| Porous carbon has been widely applied in the area of electrochemical energy storage and conversion, water purification and industrial catalysis due to its high specific surface area and well-developed porous structure as well as its good electric/thermal conductivity and chemical stability. With the development of scientific technologies, however, the unique pores size dominated porous carbon is not sufficient to meet the requirement in the mentioned applications. Thus, the hierarchically porous carbon with macropores, mesopores and micropores has been proposed, which exhibits greater potential in these application areas due to its structural advantages that macropores can serve as the reactant reservoir, the mesopores facilitate the quick transfer of the reactant and the micropores provide large surface area for reactions. Generally, the templating and activation methods are treated as effective strategy for the preparation of the hierarchically porous carbon.However, these strategies still suffer from the high-expense, complicated procedure and uncontrollable porous structure of the obtained porous carbon.Therefore, it is meaningful to develop a low-cost, controllable and environmental-friendly method for the synthesis of hierarchically porous carbon.Recently, the biomass materials have been widely used as precursor for the synthesis of hierarchically porous carbon due to its availability, low-cost and renewability in nature. In this dissertation, the silkworm cocoon that is a natural bio-molecular material with rich nitrogen and oxygen content has been employed to synthesize the nitrogen-doped high-specific-surface-area hierarchically porous carbon through a pre-carbonization 'with a subsequent activation method. Through analyzing the variation of the composition and structure of the pre-carbonized silkworm cocoon at various temperatures and heating rates in the pre-carbonization and the activation process, the carbonization and pore-forming mechanism of the silkworm cocoon has been demonstrated, developing a controllable synthesis method- for the silkworm cocoon based hierarchically porous carbons. Afterwards, the obtained hierarchically porous carbon is applied as adsorbent for the removal of Cr6+ions, electrode material for supercapacitors as well as electrocatalysts for methanol electro-oxidation and oxygen reduction reaction, revealing the influence of specific surface area, porous structure and surface chemical state of the obtained hierarchically porous carbon on its performance in these areas.The results are illustrated as below:(1) A series of hierarchically porous carbon has been obtained through adjusting temperature and heating rate in the pre-carbonization and activation process. The results indicate that the carbonization temperature of silkworm cocoon is between 250 ? - 300 ?, in which the neighboring peptides chains in the protein transform into sp2-hybridized carbon hexagonal structure by aromatization. Moreover, the a large amount of defects can be produced in the pre-carbonized silkworm cocoon due to the oxidation of carbon during pre-carbonization, and the amount of defects increase with the increase of the pre-carbonized temperature. These defects are beneficial for pore-forming in the activation process, and the extent becomes severe with the increase of the activating temperature and the decrease of the heating rate. As a result, a nitrogen-doped hierarchically porous carbon (N: 1.58 at.%) has been synthesized with the specific surface area of 3514 m2 g-1 and the pore volume of 2.05 Cm3 g-1, in which the macro-/mesopore volume is of 41.9 % at the pre-carbonized temperature of 450 ? with a heating rate of 5 ? min-1 and a subsequent activating temperature of 900 ? at a heating rate of 1 ? min-1.(2) The obtained hierarchically porous carbon was used as adsorbent for the removal of Cr6+ ions from water, and the effect of specific surface area,porosity and composition of the porous carbon were investigated. The results demonstrate that the high specific surface area (3134 m g-1 ) and rich micropores provides sufficient adsorption sites for the adsorption of Cr6+,while the open meso-/macropores is favorable for the quick transfer of the Cr6+. Thus, the hierarchically porous carbon exhibits a adsorption capacity of366.3 mg g-1 and a high adsorption rate constant of 4× 1 0-2 g mg-1 min-1 at pH =2, which is higher than those of Norit CGP. Moreover, the hierarchically porous carbon presents a high cycling stability with the adsorption capacity retaining 98 % of its initial capacity after 5 times cycling. Further mechanism investigation indicates that the adsorption of Cr6+ by the porous carbon is a spontaneous endothermic process, which fits well with the Langmuir isotherm model and pseudo-second-order model.(3) The obtained hierarchically porous carbon was used as electrode material for supercapacitor, and the influence of specific surface area as well as pore size distribution on the capacitive performance, rate capability and energy density was investigated. The results indicate that the high specific surface area (3386 m2 g-1 ) and porosity (2.2 cm3 g-1 ) can provide large interface area for the charge storage, while the interconnected hierarchical pores is beneficial for the quick transfer of the electrolyte ions, promoting the rate capability of the supercapacitor. Electrohemical measurements indicate that the hierarchical porous carbon exhibits a capacitance of 156.1 F g-1 at the current density of 5 A g-1 in the organic-solvent electrolyte, and the capacitance retains at 128.8 F g-1 at the current density of 50 A g-1, with the retention ratio of 82.5 %,indicating its decent capacitance and rate capability. The energy density of this material is 34.41 Wh kg-' in organic-solvent electrolyte. When the ionic liquid electrolyte with a larger potential has been employed as electrolyte, the energy density of the hierarchical porous carbon enhanced to 112.1 Wh kg-1, and the energy density retains at 79.40 Wh kg-1 as the power density is 23.91 kW kg-1,further indicating its good rate capability and energy storage properties.(4) The hierarchicallt porous carbon was further used as support material for Pt-Co-P ternary alloy electrocatalysts towards methanol oxidation through an impregnation reduction method, in which the sodium dihydric hypophosphite is reductant and phosphorus (P) source. The high specific surface area of hierarchically porous carbon is favorable for the dispersion of the Pt-Co-P ternary alloy nanoparticles, improving the electrochemical active specific surface area, while the developed pores facilitate the transformation of the intermediates produced from the methanol oxidation, enhancing its CO-tolerance. Moreover, the incorporation of P enhance the electronic interaction among Pt, Co and P, and inhibits the growth of the nanoparticles size. As a result, a highly dispersed ultrafine Pt-Co-P nanoparticle (1.5 nm in size) supported on carbon has been obtained at the P content of 11.9 at.%.Electrochemical measurements indicate that the supported Pt-Co-P ternary alloy electrocatalysts present a comparable electrocatalytic activity and superior CO-tolerance.(5) The Co, N co-doped hierarchically porous electrocatalysts with high activity have been prepared from the pyrolysis of cobalt acetate, melamine and porous carbon at 650 ? with acid--washing and a secondary heat-treatment at 850 ?. The obtained electrocatalysts exhibit a hierarchically porous structure with the high specific surface area of 2817 m2 g-1 , the porosity of 1.42 cm3 g-1 and the nitrogen content of 3.82 at.%, facilitating the dispersion of the active sites and the quick transfer of oxygen and its products.As a results, the obtained electrocatalysts exhibit a high electrocatalytic activity, long-term stability and methanol-tolerance capability. In alkaline electrolyte, the half-wave potential of the electrocatalysts towards oxygen reduction reaction is 0.87 V vs. RHE, 20 mV positive than that of commercial Pt/C, and its kinetic current density is 6.46 mA cm-2 (@0.85 V vs. RHE),much higher than that of commercial Pt/C (4.68 mA cm-2). |
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