Formation and cathodic re-reduction of Cu(III) states in Y-Ba-Cu perovskites

Abstract YBa 2 Cu 3 O 6.5+δ was prepared at 950°C with subsequent cooling down at 100 K h −1 in the presence of oxygen at partial pressures of p O2 = 10 −4 −1 bar. Voltammetric measurements of pellet electrodes with paraffin impregnation in 1 M NaOH displayed two reduction peaks. The positive one, p, is reversible and corresponds to the intrinsic redox system Cu(II)/Cu(III). It is fully developed (δ = 0.5) only after anodic saturation, and it then corresponds to the ratio 2 Cu(II)/Cu(III). The initial charge of peak p after preparation corresponds to δ = 0.22 at p O2 =1 bar, and it decreases systematically with decreasing p O2 . The concentration ratio Cu(II)/Cu(III) could be derived from these results, and a quantitative discussion of the redox equilibrium at 285 ° C (H. Eickenbusch et al., Mater. Res. Bull., 22 (1987) 1505; Angew. Chem., 99 (1987) 1201; Angew. Chem., Int. Ed. Engl., 26 (1987) 1188) with oxygen and of the intrinsic redox equilibrium Cu(II)/Cu(III) in 1 M NaOH is possible. The standard Gibbs reaction energy for the former was found to be 15.2 kJ mol −1 at 285°C. Leaching experiments at pH -1 to 12 up to 2 h prior to the voltammetric measurements did not affect the subsequent voltammetric results in any case.