Mass Transport Characteristics Of All-Vanadium Photoelectrochemical Cell And Its Performance Enhancement

Owing to the grand challenge of energy crisis,the replacement of fossil fuels by renewable energy becomes inevitable to ensure the sustainable development.To date,a considerable number of researchers have devoted their great efforts to the development of renewable energy,including the solar energy,wind energy and tidal energy,etc.Profited from the significant advantages of clean,extensive,inexhaustible,etc.,solar energy is deemed to be one of the main renewable energy resources to alleviate the energy crisis and possibly replace the traditional fossil fuels,and has received a wide attention in the past decades.Presently,main commercial solar energy utilizations rely on the photothermal and photoelectric conversions by converting the solar energy into heat and electricity,respectively.Both solar utilization technologies face the problem of the intermittence.Hence,the energy storage is usually required in these applications.Currently,the solar energy storage techniques mainly include the photothermal conversion,electrochemical conversion and photoreaction conversion.Among them,vanadium redox flow battery?VRB?has been regarded as one of the most competitive candidates for large-scale energy storage.The VRB can offer lots of advantages,such as long life cycle,large storage capacity,high efficiency and safety.Moreover,the electrolyte metal ions are all vanadium,which can effectively avoid the cross contamination in the electrolyte solution and be easily recycled.In recent,some scholars proposed the all-vanadium photoelectrochemical cell by combining the photocatalytic technology with all vanadium redox flow battery to realize the conversion of solar energy to chemical energy.This new technology just emerged.Existing all-vanadium photoelectrochemical cells still face some problems.For example,the batch reaction design is usually adopted for the all-vanadium photoelectrochemical cell,leading to low capacity and a large concentration loss after a long-term operation.On the other hand,existing all-vanadium photoelectrochemical cells are usually large scale,increasing the internal cell resistance.Moreover,pure TiO₂ is often used to fabricate the photoanode,which can only respond to UV light.All these negative effects lead to the lowered performance.To resolve the above problems,a conventional large-scale all-vanadium photoelectrochemical cell was firstly designed and fabricated in this thesis.With this cell,the transport characteristics and performance were studied.Aiming at the mass transport issue encountered in conventional large-scale all-vanadium photoelectrochemical cell,a microfluidic all-vanadium photoelectrochemical cell??VPEC?was then proposed for the solar energy storage.By changing the cell size and the electrolyte flow,the performance of the all-vanadium photoelectrochemical cell was improved.Subsequently,the microfluidic all-vanadium photoelectrochemical cells with nanostructured TiO₂photocatalyst and N-doped TiO₂ photocatalyst as the photoanode materials were developed,both of which could greatly improve the conversion efficiency.Main outcomes of this thesis are summarized as follows.?1?A batch large-scale all-vanadium photoelectrochemical cell was developed.To enhance the electron transport,the photoanode in the developed cell included two layers:one was the dense layer at the bottom and the other was the top porous layer.The dense layer was prepared by spin coating and calcination,and the porous titanium dioxide layer was prepared by wet spraying and calcination.The materials characterization,including the XRD and SEM were were performed.The results showed that the photoanode had good crystal structure and porous morphology.The photoresponse of the photoanode was tested,and it was proved the the photoanode in the cell had a good photo-response.Long-term operationshowed that the cell had good stability.After that,the performance of the batch large-scale all-vanadium photoelectrochemical cell was evaluated under different light intensities and vanadium ion concentrations.It was found that the performance was improved with the increase of light intensity and vanadium concentration.However,the overall performance was not high due to the large internal cell resistance,which caused low capacity and low conversion rate.These issues can be improved by changing the cell structure and the photoanode.?2?Towared the problems encountered in the batch large-scale all-vanadium photoelectrochemical cell,a microfluidic all-vanadium photoelectrochemical cell was developed.This cell adopted the continuous flow mode,which could reduce the concentration polarization and improve the the capacity.Here,the photoanode was the same as batch large-scale all-vanadium photoelectrochemical cell.The photo-response characterization results showed that the photoanode still had good photo-response behaviors.By comparing the EIS results of the batch large-scale and microfluidic cells,it was shown that the microfluidic all-vanadium photoelectrochemical cell had smaller internal cell resistance.The influence of the proton exchange membrane thickness on the performance was studied and the results showed that as the thickness of the proton exchange membrane was increased,the permeability of vanadium ion was decreased,but the proton transfer was relatively inhibited,leading to poor performance.Through study on the influence of the light intensity and vanadium concentration,it was found that more electron/hole pairs were generated by increasing the light intensity,then the performance also increased.The increased vanadium concentration could enhance the mass transport and thereby the performance.?3?A microfluidic all-vanadium photoelectrochemical cell with the multiply nanostructured TiO₂ as the photoanode was developed.The multiply nanostructured TiO₂were synthesized by the hydrothermal method.Then,the physical/chemical properties were characterized by the XRD,SEM and BET.It was shown that the multiply nanostructured TiO₂ as the photocatalyst offered many advantages over the convention pure TiO₂ nanoparticles,including large surface area and vigorous pore structure.The effect of the light intensity and vanadium ion concentration were studied.It was found that the performance of the microfluidic all-vanadium photoelectrochemical cell with the multiply nanostructured TiO₂ increased with the increase of the light intensity and vanadium concentration.More importanly,the microfluidic all-vanadium photoelectrochemical cell with the multiply nanostructured TiO₂ yielded much better performance than did the cell with the TiO₂ nanoparticles,and the performance of the cell improved 36%.?4?To resolve the problem of conventional bare TiO₂ that can only respond to UV light,the N-doped TiO₂ as the photoanode was developed,which was prepared by the sol-gel method.Both the physical and chemical features was characterized by the TEM,FE-SEM,XRD,XPS,BET and UV-Vis spectrum analysis.It was shown that the N-doped TiO₂ photoanode had many advantages over the bare TiO₂ photoanode,including the extention of light response to the visible light,and large specific surface area and vigorous pore structure.The photoresponse results showed that the N-doped TiO₂ photoanode had good photoresponse.The effect of the light intensity and vanadium ion concentration were explored,and the performance of the cell improved 40%.The performance of the cell with the N-doped TiO₂ photoanode increased with the increase of the light intensity and vanadium ion concentration.The microfluidic all-vanadium photoelectrochemical cell with the N-doped TiO₂ photoanode exhibited much better performance than did the cell with the bare TiO₂ photoanode.