Synthesis And Electrochemical Capacitance Of Biomass-derived Nanoporous Carbon Composites

In recent years, with fast development of industrialization and rapid growth of population, fossil fuels（such as coal, oil and natural gas） energy has consumed, and meanwhile the increasingly pollution of the environment has became more and more seriously. Faced with these threats, cheap, high specific energy and power of the development of electrochemical capacitor carbon electrode materials has became one of the major problems must be solved for human sustainable development strategy. Present study shows porous carbon has been considered as one of the most important electric double layer capacitor material with higher power density, longer cycle life and faster charging and discharging performance for their hierarchical porous structure and large specific surface area. Unfortunately, their specific surface area, pore distribution, heterogeneous elements and the surface functional groups limit the specific capacitance(<200Fg^-1) and the energy density. In order to improve the capacitance performance, some nano functional phase（transition metal oxides, transition metal hydroxide and conductive polymers, etc.） have introduced into various carbon supports（carbon nanotubes, graphene or porous carbon, etc.） to add some pseudocapacitance to prepare synthetic binary or ternary hybrid capacitor electrode material. However, restricted by the present synthesis technology, pores were most filled or interlayers were enormously covered after composition result in an increase of charge transfer resistance and a decrease of large current charging and discharging of capacitor. Furthermore, construction of binary and ternary hybrid structure still exists a series of bottleneck problems, such as expensive synthesized substrates, difficult synthetic method, simple structure, unitary species, unsatisfactory distribution of functional phase and poor binding force between them.In view of this, this paper puts forward the synergistic effect between biomass porous carbon（bamboo activated porous carbon, BAC） with double layer capacitance characteristics and functional phase（metal hydroxide, metal oxide and carbon nanotubes） with pseudocapacitance characteristics. A series of organisms in nature are the perfect unities of highly delicate structures with amazing multi-level, multi-scale and multi-dimensional microstructure through millions of years of evolution and natural survival law. These natural fine porous structures can be used to preparing the structure-function coupled advanced functional materials via simple, cheap, environmental and controllable morphology synthetic method. On the one hand, making full use of the porous structure and surface characteristics of BAC intrinsically can provides more deposition sites for introduction of nano functional phase. Furthermore, based on the composite applicability analysis of biomass porous carbon, we have obtained the effective method to introducing nano functional phase with a highly homogeneous dispersed distribution and tested the capacitance performance of the binary or ternary nanocomposites. This study will benefit for the development and application of biomass porous carbons super capacitor materials and will provide an extensive idea for their structural design, prepared solution and component optimization. The major results are listed as follows:1. Realize the dispersive distribution of nano functional phases on biomass porous carbon.--Firstly, four kinds of plant-based porous carbon were choose as anchored supports and their intrinsic surface microstructure and properties for loading effects have been discussed. Then, bamboo porous carbon（BAC） were used as support to study and compare the effects on size and distribution of nano phases result from different introducing method- Wet Impregnation（WIM）, Water bath Deposition Precipitation（WB-DP） and Microwave Deposition Precipitation（MW-DP）. Ultimately, due to the uniform pore structure distribution and abundant oxygen containing functional groups on the BAC surface, together with the effective MW-DP deposition method, porous carbon/metal nanocomposites with a highly homogeneous nanoparticles distribution have realized.2. A facile low-temperature synthesis of highly distributed and size-tunable cobalt oxide nanoparticles anchored on activated carbon for supercapacitors.--A facile microwave-assisted deposition-precipitation method is utilized to prepare uniformly distributed Co₃O₄ nanoparticles on the surface of bamboo activated carbon（BAC） for electrochemical capacitors. The size of the Co₃O₄ particles is well controlled ranging from several to tens of nanometers by manipulating the pre-oxidization of the BAC. The capacitive performance of the obtained Co₃O₄/BAC hybrids is determined by the synergistic effects of electric double layer capacitance from the BAC support and psedocapacitance from the anchored Co₃O₄ nanoparticles, which is highly dependent on their particle size and loading. The hybrid materials can provide specific capacitance up to 491 F g^-1 at 0.1 Ag^-1 in a 6 M KOH electrolyte as the average size of the Co₃O₄ is 7 nm and the loading is 16.4 wt%. In addition, an excellent electrochemical stability is achieved with an only 11% capacitance degradation after 5000 charge-discharge cycles at a large current density of 5 Ag^-1, demonstrating that solid bindings between the Co₃O₄ nanoparticles and BAC support have been achieved during the microwave-assisted preparation.3. A3D hierarchical hybrid nanostructure of carbon nanotubes and activated carbon for high-performance supercapacitors.--Novel carbonaceous hybrid materials are fabricated through the in situ growth of open-tipped mesoporous carbon nanotubes（CNTs） on low-cost bamboo activated carbon（BAC） substrates with cobalt（Co） nanoparticles as the growing seeds via the chemical vapor deposition process. The CNTs are strongly bonded with the surface of the BAC supports using the fine Co nanoparticles（<10 nm） as the joints. The unique three-dimensional hybrid architectures enable the resultant materials to exhibit a considerable specific capacitance up to 440 Fg^-1 at 1 Ag^-1 as well as an excellent rate performance(97 % retention ratio at 5 Ag^-1 compared to 1 Ag^-1). In addition, the hybrid materials have an impressive cycling stability with 98.4 % initial capacitance retention after 3000 cycles at 5 Ag^-1. Besides a high specific surface area, such an excellent capacitive performance is mainly attributed to the combination of（i） the well-dispersed open-tipped CNTs(5^-12 nm) that could provide more effective ion channels,（ii） the interconnected CNT conducting networks facilitating the transport of electrons, and（iii） superfine Co nanoparticles（3-9 nm） offering pseudocapacitance, indicating the synergistic effect of both the electrical double layer capacitance and pseudocapacitive reactions.4. Microwave-assisted anchoring of flowerlike Co（OH）₂ nanosheets on activated carbon to prepare hybrid electrodes for high-rate electrochemical capacitors.-- Flower-like α-Co（OH）₂ nanosheets are in-situ grown on the surface of activated carbon supports for supercapacitors via a facile and cost-effective microwave-assisted method. The existence of carbon supports facilitates the dispersion of pseudocapacitive Co（OH）₂ nanosheets and also provides appropriate pathways for charges while charging and discharging. The obtained hybrid architectures can provide a high specific capacitance up to 345 Fg^-1 at 0.1 Ag^-1, and still retain 284 Fg^-1 at a current density as high as 5 Ag^-1. In addition, benefited from the stable hybrid nanostructures, an excellent electrochemical stability is achieved with an only 14% capacitance degradation after 5000 charge-discharge cycles at a high current density of 5 Ag^-1. The impressive electrochemical performance is ascribed to the synergistic effect of porous carbon substrates and pseudocapacitive Co（OH）₂ nanosheets.It also puts up a new concept and method for preparation of multi-dimensional biomass-derived nanoporous carbon composites with porous carbon structure and nano phase. The synthetic hierarchical structures have more promising potential applications and important academic significance in energy storage device.