Synthesis of Au@Pd core/shell nanomaterials for high-performance ethanol oxidation reactions

Abstract

This thesis summarizes my postgraduate research on the studies of Pd-based electrocatalysts for ethanol oxidation reactions, mainly on two directions: (1) the fabrication of a high-surface-area electrode, Pd coated Au nanowire (AuNW) forest on porous nickel foam substrate, to enhance the catalytic activity of ethanol oxidation reactions, and (2) cyclic voltammetry (CV) guided deposition of submonolayer Pd on Au surface with fine-tuning Pd thickness and real-time monitoring of the electrochemical properties. In Chapter Two, a modified procedure for the growth of AuNW forest on porous nickel foam substrate (previously only on oxide substrate), was proposed by changing the surface ligand from amino to cyano functionalized silane and applying the stirring condition. As a twopronged strategy, the combination of AuNW and nickel foam could enhance the electrochemical active surface over 20 times, compared with Au nanoparticles decorated flat substrate. After the pulsed-current deposition of 1 nm Pd shell on Au, this flexible electrode showed significantly improved catalytic activity on the electro-oxidation of ethanol with high tolerance to hydroxyl and ethoxyl poisoning. The peak current density of as-prepared electrocatalyst was 3, 11 and 48 times higher than that of Pd coated bare nickel foam, Pd coated AuNW on fluorine doped tin oxide (FTO), and the state-of-the-art catalyst Pd on activated carbon (Pd/C), respectively. In Chapter Three, one convenient and effective deposition method for the coating of Pd overlayer on Au surface was realized by mimicking the primary cell in the 10 mM PdCl2 deposition solution. The deposited amount of Pd can be tuned by the concentration of Pd precursor. When the concentration of PdCl2 was reduced to 2.5 mM, the Au surface would be partially coated with Pd domains and the CV scan could facilitate the physical movement of Pd atoms to realize full coverage of the exposed Au surface. The mass-specific current of the Pd coated AuNW nickel foam electrode on ethanol oxidation can be enhanced to 6464 A/g, over two times higher than that of typical electrochemical deposited one. On the basis of a series of control experiments, the Pd movement mechanism was proposed that the Pd atoms may firstly get oxidized to free Pd ions in the electrolyte, which are then reduced back to cover the Au surface. In Chapter Four, to solidify our proposed Pd movement mechanism, solution form of Pd precursor was added into the system, replacing previous solid state Pd. With potential cycling in the 16 μM PdCl2, 1.0 M ethanol and 1.0 M NaOH electrolyte, sub-monolayer of Pd deposition on Au can be readily achieved. During CV scans, the onset potential and peak current density of backward peak reveals the binding affinity of –OH group and the catalytic activity, respectively. With such real-time monitoring, the thickness of the Pd overlayer can be continuously and precisely tuned by the number of CV cycles and directly correlates with the electrochemical properties. XU WEICHANG