Study of hydrogen transport in metal hydride electrodes using a novel electrochemical method

A novel and relatively simple electrochemical method is described to determine the hydrogen diffusion coefficient (D) and the surface area (A) of hydrogen-absorbing alloy particles. In this method, which is called `potential step chronoamperometry (PSCA)', hydrogen is dissolved into the alloy until the hydrogen concentration is uniform, and then the dissolved hydrogen is extracted electrochemically at a sufficiently high potential step (larger than +0.2 V vs. Hg|HgO) with an anodic hydrogen ionization current that changes with time. From an electrochemical kinetic analysis and Fick's law for diffusion to a spherical particle, the variation of j (current density, A g^-1) as a function of t^-1/2 (t is time, s) is found to be linear over a small time range, i.e. t<a²/D (a is the particle radius) (less than about 7 to approximately 500 s) and the diffusivity and surface area can be determined from the intercept and slope of this line. The value of the diffusion coefficient of hydrogen in a LaNi_4.7Al_0.3 alloy is found to be in the range 3.1×10^-14-8.6×10^-13 m² s^-1. In the hydrogen concentration region that was examined, i.e. H/M>0.06, the value of the diffusion coefficient declines sharply with increasing hydrogen concentration in the alloy particles. This dependence of the diffusion coefficient of hydrogen on its concentration is discussed in terms of the nature of both the diffusing species and the diffusion medium. It is also found that the hydrogen transport is governed solely by solid-state diffusion in the alloy particles only after a certain period of time (approximately 40-70 s).