Preparation And Performance Of Carbon Fiber/Epoxy Resin Matrix Structural Energy-Storage Composites

With rapid development of advanced science and technology, structural-functional integration composites have been becoming an important research direction. Its high performance and multifunction are of great importance for realizing the high efficiency, light weight, high reliability of modern devices and equipments. As one kind of high performance fibers, and due to high strength, high modulus and light weight, carbon fiber (CF) has been widely used as reinforcement material. Carbon fiber reinforced resin matrix composite has become important structural material in aerospace field because of its excellent overall performance. Due to excellent conductivity and properties of carbon material in addition to excellent mechanical properties, carbon fiber is being used as functional materials. Recently, the research on carbon fiber/resin matrix structural energy-storage composites has received much attention.In view of the idea of structural-functional integration composites, CF/Epoxy resin matrix structural energy-storage composites were designed, in which carbon fiber acts as reinforcement and energy-storage electrode, and epoxy resin acts as matrix and structural electrolyte. The structure and performance of epoxy resin based gel electrolyte and the surface modification of structural carbon fiber were investigated. Microstructure, electrochemical performance and mechanical property of materials were characterized by modern instrumental analysis methods. The obtained results were as follows:The structure and property of epoxy resin based gel electrolyte were investigated deeply. The epoxy-based gel polymer electrolyte was synthesized using poly(ethylene glycol) diglycidyl ether (PEGDGE) as the matrix, triethylenetetraamine as hardener, propylene carbonate as plasticizer and tetrabutylammonium hexafluorophosphate as the supporting electrolyte salt. The effects of polymer matrix and supporting electrolyte on the structure and properties of electrolyte were investigated. The electrolyte was modified by using nano-sized TiO2, nano-sized SiO2 and mesoporous TiO2. Results showed that the ionic conductivity of electrolyte decreased and its mechanical strength improved with increasing polymer content. When the concentration of electrolyte salt was 0.8～1.0 mol/l, the degree of crystallinity of polymer matrix was decreased significantly and the ionic conductivity of electrolyte was obtained as maximum. The microstructure and performance of electrolyte could be influenced by nano-sized TiO2, SiO2 and mesoporous TiO2, which could form the crossinglinking network structure with polymer chain segment and the hydroxyl bonded on their surface could promote the cross-linking reaction of epoxy resin. The modification effect of fillers was influenced by their content and dispersibility in the polymer matrix. Owing to the microstructure and larger number of hydroxyl bonded on the surface of mesoporous TiO2, which exhibited better modification effect. When the content of mesoporous TiO2 was 6 wt.% in the electrolyte with 65 wt.% polymer content and 1.0 mol/l TBAPF6, the ionic conductivity of electrolyte could be increased from 6.81×10-5 S/cm to 11×10-5 S/cm, increased by 61.5%. The electrochemical stability window of the electrolyte was increased significantly, from 2.75 V to more than 5 V. the compressive strength of electrolyte was improved from 3.91 MPa to 5.06 MPa, increased by 29.4%. The comprehensive property of electrolyte was improved effectively.The influence and mechanism of oxidation modification on the structure and property of carbon fiber and carbon fiber/epoxy resin composites were researched. CFs were modified by using concentrated HNO3 and air oxidation. Results showed that after HNO3 treatment, the mechanical strength of carbon fiber decreased, and the specific surface area was increased slightly (around 1.1～4.5 fold), but the active functional groups were increased obviously (around 13-60 fold), consequently the surface activity of carbon fiber was improved. Compared with the composites based on untreated CFs, the specific capacitance of composites was increased (around 2.3～10.5 fold), the shear strength increased about 3～20%, the compressive strength increased about 6～31.9%, and the compressive modulus increased about 6.5～28.4%. By the optimum conditions of oxidation process, the tensile strength of carbon fiber was increased by removing the surface defect, and the surface characteristics was improved simultaneously by enlarging the surface area (around 11～48 fold) as well as the number of active functional groups (around 37～80 fold) on its surface. Subsequently, the energy storage capacity and mechanical