Synthesis, Self‐Assembly, and Electrocatalysis of Polyallylamine‐Functionalized Platinum Nanocubes

Abstract

Low-temperature polymer electrolyte fuel cells (LPEFCs), such as proton-exchange membrane fuel cells, direct alcohol fuel cells, and direct formic acid fuel cells are regarded as promising power sources for automotive, portable and stationary systems. [1] Platinum-electrocatalytic oxygen reduction reaction (ORR) is one of the key reactions in LPEFCs. However, the sluggish kinetics of ORR and poor stability of electrocatalysts holds back the commercialization of LPEFCs. [1, 2] To address this issue, numerous efforts have focused on the design and manipulation of morphology, [3] composition, [4] and chemical modification [5] of Pt electrocatalysts, where rather significant progresses have been achieved. For example, Pt{100}-terminated Pt nanocubes (Pt-NCs) exhibited significantly enhanced specific activity towards the ORR in H 2 SO 4 electrolyte resulting from the facet effect. [6] In particular, the cyanide-functionalized Pt{111}-CNad showed a 25-fold increase towards the ORR compared with naked Pt{111} facets in the H 2 SO 4 electrolyte. [6] To make the most use of the Pt metal and lower the cost of LPEFCs, Pt nanoparticles are loaded on carbon nanotubes (CNTs) because of their extraordinary physical, chemical, and mechanical properties, such as large surface area, high electrical conductivity, special chemical stability, and good corrosion resistance. [7] Herein, the polyallylamine (PAH)-functionalized Pt nanocubes (Pt-NCs) are successfully synthesized through a facile hydrothermal method. The positively charged Pt-NCs are directly assembled onto the negatively charged carboxylated CNTs through electrostatic interaction. The as-prepared Pt-NCs/CNTs nanohybrid show enhanced electrocatalytic activity, durability, and alcohol tolerance towards the ORR compared with commercial Pt/C catalyst.Solution-phase growth is an effective route for the synthesis of single-crystal Pt nanostructures having high quality. In a typical synthesis of the Pt-NCs, solutions of K 2 PtCl 4 , PAH, and HCHO were mixed together with water, and the pH of the resulting mixture was adjusted to 3.0. The homogeneous solution was heated at 140 8C for 6 hours to obtain the PAH-functionalized Pt-NCs. Then, the carboxylated CNTs were added into the above Pt-NCs suspension and sonicated for 2 hours, then separated by centrifugation. Finally, the as-prepared Pt-NCs/CNTs nanohybrid was consecutively treated with UV/ ozone and electrochemical cleaning to remove the majority of capping agent (i.e. PAH) prior to electrochemical testing (see the Experimental Section for details).The morphology of the Pt nanocrystals was investigated by transmission electron microscopy (TEM). As observed, the asprepared Pt nanocrystals are well-defined and preserve a cubic shape (87 % cubes, 4 % triangles, and 9 % irregular shapes, Figure 1 A). The average edge length of the Pt-NCs is 4 nm. The well-resolved lattice fringes and selected area electron diffraction (SAED) pattern indicate the single crystal structure of the Pt-NCs (Figure 1 B). The d spacing of adjacen...

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