Novel Porous Carbon Materials: Syntheses And Applications

Owing to high specific surface area, large pore volume, good electronicconductivity and thermal conductivity, well controlled pore size and surfaceproperties, porous carbon materials have great potential applications in manyareas, such as catalyst supports, supercapacitors, catalyst, absorbent and gasstorage. Generally, synthesis of porous carobn materials was realized throughhard template, soft template methods or activation of carbon; however, thesemethods suffer from high costs, complex synthetic procedure and impurity,which significantly hinder their wide applications. Therefore, this thesisattempts to prepare porous carbon materials based on molecule designing toreduce the cost and enhance the purity. Moreover, this thesis presentssystematic research on N-doping, loading noble metal nanoparticles,embedding transition metal-carbon composites in the porous carbon materials,and focuses on their applications in catalyst support, supercapacitor electrodematerial and oxygen reduction reaction （ORR）. In Chapter2, glucose was selected as the carbon source andethylenediamine （EDA） as the nitrogen source. By a simple one-pothydrothermal treatment, N-doped mesoporous carbonaceous materials wereobtained. Addition of EDA changed routine generation pattern ofcarbonaceous microspheres to the N-doped carbonaceous framework throughhydrothermal carbonization of glucose. Compared with the conventionalcarbon spheres, mesoporous carbon materials possess larger specific surfacearea （SSA） and pore volume. By analyzing the influences of reactants ratio,concentration and reaction time on the morphology and yield of the products,a formation mechanism of N-doped mesoporous carbon materials wasproposed. It was considered that EDA in the solution could effectivelyimprove the glucose carbonization speed and yield, resulting athree-dimensional framework formed by cross-linking of initial carbon coreswhen the carbonization step started. Nitrogen spicies on the carbon surfacecould make noble metal （Pt, Pd） nanoparticles uniformly dispersed on thecarbon supports and enhance the interaction between them. Together with themesopores facilating the diffusion of reactants, catalysts with an N-dopedmesoporous carbon support exhibited better catalytic performance. In thisreaction system, other N sources, such as1,6-diaminohexane, melamine andformamide, couldn’t transform the microspheres into mesoporous structures.In Chapter3, carbon coated self-assembled TiO₂microspheres wereprepared through a one-pot solvothermal method coupling the growth of TiO₂ and carbonization of glucose. The results showed that hydrothermaltemperature significantly effected the sizes of TiO₂nanorods in themicrospheres. Carbonization of glucose accelerated the hydrolysis of TiCl₄,thus enhanced the yield. Moreover, carbon coating prevented amalgamation ofTiO₂nanorods during the heat treatment, and carbon also acts as an electrontransport highway for improving the electrical conductivity. In addition,carbon coated graphene oxide （GO） was prepared by coupling hydrolysis ofTiCl₄with hydrothermal carbonization of glucose/EDA solution. Addition ofEDA made the hydrothermal carbonization of glucose take place on thesurface of GO, resulting a uniform carbon layer coated on GO.In Chapter4, a one-step pyrolysis method was developed to synthesizeN-doped porous carbon materials （PCM）. Ethylenediamine-tetraacetic acid（EDTA） was selected as the carbon precursor and KOH as the activating agent.Carbonization and activation were combined in this reaction, and theresultants inherited N species from EDTA. PCM has a high SSA of2014m²Â·g^-1. Owing to the high SSA and functionalized surface, PCM has a specificcapacitance of260F·g^-1at the discharge current density of500mA·g^-1andhas good cycle performance. Results showed that the reactant ratio andpyrolysis temperature play important roles in the capacitances.The N content could be enhanced by adding melamine into the startingmaterials. Addition of melamine increased the N content of the products to12.6at%and improved the yield to10times higher. The N-rich porous carbon materials （NPC） have a SSA of1678m²Â·g^-1and a specific capacitance of160F·g^-1at the discharge current density of10A·g^-1. NPC could also be used asmetal-free catalysts for ORR with an electron transfer number of3.23. Due tothe high SSA and high N content, NPC also have great potential applicationsin CO₂capture and storage, lithium ion batteries and dye-sensitized solar cell.Metallic Co was introduced into the porous carbon by addingCo（NO₃）₂·6H₂O in the initial reactants. The graphitic degree of Co-decoratedN-rich porous carbon materials （NPC-Co） was higher than NPC, which wasconfirmed by transmission electronic microscope （TEM）, X-ray diffraction（XRD） and Raman spectrum. The combination of the high specific area (1485m²Â·g^-1), high nitrogen content （10.8%） and suitable graphitic degree results incatalysts exhibiting high activity （with onset and half-wave potentials of0.89and0.80V vs the reversible hydrogen electrode （RHE）, respectively） andfour-electron selectivity for the ORR in alkaline medium—comparable to acommercial Pt/C catalyst, but far exceeding Pt/C in stability and durability.Adding conductive carbon （Ketjentblack, acetylene black） could enhance theyield without any loss in activity. Owing to their superb ORR performance,low cost and facile preparation, the catalysts have great potential applicationsin fuel cells, metal–air batteries, and ORR-related electrochemical industries.Many other carbon precursors, N-rich reactants, transition metal salts andcarbon supports have been used attempting to prepare catalysts with betterORR activities. However, results demonstrated that their performance were not better than the catalyst prepared with EDTA or EDTA-2K as the carbonprecursor, melamine as the N-rich reactant and Co（NO₃）₂·6H₂O.In this thesis, different small organic molecules were selected as theprecursors. Fabrication of novel N-doped carbon materials with controlledstructure and composition were realized through synthetic route design andoptimization. Such N-doped carbon materials could dramatically enhance thesupercapacitor and electrocatalysis performances. Moreover, controlledsyntheses of carbon nano-composites were achieved by couplingofcarbonization and hydrolysis processes. Synergetic effects between carbonand metal oxide enhanced the lithium storage performance of thenano-composites.