| Nanomaterials used as the direct formic acid fuel cells (DFAFCs) anode have been studied for nearly 10 years. The DFAFCs is considered to be one of the most promising options for the power source in portable electronic devices. Compared with the direct methanol fuel cells (DMFCs), formic acid has lower crossover through Nafion membrane and allows higher fuel concentrations or more thin film. Formic acid is a liquid at room temperature of material, and is a safe food additive. Since 2002, Pt catalysts anode catalyst has been widely used for DFAFCs. CO "poison" and Pt loading are the problem need to be solved to realize the commercialization of DFAFCs. Pt-based alloy catalyst with a second metal, such as Pt/Ru and Pt/Pd, Pt/Au has been studied as DFAFCs anode catalyst. Among them Au-Pt electrocatalysts have attracted much attention because of their higher stability and electrocatalytic. Nanowires synthesized chemical reduction method and the anodic aluminum oxide deposition are both need secondary processing step or be time-consuming process. Direct electrodeposition assemble nanowires can deposite nanowires form solution. In this method, an alternating electric field applied to a salt solution induces the crystallization of metallic wires or dendrites with metal bond. Metal ions reduce into metal nuclei which can assemble into continuous nanowires with high mechanical strength by electric field force. The nanowires can be used as anodic for formic acid oxidation without be connected renewably before applications.In this thesis, PtAu alloy nanowires were synthetized by direct electrodeposition assembly between gold microelectrode. The morphology and composition of PtAu alloy nanowires were controlled by parameters of a lternating current and electrolyte composition. SEM, TEM, EDS and XRD were used to characterize the morphology, composition and structure of the nanowires. The effect of AC parameters and electrolyte on the synthesis of gold platinum alloy nanowires were discussed. PtAu alloy nanowires with different composition were used to study the activity and stability of formic acid electro-oxidation. The optimum composition ratio were determined. The results are obtained as following:(1)The additive in the electrolyte has direct influence of the branching degree and composition range of the nanowires.0.5 mol-dm-H3BO3 can stabilize the electrolyte. The conductivity and the deposition rate are both increased, which lead to dendritic nanowires. The addition of NH4CI to the electroplating solution results in a dramatic suppression of side branch formation. NH3-H2O as the addition can form new complexes, that decrease the reduction rate of platinum. And the surface is roughness.(2)Change HAuCl4 and H2Ptl6 concentration can adjust the range of Pt/Au ratio of the alloy nanowires. The higher concentration result in bigger diameter of nanowires. Nanowires with more Pt component display more smoother.(3) Voltage and frequency are the key factors of direct electrodeposition. The voltage threshold can provide power for the reduction of metal ions. The electroplating solution with 13mmol·dm-3 H2PtCl6·6H2O,0.2 mmol·dm-3 HAuCl4·4H2O and 0.5 mol·dm-3 H3BO3 was uesd to deposited PtAu alloy nanowires. When the deposition frequency remained at 20 MHz, low voltage lead to rough alloy nanowires. The voltage range is 16-18 Vpp can obtained PtAu alloy nanowires with distinct structure.(4)The voltage and frequency provide sufficient overpotential to fabricate nanowires. With the frequency increased from lto 20MHz at 16 Vpp, metal ion reduction rate and nanowires growth rate is faster. Highly branched nanowires with smaller diameter were obtained. The Pt atomic fraction increased witn the increased of the frequency.(5)Nanowires grow along the direction of electric field, and dendritic is more likely to occur in a strong regional and a smaller field strength is easy to form one-dimensional structure. The growth mechanism of PtAu alloy nanowires is explained by electrodeposition direct assembly theory. The formation of dendrites is a typical product of diffusion-limited process.(6)The crystalline structure of Au, Pt, PtAu nanowires were characterized by XRD. The peaks of PtAu were located between Au and Pt X-ray diffraction patterns. And no single metal characteristic peak appeared. TEM and FFT results showed that alloy nanowires were single-phase alloy structure.(7)The obtained Au, Pt, Pt35Au65 and Pt79Au21 nanowires were measured in 0.5 mol-dm-3 HCOOH and 0.5 mol-dm-3 H2SO4 by by cyclic voltammetry. The results showed that the oxidation current density of Pt35Au65 and Pt79Au21 nanowires were 3 and 1.64 times higher than Pt nanowires, respectively. The oxidation potential of Pt35Au65 was the most negative one. There was a linear relationship between square root of the oxidation current density and scan rates. And it suggest that the reaction is a diffusion controlled process in the electrode surface. |
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