The electroless co-deposition of Pd and Ag as membrane preparation method offers advantages such as fast synthesis procedure and easy annealing of the deposits in comparison to the sequential plating method. Since the electroless deposition involves redox reactions, the electrochemical technique such as linear sweep voltammetry (LSV) could be very useful to understand the key aspects of electroless plating bath and its components and their effect on the deposit characteristics. For the co-deposition, the deposition potential of co-depositing metals should be close to each other. Complexing agents in the bath influence the metal deposition potential and other deposits characteristics such as morphology and deposits compositional homogeneity. The objective of this study was to investigate the effect of complexing agents (NH3, Na2EDTA and Glycine) in the bath on the Pd-Ag co-deposits characteristics such as morphology and compositional homogeneity using the LSV technique for the synthesis of Pd-Ag composite membrane.
The LSV scans were obtained using the BAS 110B/W electrochemical station. Ag/AgCl (3 M NaCl) was used as a reference electrode. Stainless steel wires plated with Ag or Pd acted as working electrode. For the LSV scans, 3 mM concentration each for Pd and Ag was maintained in the bath. Porous stainless steel coupons were used for the morphology and compositional homogeneity characterization. The deposits were characterized by using SEM, EDX and X-ray differactometer. For the coupon and membrane preparation, the total metal concentration of 4 mM (10 wt% Ag) was maintained in the baths. The coupons with Pd-Ag deposits were annealed at 500 oC for 10 h in H2 to study the annealing characteristics of the co-deposits. Based on the LSV and coupon study, a composite Pd-Ag porous Inconel membrane was synthesized using glycine bath and was characterized for the H2 permeation at successive temperatures of 250 (40 h), 300 (60 h), 350 (70 h) and 400 oC (110 h).
The LSV scans of the NH3 bath showed that the Pd deposition occurred at approximately 650 mV higher cathodic potential than the Ag deposition. The Ag deposition was limited by the mass transfer of Ag ions to the sample surface resulting in inhomogeneous deposits with Ag concentrated more on the tip of the pores of the substrate than inside the pores. The LSV scans obtained from glycine and Na2EDTA baths showed characteristics similar to the NH3 bath. The Pd deposition occurred at 550 (Na2EDTA) and 510 mV (glycine) higher cathodic potential than that for Ag. The deposits obtained from NH3 and Na2EDTA baths were dendritic and porous in nature in comparison to those obtained from glycine bath which were relatively smooth with compact morphology. Therefore, glycine bath was further used to synthesize a Pd-Ag membrane.
The X-ray patterns of the deposits showed that the deposits were bimetallic in nature i.e. the deposits consisted of Pd and Ag particles and required further annealing for alloy phase formation. The annealing of the coupons with approximate 3.5 µm thick deposits obtained from the glycine bath showed almost complete alloying of Pd-Ag deposits when annealed at 500 oC for 10 h in H2 atmosphere as was evident by the X-ray pattern of the deposits which showed only single phase present after annealing. The annealing time and temperature for the co-deposits was considerably lower than that required for the deposits formed by the sequential coating and diffusion method which requires temperature of 550 oC or more.
No He flow was observed for as synthesized 8 µm thick membrane. The membrane on heating showed increase in the H2 permeance at 250, 300 and 350 oC due to the alloying of the Pd-Ag deposits. The EDS analysis of the cross-section of the membrane (with 6 wt% Ag overall) indicated more Ag concentrated towards the region away from the substrate similar to the coupon study. Also the deposits showed decline in H2/He selectivity (DP = 1 atm) on heating between 250-400 oC. The membrane at 400 oC showed H2 permeance of 36.7 m3/m2-h-atm0.5 and selectivity value of 175. The membrane showed slightly less permeance than that of pure Pd foil of the same thickness (46 m3/m2-h-atm0.5) which could be due to the error in the measurement of the thickness of the membrane. Also Ag wt% in the membrane was not significant enough to see the effect of Ag on the membrane permeance. The Ag wt% in the membrane could further be increased by simply increasing the Ag% concentration in the bath.
In conclusion, the large difference in the deposition potential between Pd and Ag resulted in inhomogeous composition of the deposits. Therefore modifying the complexing agents in the bath which provides close deposition potential of Pd and Ag could help to produce homogeneous deposits and requires further investigation. The glycine based bath showed relatively compact morphology and the deposits showed increase in the H2 permeance in the temperature range of 250-400 oC due to the alloying effect. The co-deposits also showed decline in H2/He selectivity on heating in the same temperature range.