Surface of powdered LaNi 5 intermetallic compound has been modified by active particle coverage with electroless nickel (Ni-P). The electrode degradation process in 6 M KOH solution has been tested across 70 charge/ discharge cycles at −0.5 C/+0.5 C rates. It has been established that after approx. 25-35 initial cycles, the electrode degradation process fulfills first order chemical reaction kinetics law: logarithm of discharge capacity linearly decreases with cycle number. The rate constant for the Ni-P protected material is over 20 % lower than that of as received one. The surface modification also improves the alloy hydrogenation kinetics: exchange current densities of H 2 O/H 2 system are generally greater for modified material and, contrary to uncovered material, do not practically decrease with long-lasting cycling.
Effect of small addition of tin (1.7 at.%) into LaNi4.5Co0.5 alloy on gas phase and cathodically charged hydrogen absorption ability as well as its corrosion resistance in 6M KOH solution is discussed. To reveal the effect of Sn doping three alloys have been selected: LaNi4.5Co0.5 (precursor), LaNi4.5Co0.5Sn0.1 and LaNi5 - as a parent compound. The room temperature p-C isotherms indicate to beneficial effect of Sn addition which causes decrease of H2 equilibrium pressure and does not limit atomic hydrogen solubility. Discharge capacities (Qdisch), exchange current densities of H2O/H2 system () as well as corrosion rates () have been determined for the tested alloys on the basis of cyclic galvanostatic measurements (at –0.5C/+0.5C rates). It has been shown that for N > 3 cycle the discharge capacity of LaNi4.5Co0.5 is ca twofold greater than that for LaNi5 reference. Addition of 1.7 at.% Sn into Co-containing alloy expands the discharge capacity by 30-40%. The Co containing alloys reveal twice as great exchange current densities of H2O/H2 system compared to LaNi5, however, Sn addition slightly decreases the , especially for latest cycles. The partial cobalt substitution for Ni accelerates alloy corrosion in alkaline solution, however, tin addition fully eliminates this effect.
The Randles-Sevčik relationship has been applied to evaluate atomic hydrogen diffusivity in massive LaNi 5 intermetallic compound. The electrode was cathodically hydrogenated in 6 M KOH solution (22°C), and then voltammetry measurements were carried out at various, very slow potential scan rates (υ =0.01-0.1 mV·s
−1). At potentials more noble than the equilibrium potential of the H 2 O/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 α =½. 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(υ 1/2 ) straight line is a measure of the atomic hydrogen diffusion coefficient. Assuming the hydrogen concentration in the LaNi 5 material after cathodic exposure to be C 0,H =0.071 mol·cm −3 (63 % of theoretical value), the hydrogen diffusion coefficient equals D H =2.0· 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 H 2 O/H 2 system on the tested material to be determined. The exchange current densities on initially hydrogenated LaNi 5 alloy are close to 1 mA·cm −2
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