- Title
- Supramolecularly engineered nanomaterials for photo and electro catalytic water splitting
- Creator
- Panangattu Dharmarajan, Nithinraj
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2024
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Unpredictable climate change, driven by global warming, poses a serious threat to the sustainability of future generations. Since the wake of the Industrial Revolution, the unchecked use of fossil fuels as a primary energy resource has been a major contributor to these disasters. Therefore, it is necessary to develop alternative, sustainable, and renewable energy production resources. Renewable energy sources, such as solar, wind, tidal, and geothermal energy, can be harvested and stored using various sustainable methods for future use, although these sources can only provide an intermittent supply of energy. Hydrogen production through water splitting is considered one of the most suitable options for current predicaments since the green hydrogen produced using sunlight and water is an inexhaustible and eco-friendly energy solution. Water splitting can be carried out via electrochemical or photochemical processes, where the thermodynamic barrier is overcome by an external bias or photogenerated electron– hole pairs. The key point is to design and develop new catalysts with specific properties to enhance the rate of catalytic reactions. For instance, photocatalytic performance can be enhanced by manipulating the bandgap and modifying the catalyst properties to reduce the energy barrier. The material properties can be engineered using heteroatom doping, stoichiometric modulation, heterojunction construction, and co-catalyst assistance. Similarly, in electrocatalytic hydrogen production, an external bias is applied across the cathode and anode in an aqueous electrolyte, where the water molecules are decomposed to hydrogen and oxygen molecules at the interface between the catalyst electrode and electrolyte. The electrocatalyst should reduce the reaction energy barrier by facilitating efficient adsorption, desorption and charge transfer during the electrocatalytic reaction. However, photocatalytic hydrogen production is more sustainable owing to the direct use of solar energy to produce hydrogen by splitting water; however, its overall efficiency is currently limited. Electrocatalytic way of producing hydrogen, while requiring external electric energy, provides flexibility in energy sources and is scalable for large-scale production of hydrogen using the current technology. Nevertheless, industrial-level hydrogen production is limited by the lack of suitable catalysts that exhibit high stability, low cost, and minimal overpotential. Consequently, the design of a catalytic material capable of overcoming these limitations is one of the most discussed topics in the research community. In this thesis, we aim to address these challenges by developing novel photo/electrocatalytic nanomaterials through unique selfassembly and nanotemplating strategies as it will assist us in manipulating the fundamental aspects of the catalytic materials at the molecular or atomic level. The first chapter provides a brief introduction to the hydrogen economy and identifies knowledge gaps. The second chapter comprehensively reviews the use of supramolecular self-assembled carbon nitrides for photocatalytic water splitting. The review covers the need for carbon nitride, a metal-free semiconducting photocatalyst material, and its progress towards a breakthrough in the future hydrogen economy. In addition, an in depth understanding of carbon nitride synthesis and the strategies to overcome the limitations in the hard/soft templating method, heteroatom doping, designing of heterostructures, and supramolecular self-assembly assisted synthesis of carbon nitrides to improve the photocatalytic efficiency of the material are reviewed in detail. The unique advantage of self-assembled complex precursor-assisted carbon nitride synthesis in controlling the carbon nitride formation at the molecular level is explained in depth through the optical properties, and morphological and textural features. The strengthened mechanism involved in the photocatalytic water splitting is described in terms of the overall water splitting and Z-scheme photocatalysis and points out the challenges and potential future directions of using supramolecular complex derived carbon nitrides in photocatalytic water splitting. In the third chapter, we demonstrate that the photocatalytic activity of carbon nitride can be enhanced by improving its electron conductivity, exposing its active surface area, and retarding the kinetics of charge-carrier recombination via supramolecular self-assembled synthesis integrated with heteroatom doping. The π electron distributions on the carbon nitride crystal surface were modulated at the atomic level by precise carbon doping on the graphitic nitrogen sites in the C3N4 structure. Through a novel synthesis method utilizing thiourea, we synthesized a supramolecular complex and subsequently converted it into carbon nitride via thermal polymerization. Likewise, by introducing varying amounts of trimesic acid molecules into the complex structure, C-doped carbon nitride was synthesized. The conduction and valence band positions were tuned by varying the amount of doped carbon, and the optimal amount of carbon doping in carbon nitride was determined to maximize the hydrogen yield. Studies using XPS and NEXAFS revealed that the graphitic nitrogen site in the carbon nitride structure is the most probable location for carbon doping, and C-doped carbon nitride exhibits improved visible-light absorption and reduced charge-carrier recombination kinetics, which enhances photocatalytic activity. The developed C-doped carbon nitride demonstrates a substantial increase in hydrogen production under the solar simulated irradiation and a fourfold increase in photocurrent density compared to the bulk graphitic carbon nitride as a control material. In the fourth chapter, we demonstrate the development of a heavily doped iron-nickel layered double hydroxides (NiFe-LDHs) catalyst to overcome the sluggish kinetics of the OER, which limits industrial-level hydrogen production. This approach involves the use of a hollow etching method to synthesize NiFe-LDH from nickel nitrate hexahydrate and a metal-organic framework (MOF), specifically MIL88A, which serves as a sacrificial template. The derived material exhibits a unique hollow capsule-like morphology with exceptional OER performance and a remarkably low overpotential of 244 mV at 10 mAcm-2. In addition, NiFe-LDH exhibits a low Tafel slope, high electrochemical surface area, and high turnover frequency, making it an ideal OER electrocatalyst. NEXAFS studies and XPS confirmed the presence of Fe2+ states and the presence of abundant oxygen vacancies in NiFe-LDH. Subsequent post-OER XPS analysis shows a clear change in the material composition, including a decrease in oxygen vacancies, an increase in M-OH bonds, and further reveals a decrease in M-O bonds indicating that NiFe-LDH predominantly followed a lattice-oxygen-mediated mechanism instead of an absorbate evolution mechanism. Overall, the thesis highlights the key discoveries on various electro and photocatalysts based on nanostructured materials for clean hydrogen production with high efficiency. It is proposed that, in the future, it is of paramount importance to develop a heterojunction catalyst devoid of precious metals to simultaneously produce hydrogen and oxygen. One of the unique catalytic system that we can try in the future is a heterojunction system with FeNi-LDH and supramolecular sheets of carbon nitride as it can offer tunable conduction and valance band edges with more active sites required for effective overall water splitting.
- Subject
- photocatalysis; electrocatalysis; hydrogen; carbon nitride; supramolecular; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1511428
- Identifier
- uon:56494
- Rights
- Copyright 2024 Nithinraj Panangattu Dharmarajan
- Language
- eng
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