Coupled Mass Transport And Electricity Generation Characteristics Of MEA Based Photocatalytic Fuel Cell

Energy crisis and environmental pollution are two grand challenges toward the sustainable development.In particular,the water pollution greatly affects the natural water ecosystem and threatens the human health.Hence,the wastewater treatment has become one of major concerns.Conventional wastewater treatment technologies usually focus on how to remove the contaminants efficiently.However,there is plenty of chemical energy contained in the wastewater.If the chemical energy stored in the wastewater can be recovered during the treatment process,not only the water pollution but also energy crisis can be mitigated.In recent,the semiconductor photocatalytic technology became one of the focuses in the field of the wastewater treatment.The integration of photocatalytic technology with fuel cell technology results in a new technology of photocatalytic fuel cell(PFC),which can degrade the organics in the wastewater and simultaneously convert it into electricity by solar energy,realizing the recovery of the wastewater and showing promising prospect.Moreover,the noble metal catalysts used in conventional fuel cells are replaced by cheap semiconductors in PFCs,which lowers the capital cost.However,there still exist several critical issues to be addressed,such as low solar energy utilization efficiency,bulk cell structure,large mass transfer resistance,recombination of the electron/hole pairs and low specific surface area and so on.Aiming at these issues,inspired by the membrane electrode assembly(MEA)used in conventional proton exchange membrane fuel cells,the photocatalytic fuel cells based on the MEA design have been developed,which makes the system more compact and flexible and deceases the mass transfer resistance.The main outcomes are summarized as follows.(1)A MEA based PFC with the photoanode made by the transfer method.In this thesis,a MEA based PFC with the photoanode fabricated by the transfer method was developed.In this method,the TiO2 nanoparticles were firstly sprayed on a thin PTFE film to form a thin TiO2 film,which was then hot pressed onto the Nafion membrane.Finally,this half MEA was hot pressed with the cathode to form a MEA.The effect of the light intensity and liquid flow rate on the cell performance was experimentally studied.It was found that the increase of the light intensity could improve the cell performance.The increase of the liquid flow rate led to decreasing the cell performance in the testing range with small reduction.Overall,the cell performance fabricated by this method was poor.This might be because the photoanode fabricated by this method could not experience the calcination at high temperature and photosensitization limited by the use of Nafion membrane.(2)A MEA based PFC with the photoanode made by the wet spray method.Aiming at the issues encountered in the photoanode fabricated by the transfer method,a MEA based PFC with the photoanode made by the wet spray method was designed and fabricated.In this design,TiO2 nanoparticles were firstly wet sprayed on the photoanode followed by the calcination at high temperature and quantum dot sensitization.Finally,the TiO2 coated carbon paper was attached on the Nafion membrane with the hot-pressed cathode to form the MEA.Experimental results indicated that the PFC fabricated by this method yielded much better performance than did the PFC fabricated by the transfer method.The study on the electrolyte condition and quantum dot sensitization implied that the alkaline condition yielded the best performance.Quantum dot sensitization enabled the visible-light response and the improvement in the solar utilization and cell performance.The effects of the light intensity,liquid flow rate,electrolyte and organics concentrations on the cell performance were also investigated.It was shown that the increased light intensity could generate more electron/hole pairs and thus improve the cell performance.An increase in the liquid flow rate resulted in the decreased cell performance.Increasing the KOH concentration allowed the cell performance to be improved.When the organics concentration was increased,the cell performance was firstly increased and then decreased.(3)A MEA based PFC with a solar-responsive mesoporous photoanodeToward the problems of low specific surface area and the resulting poor performance in conventional PFCs,a MEA based PFC with a solar-responsive mesoporous CdS-ZnS-TiO2/SBA-15 photoanode was designed and fabricated.The developed photoanode was characterized by TEM,FE-SEM,BET and UV-vis.It was found that the addition of mesoporous SBA-15 could greatly increase the specific surface area and allowed more CdS-ZnS to be adsorbed.Besides,the photoresponse behaviors and long-term performance of the develop PFC were characterized.It was found that the developed PFC showed good photoresponse and relatively stable performance.The effects of the light intensity,liquid flow rate,electrolyte and organics concentrations and mass ratio of SBA-15 on the cell performance were also studied.It was shown that the alkaline condition showed better performance than did the neutral condition.With the increase of the light intensity and electrolyte concentration,the cell performance was improved.As the liquid flow rate increased,the cell performance was decreased.When the organics concentration and mass ratio of SBA-15 were increased,the cell performance was firstly increased and then decreased,resulting in optimal organics concentration and mass ratio of SBA-15 to maximize the performance.(4)A MEA based PFC with an anion exchange membraneIn this thesis,a MEA based PFC with an anion exchange membrane was developed.In this design,the proton exchange membrane was replaced by an anion exchange membrane.For this PFC,the effects of the light intensity,liquid flow rate,electrolyte and organics concentrations on the cell performance were explored.It was found that increasing the light intensity improved the cell performance,while increasing the liquid flow rate resulted in the decrease of the cell performance.When the KOH concentration was increased from 0.1 M to 0.3 M,the cell performance was increased.Unlike the PFC with a proton exchange membrane,when the ethanol concentration was increased from 1% in volume to 20%,the cell performance was always increased.Experimental results indicated that under the same conditions,the PFC with an anion exchange membrane showed lower performance than did the proton exchange membrane.This might be due to poor transfer of OH-resulting from large thickness of the employed anion exchange membrane.