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内容記述 |
Due to the high electronic conductivity and variety of forms, various carbon<br />nanomaterials have been developed in recent years. In particular, porous carbon<br />materials and graphitic carbon materials are very promising as electrode materials<br />in supercapacitors, lithium ion batteries, and fuel cells. Recent demands for<br />electrode materials are oriented toward the development of new porous carbon<br />materials with high conductance both for the electron and ion transportations.<br />However, it has been extremely difficult to combine the properties of porous<br />structures and graphitic ones.<br /><br /> We have synthesized silver acetylide (Ag<small>2</small>C<small>2</small> : Ag-C ≡C-Ag) with<br />3D-interconnected frameworks and converted the acetylide to a new carbon<br />material, mesoporous carbon nano-dendrites (MCNDs) with ultra-thin graphitic<br />structure. MCNDs are synthesized by controlling the highly exothermic<br />segregation reaction of silver acetylyde into carbon with porous structure and<br />silver-vapor. The dendroide acetylides were quickly warmed to 250℃ emitting a<br />brilliant flash of raddish orange light with a thunderous sound indicative of the<br />sudden jump of the local temperature to higher than 2000℃. The sudden heating<br />boils off the silver from the main body, leaving 3D-interconnected carbon<br />frameworks, the MCNDs.<br /> Raman spectra of MCNDs clearly indicate that carbon frameworks consist<br />of mainly graphitic structure of 1-3 layers. SEM and TEM images as well as<br />EELS spectra show that the main body with ～100 nm radii branches every<br />100-150 nm and are composed of cells with ultra-thin graphitic walls. The BET<br />(Brunauer-Emmett-Teller) surface area of MCNDs was estimated to be 1324-1996<br />m<sup>2</sup>/g from the nitrogen adsorption and desorption isotherm at 77 K, and the<br />adsorption-desorption curves indicate the presence of micropores (pore size: <2<br />nm) and mesopores (2-50 nm) with a continuous size distribution up to 10-20 nm.<br />These results show that the MCND combines porous structures with graphitic<br />ones.<br /><br /> One of the most suitable applications of MCNDs is to the supercapacitor<br />electrodes. In this case, the high fluidity of solvent phase is also demanded as well<br />as large surface area. The dendritic structure and the presence of mesopores on the<br />surface area of MCNDs can be well suited for these requirements. In order to<br />examine the electrochemical properties of MCNDs as supercapacitors electrodes,<br />the author assembled Sandwich-type capacitors on a platinum current collector<br />with two carbon electrode sheets consisting mainly of MCND, and<br />polytetrafluoroethylene (PTFE) porous separator were assembled. Cyclic<br />voltammetry of a supercapacitor with MCND electrodes showed good rectangular<br />curves, even at a scanning rate of 300 mV/s and peak current density higher than<br />l0 A/g, suggesting applicability for high current and high-speed charge-discharge<br />capacitors electrodes.<br /><br /> The mesopore on the surface area of MCNDs are also available as<br />impregnate sites of catalyst metals and lithium storage metals. We successfully<br />impregnated tin(Sn) and platinum(Pt) metals in the mesopores of MCND through<br />the adsorption of SnCl<small>2</small> or H<small>2</small>[PtCl<small>6</small>]・(H<small>2</small>O)<small>6</small>, and<br /> the reduction with hydrogen gas.<br /><br /> Sn nanoparticles with an average size of 10 nm were prepared in the pores<br />of MCNDs by chemical reduction of SnCl<small>2</small> with hydrogen gas for anode materials<br />of lithium ion butteries. The nanoparticles grow with increasing reduction<br />temperature and some pores were occupied with Sn almost entirely. One of the<br />major problems to prevent the practical use of Sn solid single electrodes is poor<br />cycleability due to the large volume changes during lithium alloying and<br />dealloying. However, Sn/MCND composites exhibit significantly enhanced<br />cycling performance for lithium storage. When used as a lithium ion battery, we<br />find that a first discharge capacity of 646 mAh/g and the capacity retain a value of<br />481mAh/g can be obtained after 50 charge and discharge cycles. This improved<br />cycling performance of the Sn/MCND composite could be attributed to its<br />low-density feature caused by dendritic structure of MCND, which has sizable<br />space for the large volume changes during lithium alloying and dealloying.<br /><br /> Highly dispersed Pt nanoparticles (Ca. 3 nm) in pores of MCND were also<br />easily prepared through the sonochemical process of a solution of<br />H<small>2</small>[PtCl<small>6</small>].(H<small>2</small>O)<small>6</small> with MCND. This Pt/MCND composite can be used as an<br />electrode of the direct methanol fuel cell (DMFC) for electrochemical oxidation<br />of methanol fuel. We also prepared Pt/AC (AC : high grade activated carbon,<br />Kuraray YP-17) composites for comparison of electrochemical performance.<br />These Pt/MCND and Pt/AC composites were analysed by TEM observation,<br />X-ray diffraction (XRD) and Thermo Gravimetric Analysis (TGA). These<br />structural analyses show that the average size and quantity of Pt particles on the<br />surface of carbon matrix are very similar to each other. The electrochemical active<br />surface areas and methanol electro-oxidation properties of these catalysts were<br />investigated by cyclic voltammetry. As a result, the Pt/MCND composites<br />indicated higher electrochemical activities than that of Pt/AC composites. The<br />electrochemically active surface area is estimated from the CV curve of the<br />Pt/MCND electrode in 0.5 mol/L H<small>2</small>SO<small>4</small> solution to be 62.1 m<small>2</small>g. It is 1.3 times as<br />large as that of the Pt/AC electrode. The excellent performance of the Pt/MCND<br />composite could be attributed to the dendritic structure and the graphitic structure<br />of MCND, which has sizable space for the high fluidity of the solvent and gases<br />for efficient catalytic reaction. |
要旨・審査要旨 (377.3 kB)
