Carbon nanotube-supported dendritic Pt-on-Pd nanostructures: growth mechanism and electrocatalytic activity towards oxygen reduction reaction

Abstract

We describe the growth of Pt-on-Pd dendritic nanostructures of different compositions (Pt₆₄Pd₃₆, Pt₅₂Pd₄₈ and Pt₂₁Pd₇₉) over multiwall carbon nanotubes (MWCNTs) and their electrocatalytic performance towards oxygen reduction reaction (ORR). The reduction of metal precursors (PtCl₆²⁻ and PdCl₄²⁻) was achieved using ascorbic acid (AA) in the absence of surfactants or shape regulating agents. The difference in the reduction potential of Pt and Pd precursors favors their sequential reduction. The mechanism of the growth of dendritic nanostructures involves (i) the initial reduction of Pd(II) and Pt(IV) complexes to Pd(0) and Pt(II), (ii) complexation of in situ generated dehydroascorbic acid (DHA), with Pt(II) species, and (iii) reduction of the Pt(II)–DHA complex by the excess AA and the growth of dendritic nanostructures over the surface-confined Pd seeds. The bimetallic nanostructures are 25–55 nm in size with an average branch width of 2–3 nm. The elemental composition-dependent electrocatalytic performance of the bimetallic nanostructures towards ORR was evaluated in terms of specific activity, onset potential and durability. The nanoelectrocatalyst of Pt₆₄Pd₃₆ composition has the highest area and mass specific activity at 0.9 V (342 μA cm⁻² and 36.5 μA μg⁻¹) and is highly durable compared to other compositions. The electrocatalytic activity of the bimetallic nanoelectrocatalyst is compared with that of conventional spherical nanoparticles synthesized under identical conditions as well as with the commercial catalyst. The area specific activity of Pt₆₄Pd₃₆ composition was found to be ∼7.5 times higher than that of the commercial Pt black, reflecting its superior electrocatalytic activity.