Research On Preparation And Performance Of Graphene Based Electrode Materials For Supercapacitors

The effective and practical technology for electrochemical energy conversion and storage is the supercapacior used in hybrid electric vehicle, electric vehicle, portable electronic device and backup power. In these applications, electrode materials are crucial for the performances of supercapaciors. Graphene, as a unique two-dimensional carbon material, has been not only applied in the electrochemical double-layer capacitors as ideal electrode materials, but also combined with metal oxides/hydroxides for preparing the electrode materials of pseudocapacitors as the substrate materials, due to its fast electron mobility and large surface area. In this work, the subject has been researched as following.Graphite oxide was firstly synthesized from flake graphite by a modified Hummers method, afterwards it was reduced to graphene sheets via a solvothermal route in ethanol. In the meantime, the effects of reaction time and temperature on the structure and on the electrochemical performance of graphene sheets have been studied. With the increasing reaction time and enhancing temperature, more oxygen-containing groups could be removed to restore π-bonding, leading to the increase of electron transport and specific capacitance of graphene sheets as the electrode materials of supercapacitors. Graphene sheets prepared at 493 K for 10 h were transparent, curled and partially folded, a mixture of few-layer graphenes(2-6 layers). At the current density of 0.1 A/g, the specific capacitance of graphene sheets could reach 186 F/g in the 6 mol/L KOH solution. After 50 cycles at each current density(0.1, 0.4, 0.8 and 1.2 A/g), the retention of specific capacitance was 78%, then its capacity decreased only 2.5% after 2000 cycles at 1.2 A/g. The EIS analysis of the electrode after 1st cycle and 2000 th cycle at 1.2 A/g displayed that the fitting values of inner resistance and faradic charge transfer resistance of the electrode were low and did not change too much after cycling. The result illustrates that the as-prepared graphene sheets have the superior electrochemical performances as a suitable and promising electrode material used for electrochemical double-layer capacitors.By chemical precipitation, nano-Ni(OH)2 were loaded on the surfaces of graphene sheets for preparing the Ni(OH)2-graphene sheet composite. Due to redox reactions of Ni(OH)2 in the alkaline electrolyte, a higher capacitance of composite can be obtained. Besides, graphene sheets were adopted as a substrate to combine with Ni(OH)2 for making up their shortcomings of poor electrical conductivity and lack of cycling stability at a high current density. The specific capacitance of Ni(OH)2-graphene sheet composite was 959 F/g at 0.2 A/g in the 6 mol/L KOH solution and the retention of was 68.45% after 20 cycles at each current density(0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 A/g).Because of a longer reaction time of solvothermal reduction of graph ite oxide to graphene sheets in ethanol, this might cause the reduced graphene sheets to restack. To solve the problem, acid-treated carbon nanotubes and graphite oxide were uniformly dispered in ethanol, then graphene sheet-carbon nanotube composite was obtained with solvothermal treatment at 473 K for 10 h. The added carbon nanotubes(15 mass %) could prevent graphene sheets from restacking, so the BET surface area of graphene sheet-carbon nanotube composite(109.07 m2/g) was larger than that of pure graphene sheets(32.06 m2/g) prepared in the same reaction condition.To further increase the capacity of the electrode material, nanometer sized Ni(OH)2 were incorporated into graphene sheet-carbon nanotube composite by chemical precipitation. The Ni(OH)2 inserted into graphene sheet-carbon nanotube composite prevent not only the loss of electric contact, but also the exfoliation of Ni(OH)2 attributed to their volume expansions during cycling at a bigger current density. The specific capacitance of Ni(OH)2-graphene sheet-carbon nanotube composite was 1170 F/g at 0.2 A/g in the 6 mol/L KOH solution. After 20 cycles at each current density(0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 A/g), its capacitance remained 73.04% of initial capacitance compared to 25.48% for pure Ni(OH)2, displaying the excellent rate performance of Ni(OH)2-graphene sheet-carbon nanotube composite. At 1.2 A/g, the specific capacitance of the above composite decreased 20.03 % after 1000 cycles. This shows the excellent cycle life of Ni(OH)2-graphene sheet-carbon nanotube composite. Therefore, the as-prepared electrode material of supercapacitors has a promising application prospect.The Mn3O4 nanoparticles prepared by the decomposition reaction of Mn(Ac)2 were loaded the surfaces of graphene sheets obtained through the reduction of graphite oxide to produce Mn3O4 nanoparticle-graphene sheet composites by one-pot solvothermal reaction at 473 K for 10 h in ethanol. It is benefical that not only graphene sheets have been closely contacted with Mn3O4 nanoparticles to enhance their performance of electronic conduction, but also the capacitance of composite has been rasied by the redox reactions of Mn3O4. Moreover, the influences of the mass ratios of Mn2+/graphite oxide on the electrochemical performances of composites have been explored. At a scan rate of 2 m V/s, the composite prepared with Mn2+/ graphite oxide mass percent of 10 : 90 shows a high specific capacitance of 296 F/g in the 6 mol/L KOH solution, and the specific capacitance still retains 81% of the initial capacitance at 0.5 A/g after 1000 cycles. Hence, the composite material is a promising electrode material for supercapacitors.