Study On Controllable Synthesis And Oxygen Reduction Properties Of Pt, Pd Nanocrystals Functionalized With Polyallylamine

The synthesis of Pt-based and Pd-based nanostructures with controlled size, morphology and composition has attracted enormous interest due to the electronic effect, geometric effect and facet effect, as well as invaluable applications in heterogeneous catalysis and fuel cells. Very recently, a new trend in electrocatalyst designs has been proposed, which is based on the modification of the metal surface by nonmetallic molecules that can block the adsorption of poisoning species and spectator anions. In the paper, we report a convenient and environmentally friendly water-based route to synthesize the Pt and Pt-based nanostructures with controlled morphology, size and composition in the presence of polyallylamine (PAH). In the process, the influence of experimental parameters, such as the reaction temperature, reaction time, solution pH and the type of reducing agents, on the size, morphology and composition of nanostructures will be investigated systemically. Furthermore, the electrocatalytic activity, stability and alcohol tolerant ability of PAH functionalized Pt-based and Pd-based nanostructures towards the oxygen reduction reaction will be explored by using various electrochemical techniques. In addition, the effect of PAH layer adsorbed on nanostructures surface on electrocatalytic activity, stability and alcohol tolerant ability of nanostructures also demonstrated detailedly by analyzing experimental data.The main results and conclusions are summarized as follows:1. We successfully synthesized PAH functionalized Pt nanocubes (Pt-NCs) with ORR activity by manipulating electronic effect, geometric effect and facet effect. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements demonstrated the PAH functionalization affects significantly the geometric structure and electronic structure of Pt nanoparticles. The as-prepared Pt-NCs showed superior electrocatalytic activity and stability towards the ORR in both H2SO4and HClO4electrolytes compared to the commercial Pt black, indicating PAH functionalization is a feasible way for the electrocatalyst design.2. The positively charged pony-size Pt-NCs were synthesized and directly assembled onto the negatively charged carboxylated CNTs through electrostatic interaction for decreasing Pt cost and improving ORR activity. Compared to the commercial Pt black, the Pt-NCs/CNTs nanohybrids showed superior electrochemical activity and stability towards the ORR. Moreover, cyclic voltammetry tests showed the Pt-NCs/CNTs nanohybrids had excellent alcohol tolerant ability due to the space steric effect of PAH molecules.3. The three-dimensionally (3D) interconnected Pt nanolance assemblies (Pt NLAs) were synthesized in the novel liquid-liquid two-phase system. The PAH functionalization improved the electrocatalytic activity of the Pt NLAs towards the ORR due to high interface proton concentration on the Pt surface and excellent anti-oxidation ability of the Pt nanocrystals. Meanwhile, the PAH molecules bound on the Pt NLAs surface could act as barrier networks to restrain accessibility of alcohol, exhibiting a high ORR selectivity. In addition, the PAH functionalized Pt NLAs showed superior durability for the ORR due to their particular interconnected structure.4. PAH functionalized Pd icosahedra were synthesized by a facile hydrothermal method, which possess a lower price compared to that of the Pt-based catalysts. The protective function of PAH layers and resultant enhanced anti-etching capability of Pd atom were responsible for the formation of the Pd icosahedra. Very importantly, the as-prepared PAH functionalized Pd icosahedra exhibited superior electrocatalytic activity and ethanol tolerant ability towards the ORR compared to the commercially available Pt black in alkaline media. This results fully shows that Pd, to a certain extent, could replace Pt t as a electrocatalyst towards the ORR.5. The Pt-Pd alloy nanoflowers (Pt-Pd ANFs) were successfully synthesized through a facile co-chemical reduction method in a PAH based aqueous solution, which reducing the utilization of Pt and simultaneously improving the ORR activity due to the modified geometric and electronic structures of Pt atom. The electrochemical tests indicated the Pt-Pd ANFs exhibited superior ORR activity along with satisfactory stability and methanol-tolerant ability under acidic conditions.