The Preparation Of Graphene-based Hydrogel Composites And Their Properties

Recently,graphene has attacted increasing attention in recent years due to its extraordinary optical,mechanical,thermal properties,and large specific surface area.In order to give full play to the excellent performances of graphene,the degree of stack in layers of graphene-based macro material should be effectively decreased.The preparation of self-assembly of graphene hydrogels is an efficient solution.Combined the excellent conductivity and cycle performance of three-dimensional porous structure of graphene materials with the high capacity of metal oxides/hydroxides electrode materials,we could prepare carbon-based composite for electrode materials with high energy density and power density.It provides a new way to obtain high performance electrode material for supercapacitors.In this article,the applications of functional graphene-based materials in electrochemical energy conversion and energy storage are studied with some typical hydroxides,oxides,sulfides and carbon nitride compounds.Detailed research findings are as follows:?1?First,graphene oxide?GO?was prepared by the modified Hummers' method.Graphene hydrogels?GH?were prepared via a one-step hydrothermal synthesis with hydrazine hydrate as reducing agent.GO and GH were conducted with series of structure and morphological characterization.The results show that GO is successfully reduced to graphene,and the obtained hydrogels shows good three-dimensional structure by the reveal of TEM and SEM.In two-electrode system electrochemical tests,the GH shows higher electrochemical performance than the freeze-dried GH in symmetric supercapacitors.At a high current density of 20 A/g,GH exhibits a specific capacitance of 103.5 F/g,while freeze-dried GH shows only 26.3 F/g.Significantly,about 81.7%of initial specific capacitance of GH could be retained when the current density increased from 2 to 50 A/g,indicating a good rate capability.?2?Nickel hydroxide/graphene hydrogel composites?NGHs?were obtained via a one-step hydrothermal synthesis.The results show that three-dimensional graphene hydrogel composites were prepared by the facile one-step hydrothermal method.The uniform-sized Ni?OH?2 nanoplates grow on graphene sheets,and the Ni?OH?2/graphene hydrogel owns a three-dimensional structure significantly.Ni?OH?2 nanoplates could effectively inhibit the restack of graphene sheets and the intercalated graphene layers could also reduce the agglomeration of Ni?OH?2 nanoplates.The unique three-dimensional structure as well as thesynergistic effect between Ni?OH?2 and graphene effectively improvs the electrochemical performances.Compared to pure graphene hydrogels and Ni?OH?2 nanoparticles,the electrochemical properties of NGHs are significantly improved.NGH7.5?the calculated mass ratio of formed Ni?OH?2 with GO is 7.5:1?exhibits excellent electrochemical properties,like high capacitance,rate capability and good cycle performance?a specific capacitance of 1125.4 F/g at a charge and discharge current density of 0.5 A/g,which is nearly 2.2 and 8.1 times of those of Ni?OH?2 and GH.After 1000 cycles,the capacitance could maintain 87.3%of the initial capacitance,which is also higher than that of bare Ni?OH?2?.?3?As a cathode material of lithium ion battery,nickel oxide?NiO?has a high theoretical capacity.By calcining the freeze-dried NGHs,we can obtain the aerogel composites?NiO/G?.The SEM results show that NiO/G owns the same microstructure of NGH7.5.Thanks to the porous three-dimensional structure,the electrochemical performances of NiO/G are remarkably superior to those of NiO.The electrode based on the NiO/G composite delivers a discharge capacity of 1349 and 992 mAh/g at the 1st cycle with a coulombic efficiency of about 73.5%.Furthermore,NiO/G composite still shows a high reversible capacity of 881 mAh/g after 30 discharge-charge cycles.In contrast,the coulombic efficiency of NiO is 70.9%.When the current density returns to the initial 100 mA/g after 90 cycles,the NiO/G composite recovers 47.3%of its first capacity.For NiO,the reversible capacity of pure NiO only delivers 284 mA/g corresponding to only 30%capacity retention of the first capacity.?4?As the kin elements,oxygen and sulfur elements have many similar properties.On this basis.After the study of oxides,we investigated the preparation and electrochemical properties of the sulfides/graphene hydrogel composites.Based on cobalt sulfides,two different kinds of precipitant were used to prepare the amorphous CoS/graphene hydrogel composites?CGHNa2S?and the hexagonal phase CoS/graphene hydrogel composites?CGHthiourea?,respectively.Based on these two paths,the formation mechanism of these two hydrogel composites are discussed.The characterization results of TEM and SEM comfirm the successful combination of CoS nanoparticles and graphene,and forms a free-standing and three-dimensional structure.The hexagonal phase CoS/graphene hydrogel composites,which was prepared with thiourea as precipitant,show an excellent electrochemical performance.Compared the electrochemical performances of different samples,CGHthiourea exhibits a specific capacitance of 564.5 F/g at a current density of 1 A/g current density,which are 1.2 and 1.3 times of CGHNa2S and CoSthiourea.In addition,CGHthiourea can maintain 95.6%of the initial specific capacitance after 2000 cycles of charge and discharge at a current density of 0.5 A/g.?5?Because of the excellent chemical and physical properties,graphite carbon nitride?g-C₃N₄?can be used as a unique light catalyst and catalyst with high chemical and thermal stability.The three-dimensional structure g-C₃N₄ photocatalyst was prepared by dicyandiamide milled with NaCl.The influence of the amount of NaCl is studied on the hydrogen production of g-C₃N₄ photocatalytic performance.And the formation mechanism of the new g-C₃N₄ with three-dimensional structure is also discussed.The g-C₃N₄/graphene hydrogel composites are prepared by hydrothermal method,and the further performance of these composites will be explored.The obtained g-C₃N₄ showed excellent photocatalytic properties.The specific surface area of D90 is remarkably?7 times larger to bulk g-C₃N₄.The rate of H2 evolution of bulk g-C₃N₄ calcined from dicyandiamide is 4.1 ?mol/h.For g-C₃N₄ prepared with the assistance of NaCl,the rate of H2 production is 21.7 ?mol/h,which is 5.3 times to bulk g-C₃N₄.The free-standing macro structure of g-C₃N₄/graphene hydrogel composites provide them more possibilities in practical applications.