Coupled Mass Transport And Electricity Generation Characteristics Of Solar Responsivephotocatalytic Fuel Cells With An Air-breathing Cathode And Its Performance Enhancement

With the rapid development of the society, the energy crisis due to fast depletion of fossil fuels and environmental pollution have become two critical problems facing our planet. In particular, water pollution has become more serious. Therefore, seeking a clean energy and developing an efficient technology to recycle the energy stored in wastewater become significant. In recent, the photocatalytic fuel cell?PFC?, which is an integration of photocatalysis and fuel cell technologies, has emerged. This new technology can simultaneously degrade the wastewater and generate the electricity, which has shown a promising perspective for sustainable environment and energy development. In PFCs, the semiconductor photocatalysts replace the precious metals used in conventional fuel cells, making them more cost-effective. Moreover, many types of organic wastes can be utilized in PFCs, which overcomes the fuel limitation in conventional fuel cells. Besides, the products of PFCs are almost environmentally friendly. These features make PFCs to receive ever-increasing attention all over the world.Complex mass transport, light transmission and separation and migration of the photo-generated electron-hole pairs coupled with photoelectrochemical and electrochemical reactions occur in PFCs. This complex and coupled transport process is inherently assocated with the photoanode structure and reactor design, which thereby affect the performance of the PFC. Although PFCs have been widely investigated, the underlying characteristics of coupled transport and electricity generation remains unclear. There exist some major issues to be addressed, such as poor visible-light activation, low efficiency, high recombination and poor output power density.Aiming at these critical issues, in this thesis, a novel PFC with an air-breathing cathode and a solar responsive photoanode was proposed, which could enhance oxygen transport and improve the light utilization efficiency. With this cell, the coupled mass transport and electricity generation characteristics was investigated. Finally, from the aspect of the design of the cell structure and photoanode, new photoanodes and micro photocatalytic fuel cells were proposed to enhance the cell performance. The main research works include:?1? The performances of the PFC under different electrolyte environments and using various kinds of wastes as fuels were studied.?2? The respective electrode potential characteristics were studied. The influences of the operation parameters, including fuel concentration, electrolyte concentration, light intensity and oxygen concentration, on the potentials of each electrode were studied.?3? A novel CdS-SiO₂-TiO₂ composite photoanode was proposed and its photoelectrochemical performance was studied.?4? A novel photoanode with a cascading gradient pore microstructure was proposed, which consisted of a nanocrystalline TiO₂ layer, a microporous layer and a macroporous layer. Its photoelectrochemical and photocatalytic activities were studied. How the light and mass transfer affect the photoelectrochemical reaction was investigated.?5? A novel optofluidics-based micro PFC was proposed and its performance was studied. A novel microreactor with TiO₂-coated fiberglass was proposed. The characteristics of the mass transport coupled with the photocatalytic reaction was investigated. The main outcomes are summarized as follows:?1?A PFC with a solar responsive photoanode and an air-breathing cathode was developed, by which the performances of the PFC with different electrolyte conditions and fuels were visited. It was found that the alkaline electrolyte generated a better discharging performance than did the neutral one as a result of the enhanced kinetics and charge transport. Moreover, the photosensitizer photolysis would be comparably retarded. When operating with different fuel sorts, the PFC demonstrated favorable performance with various types of alcohols and saccharides. Methanol and glucose, with respect to short carbon chain length and simple molecule structures, yielded the best discharging performances. Artificial sewage and some real wastewater were also tested and the results showed that the satisfactory PFC discharging performance could be achieved.?2? Utilizing the three-electrode system, the respective electrode potential characteristics were investigated. The effects of the operating parameters on the photoanode and air-breathing cathode potentials were also studied. It was shown that the photoanode benefited from an increase in the fuel concentration?from 1% to 10%? due to enhanced mass transfer but the cathode performance decreased as a result of the increased mixed potential. However, because the overall cell performance was mainly affected by the photoanode, the PFC still showed an increasing trend. When the fuel concentration was further increased to 40%, the ionic conductivity was decreased, leading to a decrease in the cell voltage. For the electrolyte concentration testing, higher electrolyte concentration was more favorable because both the photoanode and cathode performances were enhanced and ionic conductivity was also increased, thereby improving the PFC overall performance. Besides, it was also demonstrated that the photoanode potential was greatly affected by the light intensity. High light intensity could accelerate the photoelectrochemical reaction rate because more excited electron-hole pairs were generated. Regarding air condition, increasing the oxygen concentration could accelerate the oxygen reduction reaction and thus improve the cell performance.?3?A novel CdS-SiO₂-TiO₂ composite photoanode was developed to improve the photoelectrochemical performance. In this design, CdS could not only broaden the spectral absorption range but also facilitate the transfer of photo-induced electron. Adding SiO₂ nanoparticals in the catalyst layer could change the distribution of electron, restraining the recombination of electron-hole pairs. In addition, the absorption property of SiO₂ nanoparticals is strong, which allows more CdS to be loaded under the same conditions and thus increased the photocurrent. In this part, the effect of the SiO₂ content was studied. It was found that the photoanode performance was improved with increasing the SiO₂ content from 2% to 10%, while it dropped with further increasing the SiO₂ content to 30%.?4?Unlike conventional TiO₂ photoanode with only one characteristic pore size, a novel photoanode with a cascading gradient pore microstructure was developed. Because such design was able to greatly improve the photon and mass transfer, reduce the electron transfer resistance and prevent the recombination of the photo-generated electron-hole pairs, the photoelectrochemical photocatalytic activities were significantly enhanced. Results also indicated that the photoelectrochemical and photocatalytic activities of the proposed photoanodes were significantly influenced by the PMMA/TiO₂ ratio. An increase in the PMMA/TiO₂ ratio from 1:5 to 1:1 led to the improved performance because more macropores generated could enhance the mass transport and light harvesting, while too high PMMA/TiO₂ ratio worsened the performance because an excessive quantity of the PMMA template resulted in the lowered light utilization efficiency.?5?The optofluidics-based micro PFC was developed, which this type of fuel cell could not only enhance mass transport but also enhance the light transmission and enable more uniform light distribution. Because of these reasons, the electricity generation of this type of fuel cell was evidently improved. In addition, the developed optofluidic PFC could also yield a good degradation efficiency with a maximum value reaching 83.9%. To further enhance the mass transfer in the optofluidic PFC, a novel optofluidic microreactor with photocatalyst coated on the fiberglass was developed. Results revealed that, compared with the conventional planar film-designed microreactor, the optofluidic microreactor with the catalyst-coated fiberglass could dramatically improve the performance as a result of enhanced mass transport and shortened light path. The effects of different parameters including the flow rate, MB concentration, and fiberglass quantity were also visited to evaluate the performance of the proposed optofluidic microreactor. Results showed that low flow rate and MB concentration and large fiberglass quantity could yield high degradation efficiency. Besides, with the proposed optofluidic microreactor, the reaction rate constant could be increased 2-3 times. In addition, this design of the catalyst-coated fiberglass could significantly enlarge the reaction area in the microreactor.