摘要
石墨烯是一种具有单个碳原子厚度的二维碳材料，由于这种碳材料具有良好的导电性和大的比表面积，而被认为是一种理想的双电层电容材料。但因其比电容低等不足限制了实际应用。赝电容材料虽然具有高的比电容，但又因其导电性差而降低了它们在电化学反应过程中的活性物质利用率、倍率性能和循环性能。本研究者利用石墨烯特殊的物理和化学性质，将赝电容材料作为第二相引入到石墨烯中，以获取的复合材料可以具有突出的电化学性能。结合Hummer法与热还原制备了石墨烯，并以石墨烯为载体，制备了MnO_2、MnOOH、Co和Co_3O_4与石墨烯的超级电容器复合电极材料，研究了它们的电化学性能。
在石墨烯的合成方法研究中，发现当石墨粉具有规则的薄片状结构时，制备的石墨烯也具有相对小而且规则的石墨烯簇。在石墨烯的电化学性能研究中发现，在Na_2SO_4和KOH电解液中，石墨烯表现出不同的比电容。在Na_2SO_4电解液中，石墨烯在循环伏安扫描速度为5mV s~(-1)下的比电容为56.3F g~(-1)；扫描速度为500mV s~(-1)时，石墨烯的比电容降低到18.1F g~(-1)。在KOH电解液中，石墨烯在循环伏安扫描速度为5mV s~(-1)下的比电容为146.0F g~(-1)；扫描速度增加到500mV s~(-1)时，石墨烯的比电容降低到95.8F g~(-1)。石墨烯在KOH电解液中表现出了更高的比电容。此外，以石墨烯作为电极活性材料的对称电容器的比电容不会随着工作电压区间的变化而变化，说明石墨烯对称电容器在Na_2SO_4和KOH电解液中具有稳定的双电层电容性质。利用水热反应移除石墨烯中的羰基，通过红外和电化学阻抗分析发现，羰基的减少会破坏石墨烯的共轭结构和降低石墨烯的亲水性，使石墨烯的比电容发生降低。
在石墨烯基复合材料的研究中，采用KMnO4与石墨烯之间的氧化还原反应，在石墨烯的表面生成了MnO_2颗粒，并通过水热反应将这种MnO_2转化成为了一维的MnOOH纳米丝。MnO_2/石墨烯和MnOOH/石墨烯复合材料的电化学性能研究表明，在Na_2SO_4电解液中MnO_2可以显著的提高超级电容电极材料的比电容；在制备MnOOH/石墨烯复合材料的水热反应过程中加入氨水可以明显的降低MnOOH/石墨烯复合材料的欧姆阻抗和电化学阻抗，有利于提高这种复合材料的比电容。
通过沉淀法将Co(OH)_2纳米片嵌入到石墨烯的交错片层结构中，并采用热处理方法将石墨烯中的Co(OH)_2纳米片分解成为Co_3O_4纳米片，制备了纳米片Co_3O_4/石墨烯复合材料。另外，采用水热反应将石墨烯中的Co(OH)_2纳米片转化成为了均匀分布在石墨烯表面的金属Co颗粒，制备了金属Co/石墨烯复合材料。电化学性能研究表明，在充放电电流密度为0.2A g~(-1)下，Co_3O_4纳米片和纳米片Co_3O_4/石墨烯复合材料的比电容分别为203F g~(-1)和338F g~(-1)；在充放电电流密度为1.5A g~(-1)下，金属Co和金属Co/石墨烯复合材料的比电容分别为460F g~(-1)和1373F g~(-1)。另外，金属Co/石墨烯复合材料的电化学反应电极电位为-0.70V和~(-1).07V (vs. Hg/HgO)，因此该复合材料适合作为不对称电容器的负极材料。以金属Co/石墨烯复合材料为负极，石墨烯为正极的不对电容器在充放电电流密度为0.5A g~(-1)下的比电容为197F g~(-1)。
Graphene which has a two dimensional carbon plane with one-atomicthickness is considered as a new carbon material for supercapacitors, due to itsextraordinary electrical and high surface-to-volume. However, the lowcapacitance of graphene hinders its practical application. Additionally,pseudo-capacitor materials have high theoretical capacitances, but the poorelectric conductivity and low specific surface area of these materials reduce theutilization of active materials in the electrode, rate capability and cycleperformance. With the special chemical and physical properties of graphene, thecombination of graphene with a second phase material can further obtain thecomposites with excellent electrochemical properties. Herein, graphene wasprepared through Hummer method followed by thermal treatment. AndMnO_2/graphene composites, MnOOH/graphene composites, Co/graphenecomposites and Co_3O_4/graphene composites were synthesized, and theirelectrochemical performance was studied.
Researches on the preparation of graphene show that the graphene preparedfrom graphite powers with regular morphology possesses small and regulargraphene clusters. However, the electrochemical characterization of grapheneindicates that this kind of carbon material exhibits different electrochemicalperformance in the terms of the specific capacitance and rate performance inNa_2SO_4electrolytes and KOH electrolytes, respectively. In Na_2SO_4electrolytes,specific capacitances of graphene are56.3F g~(-1)and18.1Fg~(-1)at CV scan rates of5mV s~(-1)and500mV s~(-1), respectively. Moreover, in KOH electrolytes, specificcapacitances of graphene are146.0Fg~(-1)and95.8F g~(-1)at CV scan rates of5mVs~(-1)and500mV s~(-1), respectively. The results show that graphene has a higherspecific capacitance in KOH electrolytes rather than that in Na_2SO_4electrolytes.The specific capacitance of the symmetrical capacitor consisting of graphene asthe active materials is steady when the cell potential range of symmetricalcapacitor is changed, indicating that graphene has ideal electrochemical stabilityin Na_2SO_4electrolytes and KOH electrolytes. Through hydrothermal reaction,the number of carbonyl groups within graphene was decreased. The results fromFT-IR and EIS show that the reduction of carbonyl groups can damage theconjugate structure of graphene and increase its hydrophobic, which reducedspecific capacitances of graphene.
In the researches on the graphene based composites, MnO_2particles wereintroduced to the surface of graphene through the redox reacton between KMnO4 and graphene. And then, MnO_2was transformed to MnOOH nanowire under thehydrothermal condition. The electrochemical characterizations ofMnO_2/graphene and MnOOH/graphene show that the addition of MnO_2canenhance obviously specific capacitances of the electrode materials forsupercapacitor in Na_2SO_4electrolytes; aqueous ammonia added into the reactionsystem can decrease the electrochemical impedance of as preparedMnOOH/graphene and increase the specific capacitance.
Co(OH)_2nanoplates can be inserted into the intercalated structure of graphenethrough the precipitation method. Co_3O_4nanoplates/graphene were preparedthrough thermal treatment of Co(OH)_2nanoplates within graphene-basedcomposites. Moreover, under the hydrothermal condition, Co particles can beobtained within graphene-based composites and dispersed well on the surface ofgraphene. The research on electrochemical performance indicates that specificcapacitances of Co_3O_4nanoplates and Co_3O_4nanoplates/graphene are203F g~(-1)and338F g~(-1)at a current density of0.2A g~(-1); specific capacitances of Co andCo/graphene are460F g~(-1)and1373F g~(-1)at a current density of1.5A g~(-1). Theelectrochemical reaction of Co/graphene occurs at-0.70V and~(-1).07V (vs.Hg/HgO). Hence, this composite material can be used as an anode material forsupercapacitors. The specific capacitance of the asymmetry capacitor usingCo/graphene as the anode electrode and graphene as the cathode electrode is197F g~(-1)at a current density of0.5A g~(-1).
引文
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