The electrochemistry of Pd-coated MgySc(1 − y) thin film electrodes: A thermodynamic and kinetic study

The thermodynamic and kinetic properties of Pd-coated Mg_ySc_(1 − y) thin film electrodes are investigated. These thin film electrodes can be described as a two-layer structure, in which the Pd and Mg_ySc_(1 − y) layer contribute to the overall electrochemical response. In order to identify the response of the Pd layer in the two-layer system, thin films consisting of solely Pd, with identical thickness and orientation, were measured. Based on the fact that the chemical potentials of the individual layers of the Pd-coated Mg_ySc_(1 − y) thin films are equal at equilibrium, the exact hydrogen concentration in each layer could be determined. It is shown that during the major part of the hydrogen extraction process of the Mg_ySc_(1 − y) thin films, the composition of the Pd topcoat is close to PdH_0.001. The kinetics of the surface reactions was investigated using electrochemical impedance spectroscopy and showed that, when cross-correlating the results of Mg_ySc_(1 − y) thin films (y = 0.65 − 0.85) and pure Pd films, the surface kinetics are completely dominated by the Pd topcoat. Additionally, it was shown that the charge transfer reaction, and not the absorption reaction is the rate-determining step. The impedance response, dominating the overall kinetic impedance at the hydrogen-depleted state, could be linked to the transfer of hydrogen across the Pd/Mg_ySc_(1 − y) interface in the two-layer thin film electrode.