Theoretical analysis of hydrogen transport through an electrode at the coexistence of two hydrogen-poor and -rich phases based upon the concept of hydrogen trapping

The potentiostatic current transient equations for hydrogen transport through an electrode at the coexistence of two hydrogen-poor alpha and -rich beta phases under impermeable boundary conditions were derived based on McNabb and Foster's physical model of hydrogen trapping, considering the interstitial sites in the beta phase and phase boundaries between the alpha and beta phases as two kinds of reversible and irreversible trap sites, respectively. The decay transients characterised by hydrogen transport through the electrode in the simultaneous presence of the reversible and irreversible traps show a linear relationship between reduced hydrogen flux and reduced time on the logarithmic scale with a slope of -1/2 and then a steep exponential decay with reduced time, followed by a concave downward curve (three-staged transient). The decay transients for hydrogen transport through the electrode in the presence of both trap sites and the trapped hydrogen concentration profile across the electrode at various times were simulated as functions of such parameters as capture rate, release rate, irreversible trap strength and fraction of trap occupied in the electrode, in order to characterise the influence of each parameter on the decay transients and concentration profile. (C) 1998 Elsevier Science S.A.