Preparation of Ultrathin Palladium Nanosheet and Its Application in Pd@Pt Core-Shell Catalyst for Oxygen Reduction Reaction

Abstract

High cost, sluggish kinetics, and poor durability of Pt catalysts predominantly hinder the wide-spread commercialization of fuel cells. Pt-based core-shell nanosheet catalyst may resolve　these problems listed above because of the following 3 advantages. 1) The Pt content can be greatly reduced via core−shell nanostructures consisting of a Pt shell on appropriate monometallic or alloy cores; 2) Nanosheets have high surface-to-volume ratio and terrace sites. The active sites for the oxygen reduction reaction (ORR) have been suggested to be located on the terrace sites of the nanocrystals.[1] 3) Nanosheets should dramatically reduce Pt dissolution due to few edges and corners with their low coordinate sites. Terraced facets have been reported to be more stable than edges and corners.[2] Here, we present a novel method to synthesize palladium (Pd) nanosheets by a wet-chemical preparation at room temperature (Fig.A). The Pd precursor (Pd(acac)2) is reduced by CO in the presence of decylamine (DA). The protection agent (DA) was easily removed by washing with acetic acid. Using this nanostructure as a core, Pd@Pt core-shell nanosheets were prepared by surface limited redox replacement (SLRR). AFM indicates Pd nanosheets are synthesized with a thickness around 0.9 nm to 1.6 nm and lateral size from tens to several hundred nanometers (Fig.B). After dispersing the Pd nanosheet on a carbon support, Pd@Pt core-shell nanosheets with different Pt shell thickness were prepared, i.e. Pd6.0ML@Pt2.7ML, Pd6.0ML@Pt4.4ML, and Pd6.0ML@Pt5.9ML. The electrochemically active surface area (ECSA) of Pd@Pt nanosheets are 56, 48, 60 m2/g-PGM (90, 66, 77 m2/g-Pt), respectively (Fig. C). The linear sweep voltammetry is applied by using a rotating disc electrode (RDE) in 0.1 M HClO4 at room temperature. The ORR activity is evaluated from jk values at 0.9 V (vs RHE) estimated from Koutecky-Levich Plots. The Pd@Pt nanosheets demonstrated mass activity of 364, 685, 598 A/g-Pt (227, 501, and 492 A/g-PGM), which is much higher than the benchmark Pt/C catalyst (177 A/g-Pt) for the oxygen reduction (Fig. D). This research was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Fig. A) TEM of Pd@DA nanosheets; B) AFM image of Pd nanosheets; C) CV profiles of the Pd@Pt nanosheets recorded in N2-saturated 0.1 M HClO4 solution at a sweep rate of 50 mV/s; and D) The mass activity of Pd@Pt nanosheets and commercial Pt/C. Reference F. J. Perez-Alonso, D. N. McCarthy, A. Nierhoff, P. Hernandez-Fernandez, C. Strebel, I. E. L. Stephens, J. H. Nielsen, I. Chorkendorff, Angew. Chem. Int. Ed. 2012, 51, 4641–4643.F. N. Büchi, M. Inaba, T. J. Schmidt, Polymer Electrolyte Fuel Cell Durability, Springer Science+ Business Media, New York, 2009. Figure 1