| Recently,supercapacitors have drawn great attention because of their high charge/discharge rate,long life cycle,and outstanding power density.Supercapacitors,also known as ultra-capacitors or electrochemical capacitors,can store electrical energy faradaically(pseudocapacitor)and electrostatically(electrical double layers).Pseudo-capacitor materials offer much higher specific capacitance and energy density than electrical double layers(EDLCs)via reversible faradaic redox reactions.However,currently,one major challenge that keeps supercapacitors from their promising applications is their low energy density.The objective of this thesis research is to develop supercapacitors with simultaneously high power density and energy density.The transition metal oxides and transition metal sulfides(TMSs)as pseudo-capacitors are promising electrode materials for supercapacitors owing to their superior electrochemical properties.Metal sulfides have attracted significant research interest for their high specific capacitance,environmental friendliness,chemical and thermal stability,and much higher electrical conductivity than their metal oxides counterparts.Ternary metal sulfides can offer better electrochemical performance than bimetal sulfides or single component metal sulfides by using all 3 metal ions in the redox reactions.Based on these considerations,the ternary metal sulfides have been synthesized,characterized,their electrochemical performances tested and used to assemble the asymmetric supercapacitors as follow:Hierarchical mesoporous Zn-Ni-Co-S-rGO/NF(nickel foam)microspheres have been prepared by hydrothermal,sulfurization,and calcination process.The effect of different sulfurization time on the morphology and capacitance of composites was tested.The high electrochemical performance of(Zn-Ni-Co-S-rGO/NF)composite was obtained when the sulfurization time was 3 h(Zn-Ni-Co-S-rGO/NF-3h),where a specific capacitance of 627.7 F g-1 at 0.25 A g-1 and excellent rate capability of about 97.8%capacitance retention at 2 A g-1 after 4,000 cycles were achieved.Moreover,an asymmetric supercapacitor fabricated by(Zn-Ni-Co-S-rGO/NF-3h)composite as the positive and activated carbon(AC)as negative electrodes,showed a high energy density of 75.96 Wh kg-1 at a power density of 362.49 W kg-1 with a remarkable cycle stability performance of 91.2%capacitance retention over 5,000 cycles.Manganese-nickel-cobalt ternary sulfide(MNCS)nanosheets deposited on reduced graphene oxide/nickel foam(rGO/NF)have been synthesized via a simple electrochemical approach.The obtained ternary metal sulfide(MNCS@rGO/NF)showed better electrochemical performance than corresponding bimetal nickel-cobalt sulfides on reduced graphene oxide/nickel foam(NCS@rGO/NF),which can be attributed to the addition of manganese to increase the oxidation state in the electrodes.The MNCS@rGO/NF electrode material achieved a significant specific capacitance of 1,302 F g-1 at 0.5 A g-1,with good rate performance and cyclic stability.In addition,asymmetric supercapacitor was fabricated using the MNCS@rGO/NF and rGO/NF as positive and negative electrodes respectively and it managed to deliver a high energy density of 103.62 Wh kg-1 at a power density of 562.47 W kg-1Molybdenum nickel-cobalt sulfide(MoNiCoS)microspheres have been electrodeposited on the Ni foam with a different concentration for evaluating its effect on the morphology and capacitance of the composites.The microspheres structure has been characterized using techniques such as XRD,SEM,TEM,and XPS to understand their growth mechanism.MoNiCoS microspheres exhibit a high specific capacitance of 1,472 F g-1 at 0.5 A g-1 along with excellent cycling stability of 98%after 5,000 cycles.Moreover,an asymmetric supercapacitor composed by MoNiCoS as the positive electrode and AC/rGO as the negative electrode showed a high energy density of 146.8 W h kg-1 at a power density of 482.2 W kg-1 and excellent cycle stability. |
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