Systematically controlled synthesis of rGO/nano-MnO_x materials for supercapacitors

The effects of hydrothermal production conditions on crystal forms, morphology and electrochemical performance of in situ reduced graphene oxide/manganese oxides (rGO/MnO_x) hybrid electrode materials were studied. The XRD/SEM results show that the ratio of precursors, reaction temperature and type of solvent significantly affected the crystal form obtained and nano-structure of MnO_x. Varied precursor ratios of KMnO₄:GO (5:1, 7:1, 9:1 and 11:1) yielded the hybrids of rGO/Mn₇O₁₃5H₂O, rGO/β-MnO₂, rGO/Mn_.98O₂, and rGO/K_0.27MnO₂(H₂O)_0.54, respectively, having corresponding nano structures of nano-sheets composed petal-like shape, petal-like shape based nano-wire clusters, petal-like shape with backwardly arched top and bigger mesoporous petal-like nano structures. Hybrid prepared precursor with the ratio of 5:1 exhibited the highest specific capacitance of 320 F/g at 0.2 A/g and the smallest diffusion impedance. However, hybrid prepared precursor with the ratio of 7:1 presented the lowest electron transfer impedance. Varying hydrothermal treatment temperatures (140, 160 and 200 °C) yielded hybrids: rGO/MnO(OH), rGO/Mn_0.91O_1,82(H₂O)_0.18, and rGO/Mn₇O₁₃5H₂O, respectively. The nano structures of the hybrids were nano-sheets having petal-like mesoporous structure, but having larger dimensions as temperature increase. The hybrid prepared at 160 °C showed the highest specific capacitance of 214 F/g at a current density of 0.2 A/g and the lowest solution resistance of 1.61 Ω, whiles, the hybrid prepared at 140 °C inferred the best cyclic reversibility.