Analysis and engineering of advanced materials for visible light water splitting

Abstract

Solar energy is the most abundant natural resource on the planet with added advantages of being a renewable and clean source. Approximately 36000 TW of solar energy reaches the surface of our planet on yearly basis, which is a significant figure that gives great possibilities to solve the energy crisis of the current generation.[1] The ability to harness and convert solar energy into more useful forms has attracted great importance and interest in the current day and age due to the rising energy demand. Photoelectrochemical water splitting, a process inspired by the photosynthetic processes in plants, has garnered interest in recent years due to its potential uses not only as energy conversion system but also for the purposes of energy storage by means of producing energy rich chemical fuels such as hydrogen gas (H2).[2] Commercial electrolysers rely on the electricity from conventional sources of energy to generate H2 gas for various industrial purposes. The possibility to replace the conventional source of energy with a renewable source such as solar is highly promising for a number of industrial and engineering chemistry reactions alongside the applications in energy conversion and storage. On the other hand, there are significant challenges towards achieving a good Solar-to-Hydrogen (STH) efficiency with a low reaction overpotential and a simple, cost effective and space saving design. The development of direct photoelectrochemical cells with photoactive anodes and/or cathodes is thus necessitated, which drives down the device design costs, and consequently, the cost of operation.[3] However, the development of an ideal photoelectrode for either the Oxygen Evolution Reaction (OER) or the Hydrogen Evolution Reaction (HER) presents several considerations for researchers, with an ideal band gap of 1.8 – 3.2 eV, high photovoltage of greater than 1.61 V, high current densities corresponding directly to the amount of output gas, and fast charge transfer kinetics with minimum recombination of charge carriers.[4,5] Furthermore, the band positions of the materials as well the charge transfer kinetics have a pH dependency, and it is to be ensured that a given combination of anode and cathode material is to be stable in a particular electrolyte over an extended period of time.[6,7] Keeping these factors in mind, this thesis deals with the analysis and engineering of advanced materials and/or heterojunctions that are able to efficiently catalyse the Oxygen Evolution Reaction (OER) and/or Hydrogen Evolution Reaction (HER), with the following broad goals: Design and synthesis of light-harvesting novel nanostructured materials for use as photo-electrodes to aid Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER) in photoelectrochemical water splitting process; • Identification of traits/properties associated with mechanistic processes in OER and HER and evolving mechanisms of selection of materials and nanostructure/film optimization to achieve best possible performance and stability; • Identification of materials with optimally placed band energies and/or heterostructures working in complementary fashion to enhance the photocatalytic performance for OER or HER respectively; • Identification of novel, cost-effective and earth abundant materials with ease of synthesis and production for mass use as effective and stable photocatalysts to facilitate H2 production • Solid-state and electrochemical characterization of inorganic and metal organic framework-based materials for use as photoelectrodes in the photoelectrochemical water splitting process The thesis consists of five chapters and begins with a general introduction indicating the requirement for new energy sources, the potential of harvesting solar energy, the impact of hydrogen energy for a sustainable future, the challenges associated with the design of materials for electrochemically producing hydrogen and utilizing solar energy for the same, and the current literature review in the field. This is followed by four chapters based on the design and identification of novel materials, heterostructures and a new metal organic framework. Finally, the thesis concludes with views on the future prospects of optimizing materials properties and design parameters for electrode design w.r.t. photoelectrochemical cells for the purpose of sustainable and cost-effective solar light assisted water splitting, as well as potential applications of the findings in this thesis towards other electrical and/or electrochemical applications where the findings may be applicable and useful.