Preparation And Performance Characterizations Of Graphene Supported Metal Nanostructures

Fuel cells are a kind of devices that can directly convert the chemical energy into electricity, and they have several advantages including no pollution, high conversion efficiency and a wide range of applications etc. Currently, several disadvantages e.g. high prices and low activity of traditional Pt anode catalysts have affected their commercialization processes. Therefore, the preparation of high-performance electro-catalysts is an important research topic of the fuel cell development. Small-sized metal particles have higher catalytic activity, but they are prone to agglomeration and catalyst poisoning and can not be reused. Taking this as a starting point, in this dissertation, we have synthesized a series of graphene (oxide) supported highly dispersed mono-, bi- and multi-metal nanoparticles composites using graphene (oxide) with high conductivity and high specific surface area as supporting materials, and systematically studied their electro-catalytic performances for the oxidation reaction of small organic molecules (methanol and formic acid) with the discussion of the metal-metal and metal-substrate synergistic effects as well as the morphological effect on the performance. The main contents are as follows:1. Graphene oxide (GO) nanostructures with different morphologies are synthesized by adjusting the content of the introduced Ni2+using a simple hydrothermal method. The three dimensional network nanostructure (3DPN-GO) is obtained without adding Ni2+, and a change of nanostructures from single-(SL-) to few-layer (FL-) GO occurs with Ni2+ amounts increasing.3DPN-GO has an excellent capacitive performance among them, which should be attributed to the highest specific surface area. The electrode prepared by 3DPN-GO has a mass specific capacitance of 352 F g-1 at 5 mV s-1, which is much better than those of SL-and FL-GO and higher than the recently reported value (240 F g-1) of 3D GO nanostructures. The GO controllable synthesis is of great significance for the design of composite nano-functional materials.2. Reduced graphene oxide (RGO) nanosheets supported Pt nanoparticles (NPs) composites have been synthesized via the one-step procedure with ethylene glycol (EG) as the reducer at constant 180℃. The obtained Pt NP is the monocrystal with an average size of 12 nm in diameter and the shape is irregular with less smooth edges. The presence of oxygen-containing functional groups on the GO surface is suggested to be responsible for the selective nucleation and growth of Pt NPs, and the aggregation of NPs does not occur, which results in the formation of small size Pt NPs dispersed uniformly on RGO nanosheets. The composite has a higher electrochemical active surface area value of 31.7 m2 g-1 with a larger ratio (If/Ir= 0.96) of the forward anodic peak current (If) to the reverse anodic peak current (Ir) towards the formic acid electro-catalytic oxidation reaction, and shows the excellent electro-catalytic stability.3. The uniformly dispersed bimetallic Co-Pd NPs with an average particle size of 5 nm supported on the RGO surface (CoPd-RGO) are synthesized via a two-step method, where firstly formed Co NPs are used as seeds for the subsequent growth of Pd and Pd is added to one side of Co NPs, forming Co-Pd bimetallic interfaces. The composite’s morphology, microstructure, size and composition are characterized, and the involved formation mechanism is also discussed. The CoPd-RGO composite has been studied for the electro-catalytic performance of the formic acid oxidation reaction in the alkaline condition. The results show that the CoPd-RGO has the highest catalytic activity and the best stability than those of the single-component Co-RGO and commercial Pd black. This is due not only to the small size Co-Pd NPs with bimetallic interfaces providing more active atoms accessible for reactants, but also the synergistic effect between metals and graphene. This method will be extended to synthesize other bimetals/RGO composites.4. The trimetallic Pt/Au@Pd-G composite has been successfully prepared by a two-step method. Firstly, graphene nanosheets supported core-shell structured Au@Pd bimetallic NPs can be synthetized by a one-step thermal reduction method, and the NPs with an average diameter of 11 ran can be homogeneously dispersed on the graphene surface. The particle number density as well as the Pd shell thickness can be controlled by changing concentrations of metallic precursors, and combined with experiment results, the one-step formation mechanism for this core-shell structure has been discussed. Then, the Pt can be further grown on the surface of Au@Pd bimetallic NPs. The Au@Pd-G bimetallic composite exhibits significantly higher catalytic activity and stability than monometallic counterparts (Pd-G and Au-G) toward the methanol electro-oxidation reaction in acidic media, which indicates that the Au@Pd-G has the important application prospect in methanol fuel cells.5. In order to study the effect of the metal particles’morphology on the catalytic performance, RGO nanosheets supported spherical dendritic Pt particles have been prepared by the microwave-assisted one-step method. The Pt particles consist of 5-7 nm grains and are well dispersed on the surface of RGO. Adjusting the precursor concentration can change the nanograins’number inside an individual Pt particle, and the Pt particles’diameters can be controlled in a wide range. The presence of oxygen-containing functional groups on the surface of nanosheets and microwave selective heating are suggested to be key roles for the selective nucleation and growth of Pt, and a fast spontaneous nanoparticle assembly leads to the dendritic morphology. Due to the morphology effect and the interface effect between grains inside an individual particle, the RGO-Pt obtained with Cpt-ion=3.1 mM has the highest electrochemical active surface area value of 56.9 m2 g-1 with the largest ratio (If/Ir= 0.97) towards the formic acid electro-oxidation. The corresponding current density of the formic acid oxidation is 2.95 mA cm-2, which is higher than that of the reported Pt/graphene (2.39 mA cm-2).