ELECTROCHEMICAL INTERCALATION OF OXYGEN IN ESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.18) AT 298-K(X (0)

Electrochemical intercalation of oxygen into Nd2NiO4+x has been invest igated by potential step experiments (5 mV steps, 10 muA cutoff curren t) in aqueous 1 M KOH at 298 K. Stoichiometric Nd2NiO4 is obtained on the initial reduction of the starting material Nd2-NiO4.18. On reoxida tion, a maximum composition of Nd2NiO4.1 is obtained below the potenti al where oxygen evolution is observed. In the composition range 0.0 le ss-than-or-equal-to x less-than-or-equal-to 0.08 intercalation is reve rsible. The electrochemical data show two-phase behavior in the compos ition range 0.01 less-than-or-equal-to x less-than-or-equal-to 0.04 an d a single-phase region 0.04 less-than-or-equal-to x less-than-or-equa l-to 0.08. Parallel X-ray diffraction studies show that Nd2NiO4+x is o rthorhombic for 0 less-than-or-equal-to x less-than-or-equal-to 0.01, tetragonal for 0.04 less-than-or-equal-to x less-than-or-equal-to 0.08 and orthorhombic at Nd2NiO4.18. An enthalpy of oxidation of -204 kJ/m ol of O2 was obtained from the electrochemical data in the single-phas e region.