Study On Design And Electrochemical Performance Of Co-based Supercapacitors Electrode Materials

Supercapacitor is a new kind of energy storage device with high power density,long cycling life and fast charge and discharge,which can be applied in the filed of storage and conversion of renewable energy,electric vehicles and portable electronic equipment.However,the low energy density limits the large-scale applications of supercapacitors.As we all know,electrodes are the most important part in supercapacitors,which play a crucial role in the performance of supercapacitors.Therefore,developing the high-performance electrode materials is significant to greatly improve the energy density of supercapacitors.Cobalt-based oxides,sulfides and hydroxides are common materials that are easy to synthesize.When used as the electrode materials for supercapacitors,they all exhibit good electrochemical activity and have high theoretical capacity resulting from the rapid redox reactions with electrolyte,which make them become promising pseudocapacitor materials.However,cobalt-based oxides,sulfides and hydroxides suffer the bad conductivity,poor practical capacitance and easy to swell during charging and discharging,which hinder their commercial application.Hence,in order to obtain high-performance cobalt-based electrode materials,this subject aims to enhance the conductivity,capacity and long cycling stability of cobalt-based electrode materials through synthesizing nano-scale electrode materials,controlling morphology and composing with highly conductive graphene.Moreover,the cobalt-based cathode materials are used in aqueous asymmetric supercapacitors to widen the operationg voltage for further improving the energy density.The main research works are as follows:(1)ZIF-67 was successfully fabricated on the surface of graphene to form ZIF-67/graphene composites(ZIF-67/G),owing to the Co2+ were adsorbed on the GO through the surface functional groups.After the high temperature calcination in the air,the ZIF-67 on the surface of graphene was transformed into the nanparticle-assembled Co3O4 polyhedron to form hierarchical Co3O4/G composites.The specifical structure and composition of Co3O4/G composites could possess variety of oxidation sites and enhance the conductivity and long cycling stability.The electrochemical tests indicted that Co3O4/G composites shows higher capacitance and cycling life than Co3O4.Moreover,an asymmetric supercapacitor was assembled by Co3O4/G composites(cathode)and commercial activated carbon(anode),which shows an energy density of 19 Wh kg-1 and an excellent long cycling stability with 95.2%capacity retention after 5000 cycles.(2)To obtain higher electrochemical performance cobalt-oxides electrode materials,the ZIF-67/G was transformed into Co2V2O7/G composites by transforming the ZIF-67 to Co2V2O7 via an in situ ion exchange reaction with the VO3-under alkaline.The results of morphological and compositional characterizations confirm that the ZIF-67 has been successfully transformed into the hollow Co2V2O7 nanocages assembled by nanoparticle,which are densely distributed on graphene.The nanoparticle-assembled hollow Co2V2O7 nanocages could shorten charge-transport lengths,provide more active sites for redox reactions and accommodates the possible volume expansion during long-term cycling test.In addition,graphene,with the merits of high electrical conductivity and stability,can remarkably enhance the conductivity and served as the buffering substrate to counteract the internal stress and alleviate pulverization issues during cycling.The electrochemical tests show that Co2V2O7/G electrode material exhibits excellent electrochemical performance with specific capacitance of 576 F g"1 at a current density of 1 A g-1,which is about 2.3 times that of Co3O4/G electrode material.Moreover,the Co2V2O7/GG electrode material exhibited a remarkable long cycling stability with 93%capacitance retention after 10000 cycles.The asymmetric supercapacitor devices assembled with Co2V2O7/G and reduced graphene oxides(rGO)possess a high voltage window of 1.5 V and deliver a high specific capacitance of 86 F g-1 at 1 A g-1,a high energy density of 25.7 Wh kg-1 at a power density of 663.5 W kg-1 and excellent long cycling life with 88%capacitance retention after 10000 cycles,indicating that the CO2V2O7/G composites have a promising application in supercapacitors.(3)To avoid the "dead mass" typically encountered by using conductive agents and binders in the conventional slurry coating method,which will affects the electrochemical performance of electrode materials,hierarchical core-shell hollow iron-cobalt sulfides nanoarrays supported on conductive nickel foam(Fe-Co-S/NF)were fabricated using cobalt-based metal-organic frameworks conpounds(Co-MOFs)nanoarrays as the sacrificial templates.Delicately manipulating the ion-exchange reaction between Co-MOF and FeSO4 to yield FeCo-LDH nanosheets on the surface of Co-MOFs nanoarrays and the subsequent solvothermal sulfurization led to the formation of hierarchical core-shell hollow nanostructure with FeCo2S4-nanosheets shell assembled on the Co3S4 hollow nanoarrays.The FeCo2S4 nanosheets attached on the surface exhibit better conductivity and can greatly enlarge the electrode/electrolyte contacting area,which provides abundant electrochemical active sites to improving the specific capacitance.Moreover,the hollow structure can effectively alleviate volume expansion during cycling charge/discharge to enhance the cycling stability.Consequently,the obtained Fe-Co-S/NF electrode exhibits a specific capacitance of 2695 F g-1 at 1 A g"1,a favorable rate capability(69.8%capacitance retention at 10 A g-1),and retention of 84%over 1000 cycles.Moreover,an asymmetric supercapacitor was assembled by engaging Fe-Co-S/NF and reduced graphene oxide(rGO)as cathode and anode,respectively.The obtained device possess a high voltage of 1.6 V and delivers a good specific capacitance of 131 F g-1 at 1 A g-1 and a high energy density of 43.6 Wh kg-1 at a power density of 770 W kg-1,while maintaining a capacity retention of 89.6%after 5000 cycles.The robust electrochemical properties indicate that the Fe-Co-S/NF is a promising electrode material for supercapacitor.(4)NiCo layered double hydroxides(NiCo-LDH)nanosheets were vertically grown on the rGO sheets through a one-step hydrothermal method to obtain LDH/rGO composites.The SEM and TEM characterizations confirmed that as-prepared material have LDH/rGO/LDH sandwich-like nanostructure.The conductive and flexible graphene,as a substrate,can significantly reduce the agglomeration of NiCo-LDH,improve the structural stability and serve as a fast transportation highway for released electrons.The crosslinked NiCo-LDH nanosheets vertically grown graphene provides sufficient contact area between electrode and electrolyte for enabling fast ion migration and efficient faradaic reaction.The electrochemical performance tests suggest that the as-obtained LDH/rGO composites with Ni:Co=2:1 possessed the best performance,which shows a maximum specific capacitance of 2130 F g-1 at 2 A g-1(1.376 times that of LDH).Moreover,the LDH/rGO composites can retain 76.5%of its initial capacitance after 5000 cycles,much better than that of LDH(only 48.1%retention after 2,000 cycles).Asymmetric supercapacitors were assembled with LDH/rGO composites and rGO as cathode and anode,respectively.The as-assembled device demonstrated a wide voltage of 1.6 V,a high specific capacitance of 104 F g-1 and a maximum energy density of 34.5 Wh kg-1,while maintaining a capacity retention of 86.7%after 10000 cycles,indicating a good application prospect of LDH/rGO composites for supercapacitors.