Electrochemical Study of Secondary phases in hydride-forming AB2 alloys

F. RUIZ,; E. B. CASTRO; H. A. PERETTI; A, VISINTIN

Foz de Iguaçú

Congreso; ISE 2008 Spring Meeting; 2008

International Society of Electrochemistry

In previous works the characterization of AB2-type ZrCrNi alloy, prepared by arc melting furnace, indicated that the as-melted material had different electrochemical characteristics as compared to the annealed one. The effect was ascribed to the presence of secondary phases ZrxNiy formed during solidification of the melt. Electrochemical studies of ball milled mixtures of annealed ZrCrNi alloy with powders of the phases Zr7Ni10, Zr8Ni21 and Zr9Ni11 indicated that the main secondary phase responsible for the properties improvement is Zr8Ni21. The aim of this work is to analyze, using impedance spectroscopy,  the catalytic and transport properties of these secondary phases in order to determine the physicochemical parameters responsible for the improvement of the alloy performance. A ZrCrNi alloy was prepared by arc melting adequate proportions of the composition elements. Alloys corresponding to the secondary phases Zr7Ni10, Zr8Ni21 and Zr9Ni11 were prepared following the same procedure. The base alloy ZrCrNi was heat treated for 30 days at 1250 K plus 15 days at 1273 K, whereas the alloys corresponding to the secondary phases were heat treated for 30 days at 1273 K. Finally, powders of the each secondary phase were mixed with powder of the base ZrCrNi alloy in a 20 wt.% proportion using a RETSCH S-1/2 ball mill. Samples of these mixtures were used in the preparation of the  working electrodes. Electrochemical measurements were carried out using a 7M KOH solution, a Ni mesh as counter-electrode, and Hg/HgO as reference electrode. Different electrochemical studies were made ciclic voltametric, galvanostatic charge  discharge cicling and rate capabiliy. The hydrogen process is described in terms of the Volmer-Heyrovsky-Absorption(HAR) scheme coupled to hydrogen diffusion in the alloy material.  Experimental  impedance data were simulated in terms of the proposed model exhibiting good agreement in the whole frequency range. The parametric identification allowed to determine that electrodes prepared with alloy Zr8Ni21 exhibit the highest values for the hydrogen diffusion coefficient, D, exchange current density and HAR kinetic constant, in accordance with improved performance of the electrode. Also it was determined that the kinetic constant related to the HAR process is more than one order smaller for electrodes prepared with  Zr9Ni11, being the  diffusion coefficient, D,  3 orders smaller for this alloy than for electrodes prepared with, Zr8Ni21 or Zr7Ni10.