MASS-TRANSFER AND KINETIC PHENOMENA AT THE NICKEL-HYDROXIDE ELECTRODE

Thin-film (10 to 40 nm thickness) nickel hydroxide intercalation elect rodes were constructed using an electroprecipitation technique. Cyclic voltammetry, potentiostatic step, and galvanostatic discharge experim ents were performed and interpreted in terms of a macroscopic model tr eating the simultaneous mass transfer, kinetic, and thermodynamic phen omena occurring within the cell. The side reaction, oxygen evolution, exhibited irreversible Tafel behavior, with a proton concentration-dep endent exchange current density of 4.5 x 10(-9) [(c(0) - c)/c(0)] A/cm (2) on pure nickel hydroxide films, and a constant exchange current de nsity of 4.5 x 10(-9) A/cm(2) on cobalt hydroxide-containing nickel hy droxide films. The apparent anodic transfer coefficient for the oxygen reaction is 0.49 on pure nickel hydroxide films and 0.42 on cobalt hy droxide-containing nickel hydroxide films. The intercalation reaction is described with a Butler-Volmer-type expression with a large, proton concentration-dependent exchange current density of 9.5 x 10(-2) [c(c (0) - c)](1/2) A/cm(2), and anodic and cathodic apparent transfer coef ficients of 0.5 for both electrode types. Here c and c(0) have units o f mol/cm(3). The proton diffusion coefficient in pure nickel hydroxide was found to depend on the proton concentration, with values ranging from 1.2 x 10(-13) to 1.9 x 10(-12) cm(2)/s with a concentration-avera ged value of 3.417 x 10(-13) cm(2)/s. In cobalt hydroxide-containing n ickel hydroxide, the values ranged from 2 x 10(-13) to 1.9 x 10(-12) c m(2)/s, with a concentration-averaged value of 8.402 x 10(-13) cm(2)/s .