Synthesis And Electrochemical Capacitive Performance Of Manganese Oxide-based Composites

With the rapid development of smart grids and new energy vehicles, there is a rising demand for high-efficiency energy storage systems. Supercapacitors, characterized by high power density and excellent service life, have evolved into an important class of new-type energy storage devices. Electrode materials are fairly critical to the performance of a supercapacitor, which makes their research and development become a hot issue. As a consequence, electrochemical capacitive properties of various transition metal oxides, e.g., Ru O₂, Co₃O₄, Ni O, V2O5 and Mn O₂, have been studied in the last decades. Except for low electric conductivity and poor cycling stability, manganese oxides have attracted extensive attention due to high theoretical capacitance, low cost and environmental compatibility. In this work, the above problems can be effectively solved by combining manganese dioxide with other materials. Main contributions of this research are as follows:（1） Galvanostatic anodic deposition and electrochemical capacitive performance of nano-Co₃O₄/Mn O₂ composite. The electrolyte used for preparing such composite contains Mn（NO3）2 and suspending nano-Co₃O₄ particles. The formation of nano-Co₃O₄/Mn O₂ composite goes through the following processes. The former is diffusion and absorption of nano-Co₃O₄ particles onto the anode and the latter is embedding of nano-Co₃O₄ particles into electrodeposited Mn O₂ matrix. Co₃O₄ content in the composites can be effectively controlled through adjusting volume fraction of nano-Co₃O₄ particles in the electrolyte, which can change the proportion of active material participating in charge storage process. When Co₃O₄ volume fraction in composite is 0.23, specific capacitance of the composite reaches 317 F/g at the scan rate of 5 m V/s. Furthermore, its specific capacitance retention is 93.4% after 5000 galvanostatic charge-discharge cycles.（2） Potentiostatic anodic deposition and electrochemical capacitive performance of nano-WO₃·H₂O/Mn O₂ composite. Nucleation and steady growth of Mn O₂ matrix are accelerated after the addition of nano-WO₃·H₂O particles into the electrolyte. This composite consists of numerous WO₃·H₂O/Mn O₂ nanosheets and shows a highly porous morphology, which could promote the penetration and transport of electrolyte ions and improve the utilization of active material. Electrochemical measurements reveal that the nano-WO₃·H₂O/Mn O₂ composite has both high specific capacitance（363 F/g at the current density of 0.5 A/g） and good power capability（210.3 F/g at the current density of 10 A/g）. Capacitance retention of this composite still remains 93.8% after 5000 galvanostatic charge-discharge cycles at the current density of 2 A/g.（3） Electrochemical deposition and electrochemical capacitive performance of three-dimensional Zn O@Mn O₂ core-shell nanocables. The preparation process of three-dimensional Zn O@Mn O₂ core-shell nanocables involves potentiostastic deposition of Zn O nanorod arrays and potentiodynamic deposition of multivalent and partially hydrous manganese oxide. According to cyclic voltammetry and galvanostatic charge-discharge measurements, the specific capacitances of Zn O@Mn O₂ core-shell nanocables reach 537.8 F/g at the scan rate of 5 m V/s and 613.5 F/g at the current density of 1 A/g. Electrochemical impedance spectroscopy also confirm that electrochemical properties of manganese oxide have been greatly enhanced due to the support of Zn O nanorod arrays. Furthermore, capacitance retention of three-dimensional Zn O@Mn O₂ core-shell nanocables is 89.8% after 5000 charge-discharge cycles, which demonstrates its good cycling stability.（4） Synthesis and electrochemical capacitive performance of hierarchically porous Mn O₂/rice husks derived carbon（Mn O₂/RHC） composite. Mn O₂/RHC composite is prepared using a two-step process, including carbonization-activation of rice husks to obtain RHC and in situ chemical precipitation to incorporate Mn O₂. The RHC framework with good conductivity is suitable for charge collection and can provide rapid paths for electron transfer. More importantly, hierarchically porous microstructure consisted of abundant interconnected macropores, mesopores and micropores is basically retained in the composite due to the formation of Mn O₂ in mesopores/macropores. Specific capacitance of Mn O₂/RHC composite reaches 197.6 F/g at the scan rate of 5 m V/s and 210.3 F/g at the current density of 0.5 A/g. Mn O₂/RHC composite also displays favorable cycling stability with 80.2% capacitance retention after 5000 galvanostatic charge-discharge cycles.