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Title: Porous materials for solar-thermal conversion and catalytic water splitting
Authors: Ma, Sainan
Degree: Ph.D.
Issue Date: 2019
Abstract: With the fast growth of population and increasing environmental problems, resources shortage has drawn global concerns in recent years. Conversion of renewable energy sources like solar energy to generate other resources such as freshwater and hydrogen could be a promising strategy to solve the resources issues. Among them, interfacial solar steam generation arises to be an attractive strategy to generate freshwater by utilizing only solar energy. And photocatalytic water splitting with solar energy and photocatalysts or electrocatalytic water splitting powered by solar energy, can be promising to mass-produce hydrogen. Development of novel materials plays an important role for these energy conversions. Materials with porous structures are usually attractive in the energy conversion applications. This doctoral study focuses on crafting porous materials for applications in solar-thermal conversion for freshwater generation and catalytic water splitting for hydrogen and oxygen generation. Utilizing solar energy to evaporate water is one of the green and promising approaches in addressing the issues of global freshwater shortage and water pollution. Conventional solar steam generation systems usually involve bulk-water heating with low efficiency due to the considerable thermal energy loss. Recently, interfacial solar steam generation systems using floating evaporators to absorb heat at the air-water interface have received wide interests. A low cost, efficient and durable solar absorber with porous structure is vital for the development of solar steam generation. In my research, a recycled floating black polyurethane (PU) sponge was used as a solar absorber material. The sponge was modified by a simple one-step hydrophilic treatment with dopamine hydrochloride to increase the surface hydrophilicity. An evaporating rate of more than 3.5 times higher than that of the natural evaporation was achieved with an evaporation efficiency of above 50%. Moreover, solar ethanol distillation enabled by this recycled black PU sponge was also demonstrated, yielding up to 25 wt% concentration promotion under each distillation cycle. Another work focuses on the porous carbon materials as solar absorber. Carbon materials have gained extensive research attention for solar steam generation owing to the non-toxic nature and environmental friendliness. In this work, the metal-organic framework (MOF) derived porous carbon (MDPC) was synthesized and employed as solar absorber for enhancing water evaporation. An efficient surface heating and evaporation system was designed by coating the leaf-like 2D MOF precursor on stainless steel mesh followed by calcination (MDPC/SS mesh), and then in conjunction with a floating airlaid paper-wrapped polyethylene (EPE) foam. The prepared solar evaporator with unique light trapping structures shows a high solar energy absorption (> 97%), excellent hydrophilicity, and great surface heat localization for solar steam generation. Consequently, a photo-thermal conversion efficiency of 84.3% with an evaporation rate of 1.223 kg m-2 h-1 was achieved under one sun illumination. Both of the recycled PU sponge and MDPC/SS mesh show good recyclability and durability for solar steam generation, leading the great potential for practical water treatment for freshwater or drinkable water regeneration.
Harvesting and converting solar light into highly energetic chemical fuels like hydrogen with the assistance of efficient catalysts is another attractive way to utilize solar energy. The development of low-cost efficient non-noble-metal based photocatalysts is of great significance in practical photocatalysis. Recently, 2D transition metal dichalcogenides (TMDs) have enthralled tremendous attention in photocatalytic hydrogen evolution reaction (HER). Methods and mechanisms for improving the photocatalytic activity of TMDs are important and popular research topics. In this work, we applied a simple laser drilling method to engineer well-aligned pore arrays on magnetron-sputtered WS2 nanofilms. The arrays of well-aligned WS2 porous structure obtained by laser drilling can offer more active edge sites for photoelectrocatalytic reactions, while the construction of WS2/WO3 heterojunction boosted the separation of photogenerated electron-hole pairs, as indicated by the open-circuit photovoltage decay (OCPVD). Raman spectrum and X-ray photoelectron spectroscopy (XPS) verified the partial oxidation of WS2 into WO3 by laser treatment. The WS2 film after typical laser treatment exhibited a photocurrent density of up to 31 times larger than that of pristine WS2 film. Except for photocatalytic water splitting, electrocatalytic water splitting for hydrogen and oxygen generation is another highly promising strategy to produce clean fuels from renewable energy resources, in which oxygen evolution reaction (OER) is an important half-reaction. The development of cost-effective, durable and high-efficient OER electrocatalysts is an extremely critical technology for the large-scale industrial water electrolysis. In my work, we fabricated an efficient porous oxygen evolution electrode by direct selenization of porous iron-nickel (FeNi) alloy foam through thermal selenization process. The obtained Fe4.4Ni17.6Se16/NiSe hybrid (FNS/NS) foam displays outstanding durability and remarkable catalytic activity in 1M KOH with low overpotentials of 242 and 282 mV to achieve the current densities of 100 and 500 mA cm-2 , respectively, which, to the best of our knowledge, exceeds most of the reported selenide-based electrocatalysts. In summary, porous materials like dopamine modified PU sponge and MDPC have been fabricated as solar absorber materials to convert solar energy into thermal energy for solar steam generation, which is promising in addressing freshwater scarcity; Non-noble-metal catalysts like WS2/WO3 heterojunction-based porous film and FeNi-based selenides foam have been synthesized and evaluated for photocatalysis and electrocatalysis, respectively, which is crucial for sustainable chemical fuels generation like hydrogen and oxygen. Our works provide the great potential to meet the high demand for clean, affordable and sustainable resources along with the fast-growing world population.
Subjects: Hong Kong Polytechnic University -- Dissertations
Porous materials
Renewable energy sources
Solar energy
Pages: xxvi, 149 pages : color illustrations
Appears in Collections:Thesis

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