Thermodynamic paths in the two-phases domain of the PdH system and a method for kinetic analysis

Electrochemical impedance spectroscopy is commonly used to analyze phase transformation mechanisms in metal hydride electrodes. Surprisingly, experimental EIS measurements are, in general, not related to the thermodynamics of the system. This is the aim of the present work in which impedance spectra are obtained in relation with the thermodynamic paths observed in the two-phases domain of the PdH system. First, the general features of hysteresis loops, recorded either from the absorption or from the desorption potential plateaux, are given. Potentiostatic displacements within a few mV apart from the potential plateaux reveal the existence of reversible thermodynamic paths. For displacements exceedings 5-6 mV, irreversible paths are observed. To avoid non-linearities associated with hysteresis, non-harmonic perturbations are then used to analyze the kinetics along these paths. Impedance diagrams are obtained by Fourier transforming the current responses of the electrode submitted to 1 mV-amplitude potential steps. Experimental impedance spectra are best-fitted using classical model impedance functions derived for monophasic electrodes of finite dimensions. It is found that the hydrogen solubility and the hydrogen diffusion coefficient along the reversible paths vary smoothly with the composition H/Pd, and thus, with the amount of β phase inside the electrode. Results are discussed in relation with α-PdH and β-PdH phase distribution inside the electrode.