properties of CF/Epoxy composites were improved obviously. Compared with the composites based on untreated CFs, the specific capacitance of composites was increased (around 22～132 fold), the shear strength increased about 10.8～36%, the compressive strength increased about 22～65.4%, and the compressive modulus increased about 18.4～64%. The CF/Epoxy composites with CF electrode and epoxy resin based electrolyte exhibited typical double-layer capacitance behavior of supercapacitor. Attributed to the increase in the specific area and surface functional groups, which leads to the increase of surface wettability and sufficient utilization of the CFs, subsequently, the electrochemical performance and mechanical property of CF/Epoxy composites were improved.The mesoporous TiO2 was synthesized by evaporation-induced self-assembly method. The mesoporous anatase TiO2 with worm-like ordered mesoporous structure was prepared using P123 as template, tetrabutyl orthotitanate as titanium source, and HCl as hydrolysis inhibitor. The effects of calcination temperature, P123 and HCl on the microstructure and properties of mesoporous TiO2 were investigated. Results showed that the average mesoporous size was 4.3～ 7.8 nm, the average grain size was 8.4～12.5 nm and the specific area of sample was 92.7～ 263.6 m2/g. When the calcination temperature increased from 350℃ to 500℃, the specific surface area of sample decreased, its average mesoporous size increased, its grain size increased and the regularity of pore structure decreased. With calcining at 400℃ for 3 h, mesoporous TiO2 was obtained with the specific surface area of 190.3 m2/g, the average mesoporous size of 7.2 nm and the grain size of 10.1 nm. With P123 concentration increased from 0.3 g to 1.5 g, the average mesoporous size increased, the regularity of pore structure increased and the specific surface area of mesoporous TiO2 increased from 94.2 m2/g to 202.4 m2/g. However, the further increase of P123 concentration caused a decrease of the regularity of pore structure. When the content of Pi23 was 0.8～1.2 g the ordered mesoporous structure was obtained. With increase in HCl content, the average mesoporous size of sample increased and its specific area increased first and then decreased. The obtained worm-like TiO2 was regular with good stability and larger specific area and mesoporous size when the ratio of HCl/Ti was 0.5～1.0. The microstructure and properties of mesoporous TiO2 can be regulated by changing the reaction condition.The surface modification method of carbon fiber using mesoporous TiO2 was developed. Based on the research of the preparation of mesoporous TiO2, carbon fiber was modified by mesoporous TiO2 coating to improve its specific surface area and oxidation resistance. Mesoporous TiO2/CF reinforcements were prepared by in-situ synthesis. The specific surface area of CF was improved effectively by loading mesoporous TiO2 on its surface. The specific surface area of CF increased from 1.5 m2/g to 89.6 m2/g with the loading of mesoporous TiO2 of 65.4 wt.%. Moreover, mesoporous TiO2 coating could improve the mechanical strength of CF by repairing the surface defect and subsequently reducing stress concentration. The stability of mesoporous TiO2 coating was influenced by calcination temperature. The uniform and highly adhered layer of mesoporous TiO2 to CF surface was obtained with the lower calcination temperature. Due to the further increase of calcination temperature, there are higher internal stress in coating and shrinkage of TiO2 increase obviously, leading to a decrease in the stability of coating.The mechanical property and electrochemical performance of mesoporous TiO2/CF reinforced Epoxy composites were investigated. Results showed that the shear strength of composite increased about 40%, the compressive strength increased about 77% and the compressive modulus increased about 80%. The energy mechanism of the composite was double-layer capacitance behavior without pseudo-capacitor energy storage characteristics with good rate performances and cycling stability. Due to the mesoporous structure of TiO2 and the cooperation effect between TiO2 and CF, the energy storage capability of CF electrode was improved obviously. The specific surface area of CF was increased from 1.5 m2/g to 69.2 m2/g and the specific capacity was improved from 0.092 F/g to 3.34 F/g with the loading of mesoporous TiO2 of 31.8 wt.%. Meanwhile, the energy density and power density were enlarged too. Owing to the decrease in charge transfer resistance and the increase in ion transport and diffusion of mesoporous TiO2, the equivalent series resistance of capacitor reduced significantly, indicating that the electrochemical performance of capacitor was improved obviously.