Graphene/Manganese Oxide Nanocomposites As Electrode Materials For Supercapacitor

Manganese oxide (MnO2) as electrode materials for supercapacitor has been received the enormous worldwide attention driven by its high theoretical value of specific capacitance, structural flexibility, in addition to the low cost and more friendly environmental nature. However, MnO2nanoparticles tend to form big agglomerates, which would degrade the electrochemical performance. Thus, incorporation of nanostructure MnO2into conductive supports with large surface areas has been demonstrated to be an effective method for preparing electrode materials for high performance of supercapacitors. In our work, the GOs/MnO2and GSs/MnO2nanocomposites were fabricated by using graphene sheets (GSs) and graphene oxides (GOs) with high surface area and good electrical conductivity as conductive supports to anchor transition metal oxide nanocrystals with special morphologies to enhance their capacitance properties, respectively. Our main results are listed as follows:(1) GOs/MnO2nanocomposites:The formation of MnO2nanocrystals on the surfaces of GOs were achieved through a facile chemical deposition method. It is found that flower-like MnO2nanocrystals were firmly anchored on the GOs. The effects of the reaction time and feeding radio on the morphologies and microstructures of MnO2nanocrystals were discussed. With the increase of the time or the feeding ratio of MnO2, the size of flower-like MnO2nanocrystals increased and finally the change from flower-like to rod-like nanostructure can be observed. More attentions were paid to their electrochemical properties. It is found that with the increase of the time or the feeding ratio of MnO2, specific capacitance for the composite increased firstly and reached to a maximum value of about366F/g for GOs/MnO2-3h with capacity retention of87%after500continuous charge/discharge cycles in0.5M Na2SO4, and then decreased. The high electrochemical performance benefited from the unique nanostructure of composite electrode, in which GOs with large surface areas provide an effective supports for the formation of the flower-like MnO2nanocrystals and good conductive pathway, and flower-like MnO2nanocrystals provide large effective electrochemical reactive area, facilitating the contact of the electrolyte and active materials.(2) GSs/MnO2nanocomposites:The graphene sheets were firstly obtained through chemical conversion of GO, and then the GSs/MnO2composite was fabricated by chemical deposition. It is found that with the increase of the reaction time or feeding radio, the size and density of flower-like α-MnO2nanocrystals increased gradually. Furthermore, better electrochemical performance can be obtained by coating method with conductive additives than hot-pressing method with pure composite. The maximum value of414F g-1for the MnO2/GSs can be achieved at1mVs-1, which was slightly larger than394F g-1for the Mn02/Gos. In addition, large specific capacitance of356F/g can be remained after300continuous charge/discharge cycles in0.5M Na2SO4, attributing to more flower-like MnO2nanocrystals anchored on the GSs.(3) Ternary GSs/CNT/MnO2composites:The mixture of GSs and CNT was firstly prepared under ultrasonic irradiation with mechanical stirring, and then GSs/CNT/MnO2composite was fabricated by chemical deposition. It is found that MnO2nanoparticles covered simultaneously on the surface of both GSs and CNT, and CNT/MnO2can effectively isolated the CNT/MnO2sheets. An ideal three-dimensional structure can be achieved with the feeding ratio of CNT/GS of2:1and under the above-mentioned preparing condition for MnO2nanocrystals. The specific capacitance value of GSs/CNT/MnO2was about566F/g at1mVs-1, which was larger than414F g-1for the GSs/MnO2, and capacity retention only decayed to97%after500continuous charge/discharge cycles in0.5...