Hydrogen diffusivity in the massive LaNi5 electrode using voltammetry technique

The Randles-Sevik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi5 intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22 A degrees C), and then voltammetry measurements were carried out at various, very slow potential scan rates (upsilon = 0.01-0.1 mV A center dot s(-1)). At potentials more noble than the equilibrium potential of the H2O/H-2 system, the anodic peaks were registered as a consequence of oxidation of hydrogen absorbed in cathodic range. The peak potentials linearly increase with the logarithm of the scan rate with a slope of 0.059 V. The slope testifies to a symmetric charge transfer process with symmetry factor alpha = A1/2. The peak currents linearly increase with the square root of the potential scan rate, and the straight line runs through the origin of the coordinate system. The slope of the I (a) ((peak)) = f(upsilon (1/2)) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi5 material after cathodic exposure to be C (0,H) = 0.071 mol A center dot cm(-3) (63 % of theoretical value), the hydrogen diffusion coefficient equals D (H) = 2.0 A center dot 10(-9) cm(2)s(-1). Extrapolation of rectilinear segments of potentiodynamic polarization curves with Tafel slopes of 0.12 V and linear polarization dependencies from voltammetry tests allowed the exchange current densities of the H2O/H2 system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi5 alloy are close to 1 mA A center dot cm(-2), irrespective of the electrode potential scan rate.