Tailoring photo-responsive materials for solar desalination and photoelectrochemical water splitting: optimizing light harvesting and charge dynamics

Abstract

This thesis focuses on the design of photo responsive nanomaterial-enabled technology in achieving sustainable production of fresh water and green hydrogen via solar desalination and photoelectrochemical water splitting applications. The solar desalination aspect explores the utilization of nanomaterials and design of the device in enhancing the efficiency and cost-effectiveness of the processes. Solar desalination is a promising method that can produce desalinated water utilizing solar energy via photothermal material. However, the solar-to-vapor efficiency has been limited due to the lack of a proper design for the evaporator to deal with either a large amount of heat loss or salt accumulation. First, these issues are addressed via two cost-effective approaches: I) a rational design of a concave shaped supporter by 3D-printing that can promote the light harvesting capacity via multiple reflections on the surface; II) the use of a double layered photoabsorber composed of a hydrophilic bottom layer of a polydopamine (PDA) coated glass fiber (GF/C) and a hydrophobic upper layer of a carbonized poly(vinyl alcohol)/polyvinylpyrrolidone (PVA/PVP) hydrogel on the supporter, which provides competitive benefit for preventing deposition of salt while quickly pumping the water. The 3D-printed solar evaporator can efficiently utilize solar energy (99%) with an evaporation rate of 1.60 kg m–2 h–1 and efficiency of 89% under 1 sun irradiation. Second, a three-dimensional-graphene-network (3DGN) is synthesized as an effective photoabsorber material, which is capable of high light absorption and ease of water supply due to rational design of porous networking structure. A tall cylindrical wooden piece is used as a supporter and 3DGN is placed on top of the supporter to construct a three-dimensional (3D) evaporator. Finally, cotton was wrapped over the evaporator for smooth water supply. This device achieves a balance between supplied solar energy and the heat energy required for interfacial evaporation (demand energy) by optimizing the height of the supporter structure. With the specific design of the proposed 3D solar evaporator, our device achieves a remarkable efficiency, approaching its performance limit (94%) with an evaporation rate of 2.30 kg m-2 h- 1, along with efficient salt collection and scalability. In addition, an efficient photoanode, Al:Ti codoped hematite (Fe2O3) is fabricated for enhancing the performance of photoelectrochemical water splitting (PEC). The substitution of Al3+ for Fe3+ induces local strain within the lattice, reducing interionic distances and thereby enhancing the charge carrier transport properties. However, theoretical findings revealed initially unfavorable formation energy when Al3+ is doped into hematite, leading to significant lattice distortion due to size mismatch and thus limiting PEC activity. Co-doping Al3+ with Ti4+ in Fe2O3 restored the lattice symmetry by alleviating strain, resulting in a favorable formation energy. Additionally, Ti4+ contributes excess electrons, further increasing the electrical conductivity. By leveraging formation energy control through Ti doping, our optimized Al:Ti–Fe2O3 with a cocatalyst exhibited a photocurrent density of 4.00 mA cm–2 at 1.23 VRHE, representing a 6.5-fold improvement over Fe2O3 alone.