| dc.description.abstract | 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. | en |
