Development of a new hybrid photochemical/electrocatalytic water splitting reactor for hydrogen production: design, analysis and experiments

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Solar-driven water splitting combines several attractive features for sustainable energy utilization. The conversion of solar energy to a type of storable energy has crucial importance. An alternative method to hydrogen production by solar energy without consumption of additional reactants is a hybrid system which combines photochemical and electro-catalytic reactions. The originality of this research lies in the engineering development of a novel photo-catalytic water splitting reactor for sustainable hydrogen production, and verification of new methods to enhance system efficiency. The scope of this thesis is to present a thorough understanding of complete photocatalytic water splitting system performance under realistic working conditions. In this dissertation, an experimental apparatus for hydrogen and oxygen production is designed and built at UOIT to simulate processes encountered in photo-catalytic and electro-catalytic water splitting systems. The hybridization of this system is investigated, and scale-up analysis is performed based on experimental data using a systematic methodology. The hydrogen production rate of approximately 0.6 mmol h-1 corresponds to a quantum efficiency of 75% is measured through illumination of zinc sulfide suspensions in a dual-cell reactor. Utilization of ZnS and CdS photo-catalysts to simultaneously enhance quantum yield and exergy efficiency is performed. The production rate is increased by almost 30% as compared with ZnS performance. In the next step, an oxygen production reactor is experimentally investigated to simulate processes encountered in electro-catalytic water splitting systems for hydrogen production. In this research, the effects of ohmic, concentration and activation losses on the efficiency of hydrogen production by water electrolysis are experimentally investigated. The electrochemical performance of the system is examined by controlling the current density, temperature, space, height, and electrolyte concentration. The experimental results show that there exists an optimum working condition of water electro-catalysis at each current density, which is determined by the controlling parameters. A predictive mathematical model based on experimental data is developed, and the optimized working conditions are determined. The oxygen evolving half-cell is also analyzed for different complete systems including photo-catalytic and electro-catalytic water splitting. An electrochemical model is developed to evaluate the over-potential losses in the oxygen evolving reaction and the effects of key parameters are analyzed. The transient diffusion of hydroxide ions through the membrane and bulk electrolyte is modeled and simulated for improved system operation. In addition, a new seawater electrolysis technique to produce hydrogen is developed and analyzed from energy and exergy points of view. In this regard, the anolyte feed after oxygen evolution to the cathode compartment for hydrogen production is examined. An inexpensive and efficient molybdenum-oxo catalyst with a turn-over frequency of 1,200 is examined for the hydrogen evolving reaction. The electrolyte flow rate and current density are parametrically studied to determine the effects on both bulk and surface precipitate formation. The mixing electrolyte volume and electrolyte flow rate are found to be significant parameters as they affect cathodic precipitation. Furthermore, a new hybrid system for hydrogen production via solar energy is developed and analyzed. In order to decompose water into hydrogen and oxygen without the net consumption of additional reactants, a steady stream of reacting materials must be maintained in consecutive reaction processes, to avoid reactant replenishment or additional energy input to facilitate the reaction. Supramolecular complexes [{(bpy)2Ru(dpp)}2RhBr2](PF6)5 are employed as the photo-catalysts, and an external electric power supply is used to enhance the photochemical reaction. A light-driven proton pump is used to increase the photochemical efficiency of both O2 and H2 production reactions. The maximum energy conversion of the system can be improved up to 14% by incorporating design modification that yields a corresponding 25% improvement in exergy efficiency. Moreover, a photocatalytic water splitting system is designed and analyzed for continuous operation on a large pilot-plant scale. A Compound Parabolic Concentrator (CPC) is presented for the sunlight-driven hydrogen production system. Energy and exergy analyses and related parametric studies are performed, and the effect of various parameters are analyzed, including catalyst concentration, flow velocity, light intensity, reactor surface absorptivity, and ambient temperature. Two methods of photo-catalytic water splitting and solar methanol steam reforming are investigated as two potential solar-based methods of catalytic hydrogen production. The exergy efficiency, exergy destruction, environmental impact and sustainability index are investigated for these systems, as well as exergoenvironmental analyses. The results show that a trade-off exists in terms of exergy efficiency improvement and CO2 reduction of the photo catalytic hydrogen production system. The exergo-economic study reveals the maximum hydrogen exergy price of 2.12, 0.85, and 0.47 $ kg-1 for production capacities of 1, 100, and 2000 ton day-1, respectively. These results are well below the DOE 2012 target and confirm the viability of this technology.
Hydrogen, Photo-catalysis, Electro-catalysis, Efficiency, Hybridization