Synthesis Of Novel Mesoporous/Microporous Carbon/Carbon Composites For Electrochemical Application

As the fast development of modern society, the energy crisis became a huge problem to be solved. The reducing of the fossil oils makes it urgent to develop new alternative energy. On the other hand, enhanceing the storage efficiency of the existing electric energy lead the fast development of electrode material. Electric energy can be stored in two different ways, one is storage in the battery or fuel cell in the form of chemical energy, the other is storage in the capacitor in form of electrostatics. While traditional graphite electrode is fail in meeting the needs of new generation of electrical energy storage/conversion devices such as electric car. The nanoporous (mesoporous and microporous) carbon materials has large specific surface, uniform and tunable pore size, good electrical conductivity, become a research hot topic in the field of electrochemical energy storage. The nanoporous carbon has palyed an important role in lithium ion batteries, supercapacitors, fuel cells and energy storage and conversion of the relevant electrochemical area occupies the important position. A larger number of nanoporous carbon materials with different structures, morphologies and porosities can be obtained through either hard or softtemplating methods. However, there are still several challenging issues in electrochemical application that need further investigation. In this thesis, we are focus in design nanoporous carbon with excellent electrochemical performance, including (1) the optimization of porous structure for energy storage, (2) exploration of core-shell or york-shell structure for energy storage or electro-catalysis application.In chapter 2, activation of ordered mesoporous carbon orientates the development and application of new carbonaceous supercapacitor materials with high energy density and power density. Ordered mesoporous carbons FDU-15 are synthesized in large scale via a soft template method through evaporation induced self-assembly of mesostructure on the sacrificed polyurethane foam. Common activating agent potassium hydroxide (KOH) is utilized to improve the surface area and tailor the pore texture of the ordered mesoporous carbon by adjusting KOH/carbon mass ratio as well as activation time. At low KOH/carbon ratio, the generated micropores increase in volume and either connect to other micropores or eventually become mesopores. At high KOH/carbon ratio, an excess amount of micropores would be generated. Meanwhile, the continuous shrinkage of carbon framework is carried through as prolonged time at high activation temperature. Competition between KOH etching and shrinkage of mesopores is existed during the activation. The latter obviously preponderates over the former at low KOH/carbon ratio, which is reversed at high KOH/carbon ratio. Thus, an optimized micro-mesostructure is achieved under certain activation conditions:maintained ordered mesostructure, suitable microporosity, high surface area (1410 m2/g) and large pore volume (0.73 cm3/g). The activated sample exhibits improved electrochemical behavior with a gravimetric capacitance of 200 F/g, excellent rate performance and good cycling stability with capacitance retention of about 98% over 300 cycles.In chapter 3, highly ordered nanoporous graphitized carbons with three-dimensional ly arrayed micropores and homogeneously distributed mesopores were successfully synthesized by using commercial mesoporous zeolite NaY as template through a simple one-step chemical vapor deposition (CVD) process by employing methane as a special carbon precursor. The obtained nanoporous carbons have highly ordered microstructure inherited from zeolites, nanographene constructed frameworks, high surface area of about 2000 m2/g, and suitable micro/mesopores distribution, which can be adjusted by carbonization temperature (700-900℃) and time (1-4 h). In the nano-sized graphitized nanoporous carbon (～700 nm), the homogeneously distributed mesopores are favorable for electrolyte diffusion even at high current density. While ordered arranged micropores provide high surface area to accumulate lithium ions. Importantly, the thin frameworks constructed by curved graphene nanosheets, facilitate the collection and transfer of electrons during the cycling process. Therefore, expected excellent electrochemical performance of the graphitized nanoporous carbon as anode material for lithium ion batteries is demonstrated:a superior lithium ion storage (1540 mAh/g for the initial discharge cycle,1000 mAh/g for the reversible discharge cycle at the current density of 40 mA/g), high-rate performance (with retained capacity of 500 mAh/g at the