Preparation of Extremely Small Pt Nanoparticle Catalysts and Their Electrocatalytic Activity for Oxygen Reduction Reaction

Introduction

Polymer electrolyte fuel cells (PEFCs) have attracted great attention as the next generation energy device for electric vehicles due to their high energy conversion efficiency and low emission of pollutants. Pt nanoparticles-loaded carbon black (Pt/CB) is widely used as an electrocatalyst in PEFCs. However, it is necessary to minimize the consumption of Pt because of its high cost and diminishing reserves.

CO is known to be strongly adsorbed on Pt atoms. If Pt nanoparticles are prepared in a CO atmosphere, the growth of Pt may be suppressed. Based on the above idea, we have prepared a Pt-carbonyl cluster complex ([Pt₃(CO)₃ (μ-CO)₃]₃²⁻) by bubbling CO in an acetonitrile-water (1:1 in volume ratio) mixed solution containing PtCl₆²⁻.¹ The complex spontaneously adsorbed on CB, and oxidatively broke and aggregated to form Pt nanoparticles in air. The resultant Pt nanoparticles had the monodispersed particle size of 1.8±0.3 nm. If the Pt-carbonyl cluster complex were fixed on CB with a stabilizer, the aggregation of the resultant Pt nanoparticles could be suppressed. In this study, we designed to inhibit the growth of Pt nanoparticles by electrostatically binding several kinds of stabilizers with the anionic complex on CB before converting to Pt. We characterized the resultant extremely small Pt nanoparticles-loaded CB and evaluated its catalysis for oxygen reduction reaction.

Experimental

Several cationic molecules, polydiallyldimethylammonium-chloride (PDDA), methyl-viologen(MV), benzyl-viologen(BV), cetyltrimethylammonium-bromide(CTA), pyridine(Pyr), bipyridine(Bpy) and terpyridine (Tpy), were used as a stabilizer in this study. They were mixed after the Pt-carbonyl cluster complex dianion was adsorbed on CB, leading to the production of ionic couples due to electrostatic interaction.

Results and Discussion

Pt nanoparticles prepared with stabilizers except CTA were smaller in mean size than conventional Pt nanoparticles (ca. 1.8 nm) prepared without stabilizer, suggesting that each stabilizer inhibited the growth of Pt nanoparticles on CB.

In ORR, specific activity (SA), activity per unit electrochemical surface area of Pt, and mass activity (MA), activity per mass of Pt, for the Pt/CB catalyst prepared with bipyridine (Pt/CB+Bpy) were ca. 1.4 and 1.6 times higher than those of the commercial catalyst (Pt/CB-TKK(Tanaka Kikinzoku Kogyo TEC10E50E)), respectively. X-ray photoelectron spectra exhibited that in Pt(4f) core level spectra of Pt/CB+Bpy, a peak assigned to metallic Pt shifted to higher binding energy because of the final state effect and a decrease in electron density of Pt surface atoms due to adsorption of electron-withdrawing stabilizers, which seems to weaken Pt-O bond and increase SA and MA.

Reference

1. E. Higuchi, A. Taguchi, K. Hayashi, H. Inoue, J. Electroanal. Chem., 663, 83-89 (2011).