Abstract
Abstract The general practice for the synthesis of Pd-Ag alloy membrane consists of the electroless deposition of layers of Pd and Ag alternately, which are then annealed to form Pd–Ag alloy. However, there are some disadvantages in the above manufacturing process, such as complicated steps, time-consuming and difficult to control the membrane thickness. On the other hand, the most common reducing agents used in electroless method are hydrazine (N2H4) and sodium hypophosphite (NaH2PO2), which are very toxic to human body and environment, so it must be avoided to use as much as possible. Hence, the goal of this study is to prepare the alloy by using electroplating method without any toxic reducing agent. Firstly, preliminary results of this study found that the composition of Pd and Ag can be varied with additives and plating parameters. Because most of literatures have reported that Pd-Ag membrane composed of approximately 75 % Pd content, which is suitable for hydrogen permeation and stability. Furthermore, the fractional factorial design (FFD) was carried out in order to efficiently find out the significant factors during the electroplating bath system, which is used to control the composition of Pd-Ag deposition. In this study, the Pd-Ag ratios in all the deposits are almost equivalent to its proportion in plating solution, which shows the electroplating process of Pd-Ag system is approaching equilibrium codeposition. On the other hand, the operational equation and relation between Pd/Ag ratio in Pd-Ag deposits and Pd2+/Ag+ ratio in plating solutions were obtained by fixing the total concentration of the plating bath and varying the Pd2+/Ag+ concentration ratios. However, in order to reduce the dendritic structures during the deposition process on the surface, the additives containing amino-group, such as Lugalvan G35, Lugalvan IZE and Lugalvan P were added to inhibit the formation of dendrites efficiently. These results show that the relatively smooth surface morphology could be achieved with plating bath containing Lugalvan G35 and Lugalvan P simultaneously. The ratio of Pd/Ag in this deposit was almost equal to 3. The surface morphology, material compositions, and crystalline structure of the as prepared alloy were characterized by using scanning electron microscopic (SEM), energy-dispersive X-ray (EDX) spectroscopic, and X-ray diffraction (XRD) analysis respectively. For study of hydrogen adsorption/desorption, the Pd-Ag electrodes with best adhesion were chosen and electrochemical analyses were carried out. The hydrogen diffusion coefficient in the deposition with various Pd-Ag compositions can be evaluated according to the theoretical models in the previous reported literatures. The order of diffusion coefficients is listed below: Pd3Ag1＞Pd5Ag1＞Pd＞Pd2Ag1＞Pd1Ag1, indicating that the Pd-Ag membrane with Pd/Ag ratio of 3 is more suitable for application of hydrogen permeation membrane. Finally, based on the relation between electrodes with different Pd-Ag compositions and hydrogen diffusion coefficients, the pure Pd electrode was chosen for application of electrochemical reference. From the potential stability test in normal plating bath condition, the stable potential of palladium-hydride is about 50～85 mV(vs. RHE) and last several hours. Furthermore, the palladium-hydride electrodes can be practically used as an electrochemical reference electrode even at high temperature or in supercritical fluid conditions. Keywords: Pd-Ag alloy membrane, electroplating; fractional factorial design; additives containing amino-group; palladium-hydride electrode; electrochemical reference applications