الفهرس 
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Abstract
The rapid growth of the global economy, along with the growing worldwide population, has resulted in an energy crisis due to the rapid depletion of fossil fuel sources as well as a negative impact on the environment due to gas emissions. Therefore, a lot of researches have been performed to achieve highly productive, eco-friendly, and inexpensive sustainable energy storage systems. Although the supercapacitors have outstanding properties, they can’t be used in a variety of applications due to the lower values of specific energy. So, the main focus in this field of research is to create innovative electrode materials for supercapacitors while maintaining their low cost, high specific power, and durability. In this regard, ternary transition metal sulfides, especially the NiCo2S4, are characterized by their high electrical conductivity, multiple redox reactions, and outstanding rate capability.
In this thesis, a facile, one-step hydrothermal technique has been used to prepare porous NiCo2S4 with a hollow microsphere-like morphology. It has a moderate porosity and surface area of about 4.6 m2g−1, which provides electro-active sites and short diffusion paths for ions and electrons, resulting in better Cs of 588 F g−1 at a density of 1 A g-1. The as-prepared NiCo2S4 has been improved through a series of studies by manipulating their morphology and increasing their surface area and porosity, which provide more electro-active sites and short diffusion channels for both ions and electrons. This produces a material with high conductivity, cycling stability, and specific capacitance.
In the first study, the NiCo2S4 was integrated with the reduced graphene oxide (rGO). In this experiment, the NiCo2S4 was happened on the surface of the rGO during the hydrothermal reaction. The spherical NiCo2S4 that has been synthesized is uniformly dispersed and firmly attached to the surface of the wrinkled rGO. The better attachment of the NiCo2S4 to the rGO surface provides a porous structure of NiCo2S4/rGO nanocomposite with an improved surface area of 14.21 m2g−1 compared to the pure NiCo2S4. The NiCo2S4/rGO nanocomposite also has outstanding electrochemical characteristics, since it reveals a high Cs of 1072 F g−1 at a current density of 1 A g-1. Therefore, the NiCo2S4/rGO electrode has a capacitance about 1.8 times greater than the NiCo2S4, indicating a remarkable enhancement of storage capability due to supporting NiCo2S4 on rGO sheets. A hybrid supercapacitor of NiCo2S4/rGO as a positive electrode and a negative electrode of activated carbon was assembled. It revealed an outstanding energy density of 41.52 W h kg−1 at a density of 1067 W kg−1 relative to previously reported NiCo2S4-based supercapacitors.
In the second study, the NiCo2S4 was decorated on porous electrospun carbon nanofiber (CNF) hydrothermally using the same reaction conditions that were used in the previous work. Firstly, the PAN nanofibers were fabricated using an electrospinning method, subsequent by heat treatment steps to be converted into porous CNF. The NiCo2S4 was grown on the surface of the CNF during the hydrothermal reaction, resulting in a hierarchical structure of NiCo2S4@CNFs. The NiCo2S4 is homogeneously distributed and tightly anchored to the surface of the CNFs. The NiCo2S4@CNFs have a highly porous structure with superior surface area of 77.85 m2g−1. It has enhanced electrochemical properties compared to the NiCo2S4; it reveals a Cs of 754.4 F g−1 at a current density of 1 A g-1. Furthermore, a NiCo2S4@CNF//CNF hybrid supercapacitor of a positive electrode of NiCo2S4@CNFs and a negative electrode of CNF was assembled and revealed a superior energy density of 65.6 Wh kg−1 at a power density of 665 W kg−1.
Another study has been performed by tuning the composition of the NiCo2S4 with some metals such as Cu and Zn. The effect of partial substitution of two ratios (0.1 and 0.5 moles) of Cu and Zn elements in place of the Ni element has been investigated on the morphology and electrochemical properties of the NiCo2S4. The NiCo2S4 morphology was changed from spherical into a micro flower-like structure upon substitution process with Cu and Zn elements. The Cu-and Zn- substituted NiCo2S4 nanoflowers were evaluated in a three-electrode configuration to assess their electrochemical behavior. The substitution ratio of 0.1 mole gives Cs of 818.56 and 654.56 F g-1 at a current density of 2 A g-1 for the Cu- and Zn- substituted NiCo2S4, respectively. When using a substitution ratio of 0.5 mole, the Cs values of Cu- and Zn- substituted NiCo2S4 decreased to 443.21 F g-1 and 395.5 F g-1, respectively. Therefore, tuning the composition of the NiCo2S4 with a ratio of 0.1 mole of Cu and Zn elements in place of the Ni element increases the Cs value in comparison with the NiCo2S4, which has a Cs value of 462.24 F g-1 at a current density of 2 A g-1. The constructed Cu0.1Ni0.9Co2S4//CNF HSC shows higher Ed of 31.25 Wh kg−1 at a Pd of 823.57 W kg−1. However, it reveals poor stability with capacitance retention of 14% after 3000 charge/discharge cycles, due to the degradation of the material during the redox processes. As a result, the cycle stability needs to be increased in order to be used in energy storage applications.
In the final study, the rGO and Cu were selected from our study to be combined with the NiCo2S4 in one composite and directly grown on a 3D hierarchical framework of nickel foam without using binding material to increase electrical conductivity and lower the interfacial resistance of the NiCo2S4. The free-standing Cu- substituted NiCo2S4@rGO nanoflowers have a porous structure with a high surface area of 27.75 m2g−1. The binder-free electrode of the as-prepared material reveals a Cs value of 3052 F g−1 at 2 A g−1. However, in comparison to the previously published NiCo2S4@rGO-based SCs, the Cu-NiCo2S4@rGO//Bi2Se3 HSC reveals a lower Ed value, which might be due to the lower durability of the Cu-NiCo2S4@rGO/NF electrode, as well as the exfoliation of the active material from the NF surface during the charge/discharge process, which leads to rapid deterioration of the Ed and Pd of the as-fabricated device.