Study On Underwater Photoelectricity Conversion Based On Coordination Crystals

Ocean electricity supply becomes an important part of ocean development with the promotion of marine strategy for strengthen state.Marine solar energy is an ideal source of clean energy.However,the working environment and intermittence limited application of this energy for continuously day-night electricity supply in the ocean.On land,the intermittent problem can be effectively solved by combining artificial photosynthesis and secondary ion battery,thereby the goal of sustainable solar energy conversion and utilization can be achieved.However,how to realize the sustainable utilization of solar energy underwater in marine environment is still a problem to be solved.Aiming at underwater solar energy conversion and continuously day-night electricity supply in the marine,this dissertation designed a new principle,developed the integration technology including underwater solar energy conversion,storage and power supply,and constructed a new type of solar seawater battery device.The main contents of the dissertation are as follows:1.Study on photoelectric conversion mechanism in seawaterA novel underwater photoelectric conversion system was proposed and designed by combining photolysis of water and aqueous sodium ion battery.The TiO₂ film was used as an end for capturing and converting solar energy.The coordination crystals,Ni₃[Fe?CN?₆]₂,were selected as the electron receiver with ion and electron storage capacity.The TiO₂-Ni₃[Fe?CN?₆]₂ underwater photoelectric conversion system was constructed by using both electrodes.Transient generation of photocurrent confirmed successful underwater photoelectric conversion.Analytical methods such as X-ray photoelectron spectroscopy,X-ray diffraction,and Fourier transform infrared spectroscopy indicated that when the system was illuminated in a seawater environment,electron-hole pairs were generated in the TiO₂ film.The holes caused oxidation of the water,generating oxygen molecules and protons.The electrons immigrated into the Ni₃[Fe?CN?₆]₂ terminal through an external circuit,forming a current in the external circuit.Ni₃[Fe?CN?₆]₂ accommodated sodium ions from the seawater into the crystal lattice to maintain the electrical neutrality while capturing the photo-electrons.In short,the key point for realizing continuous separation of electron-hole pairs was due to ion-intercalation of the coordination lattice assisted storage of the electrons and hole-capture with the water molecules.2.Study on joint generation of electricity and oxygen by prototype day-mode solar seawater batteriesBased on our photo-electricity conversion mechanism,we constructed a prototype device of day-mode solar seawater batteries,including three systems,TiO₂-Ni₃[Fe?CN?₆]₂,TiO₂-Co₃[Fe?CN?₆]₂ and TiO₂-Cu₃[Fe?CN?₆]₂.We investigated synergistic interactions which happened among electrons,ions,molecules,and the crystals during electricity supply.Photoelectric conversion performances of the devices were evaluated,and the characteristics of the currents formed by charge transferring were measured.The TiO₂-Co₃[Fe?CN?₆]₂ system exhibited the highest photoelectric conversion efficiency of 3.463%.The TiO₂-Cu₃[Fe?CN?₆]₂ system showed the strongest photo-current with a current density of about 13 mA cm^-2.During electricity supply,oxygen molecules were generated at the photoanode.The continuous production of the oxygen can be used to supply oxygen in seawater or suspension of seawater,which was valuable for restoration of hypoxic seawater and tidal flat.3.Assembly and evaluation of prototype day-night solar seawater batteryBased on the prototype day-mode solar battery,we chose Fe₄^III[Fe?CN?₆]₃coordination crystals,which had suitable redox potential,as the energy transition pot.These crystals receive electrons and ions upon light illumination,therefore can be used to assemble the day-mode part with TiO₂ photoelectrode.In the night mode,the electrons stored in the day-mode were transferred to the dissolved oxygen in the seawater.By coupling with the carbon electrode,two working modes were integrated,leading to a prototype day-night solar seawater battery,TiO₂-Fe₄^III[Fe?CN?₆]₃-C_MnO2.In the day mode,the photo-electrons and sodium ions were inserted into the Fe₄^III[Fe?CN?₆]₃ crystals,converting the crystals into Na₄Fe₄^II[Fe?CN?₆]₃.In the night mode,the electrons stored in Na₄Fe₄^II[Fe?CN?₆]₃ were further transferred to the carbon electrode,accepting by the dissolved oxygen.In the meanwhile,the sodium ions in the crystal of Na₄Fe₄^II[Fe?CN?₆]₃ were released back to the seawater.The whole process was similar to the joint action of photosynthesis and respiration in algae cells.The density of the photocurrent was up to 4.7 mA cm^-2.After 100 day-night cycles,the output voltages of the device showed no significant attenuation.The maximum output power of the device was 0.35 mW cm^-2 in night mode.The photoelectric conversion efficiency was 0.62%,which was close to the photosynthetic efficiency of Ulva lactuca and Gracilaria lemaneiformis.4.Influence of environmental parameters on the performance of the prototype solar seawater batteriesWe studied influence of the effects including salinity,water depth,light intensity,dissolved oxygen and flow of the seawater on the performance of our batteries.?1?The relationship between the salinity and output voltage of the device was measured both in the day and night modes.In the day mode,the output voltage of the device was positively correlated with salinity.The TiO₂-Fe₄[Fe?CN?₆]₃ system exhibited a linear relationship with salinity in the range of 0.7%³.5%in salinity.In the night mode,the output voltage of the device increased first,then decreased with the increase of salinity.?2?The photoelectric conversion at different water depths during operation was tested.As the water depth increased from 0 cm to 90 cm,the output voltage of the device decreased from 0.38 V to 0.25 V.By fitting the current density with the water depth,the photocurrent value has a linear relationship with the water depth in the range of 0cm⁵0 cm.?3?In the night mode,concentration of the dissolved oxygen strongly correlated to electrical capacity of the device.In oxygen-rich seawater(>5 mg L^-1),the initial discharge voltage was about 0.42 V,and the discharge curve exhibited a redox characteristic.In hypoxic seawater(<0.5 mg L^-1),the initial voltage was only 0.17 V,and vanished in a very short period.?4?The output voltage of the device correlated to the irradiation intensity.The voltage increased with the strengthening of the light intensity.?5?The electricity generation performance of the device was influenced by flow of the seawater both in the day and night modes.With the increase of the flow velocity,both the output voltage and capacity of the device were optimized in the day.In the night mode,the discharge process was affected by impact of the flow rate on concentration of the dissolved oxygen,because polarization occurred at the early stage.In the later discharge stage,the flow condition could effectively inhibit the polarization.