Preparation And Pseudocapacitive Properties Of Transition Metal Compounds-Based Composite

As for supercapacitor electrode materials,transition metal compounds（oxide/sulfide）have attracted significant attention due to the involvement of faradic redox reactions which contribute to higher energy storage capacity compared to electrochemical double layer capacitance.The development of these materials should no only focus on the explorement of new materials delivering excellent performance,but also a detailed understanding of the fundamental energy-storage processes.Moreover,the properties of transition metal compounds（such as electronic/ionic conductivity）limit their supercapacitor performance.Graphene,a typical two-dimentional material,has been usually employed as substrate for the formation of transition metal compounds,which can improve the electronic/ionic conductivity and increase the electrochemical active sites at the same time,resulting in the boost of overall performance.This work will firstly investigate the effect of graphene substrates on the structure and performance of MnO₂-based materials,exploring the factors influencing the capacitive performance.Then,the structurefeatures and pseudocapacitive properties of cobalt-sulfide system are well studied.Graphene is used as a substrate to grow up the Co9S8 crystals,and the related electrochemeical performance is investigated,during which the key parameters which can greatly influence the capacitive properties of the composites are explored:（1）A serial of graphene/MnO₂ composites are prepared using GO,graphene and nitrogen-doped graphene as substrates.The MnO₂（belongs toδ-MnO₂ crystalline phase）with sheet-like shape,is homogeneously and tightly attached on the graphene surface.Different graphene substrates result in the differences in structure,morphology and chemical component of the composites.Abundant oxygen-containing groups on graphene oxide enable higher mass loading of Mn O₂,however,resulting in smaller sheets size at the same time,which leads to low capacity,poor rate performance,high charge-transfer resistance and low ionic conductivity.While using reduced graphene oxide as substrate,large sheets of the hybrids are obtained with a sacrifice in MnO₂ mass loading,which favors better capacitive and rate performance.Kinetic analysis is used to verify the changes in the charge storage process,confirming that higher contribution of the Faradic surface adsorption can lead to the enhanced electrochemical performance of NG-Mn.Furthermore,to further evaluate the electrochemical properties of NG-Mn in the practical capacitor,asymmetric supercapacitor（ASC）device is fabricated using NG-Mn as positive electrode materials and AC as negative electrode materials.The operation potential window reaches 1.6 V,and the CV curves possess rectangular shapes,even under a high scan rate of 300 mV s^-1,exhibiting the ideal capacitive behavior and fast charge/discharge property of the ASC.Besides,the ASC device delivers a maximum gravimetric energy density of 20.1 Wh kg^-1 and power density of 16 k W kg^-1.（2）Co₉S₈ is prepared using L-cysteine and sodium disulfide as sulfur source respectively via a two-step hydrothermal method.The results show that using L-cysteine as sulfur source favors the formation of Co₉S₈ crystal,accompanied with a relatively low specific surface area（SSA）.While sodium disulfide results in low crystallinity and higher SSA.The following high-temperature treatment contributes to the recrystallization of Co₉S₈ crystal with a decrease in specific surface area value.Further electrochemical tests indicate that the capacitive performance is highly related to the SSA,with the highest capacitance in low-crystallinity Co₉S₈ sample.Besides,the Co₉S₈ electrodes with High degree of crystallization show better rate performance.The featured electrochemical characteristics indicative of significant pseudocapacitive contribution are clearly demonstrated throngh kinetic analysis,which belongs to surface-dominated pseudocapacitance.After the incorporation of graphene,the formation of Co₉S₈ crystal is hindered,accordingly,minor-sized Co₉S₈ was formed,with the oxygenic groups on graphene surface serving as anchoring sites for Co ions.A high SSA value of 258 m²/g is obtained due to the contribution of cross-linked graphene framework and well-distributed Co₉S₈ in hybrid.A significant enhancement in capacitance is observed after the incorporation of graphene due to the expanded exposure of Co active site,which benefits the fast ion/electron transportation.A maximum capacitance of 1140 F/g can be achieved at 4 A/g,and around 90%and 74.5%of the capacitance can be maintained at a high current density of 10 A/g and 30 A/g,respectively.The cycling performance was studied by GCD curves at a current density of 8 A/g,and a good capacitance retention of 93.9%was finally remained after 1000 cycles.Moreover,graphene-coupled Co-based sulfide gives many similar kinetic features（b=0.8¹ and 91.2%at a scan rate of 8 mV/s）.For the charge storage ability within 1min(Q_1min),the hybrid is better than traditional redox-type pseudocapacitive materials（hydrated RuO₂ and MnO₂）and is comparable to the well-known Li⁺-intercalation pseudocapacitive materials（T-Nb₂O₅,V₂O₅ and MoS₂）.These can reasonably account for the good capacitive behavior and excellent rate-performance demonstrated in our previous discussion,moreover,suggesting a promising candidate of CoSG hybrid for high power-and energy-density supercapacitor electrode materials.