Photocatalytic Reduction Of Carbon Dioxide And Photocatalytic Fuel Cell Coupled System For Electricity Generation

With the development of industrialization,the consumption of fossil fuels（coal,oil,natural gas,etc.）is rapidly increasing.Fast depletion offossil fuels directly leads to energy crisis.At the same time,the overuse of fossil fuels emits a large amount of carbon dioxide,resulting in serious greenhouse effect.Under such a circumstance,it is important to explore a new technology to address the environmental and energy issues,which can reduce carbon dioxide and directly use the products of carbon dioxide reduction to generate electricity.Photocatalysis technology is a new renewable energy utilization technology,which offers the advantages of the solar energy utilization,low cost and mild operation conditions and so on.Because of these advantages,photocatalysis technology has been widely used for water splitting,photocatalytic reduction of CO₂ and photocatalytic fuel cell,etc.Although the photocatalysis technology can be used for the reduction of CO₂,photocatalytic reduction of CO₂suffers from poor yield and high cost for post utilization and so on.Therefore,a photocatalytic reduction of CO₂ and PFC coupled system was proposed in this thesis for electricity production,in which the products of the photocatalytic reduction of CO₂could be used as the fuels for the PFC.The effects of the operation parameters on the system performance were studied.According to the phenomenon that the product concentrations at the system outlet was larger than that of at the PFC entrance,the role of CO₂ in the PFC was investigated.Afterwards,the nickel foam supported TiO₂three-dimensional photocatalyst layer was proposed to enhance the photocatalytic reduction of CO₂ and so as to improve the system performance.Main outcomes of this thesis are summarized as follows.（1）Evaluation of the photocatalytic reduction of CO₂ and PFC coupled systemA photocatalytic reduction of CO₂ and PFC coupled system was constructed for electricity generation.In this system,a photoreactor for the photocatalytic reduction of CO₂ and a PFC are integrated,which can utilize the products of the photocatalytic reduction of CO₂ as the fuels for PFC to generate the electricity.The feasibility of the system was confirmed via the performance comparison between the presence of CO₂and the absence of CO₂.The effects of the light intensity,residence time and H₂SO₄concentration on the system performance were investigated.It was shown that higher light intensity generated more electron-hole pairs,facilitating the photocatalytic reduction of CO₂ and photoeelectrochemical oxidation of fuels and thereby improving the electricity generation.Higher H₂SO₄ concentration was beneficial for all photoreactions in the system and reducing the internal resistance of PFC,which could then improve the system performance.Increasing the residence time resulted in the higher product concentration,but did not exhibit obvious effect on the power generation.In addition,it was found that the product concentration at the system exit was larger than that at the PFC entrance,indicating that the remaining CO₂ from the upstream still participated in the reduction reactions in the PFC.（2）Simultaneous CO₂ photocatalytic reduction and power generation in the PFC.Towards the phenomenon that the remaining CO₂ from the upstream still participated in the reduction reactions in the PFC,using the typical product of methanol from CO₂ photocatalytic reduction to study the electricity generation performance of the PFC in presence of CO₂ and methanol.Experimental results confirmed that CO₂ was reduced to fuels in both the presence and absence of methanol and the products could be reduced to organics for electricity generation.It was also found that when the added methanol concentration was similar to the products,the power generation performance was improved significantly.However,when the products was negligible comparing to the added methanol concentration,the enhancement of performance was insignificant.Besides,the effects of the light intensity,residence time and H₂SO₄ concentration on the performance were investigated.The results showed that the increase of the light intensity and H₂SO₄ concentration were both beneficial for photocatalytic reactions.Increasing the residence time would increase the methanol concentration at the PFC exit,reduce the methanol yield,and boost the performance of PFC slightly.（3）Coupled system with nickle foam supported TiO₂ for the CO₂ photoreduction and PFCIn this thesis,nickel foam as the TiO₂ supporting materials for the photocatalytic reduction of CO₂ was proposed,which could provide large specific surface area and vigorous porous structure.As a result,the performance of the CO₂ photoreduction could be enhanced and the system performance was improved.The photocatalyst was fabricated via sol-gel method,which was consisted of a compact TiO₂ layer followed by three TiO₂ thin films.Such a three dimensional structure was inserted in the system.During the operation,the reactants passed through it so that the mass transfer could be enhanced.As compared the above-mentioned coupled system,nickle foam supported TiO₂ was able to greatly improve the system performance,which was attributed to the large specific surface area,more active sites,enhanced mass transport,and more effective light utilization.In addition,the effects of the light intensity and pore density of nickel foam on the system performance were investigated.The results showed that higher light intensity generated more electron-hole pairs,facilitating the photocatalytic reduction of CO₂ and photoelectrocatalytic oxidation of fuels.Decreasing the pore density reduced fuel production,and thus lowered the system performance.