Mass transfer and kinetic phenomena at the nickel hydroxide electrode

Thin-film (10 to 40 nm thickness) nickel hydroxide intercalation electrodes were constructed using an electroprecipitation technique. Cyclic voltammetry, potentiostatic step, and galvanostatic discharge experiments were performed and interpreted in terms of a macroscopic model treating the simultaneous mass transfer, kinetic, and thermodynamic phenomena occurring within the cell. The side reaction, oxygen evolution, exhibited irreversible Tafel behavior, with a proton concentration-dependent 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 density of 4.5 x 10(-9) A/cm(2) on cobalt hydroxide-containing nickel hydroxide films. The apparent anodic transfer coefficient for the oxygen reaction is 0.49 on pure nickel hydroxide films and 0.42 on cobalt hydroxide-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 coefficients of 0.5 for both electrode types. Here c and c(0) have units of 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-averaged value of 3.417 x 10(-13) cm(2)/s. In cobalt hydroxide-containing nickel hydroxide, the values ranged from 2 x 10(-13) to 1.9 x 10(-12) cm(2)/s, with a concentration-averaged value of 8.402 x 10(-13) cm(2)/s.