No abstract

Hydrogen absorption into palladium thin films ͑10 monolayers and 100 m͒ was studied using cyclic voltammetry. The behavior of thin layers prepared by electrodeposition is different from that of the bulk Pd. On very thin films hydrogen adsorption, absorption, and evolution processes are separated. Adsorption kinetics depends strongly on applied potential and layer thickness. A new method is proposed for the determination of the quantity of adsorbed hydrogen in the presence of the absorption process.

Models of insertion of hydrogen in metal films coating a gold electrode are presented in terms of faradaic impedance. They take into account adsorption of the protons from the solution on the electrode surface ͑two-step mechanism͒ or direct absorption in the metal ͑one-step mechanism͒ and diffusion in the metal. In addition, trapping of the hydrogen atoms and direct exchange of the trapped hydrogen with the solution are considered. The main feature discussed here is the change of the charge-transfer resistance with the thickness of the metal film. This surprising behavior is related to the exchange reaction of trapped hydrogen with the solution. The impedance diagrams are calculated for a palladium film both in the underpotential deposited ͑UPD͒ domain and the hydrogen evolution domain. In the UPD domain an impedance of the ion-blocking electrode is found. In the hydrogen evolution domain two capacitive loops are found for a film whose thickness is less than 1 micrometer.