الفهرس 
AbstractOnly 14 pages are availabe for public view
 |  | from | 99 |  |  |
| | | from | 99 | | |
Abstract
The global demand for energy, combined with the depletion of fossil fuels and the resulting environmental damage, has caused substantial attempts to develop green, efficient, and sustainable energy sources. Due to its characteristics of low toxicity, low operating temperature, eco-friendly, high energy efficiency, low exhaustion, and ease of storage and transport, direct methanol fuel cells (DMFCs) are promising alternative energy sources for portable devices and stationary applications.
The goal of this thesis is to develop low-cost carbon xerogel based materials supported on nickel foam. High porosity, high surface area, high density, and superior electrical conductivity are all advantages of carbon xerogel. Carbon xerogels have a unique porosity that allows for a large number of active sites for ions movement and electron transfer, resulting in incredible electrochemical performance for the methanol oxidation reaction.
Layered double hydroxides (LDHs) have emerged as promising electrodes materials for the methanol oxidation reaction. nanocomposites of LDHs and carbon xerogels (CX) supported on nickel foam (NF) substrate were prepared to investigate the role of carbon xerogel. The results show that NiFe-LDH/CX/NF is an efficient electrocatalyst for methanol oxidation with a current density that reaches 400 mA•cm−2 compared to 250 and 90 mA•cm−2 for NiFe-LDH/NF and NF, respectively. In addition, all LDH/CX/NF nanocomposites show excellent stability for methanol oxidation. A clear relationship is observed between the electrodes crystallite size and their activity to methanol oxidation. The smaller the crystallite size, the higher the current density delivered. Additionally, the presence of carbon xerogel in the nanocomposites offer 3D interconnected micro/mesopores, which facilitate both mass and electron transport.
Extending to our firstly published work reported on synthesis of the NiFe-LDH/CX/NF
nanocomposites for methanol oxidation reaction resulting highly improved in the current density and stability. In continuation of our interest in DMFC, A promising electro-catalyst material composed of 2D layered MoS2 with highly interconnected mesoporous network of carbon xerogel assembled on nickel foam was facilely fabricated in-situ with the hydrothermal process.
This 3D nanocomposite electro-catalyst delivered high current density of about 550 mA‧cm−2 and long-term stability. These enhancements are due to the unique structure of MoS2, which increases mesoporosity and active surface area, allowing for sufficient electrical conductive channels and pathways for electrons with outstanding electro-catalytic performance towards Methanol oxidation reaction (MOR). With this exceptional electrochemical behavior, this work could introduce a promising electro-catalyst for large scale commercial fuel cell applications. All of the electrochemical data were connected with the textural and chemical features of the produced doped carbon materials, which were organized into the following Chapters:
Chapter 1: Introduction and objectives. This chapter summarizes the state of the art in terms of environmental and energy issues related to the investigated electrochemical applications, as well as the types of materials that have been produced. Finally, the work’s aims are explained.
Chapter 2: Materials synthesis and characterization procedures. The preparation of all produced materials, as well as the experimental conditions of the common characterization procedures used in Chapters III and IV, are detailed in this chapter.
Chapter 3: Design of a Self-Supported Flexible Nanostars MFe-LDH@ Carbon Xerogel Modified Electrode for Methanol Oxidation. This chapter contains the article published in Materials journal with its publishing format, in Which Six nanohybrid composites of Layered double hydroxides supported on nickel foam denoted as NiFe-LDH/NF, NiFe-LDH/CX/NF, CoFe-LDH/NF, CoFe-LDH/CX/NF, ZnFe-LDH/NF and ZnFe-LDH/CX/NF were prepared by the hydrothermal technique with and without carbon xerogels to investigate the influence of carbon xerogels on electrode activity. Furthermore, the role of divalent cations on the electrochemical activity of methanol oxidation was investigated using three divalent cations, MII = Ni, Co, and Zn, with the trivalent cation, Fe, remaining fixed.
The results reveal that there is a clear relationship between the crystallite size of electrodes and their activity to methanol oxidation. The smaller the crystallite size, the higher the current density. In addition, NiFe-LDH/CX/NF is an efficient electrocatalyst for methanol oxidation with a current density reaches 400 mA.cm–2 compared to 250 and 90 mA.cm–2 for NiFe-LDH/NF and NF, respectively.
Chapter 4: 3D Nanohybrid of MoS2 Nanocoils Functionalized Carbon for Methanol Electro-oxidation. This Chapter describes the development of different types of carbon gels doped with X ( X: Mo, MoS2,W and WS2 ) were prepared, exhaustively characterized, and tested as electrocatalysts for MOR by hydrothermal method. The results show that MoS2/CX/NF is an efficient electrocatalyst for methanol oxidation with a current density reaches 550 mA.cm–2 compared to 90 mA.cm–2 for bare NF. The unique structure produced in MoS2/CX/NF may be responsible for the increased electrocatalytic activity.