Electrochemical behavior of nickel hydride in sodium hydroxide solutions

Electrochemical behavior of nickel hydride Ni2H (β-phase) is studied in 0.01-1 N NaOH by using common (VA) and cyclic (CVA) voltammetry, chronocoulometry, amperometry, and potentiometry. The limiting anodic and cathodic currents in VA and CVA curves are caused by the hydride decomposition via the following scheme Ni2H &rarr α'-phase &rarr Ni + Habs, where the intermediate α'-phase contains 0.003 at. % H, that is, one tenth that in the saturated α-phase (0.03 at. %). At open circuit, the hydride maintains the equilibrium hydrogen potential. In the first 30 min, the hydrogen ionization from hydride is limited by solid-state diffusion and, later, the hydride decomposition. The anodic process involves ionization of sorbed hydrogen, while the cathodic process represents its electrochemical desorption: H2O + Hads + e &rarr H2 + OH-. The hysteresis observed in the cathodic CVA and open-circuit chronograms of the hydride potential in the beginning of anodic dissolution reflect the changes in the surface coverage of hydride with adsorbed hydrogen. The rate constant of hydride decomposition k, the rate V itself, and the equilibrium constant K are as follows: k = k = 8 × 10-5 s-3, V= 3 × 10-5 C/cm2 , and K = 10. The kinetic parameters of hydrogen electrochemical ionization from the hydride are ba = 0.12 V and β = 0.5.