Electrophoretic Deposition Of Graphene Oxide On Graphite Fiber Cloth And The Electrochemical Performance Of Composites

Electrochemical supercapacitors have received huge attention due to their quick charging, high energy density, excellent energy conversion efficiency, long cycle life, good chemical stability and ultralarge capacitance. Generally, supercapacitors can be divided into two broad categories according to the different mechanisms: the electric double-layer capacitor(EDLC) and the pseudocapacitor.The improvement of the supercapacitors’ performance mainly depends on the development of excellent electrode materials, and the electrical conductivity of the electrode materials and ion diffusivity inside materials are the main factors influencing the performance of capacitance. Graphene oxide(GO), produced by oxidating and exfoliating graphite, is a single or multi-layer atom thick sheet-like carbon material which possesses some characteristic of graphene. However, during all the phases of GO preparation and the electrochemical reaction, GO sheets are inclined to form irreversible agglomerates or even restack through van der Waals interactions, leading to the loss of its high specific surface area. Thus, it’s a pivotal issue to prevent the irreversible agglomerates of GO sheets as electrode materials in supercapacitors. To our knowledge, graphite fibers have high electrical conductivity and three-dimensional network structure, based on which a new structured electrode made from GO and graphite fiber cloth(GFC) was constructed.Electrophoretic deposition(EPD), which refers to the colloidal particle under the function of electric field move to the electrode directionally and deposit to films on the surface of electrode, is believed to be an effective and versatile approach for this purpose. In the process of electrophoretic deposition, GFC was used as the anode. Owing to GO has lots of oxygen-containing functional groups, such as carboxyl and hydroxyl, the dispersion showed negative Zeta potential, so GO solution should move to the anode under the effect of an electric field. GO has little ionization degree in aqueous solution which leads to the low loading ratio on GFC, so we inserted Na+ into GO molecule to increase the loading ratio and improve the electrochemical performance of the composites.In this report, the preparation of graphene oxide/graphite fiber(GO-GFC) and modified graphene oxide/graphite fiber(GONa-GFC) and the relationship between the loading ratio and different depositional conditions were studied. The structure and morphology of the products were characterized by scanning electron microscope, atomic force microscope and Fourier transform infrared. Raman spectrum has been used to characterize the molecular structure. Galvanostatic Charge/discharge test and Cyclic voltammetry test were used as the methods of investigating the electrochemical performance of composites.The results show that: GO-GFC and GONa-GFC were successfully synthesized with an electrophoretic deposition method. The shape of GO sheets adhered on GFC was scale-like when the loading ratio of GO was low. With the loading ratio adding, the shape turned to be colloidal-like and GO adhered on GFC with film-like shape at last. GONa have similar morphology changes on the surface of graphite fiber, but GONa had higher speed of film form. Based on the calculation of loading ratio, it can be found that the number of GONa is higher than GO’s under the same depositional conditions, and the maximum loading ratio can reach to 20.58 wt %.Through the researches of electrochemical performance of composites, found that the best electrophoretic deposition condition for producing GO-GFC is at 27 V for 2.0 h in 0.35 g/L GO solution, with Cm of 20.46 F/g at the current density of 0.5 A/g. As a contrast, the maximum Cm of GONa-GFC is 27.32 F/g which is higher than the maximum Cm of GO-GFC and the electrophoretic deposition condition is at 27 V for 2.5 h in 0.35 g/L GONa solution. Because of the influence of the internal resistance, both of the specific capacitance of GONa-GFC and GO-GFC increased with the adding of the loading ratio and decreased after the number were 8.54% and 6.02%, respectively. It is noted that the specific capacitance of GFC, GO and GO-GFC at 5 A/g decreased to 29.64%, 24.92% and 34.83% of which were tested at 0.5 A/g while the number of GONa-GFC at 5 A/g only decreased to 47.14% of which was tested at 0.5 A/g, indicated GONa-GFC has better electrochemical stability than others under high current working environment.To investigate the performance of composites deeply, the relative specific capacitance of GO or GONa was estimated. It is noted that the relative specific capacitance of GO in GO-GFC can reach 427.63 F/g maximumly at 0.5 A/g and GONa is 355.41 F/g in GONa-GFC, both of them valued at more than a dozen times of GO alone(33.39 F/g). So GO molecules in the composites have better electrochemical behavior than GO alone according to our experiments. The specific capacitance the composites of reduced GO-GFC was higher than GO-GFC while reduced GO was lower than GO, indicating that the composites prevented the agglomerates or restack of most of GO molecules.