Hydrogen Transport through $Mm(Ni_{3.6}Co_{0.7}Mn_{0.4}Al_{0.3})_{1.12}$ and $Zr_{0.65}Ti_{0.35}Ni_{1.2}V_{0.4}Mn_{0.4}$ Electrodes by Analysis of Anodic Current Transient

Abstract

The present work involves hydrogen transport through such metal-hydride electrodes as $Mm(Ni_3.6Co_0.7Mn_0.4Al_0.3)_1.12$ and $Zr_0.65Ti_0.35Ni_1.2V_0.4Mn_0.4$ by using potentiostatic current transient technique. Especially, the anodic current transient was analysed to establish the boundary conditions at the electrode surface during hydrogen extraction from the electrode. In chapter IV-1, hydrogen transport through the $Mm(Ni_3.6Co_0.7Mn_0.4Al_0.3)_1.12$ electrode was investigated in 6 M KOH solution. From the comparison of the initial current density measured experimentally with those initial current densities calculated theoretically from the Butler-Volmer equation and from the modified Cottrell equation, it was confirmed that when the electrode surface is subjected to a constant discharging potential, the ‘constraint by Butler-Volmer behaviour’ is effective at the electrode surface during hydrogen extraction below the transition discharging potential, whereas the ‘constraint of constant concentration’ is valid at the electrode surface above the transition discharging potential. In chapter IV-2, hydrogen transport through the $Zr_0.65Ti_0.35Ni_1.2V_0.4Mn_0.4$ electrodes in the presence and absence of the surface oxide scales was examined in 6 M KOH solution. From the analysis of the anodic current transient, it was proposed that in the presence of the surface oxide scales the transition of the boundary condition at the electrode surface from the ‘constraint by Butler-Volmer behaviour’ to the ‘constraint by hydrogen transfer of absorbed state to adsorbed state’ occurs if the discharging potential exceeds the transition discharging potential. In the absence of the surface oxide scales, on the other hand, it was found that the ‘constraint by Butler-Volmer behaviour’ is changed to the ‘constraint of constant concentration’ when the discharging potential is above the transition discharging potential. Consequently, it was recognised that the boundary condition ...