Preparation And Properties Of Graphene Films With High Volumic Specific Capacitance

Electrochemical capacitors（ECs）,also known as supercapacitors,have attracted a lot of attention due to its advantages of high power density,long cycling life and fast charging and discharging rate.According to the principle of energy storage,ECs can be divided into pseudocapacitors and electrical double layer capacitors（EDLCs）,and the EDLCs have better cyclic stability than pseudocapacitors.The experiments show that the electrode material is the decisive factor for high-performance EDLCs.In recent years,carbonaceous materials such as carbon aerogel,activated carbon,carbon nanotube and graphene are considered as the most promising electrode materials.Graphene has became the important electrode material for EDLCs because of its high theoretical surface area,high thermal conductivity,and good electrical conductivity.Although graphene has a larger gravimetric specific capacitance,the low density results in a smaller volumetric specific capacitance.In this paper,in order to prepare the graphene electrode materials with high volumetric capacitances,the higher volumetric specific capacitance graphene film electrode materials with different morphology and density have been prepared.The details are as follows:（1）The thermally treated dense rGO films are further treated with concentrated sulfuric acid to achieve the high C_g,C_v and cycling stability.The optimal film possesses high density of 2.04 g cm^-3,exhibits the maximal C_g and C_v of about 242.3 F g^-1 and 494.3 F cm^-3 in 1 M H₂SO₄ electrolyte based on the two-electrode testing system.Furthermore,the optimal film shows excellent cycling stability in 1 M H₂SO₄（90.2%capacitance retention after 10000 cycles）and excellent rate capability.（2）Using oxalic acid as a template,the graphene electrode materials obtained by mixing oxalic acid with graphene oxide,then the samples are freeze-dried,pressed into films and hydrothermally reduced in a saturated solution of oxalic acid.We have researched the effects of contents of oxalic acid and reaction temperatures on the samples’capacitve properties.The obtained graphene electrode materials with capsulate morphology have gravimetric specific capacitance of about 234.9 F g^-1 at 1 mV s^-1,and can retain 63.9%of its initial capacitance at high scan rate of 1 V s^-1.At the same time,the material shows excellent cycling stability with 91.2%capacitance retention after 21000 cycles at 200 mV s^-1.（3）The binder-free composite films of reduced graphene oxide（rGO）and activated carbon derived from cotton（aCFC）have been fabricated and used as electrodes for electrochemical capacitors（ECs）to avoid the decrease of capacitive performance in traditional process caused by the additional binder.The optimal aCFC is prepared at 850℃when the mass ratio of carbon and potassium hydroxide is 1 to 4.The optimal composite film prepared from the mass ratio of aCFC/GO=2/1 exhibits porous structure,and has a specific surface area of 849.6 m² g^-1 and a total pore volume of 0.61 mL g^-1.Based on the two-electrode system testing in 6.0 M KOH electrolyte,the optimal composite has specific capacitance of about 202 F g^-1,374 mF cm^-2 and 116F cm^-3 in terms of mass,area and volume,and shows excellent rate capability and good cyclic stability（91.7%retention of the initial capacitance after 5000cycles）.Furthermore,the assembled solid-state ECs by using KOH/polyvinyl alcohol as electrolyte show good mechanical stability and capacitive performances after repeated bending cycles.