Characterization and evaluation of Pt-Pd electrocatalysts prepared by electroless deposition

•Electroless deposition of Pt used to prepare Pt shells on Pd particles.•Thickness of Pt shells controlled by amount of Pt salt in ED bath.•STEM and EDS confirm formation of controlled-thickness Pt shells.•Core-shell Pd-Pt/C catalysts very active for oxygen reduction reaction.•Sample containing 0.9 ML Pt on Pd had mass activity of 329 A/g Pt.

Semi-continuous electroless deposition (ED) methods have been developed for preparation of variable and controlled coverages of Pt on Pd surfaces. The deposition of Pt occurred in an aqueous bath containing a reducible metal salt (PtCl62−), reducing agent (hydrazine) and stabilizer (ethylenediamine). To avoid electrostatic adsorption of PtCl62−, bath pH was controlled at pH 9.0, which was higher than the PZC of the carbon support, to create a negatively-charged carbon surface. Bath stability was maintained by addition of ethylenediamine and limiting the concentration of N2H4 in the bath to prevent thermal reduction of PtCl62− to form Pt0. The concentration of N2H4 was controlled by pumping N2H4 solutions at various pumping rates into the ED bath. Thus, bimetallic Pt-Pd particles with Pt loadings of 6.0, 11.7, 17.2, and 22.7 wt% were selectively deposited on Pd surfaces of 30 wt% Pd/C. The structures of the catalysts were determined by STEM and EDS as variable thickness Pt shells with Pd cores. Pt loadings of 6.0, 11.7, 17.2, and 22.7 wt% corresponded to Pt shells of 0.9, 1.7, 2.7, and 3.4 monolayers (ML) on Pd. The catalysts were evaluated for their oxygen reduction reaction activity. The core-shell Pd-Pt/C catalysts were very active, especially the sample containing 0.9 ML Pt coverage on Pd with a mass activity of 329 A/g Pt compared to 183 A/g Pt for a conventional 50.5 wt% Pt/C sample. Similarly, electrochemical surface areas (ECSA) for all Pt shell samples (72–211 m2Pt/g Pt) were higher than for the conventional catalyst (58 m2Pt/g Pt).

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