Efficient and clean synthesis of graphene supported platinum nanoclusters and its application in direct methanol fuel cell

Abstract

The graphene supported platinum nanoclusters was synthesized by an efficient and clean method, in which graphene oxide and Pt ion precursor were reduced by ascorbic acid within one-step process. The obtained Pt nanoclusters attached graphene composite (PtNCs/graphene) was characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS), which can directly show that Pt nanoclusters successfully formed on graphene and well distributed on the edges and wrinkles of graphene sheets. The further electrochemical characterizations including cyclic voltammograms (CV), current–time methods indicated that PtNCs/graphene has significantly higher electrocatalytic activity and stability for methanol electrooxidation compared to the normal Vulcan XC-72 and graphite supported Pt nanoclusters, which will lead a further application as a new electrode material in direct methanol fuel cell (DMFC).

Introduction

Direct methanol fuel cell (DMFC) in which methanol is used as the fuel has been focused intensive interests for several decades. The main advantages of DMFC compared with the other fuel cell are the portability and easy access of methanol and the high energy density, an order of magnitude greater than compressed hydrogen. However the methanol cross-over and the poison effect of intermediate, e.g. carbon monoxide (CO) to the catalysts mainly limit the performance of DMFC to the commercial market [1], [2], [3], [4]. Decreasing the effect from intermediate and enhancing the efficiency of catalyst are the main issues in the research of DMFC. Platinum is mainly chosen as the catalyst because of its highest catalytic activities of all pure metal for the methanol oxidation reactions in DMFC. But its high price and limited availability in nature hamper its using amount in DMFC [5], [6]. Unstable catalysis capability resulted from the poison effect of intermediate is another main barrier. It is well known that catalysts’ specific activity is chiefly depended on their distribution and size [1], [7]. In order to reduce the Pt loadings and enhance its catalytic activities, controlling the size of Pt in nanoscale is proved to one effective strategy [3], [8]. Owing to the high surface-to-volume ratio, platinum nanoparticles which have small size and narrow size distribution are perfect for high electrical catalyst activity. For this reason, the support materials with high surface area and good conductivity are important so as to increase Pt electrochemical activity and utilization [8], [9].

Graphene, this remarkable two-dimensional material has attracted extensive interests [10], [11], which is also looked forward to be the best candidate as the support material of Pt [12], [13], [14], [15]. Compare to traditional carbon materials, grapheme has larger specific surface area (calculated value, 2630 m²/g)[9], [16], better mechanical strength, higher conductivity (10³–10⁴ S/M)[12]. All the carbon atoms are tightly bounded by sp²-bond [15], make this special material a honeycomb lattice and atomic thickness [17]. This special nanostructure show great promise as new electrode material for its potential applications in many fields such as fuel cell, super capacitor and solar cell. Considering the high surface area, excellent mechanical and electrical properties, graphene is the most promising candidate as the supporting material to load platinum nanoparticles [5], [14], [18].

Therefore, the approaches for combining the graphene and Pt nanoparticles are widely investigated in recent years. Normally graphene and Pt nanoparticles were prepared separately and then complex them in assistant of the polymer or other glue molecular [12], [14], [19], [20]. The stepwise procedures sophisticate the operating and the bridging agents using might decrease the catalysis ability of Pt. In our previous report, it is possible to synthesis Pt nanoparticles on the substrate (ITO and GC) just by reducing the platinum precursor by ascorbic acid, which shows satisfied attachment and high catalysis performance for the methanol oxidation [21], [22]. In recent reports, it is also verified that ascorbic acid could reduces the oxygen-containing groups on the surface of graphene oxide such as carboxylic, epoxy groups [23], [24], [25]. Graphene with few layer structure and good conductivity could be prepared by this procedure. The outstanding advantage for using ascorbic acid is due to its moderate reducibility and environment-friendly reductant compared with the vigorous and poison reductant used in the chemical preparation of graphene, e.g. hydrazine and sodium borohydride [23], [26].

Ascorbic acid could reduce Pt ion to Pt nanoparticles on the substrate and the graphene oxide to graphene slide which has comparable conductivity with the graphene reduced by the hydrazine. It is possible to combine the stepwise reduction to one step by using excessive ascorbic acid. In addition, the oxygen-containing functional groups existed on the surface of graphene oxide sheets could provide more active place for anchoring Pt nanoparticles after reducing, so that Pt nanoparticles can firmly fixed on the surface of graphene and hardly to aggregate after reduction [9], [14], [27], [28], [29]. In this study, Pt nanoclusters loading and the reduction of graphene oxide were performed by one-step reducing process, in which graphene oxide and PtCl₄²⁻ were reduced together in solution by reducing agent-ascorbic acid. The as-prepared Pt nanoclusters distributed well on the surface of graphene through TEM and FE-SEM characterization. The further electrochemical experiments directly showed this composite has an extraordinary ability as the catalyst in DMFC, more stability and anti-poison ability for methanol oxidation, which strongly indicated the superiority of our method. This graphene nanocomposite was expected to work as the new electrode materials for DMFC application.