Investigation of novel ammonia production options using photoelectrochemical hydrogen
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Hydrogen and ammonia are two of the most significant clean fuels, energy carriers and storage media in the near future. Production of these chemicals are desired to be environmentally friendly. Renewable energy, in particular solar energy-based hydrogen and ammonia production technologies bring numerous attractive solutions for sustainable energy production, conversion and utilization. The energy of the sun is endless and the water is a substance which is always accessible and renewable. Ammonia is currently one of the mostly used chemicals throughout the world due to many applications, such as fertilizers, cooling agents, fuel, etc. The Haber-Bosch process is the most dominant ammonia synthesis process which requires very high temperatures and pressures to operate and consumes massive amounts of fossil fuels mainly natural gas leading a non-sustainable process in the long-term. Therefore, alternative methods for ammonia production are in urgent need of development. This study theoretically and experimentally investigates the photoelectrochemical production of hydrogen and electrochemical synthesis of ammonia in a cleaner and integrated manner. In this respect, the main objective of this thesis is to develop a novel solar energy based ammonia production system integrated to photoelectrochemical hydrogen production. The hybrid system enhances the utilization of sunlight by splitting the spectrum and combining the photovoltaic and photoelectrochemical processes for electricity, hydrogen and ammonia production. The photoelectrochemical reactor is built by electrodeposition of the photosensitive semiconductor (copper oxide) on the photocathode. The characterization of the reactor under solar simulator light, ambient irradiance and concentrated light is accomplished. Furthermore, an electrochemical ammonia synthesis reactor is built using molten salt electrolyte, nickel electrodes and iron-oxide catalyst. The electrochemical synthesis of ammonia is succeeded using hydrogen and nitrogen feed gases above 180°C and at ambient pressures. The photoelectrochemically produced hydrogen is then reacted with nitrogen in the electrochemical reactor to produce clean ammonia. The comprehensive thermodynamic, thermoeconomic, electrochemical and life cycle models of the integrated system are developed and analyses are performed. The results obtained through models and experiments are comparatively assessed. The spectrum of solar light can be separated for various applications to enhance the overall performance of energy conversion from solar to other useful commodities such as electricity, fuels, heating and cooling. The results of this thesis show that under concentrated and split spectrum, the photoelectrochemical hydrogen production rates and efficiencies are improved. The overall integrated system exergy efficiencies are found to be 7.1% and 4.1% for hydrogen and ammonia production, respectively. The total cost rate of the experimental system for hydrogen and ammonia production is calculated to be 0.61 $/h from exergoeconomic analyses results. The solar-to-hydrogen conversion efficiency of the photoelectrochemical process increases from 5.5% to 6.6% under concentrated and split spectrum. Similarly, the photovoltaic module efficiency can be increased up to 16.5% under concentrated light conditions. Furthermore, the maximum coulombic efficiency of electrochemical ammonia synthesis process is calculated as 14.2% corresponding to NH3 formation rate of 4.41×10-9 mol s-1 cm-2.