Nanoporous metal nitride based semiconductors for efficient production of hydrogen from water

Gujral, Harpreet Singh

Title: Nanoporous metal nitride based semiconductors for efficient production of hydrogen from water
Creator: Gujral, Harpreet Singh
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2022
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: The use of oil, coal, and natural gas as fossil fuels for the energy infrastructure is severely hit with the accompanying pollution in the form of greenhouse gases (GHGs) emissions which have led to the significant problems of global warming and climate change. The non-renewability and diminishing stock of fossil fuels make it significant to look for alternative sources to meet the energy demands of the rising population. Such alternative resources must be non-polluting, renewable, and be able to provide an uncomplicated transformation of the current energy infrastructure to far better energy systems with long-term sustainability. Among such alternatives, hydrogen, being a carbon-free carrier with a high energy density of 120 MJ kg-1 that is much higher than conventional fuels, is considered as one of the highly promising energy fuels. However, the most significant challenge with hydrogen is its availability as it does not exist naturally due to high reactivity. The most promising approach to produce hydrogen is through the electrolysis of water (H2O) which splits water into hydrogen and oxygen (2H2O (l) → 2H2 (g) + O2 (g)) via hydrogen evolution (HER) and oxygen evolution (OER) reactions, respectively. The efficiency of HER is largely dependent upon the electrocatalyst that drives the chemical reaction. The surface of the electrocatalyst provides a platform for HER in acidic medium via a stepwise procedure which includes Volmer/discharge (H+(aq) + e- → Had), Tafel (Had + Had → H2,ad), and Heyrovsky (H+(aq) + e- + Had → H2, ad) reactions, respectively. The rate-determining step of the HER could be any of these three reactions and is largely controlled by the physico-chemical properties of the electrocatalysts and the potential range used. Electrocatalysts based on noble metals such as platinum (Pt) are the predominant choice for HER due to their high activity and stability in acidic mediums. However, scarcity and high cost are the two major limiting factors for platinum. Therefore, finding alternative electrocatalysts that are not only cheaper, abundant, and stable but also provide competitive results for HER is an active area of research. Transition metal nitrides (TMNs) have garnered attention due to their promising potential for HER. The presence of metal imparts the required stability and activity whereas nitrogen is beneficial for tuning their electronic properties for better performance in HER. Although explored widely in recent times, there is still a lack of information on the development of porous counterparts of metal nitrides. With advanced synthesis technologies and using innovative approaches, it is possible to incorporate porous character in metal nitrides. For example, a reactive templating procedure wherein a porous template can be used to replicate porosity onto metal nitride is a novel idea. Furthermore, a nitrogen-rich template can also be used to prepare porous materials such as carbon nitride which can be mixed with metals to produce porous metal nitrides. Porous metal nitrides, owing to their high surface area along with the presence of metal and nitrogen centres could prove highly useful for HER. The research on transition metal nitrides and their porous counterparts is still in its infancy and this Ph.D. project aims to address some of the prominent issues associated with these materials. Chapter 1: The first chapter of the thesis is focused on covering the recent literature on metal nitrides. Three main classes of metal nitrides including single metal, multi-metal, and porous metal nitrides in the single and multi-metal system are covered. These three classes of metal nitrides are discussed with respect to their synthesis methods such as hydrothermal, solvothermal, templating, chemical vapour deposition, salt-template, molten salt route, ammonia thermal annealing, magnetron sputtering deposition, and physical vapour deposition. The synthesis methods were comprehensively reviewed, and relevant comparisons were made between different methods and classes of metal nitrides to illustrate the up-to-date status of the field. The second and third part of the chapter deal with the synthesis and structure and morphological control of different metal nitrides including mono, bi and mixed metal nitrides. The fourth and final part of the chapter are focused on the synthesis of porous metal nitrides and the application potential of different types of metal nitrides for hydrogen production via electrochemical and photocatalytic pathways. Chapter 2: Having reviewed the up-to-date literature in the first chapter, the thesis moves on to the second chapter which is based on the experimental research covering the synthesis of mesoporous titanium carbonitride (MTiCN) and its application for HER. The novelty of this research lies in employing a nitrogen-rich mesoporous carbon nitride (mC3N5) as a reactive template and source of carbon and nitrogen to synthesize MTiCN with unique structural and physicochemical properties. Initially, mesoporous SBA-15 was synthesized using the conventional procedure and it was further reacted with 3 amino-1,2,4-triazole at high temperature in a nitrogen atmosphere to prepare mC3N5. The mC3N5 has then used a reactive template with titanium chloride (TiCl4) as the other reactant and the mixture is carbonized at high temperature to produce a series of MTiCN materials. The synthesized materials were characterized by X-ray diffraction (XRD), nitrogen adsorption, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and NEXAFS techniques. The synthesized materials possess high surface areas (415 - 441 m2 g-1) and good crystallinity. The material synthesized using 0.50 g of the TiCl4 and 0.30 g of C3N5 showed a promising HER performance with a low onset potential of 37.6 mV vs RHE and Tafel slope of 31.2 mV.dec-1 which lies very close to the commercial platinum/carbon. This performance could be correlated with the presence of metal centers, porous features, and nitrogen functionalities. These results demonstrate that MTiCN materials are a promising candidate for HER reaction and there is scope for further improvement to enhance their efficiency which may set up a pathway for commercialization. Chapter 3: After the success of the materials synthesized in chapter 2 for HER, it was high time to vary the nitrogen content in materials to improve upon the efficiency. Chapter 3 covers the synthesis of nanoporous titanium carbonitride (NTiCN) based materials through the reactive templating approach by using mC3N6 as a nitrogen-rich template and their application for HER. The synthesis involves the nano casting of the mC3N6 using SBA-15 as a template to replicate a cylindrical rod-like structure. In the second nano-casting, the solution of TiCl4 is filled in the porous structure of mC3N6 and reacted at high carbonization temperature to convert the amorphous titanium / mC3N6 nanocomposite into NTiCN, along with some carbon residuals supporting the structure. It is found that the nano casting of the 2D template with a well-ordered porous structure is successful which is confirmed with the XRD and HR-TEM measurements. The synthesized materials displayed a high surface area (700 m2 g-1), large pore volumes (1.3 cm3 g-1), and pore diameters (3.6 nm). In acidic medium, NTiCN-1.2 loaded with 5% platinum as the electrode for water splitting shows high electrocatalytic performance with the overpotential of 27 mV at the current density of 10 mA cm-2 and Tafel slope of 44 mV dec-1 at a rotating speed of 1600 rpm on a relatively low catalyst loading of ~ 0.1 mg cm-2. This nanocomposite architecture structure gives a novel catalytic system for effective HER, which has the potential to improve the energy conversion efficiency and reduce the usage of platinum in industrial applications and may instigate the synthesis of many more efficient electrochemical systems. Chapter 4: The last chapter of the thesis includes the conclusions and provides direction for future research in the field of metal nitrides and their application for HER.
Subject: nanoporous metal nitride; semiconductors; production of hydrogen from water; global warming and climate change
Identifier: http://hdl.handle.net/1959.13/1513712
Identifier: uon:56762
Language: eng

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