Synthesis Of Novel Porous Carbon Materials And Their Applications In New Energy Technology

Porous carbon materials are widely used as electrode, electrocatalyst and catalyst support because of its high conductivity, good stability, high specific surface area and porosity. Thus, it plays an important role in the development of new energy technologies. As an inexpensive, abundant and renewable source on the earth, biomass has attracted much attention in the field of material science. In recent years, due to its mild condition (100～250℃), aqueous medium and the advantages of economic, hydrothermal carbonization of biomass is potential in synthesizing porous carbons. However, the hydrothermal carbonization and the prepared carbon materials still face challenges, including the following three aspects. First, the products prepared from a direct hydrothermal process are nonporous carbon materials, hindering their applications. Second, the hydrothermal carbonization of biomass is referred to the reactions of hydrolysis, degradation and polymerization under relatively high temperature, which make it hard to control the organic-organic self-assembly of biomass and soft templates. Therefore, it is still difficult to prepare ordered mesoporous materials by soft templating method. Third, although metal-free porous carbon materials have a few application performances in chemical catalysis and electrocatalysis to some extent, it still falls short of the requirements of the industrial application. The development of metal-carbon composites is an effective method to promote the activity of carbon materials.Considering the above questions, this thesis focuses on the synthesis of porous carbon materials and its applications in new energy technology. In the first part, a novel strategy is demonstrated to synthesize hierarchically porous carbon materials by the hydrothermal carbonization of fructose. This kind of porous carbon materials has not only micropores but also mesopores and macropores, which is favorable for mass diffusion and transformation. Besides, amphiphilic block copolymer poly(4-vinylpyridine)-block-poly (ethylene glycol) (P4VP-PEG) is introduced into the hydrothermal carbonization of fructose, which is effective to control the process of hydrothermal carbonization. On one hand, the P4VP and PEG blocks are hydrophilic, which makes fructose dissolved in the P4VP-PEG micelle. As a result, hydrothermal canbonization process is directed by the P4VP-PEG micelle. On the other hand, P4VP-PEG polymer was adsorbed on the surface of original carbon particles during the hydrothermal carbonization, which inhabites the aggregation of carbon particles and accelerates the sol-gel process. By this method, carbon nanoparticles in the size of 20 to 100 nm are synthesized, which are aggregated to carbon materials with hierarchically porous structures. The obtained carbon materials present excellent performance for electron transfer and mass diffusion. When applied in the supercapacitor, the hierarchically porous carbon materials exhibit a capacitance of 197 F g-1 at the current density of 1 A g-1, which is 4 times than the carbon material produced by the direct hydrothermal carbonization.In the second part, a soft templating method is developed to synthesize ordered mesoporous N-doped carbon materials from fructose under hydrothermal conditions. Generally, the assembly of biomass with soft template is susceptible to various reactions involved in the hydrothermal carbonization process. To date, there are few reports of effective and easy strategies to control soft templating process and synthesize ordered mesoporous carbons from biomass. In this paper, a self-transformation strategy is developed to control the soft-templating process by introducing insoluble melamine sulphates into the hydrothermal reaction system. As a result, the carbon materials that have not only a flower-like morphology but also a high specific surface area (761 cm2 g-1) and a hierarchical porous structure are synthesized. Besides, N and O element are doped in the materials simultaneously. The obtained materials present good performance for adsorbing Fe3+, and the adsorbing capacity is 2.3 mmol g-1, which is 15 times to the phenolic-resin-based porous carbon materials. Besides, the capacitance of these materials can reach to 200 F g’1.In the third part, a porous Fe-N/C material is synthesized by soft templating method using fructose as carbon source. Although the porous N-doped carbon materials have a good performance for application in supercapacitor, these materials have a low activity for electrocatalysis reactions. It is not suitable for other important new energy technology such as oxygen reduction reaction (ORR) on the cathode of fuel cells and electrochemical hydrogen evolution reaction (HER). Fe-N/C composite materials are considered as one of the most promising non-noble metal catalysts for ORR. They present good catalytic performance in both basic and acidic electrolytes. Although the Fe-N/C catalysts with high activity and stability could be prepared by hard templating method, the complicated process and high cost hinder its wide application and commercialization. In this regard, ordered mesoporous Fe-N/C materials are synthesized by introducing ferric sulfate into the system that is mentioned in the second part. By this way, Fe-modified mesoporous N-doped carbon materials are prepared in one step. It turned out that the obtained materials have improved ORR activity, high durability and good methanol oxidation resistance. Importantly, the onset potential and limited-diffusion current density for ORR are better than the commercial Pt/C catalyst.As a clean and renewable energy, hydrogen is the key field in the future energy technology. Therefore, the development of an effective and environment-friendly electrochemical method for preparation of H2 has attracted much attention. The fourth part of this thesis aims to develop metal-carbon composites with high activity for HER. The 3d transition metals are promising electrocatalyst for HER because they are abundant and cheap. In recent years, important researches for HER catalysts based on 3d-TMs have been achieved. For example, transition metals (Fe, Co, Ni etc.) composite materials encapsulated by carbon materials have high activity and durability for HER. Even so, the activity of these catalysts is still lower than the noble metal catalysts (Pt, Pd etc.), and far away from the need of practical application. Based on the results of the second part, molybdenum carbide-modified N-doped carbon vesicle encapsulating Ni nanoparticles (MoxC-Ni@NCV) are synthesized by solid thermolysis method from Ni doped melamine oxalate/molybdateas precursor. The catalytic activity of MoxC-Ni@NCV material for HER is improved because of the particular d band electron structure and similar catalytic characteristics to precious metals of y-MoC. Besides, it is demonstrated that the materials have the highest activity for HER in acidic electrolyte among the none-precious metal catalysts. The MoxC-Ni@NCV materials have not only an ultra-low overpotential of 68 mV (10 mA cm-2) that closes to the commercial Pt/C (～50 mV) but also higher durability in acidic electrolyte.In general, this thesis focuses on the preparation of porous carbon from biomass and its application in new energy technology. Optimized method is developed and the application performance is improved. First, novel hierarchical porous carbon that has good performance for supercapacitor is synthesized by hydrothermal carbonization of fructose using amphiphilic block polymer as directing reagent. By this way, the problem that only micro-sized carbon sphere and nonporous messy carbon are prepared through direct hydrothermal carbonization of biomass has been solved. Afterwards, under the assistance of insoluble melamine salt, the assembly of fructose with soft template has been controlled effectively through a self-transformation strategy. By this means, ordered mesoporous N-doped carbon materials from fructose are synthesized, overcoming the challenge for preparing ordered porous carbon from biomass by soft-templating method. In addition, porous Fe-N/C catalyst and molybdenum carbide-modified N-doped carbon vesicle encapsulating Ni nanoparticles (MoxC-Ni@NCV) are synthesized to improve the ability for oxygen reduction reaction or hydrogen evolution reaction. As a result, the activity of catalytic electrochemical reactions almost can be close to the commercial Pt/C catalyst. Thus, a significant progress on the design, improvement and application of porous carbon materials has been achieved in this thesis.