Design and synthesis of heteroatom modified highly crystalline carbon nitrides for energy storage and conversion applications

Palakkat Alihassan, Mohammed Fawaz

Title: Design and synthesis of heteroatom modified highly crystalline carbon nitrides for energy storage and conversion applications
Creator: Palakkat Alihassan, Mohammed Fawaz
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2023
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: Global warming, resulting primarily from the combustion of fossil fuels, has sparked enormous attention towards developing renewable and efficient energy sources for a sustainable future. For example, fuel cells that use hydrogen and oxygen as fuel and produce water as a by-product are currently considered to achieve a sustainable energy economy. Electro and photocatalytic pathways are two of the most researched topics for the generation of clean fuels for use in such devices, however, the efficiency of the two processes is largely dependent upon the chemistry of catalytic materials. Noble metals are a predominant choice for catalysis owing to their high efficiency, however, their scarcity and high cost are limiting factors. Therefore, the development of a material/s that could defy these limitations with noble metals is crucial. Carbon nitride, a carbon-based semiconductor, is one of the potential metal-free catalysts that possess a unique electronic structure and has shown good promise in electro and photocatalytic fields. However, the utilization of carbon nitride as a mainstream electro and photocatalyst requires significant research and development to address its poor conductivity, crystalline defects and lack of textural and charge transfer properties. This PhD thesis focuses on enhancing carbon nitride's electro/photocatalytic activity by proposing innovative synthesis strategies. Such strategies prove useful to enrich the crystallinity, improve the conductivity, visible light absorption, textural and charge transfer properties of carbon nitride. The desired level of optimization in electronic properties would be achieved by hybridization with other materials such as graphene and doping with heteroatoms such as sulfur. This is expected to increase the conductivity and at the same time expand the visible light absorption range. Crystalline features of the carbon nitride could be modified by using molten salt assisted method, which reduces decomposition during the polymerisation. Further textural features of the materials would be enhanced through the use of a hard templating process using silica. The development of crystallinity, a high surface area, and hybrid structures in carbon nitride will simultaneously facilitate the active site density, electron transport and a well-defined tunable band structure to substantially improve the electro/photocatalytic performances. This modification of the core structure will enhance the adsorption capability and reduces the recombination of the charge carriers. The catalytic activity of these novel catalysts will be tested by carrying out oxygen reduction reactions (ORR) and hydrogen evolution reactions (HER) following electrocatalytic and photocatalytic pathways, respectively. A photo-electrochemical cell would be fabricated for testing the transient absorption current of the novel carbon nitride catalysts. The proposed studies would be supported by theoretical modelling which would also be utilized for gaining insights into the mechanism of the catalytic reactions. Overall, this thesis will help to advance knowledge in the field of carbon nitride in terms of the improvement in properties such as crystallinity, surface area, conductivity and charge transfer. The first chapter of the thesis covers a comprehensive review of the recent research and developments in the field of carbon nitride and its hybrids towards photo and electrochemical applications. The review provides background information on carbon nitrides and their further evolution as promising catalytic materials. Their properties such as porosity, nitrogen content and morphology were discussed in detail by citing prominent examples from the recent literature. This highlighted the innate drawbacks of the carbon nitrides in catalysis and the strategies that could be employed to enhance their catalytic activity to better engineer the catalysts. Since the hybridisation of carbon nitride with other materials forms the core component of the thesis, it was covered extensively in the second half of the review. The relevant discussions spanned across various carbon nitride/hybrid composites such as Graphene-CN, Metal oxide- CN, Metal-sulfide-CN, MXene-CN, single atoms anchored -CN and their application for photo/electrochemical catalysis. The review concludes by presenting the remarks, perspectives, and future outlook on the development of carbon nitride and its hybrids and their potential use for electro and photocatalytic applications. The second chapter of the thesis discusses an approach to improve electrical conductivity by designing a carbon nitride electrocatalyst through hybridisation with activated graphene nanoplatelets (AGNPs) by simply integrating chemical activation and polymerising the CN precursor. The resultant electrocatalyst possesses both porosity and electrochemically active carbon nitride matrix in the graphene nanoplatelets. This approach offers not only active sites but also enhances the interaction with the oxygen intermediates, thereby promoting the easy flow of electrons in the catalytic system, which displays exceptional oxygen reduction activity without any conductive carbon support or noble metals. The combined effect of CN matrix, porosity, and the conductive graphene framework helped achieve exceptional activity towards the electrochemical oxygen reduction reaction. Such a strategy can be further extended for the design and development of carbon nitride-based hybrids with other materials as well. The third chapter of the thesis describes the development of an S-doped carbon nitride photocatalyst with intriguing structure features and performance. This photocatalyst addresses the poor catalytic activity due to the narrow absorption and the high recombination of the charge carriers in carbon nitrides. This was done by developing a heteroatom-doped carbon nitride system using a single precursor which eventually enhances the optical properties and revealed a superior performance as a potential photocatalytic hydrogen evolution catalyst. Sulfur doped high nitrogen content carbon nitride, C3N5 (SCN) synthesised from a single precursor 5-amino-1,3,4-thiadiazole-2-thiol (5-ATDT) is thermodynamically stable and contains a combined thiadiazole, triazole, and triazine framework. Density Functional Theory (DFT) and spectroscopic characterisations were used to illustrate the new structure. Due to the enlarged absorption and the fast charge transfer, the resultant SCNs exhibit a high H2 generation rate of 486 μmol g-1 h-1, which is about 60% higher than the average value of the typical g-C3N4. The fourth chapter of the thesis addresses the crystallinity and porosity of carbon nitride. A simple synthesis approach using alkali metal salt KCl reduced the decomposition of the carbon nitride precursors at high temperatures. The metal salt-assisted technique enhances the CN framework's crystallinity and reduces electron-hole recombination. Further, introducing nanoporosity using template-assisted synthesis increased the surface area while maintaining high crystallinity. Furthermore, the catalyst possesses enhanced band structure, suppressed charge transfer, and stable CN frameworks. Visible light photocatalytic hydrogen evolution performance of 269 µmolh-1 was achieved which is far exceeding about 10 times higher than the bulk graphitic carbon nitride. The newly prepared catalyst also showed enhanced performance towards transient photocurrent. These findings illustrate an encouraging method to boost the visible light photocatalytic activity for the carbon nitrides by the use of salt-assisted templating. Chapter 5 concludes by summarising the findings and contents of the thesis and providing directions for future research. The pros of carbon nitrides such as moderate band gap, facile synthesis, availability of a wide range of starting precursors and how the shortcomings including low crystallinity conductivity and porosity could be improved, were discussed thoroughly. The future directions provided insights into how the structure and properties of carbon nitrides could be further improved in the context of electro and photocatalytic applications.
Subject: carbon nitride; energy storage; photocatalysis; mesoporous
Identifier: http://hdl.handle.net/1959.13/1510901
Identifier: uon:56455
Rights: This thesis is currently under embargo and will be available from 25.07.2025, Copyright 2023 Mohammed Fawaz Palakkat Alihassan
Language: eng

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