge was followed by in situ neutron powder diffraction using a specially designed cell. For both (gamma)and(beta)-NiOOH phases, the neutron diffraction results evidenced a direct and continuous structural transformation into the(beta)Ni(OH)2 phase upon reduction, on both the first and the second plateau, with no discontinuity when encountering the second plateau. This confirms that the second plateau phenomenon is not due to any intrinsic structural properties of the active material but is related to its surface properties being prone to be strongly dependent upon the electrode preparation.