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Design and Synthesis of Electroactive Nanomaterials with Superior Energy Storage Capability based on Ion-Selective Structure for Ion-Selective Removal and their Application in Hybrid Capacitive Deionization

Sharifpour, Hanieh | 2025

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 58350 (03)
  4. University: Sharif University of Technology
  5. Department: Chemistry
  6. Advisor(s): Shahrokhian, Saeed
  7. Abstract:
  8. The existence of safe and sufficient water is of paramount importance for life on Earth and the survival of humankind. Although two-thirds of the Earth's surface is covered with water, the current demand for fresh water has not been met through these readily available sources for various reasons, including the salinity and hardness of surface waters. In the meantime, the continuous growth of the human population and emerging needs in industry and agriculture have increasingly raised the demand for water resources. Accessible freshwater resources such as rivers, groundwater, and lakes account for only a small portion of the total freshwater reserve. Therefore, today, desalination and purification of seawater, as the most abundant water source available on the planet, have gained widespread attention for producing drinking water and water required for industrial uses. Capacitive deionization (CDI), which is based on the principles of supercapacitors, is recognized as a novel technology in the desalination of surface waters. Given its high energy efficiency, low environmental impact, and cost-effectiveness, as well as the absence of the need for complex equipment, capacitive deionization is currently considered a superior and developing technology in this field. This thesis examines the application of various electroactive nanomaterials with the ability to selectively remove ions through insertion and faradaic reaction to fabricate electrodes for studies in the field of capacitive deionization. The selective performance offers significant opportunities in removing salinity, bitterness, and hardness from water, as well as in wastewater treatment in our study. The process of ion insertion and extraction by Faraday electrodes can be much more selective and efficient in terms of specific capacity compared to adsorption as an electric double layer on the surface of carbon electrodes. Another important point regarding the selective capacitive deionization technique used in this thesis is that this potential-dependent, low-cost, and energy-efficient selective removal is difficult to achieve using other alternatives in water treatment, including reverse osmosis (RO) and electrodialysis (ED), without the use of high-pressure pumps and expensive ion exchange membranes. Capacitance and charge storage capacity are closely related to the type of electrode materials used in the fabrication of electrodes. Therefore, the development of new electrode materials with high capacitance and selective performance towards a specific ion has been the most important strategy in advancing our goals in this thesis. In the first section, a symmetrical capacitive deionization cell consisting of nitrogen-doped carbon-based electrodes with a very high surface area was designed, modified by molybdenum sulfide (MoS2) nanorod structures. To this end, carbon precursors were initially deposited on a nitrogen-rich nickel substrate using a simple hydrothermal method with cost-effective raw materials. After secondary carbonization and subsequent acid eaching of the nickel content, nitrogen-doped carbon flowers (N-pCs) were obtained. Subsequently, to increase salt adsorption capacity and induce selective properties toward target ions, the carbon structures were modified with molybdenum sulfide nanorod structures through a simple hydrothermal process followed by secondary sulfurization in an inert atmosphere. As a result of this sulfurization reaction, sulfur atoms enter the structures. The presence of nitrogen increases the hydrophilicity and wettability of the electrode, reducing mass transfer resistance and increasing the permeability of electrolyte ions into the carbon structures. In addition, molybdenum sulfide structures, by inducing pseudocapacitive behavior due to redox reactions during charge-discharge processes, lead to better selectivity of these synthesized electrodes towards selective ions. The prepared composites, by creating a synergistic effect resulting from the excellent stability of carbon-based electrodes and the high conductivity of molybdenum sulfide chalcogenide structures, have imparted unique properties to this study. This cell provides compelling evidence regarding the adsorption capacity of sodium, potassium, calcium, and magnesium ions compared to other components of the solution. In the second section, a sodium-selective hybrid capacitive deionization (HCDI) cell was developed, consisting of a sodium superionic conductor (NASICON) with a carbon inner layer as the negative electrode and nitrogen-doped carbon-based electrodes in a nest-like shape (N-pCs) with a very high surface area as the positive electrode. To this end, a carbon-coated sodium superionic conductor structure was synthesized using a hydrothermal method followed by a heat treatment under an inert atmosphere. Despite the high desalination capacity of a hybrid cell, unbalanced ion adsorption and the potential rejection of co-ions are possible in this method, which were addressed by placing an ion exchange membrane (IEM) between the electrolyte and the electrode. Based on reported data, the fabricated cell can selectively remove sodium from saline water containing NaCl, along with ion recovery and energy production in the regeneration cycle. The selective removal capability of sodium ions in this study was investigated by comparing the removal capacity of Na+ in the absence and presence of interfering ions Mg2+, Ca2+, and K+. Accordingly, the sodium removal efficiency (SRE%) for this study was found to be over 50% for both the pure NaCl solution and the mixed ion solution. Therefore, the fabricated dual-function system has a high capacity for sodium removal from sodium-based solutions as well as from a mixture of different electrolytes, alongside energy recovery in the ion desorption process. This dual functionality positions the present study as a unique investigation in producing safe and sufficient water while also providing energy. In the third section, a hybrid deionization cell was designed for desalination of salt solutions, and based on the reported data, the designed system demonstrated the ability for selective deionization of chloride ions. For this purpose, bismuth oxychloride structures were initially synthesized in the form of spheres composed of interconnected nanosheets through a solvothermal method. These electroactive structures, due to their redox activity in sodium chloride and potassium chloride electrolyte solutions at negative potentials, were used as the negative electrode in a two-electrode HCDI cell configuration against nitrogen-doped carbon nests as the positive electrode. Conductivity measurements and the use of calibration curves for KCl and NaCl electrolytes showed changes in chloride ion concentrations during desalination. The reported results confirm the superior ability of the BiOCl//NCN HCDI cell to selectively absorb chloride ions from both NaCl and KCl solutions, which is consistent with the results obtained from three-electrode measurements. Therefore, the studied system has the unique ability to reduce salinity and bitterness from various water samples
  9. Keywords:
  10. Nitrogen and Sulphur Co-doped Activated Carbon ; Hybrid Capacitive De-Ionization ; Sodium Superionic Conductor (NASICON) ; Molybdenum Sulfide Structure ; Selective Ion Removal ; Ion-Selective Membrane ; BiOCl Selective Structure

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