Graphene-based Three Dimension Structure Nanomaterials: Preparation And Applications For Electrochemical Energy Systems

Development of "green" electrochemical energy devices with high performance is an efficient method to solve the problems of energy crisis and environment pollution. Since the electrode material is one of the key factors affecting the electrochemical performance of the energy device, the design and synthesis of electrochemical electrode material with excellent performance will be of great significance and academic value. In recent years, graphene and graphene based nanomaterials have attracted much attention in the energy fileds, such as supercapacitor, lithium batteries, fuel cells etc due to their excellent electrical and mechanical properties and large specific surface area. However, those structures suffer from graphene aggregation, which causes low surface area, inferior ionic accessibility and slow electron transfer, and thus modest improvement in the cell performance. Building the 3D structure of graphene or graphene based materials will effectively improve the surface area utilization, increase the electroactive sites, accelerate electron transfer and facilitate electrolyte ion diffsusion and transport, which is expected to improve the electrochemical performance of graphene based devices electrodes. Therefore, we provide the design and fabrication of several types of three dimension graphene-based nanomaterials. The application of these as-prepared materials on supercapacitors, fuel cell cathode(oxygen reduction reation) and lithium sulfur secondary battery are investigated in detailed. The relationship between the performance and structure of the materails are also studied. The main results and new findings in these works are summarized as follows:(1) The reduced graphene oxide/carbon nanotube/Ni(OH)2(RGO/CNTs/Ni(OH)2) composites were prepared by a hydrothermal method. The structural characterizations of the composite indicate that α-Ni(OH)2 nanoparticles with the size 5-10 nm are randomly decorated onto three-dimensional(3D) hierarchical structure RGO/CNTs. The electrochemical performances of the composites were investigated, and the results suggest that the electrochemical capacitance of the composites depends on the amount of CNTs and Ni(OH)2 to a large extent. This composite with optimized ratio(GC2N2) exhibits the high specific capacitance, excellent rate capability and cycle stability. At the same time, the porous RGO/CNTs(p GC) were activated by KOH as the anode material. An asymmetric capacitor based GC2N2 and p GC were fabricated and its capacitive performance was evaluated. The capacitance of the asymmetric capacitor exhibits 75 F g-1 at the voltage window of 1.7 V with a current density of 20 A g-1, and its energy and power density were 30.1 Wh kg-1 and 17.0 k W kg-1, respectively. The enhancement in specific capacitance and cycling stability is believed to be due to the 3D RGO/CNTs conductive network which promotes not only efficient charge transport and facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/ contraction and aggregation of electroactive materials during charge- discharge process.(2) A three-dimensional(3D) graphene hollow sphere(PGHS) was prepared via a hard template method and used to anchor α-Ni(OH)2 nanoparticles(PHGSN12) around 4 nm by electrochemical deposition. The electrochemical performance of the composites was investigated by galvanostatic charge-discharge(GCD) and cyclic voltammogram(CV). The composite PHGSN12 achieves the high specific capacitance of 2815 F g-1(based on Ni(OH)2 mass) or 1319 F g-1(on the basis of total active material mass) at a scan rate of 5 m V s-1, and a specific capacitance of 1950 F g-1 even at 200 m V s-1 with a capacitance retention of about 70%, indicating its high rate capability. The excellent performance of such hybrid material(PHGSNi12) is ascribed to be the smaller size of Ni(OH)2 nanoparticles and the 3D graphene hollow sphere framework facilitating electron transport and promoting efficient electrolyte ions migration.(3) Sulfur- doped 3D porous graphene hollow nanospheres framework(S-PGHS) is fabricated by directly annealing graphene oxide(GO) with benzyl disulfide(BDS), and integrating with amino-modified Si O2 nanoparticles followed by etching. The Xray photoelectron spectra and Raman spectra confirm that sulfur atoms have been successfully introduced into PGHS framework via covalent bonds. S-PGHS used as metal-free electrocatalysts for oxygen reduction reaction(ORR) exhibits excellent activity with electron transfer number of 4, and much better methanol tolerance and durability than 40 wt% Pt/C in alkaline media. The capative performance of sulfur-doped or undoped 3D porous RGO hollow sphere frameworks and sulfur-doped RGO are also investigated by the CV and GCD in detail. The results enclose that sulfur-doped 3D porous RGO hollow sphere frameworks(S-PGHS) display high specific capacitance(236 F g-1 at 2 m V s-1 and 343 F g-1 at 0.2 A g-1), excellent rate capability(120 F g-1 at 500 m V s-1) and good cyclability(only 4% decay of capacitance after 1000 cycles), which obviously superior than that of undoped 3D porous RGO hollow sphere frameworks(PGHS) and sulfur-doped RGO(S-2DG). The enhancement in ORR and capative performance is attributable to the synergistic effect from sulfur-doping enhancing the electrochemical activity and 3D porous hollow nanosphere framework structures facilitating ion diffusion and electronic transfer.(4) Sandwich-type hybrid carbon nanosheets(SCNMM) consisting of graphene and micro/mesoporous carbon layer are fabricated via a double template method using graphene oxide as the shape-directing agent and Si O2 nanoparticles as the mesoporous guide. The polypyrrole synthesized in situ on the graphene oxide sheets is used as a carbon precursor. The micro/mesoporous strcutures of the SCNMM are created by a carbonization process followed by HF solution etching and KOH treatment. The hybrid carbon nanosheets, which have a thickness of about 10-25 nm, high surface area of 1588 m2 g-1, and broad pore size distribution of 0.8-6.0 nm, are highly interconnected to form a 3D hierarchical structure. Sulfur with the content of 74 wt% is impregnated into the hybrid carbon nanosheets to generate S@SCNMM composites for the cathode materials in Li-S batteries, and the battery exhibits excellent electrochemical performance, including large capacity(1370 m Ah g-1 at 0.5C), good rate capability(510 m Ah g-1 at 10C) and good cycling stability(860 m Ah g-1 after 100 cycles at 1C), which is believed to be due to the structure of hybrid carbon materials with hierarchical porous structure, which have large specific surface area and pore volume.