Pd–Ag membrane synthesis: The electroless and electro-plating conditions and their effect on the deposits morphology

The effect of electroless and electro-plating conditions on the Pd and Ag deposits morphology obtained on porous metal substrate was investigated from the membrane synthesis viewpoint. Linear sweep voltammetry was applied for the polarization characteristics of electroless plating baths. The Pd electroless deposits ([Pd]/[N2H4] = 16 mM/5.6 mM) and Ag electrodeposits ([Ag] = 100 mM, current density = 2 mA/cm2) showed good substrate pore penetration. However, for the electroless Ag deposits ([Ag]/[N2H4] = 3 mM/5.6 mM), poor pore penetration was observed. The Ag electroless bath polarization curve indicated that within 15–25 mV electrode over potential, the entire deposition process was limited by the diffusion of Ag ions to the substrate surface. The plating conditions such as low [Ag]/[N2H4] ratio (20 mM/4 mM) or substrate stirring lowered the Ag deposition over potential and produced Ag deposits with good pore penetration, therefore suitable for the membrane preparation. Three membranes, M-1 (Pd by electroless method and Ag by electro-plating method), M-2 and M-3 (both Pd and Ag by electroless method) were prepared by the multilayer Pd–Ag sequential deposition approach. The membranes showed low activation energy for the H2 permeation (M-1, M-2 and M-3 = 4.9, 4.3 and 11.5 kJ/mole respectively) after annealing at 550 °C (24 h). The low activation energy after annealing was due to the formation of Pd–Ag alloy. The SEM and EDX analysis after annealing showed fairly uniform distribution of Pd and Ag throughout the cross-section of the membranes.