Electrochemical oxygen intercalation in La_2NiO_(4+δ) crystals

摘要

The electrochemical intercalation of oxygen from an aqueous KOH electrolyte into La_2NiO_(4+δ) has been investigated by a potential step method at potentials below oxygen evolution. The first reduction cycle occurs at nearly constant potential for the whole composition range and is characteristic of nucleation and growth behavior. On the first oxidation and subsequent reduction-oxidation cycles, the results are similar to those obtained previously for polycrystalline electrodes and show the presence of a two-phase region for 0 ≤ δ ≤ 0.06 and a single phase with the composition 0.06 ≤ δ ≤ 0.13. The P4_2cm phase with δ～0.02 and a narrow range of stoichiometry, is apparent in dδ/dV versus δ data and in the behavior of the current decay curves. Examination of the crystals at various stages of cycling show the formation of extensive cracks that apparently increase in density with cycling. Measurements of the transient currents after potential steps have been used to estimate the diffusion coefficient of oxygen in La_2NiO_(4+δ) in the single phase region. Data were obtained for both polycrystalline and single crystal electrodes.