Catalyst Support on Carbon Nanoballoon and Evaluation of Its Catalytic Activity in Direct Methanol Fuel Cells

Abstract

The reduction of CO2 emissions from cars has recently become an important issue, and the development of fuel cell vehicle (FCV) is now in progress. Two problems to be solved for its wide commercial use of fuel cells are the efficient use of platinum (Pt) and ruthenium (Ru) catalysts and the improvement of power density. Carbon black, nanometer-size carbon particles, is commercially used as a catalyst support in fuel cells owing to its high surface area, porosity, electric conductivity, low density, and low cost. Carbon nanomaterials have unique characteristics. In the previous work, we supported PtRu catalysts on various carbon nanomaterials with different geometry and evaluated the catalytic activity of the supported catalysts for direct methanol fuel cell (DMFC) [1]. In this study, we used carbon nanoballoon (CNB) as a catalyst support and measured the catalytic activity of CNB-supported PtRu catalysts. Arc black (AcB) was produced by an arc discharge of graphite in N2 atmosphere as the precursor of CNB. The twin-torch arc discharge apparatus was used for AcB synthesis. AcB is mainly composed of cocoon-shaped carbon nanoparticles with a lot of amorphous ingredients. CNB was prepared by a heat treatment of AcB in Ar gas at 2600°C for 2 h [2]. AcB and CNB comprised spherical particles of 50 nm in diameter. The particle shape of CNB is hollow. CNB is graphitic and is expected to have high conductivity. Carbon nanocoil (CNC) was synthesized using an automatic chemical vapor deposition system with consecutive substrate transfer mechanism [1]. The fiber diameters of the CNCs was ~300 nm, and the coil diameters of the CNCs was ~1000 nm. We prepared PtRu catalysts for the DMFC anode. The PtRu catalysts were loaded onto CNB by the reduction method using sodium boron hydrate (NaBH4), and counterparts employing the commercial Vulcan-supported PtRu catalyst and CNC-supported PtRu catalyst were also prepared. Hydrogen hexachloroplatinate (IV) hexahydrate (H2PtCl6·6H2O) and ruthenium trichloride (RuCl3) were used as the Pt and Ru precursors, respectively. The molar ratio of Pt and Ru was set at 1:1. Each of the carbon nanomaterials (200 mg) was dispersed in 500 mL of deionized water by sonication for 20 min. H2PtCl6·6H2O and RuCl3 were stirred in 50 mL of deionized water at 60 rpm for 10 min. The solutions were mixed and stirred at 600 rpm for 10 min. Next, a 30-fold molar excess of NaBH4 with respect to the metal precursors was added to 400 mL deionized water. This NaBH4 solution was added to the metal precursor and carbon nanomaterial mixture and stirred. The solution was then filtered, washed and dried to obtain the supported catalyst. The carbon nanomaterials were characterized using scanning electron microscopy (SEM), a laser Raman spectroscopy, Brunauer–Emmet–Teller (BET) measurements, and compressive resistivity measurements. The morphological characteristics and crystalline structure of the prepared catalysts were analyzed using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The amounts of catalyst loaded on the carbon nanomaterials were analyzed by thermo-gravimetric analysis (TGA). We measured the electrochemical property of the PtRu catalysts supported on the carbon nanomaterials with different shapes, size, and electrical properties. The same amounts of catalyst were loaded on each carbon nanomaterial. The catalyst amount was measured to be 30 wt.% by TGA. Their catalytic activities were measured by cyclic voltammetry (CV) in H2SO4 and CH3OH/H2SO4 electrolytes. The highest methanol oxidation reaction (MOR) current densities were observed for the PtRu catalysts supported on CNB. The catalyst activity of the CNB-supported catalysts was higher than that of the Vulcan-supported catalysts. This is mainly due to the higher electrical conductivity benefiting from the structure of CNB.