Synthesis of Structurally and Compositionally Tuned Two-Dimensional Materials and Their Heterostructures for Electrochemical Water Splitting

Green hydrogen generation via water electrolysis can be a sustainable approach and a potential replacement for fossil fuels. Current technologies use freshwater for the electrolysis process, which is a limited resource and can strain the earth’s freshwater resources. Seawater, on the other hand, is an unlimited resource and can be a potential feed for electrolysis. However, various ionic salts and impurities like small particulates and microbes can interfere with the electrochemical water splitting. The biggest challenge associated with seawater is the presence of chloride anions, which can lead to chlorine evolution reaction and corrosion of the working electrode. Therefore, developing efficient and stable catalysts for seawater electrolysis is critical. This PhD thesis explores the synthesis of novel transition metal-based materials and metal-organic frameworks and their potential applications in electrochemical seawater splitting. This thesis consists of the synthesis of four materials and their electrochemical applications. In project 1 (Chapter 3), the hydrothermal process followed by annealing and phosphorization is used to synthesise ultrathin and porous sheets of N-NiMo3P where nickel foam was used both as a template and Ni source, while the generated N-M bond helps in providing anti-corrosive properties to the catalyst. The ultrathin nature, along with large homogenous pores, enhances the catalytic activity of N-NiMo3P and caters to better mass transport and transfer of electrons. The developed catalyst was then used for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and it was also used to carry out full water splitting in a two-cell system. Moreover, it was also demonstrated that no chlorine was generated during seawater electrolysis. A large yield of catalysts can be obtained by using this methodology. Project 2 (Chapter 4) is an extension of the first study, which focused on enhancing the performance of the anode. A hydrothermal route followed by annealing was used to synthesise a two-dimensional molybdenum oxide@iron Oxide (MO@FO) heterostructure, which was utilised as the anode. The two dissimilar oxides, which were connected via O bridges, resulted in enhanced performance and stability. Furthermore, iron nitrate salt was added to alkaline seawater where the Fe ions from the salt integrate with the accessible edge/defect sites of the catalyst and counter the loss of Fe ions while the nitrate anions form a protective coating on the surface of the catalyst and repel the chloride anions, thus preventing corrosive chlorine chemistry. Therefore, it provides the basis for future investigations on the effects of metal salts on improved stability and performance. Project 3 (Chapter 5) explored the facile and scalable synthesis of an ionically bonded 2D heterostructure composed of an organic component i.e. Co-MOF and an inorganic component i.e. WO3.B2O3, where Co-MOF is grown vertically over the surface of WO3.B2O3 and connected via ionic Co-O-W bond. The WO3.B2O3 was first synthesised via a facile salt template method and was then used as a template for the growth of Co-MOF over it via a solid-liquid interfacial growth mechanism. Theoretical calculations were carried out to find that B assisted in the better adsorption of Co-MOF over WO3 while the hydroxyl ions modulation was achieved at the interface via the hydrolysis of B-O-B bonds from B2O3. The material was then utilised for anodic OER in alkaline seawater at industrial conditions, and the ionic bonds helped improve the catalyst’s performance and stability. Project 4 (Chapter 6) was carried out in extension to the third project to strengthen the evidence for the synthesis of ionic bonded 2D heterostructure and their role in improving the performance and stability of the catalyst in harsh seawater environments. The WO3.B2O3 was first synthesised via a facile salt template method followed by phosphorization to obtain PW8O26.B2O3, which was then used as a template for the growth of Fe-MOF over it via a solid-liquid growth mechanism. The theoretical studies demonstrated the generation of probable ionic bonds via Fe-O-W or Fe-P-W, with more probability of Fe-O-W ionic bonds due to large quantities of surface O atoms. The presence of phosphate anions also assisted in the further repulsion of chloride anions, increasing protection against corrosive chlorine chemistry. As a result, the catalyst could operate continuously in harsh seawater conditions for over 57 days. Overall, this thesis demonstrated the synthesis of efficient and stable 2D materials and their heterostructures for HER and OER in alkaline deionised water and seawater. When used as the anode, the as-synthesised catalysts displayed exceptional selectivity for OER as chlorine evolution reaction was not observed even when operated at industrial conditions (Potential = 2 V). Heterostructure formation can provide a catalyst with better stability, which could be further improved by adding metal salts to the electrolyte. Furthermore, synthesising a heterostructure with an ionic bonded heterointerface between organic and inorganic compounds demonstrated improved performance and stability in industrial conditions without the need for any additives in the electrolyte. These research projects can pave the way for the synthesis of highly efficient, stable, and scalable catalysts that can be used for industrial applications while also demonstrating the role and importance of creating ionic-bonded 2D heterostructures and their potential applications in energy storage devices.