current density of 120 mA/g) and excellent cycling stability.In chapter 4, a novel core-shell structured composite molecular sieves comprising mono-dispersed nano-sized zeolite single-crystals (i.e., nano-zeolite Y) as cores and ordered mesoporous silica as shells were synthesized via a surfactant-directed sol-gel process in basic media by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethyl-orthosilicate (TEOS) as a precursor. Uniform mesoporous silica shells are coated on the mono-dispersed nano-zeolites to form the hierarchical porous structures, the thickness of which can be tailored depending on the adding amount of TEOS. The composite molecular sieves with the thickness of 20 nm possess ultra high surface area of 1198 m2/g, ordered mesopores (～3.5 nm) from the silica shell and uniform micropores (～0.9 nm) from nano-zeolite core. The ordered mesopore channels in the shells are annularly vertical to the nano-zeolite crystal-faces. On the other hand, the nano-zeolite@mesoporous silica composite molecular sieves with such high surface area and opened hierarchical pores, can provide sufficient voids for capturing reactant molecules and also promote molecule diffusion from mesopores to micropores. Thus, the composite molecular sieves show greatly enhanced adsorption capacity (4.7 mmol/g) for large molecules such as benzene relative to that of pristine nano-zeolites (3.0 mmol/g), ascribing to the large contribution from mesopores in the shell.In chapter 5, a fascinating core-shell structured graphitic carbon material composed of ordered microporous core and uniform mesoporous shell is fabricated for the first time through a site-specific chemical vapor deposition process by using a nanozeolite@mesostructured silica composite molecular sieve as the template. The mesostructure-directing agent cetyltrimethylammonium bromide in the shell of the template can be either burned off or carbonized so that it is successfully utilized as a pore switch to turn the shell of the template "on" or "off" to allow selective carbon deposition. The preferred carbon deposition process can be performed only in the inner microporous zeolite cores or just within the outer mesoporous shells, resulting in a zeolite-like ordered microporous carbon or a hollow mesoporous carbon. Full carbon deposition in the template leads to the new core-shell structured microporous@mesoporous carbon with a nanographene-constructed framework for fast electron transport, a microporous nanocore with large surface area for high-capacity storage of lithium ions, a mesoporous shell with highly opened mesopores as a transport layer for lithium ions and electron channels to access inner cores. The ordered micropores are protected by the mesoporous shell, avoiding pore blockage as the formation of solid electrolyte interphase layers. Such a unique core-shell structured microporous@mesoporous carbon material represents a newly established lithium ion storage model, demonstrating high reversible energy storage, excellent rate capability, and long cyclic stability.In chapter 6, we present a low cost and scalable technique, via ambient pressure chemical vapor deposition (CVD) on nano-zeolite, to fabricate three-dimentional (3-D) graphene nano-box. The graphene layers continuously grow over the crystal-faces of nano-zeolite, resulting a hollow 3D nano-box. The nano-zeolite also offers a microenvironment with uniform micropores as confinement channels for synthesis of ultra-small metal nanoparticles (NPs) without any capping agent. Such rattle-like Pt NPs@graphene boxes are efficient bi-functional catalysts for fuel cells. The ultra-thin carbon shell has not only good elasticity to effectively accommodate large volume of oxygen reactant, but also excellent electronic conductivity for fast transfer of electrons. Moreover, the surfaces of Pt nanoparticles were free from any ligands, i.e., the active surface site(s) was exposed to reduce the oxygen, resulting a high efficiency in oxygen reduction reaction and long cycling life.In chapter 7, composites of CuO and a unique ordered mesoporous carbon which possess small pores inside the carbon walls have been prepared. The CuO contents varied from 9.3 to 44.6 wt%, the main mesoporess in the carbon walls are gradually filled with the incorporated CuO. The maximum specific capacitance of the CuO/MC composites is as high as 718 F/g at a high current density of 2 A/g, because of the pseudocapacitance generated by the redox reaction. More importantly, the composite with a loading amount of 33.2% exhibits excellent rate capability due to the reduced Cu during the calcination process, with 32% of capacitance retention when the discharging current density increases from 2 to 20 A/g.