Performance Study And Enhancement Of All-vanadium Photoelectrochemical Cell With The Ti³⁺/Ti⁴⁺ Photoanodes

With the rapid development of society,the depletion of disposable energy sources?coal,oil and natural gas,etc.?and overuse of them have resulted in severe energy crisis and environmental pollution issues.In order to maintain the sustainable development of social economy,the development of clean and renewable energy has become one of the concerns worldwide.As a kind of inexhaustible clean energy,solar energy has received wide attention.However,the inherent weakness of intermittence impedes its application.To address this critical defect,the photoelectrochemical conversion technology has been proposed.The photoelectrochemical conversion,which is an integration of photocatalysis and electrochemistry,has been demonstrated as a simple approach for solar energy storage in the form of chemical via photoelectrochemical cell?PEC?systems.The vanadium redox flow battery?VRB?is a promising technology for energy storage.The VRB can offer inherent advantages of decoupled and scalable power and energy density,long cycle life,reasonable storage volume,and efficient avoidance of electrolyte cross-contamination.Utilizing the advantages offered by the VRB and photoelectrochemical technology,the all-vanadium photoelectrochemical cell?VPEC?,which combines the photoelectrochemical technology and VRB,has been proposed.This method can directly convert solar energy into chemical energy.At present,TiO₂ is the most commonly used as the photoanode catalyst in VPEC due to its intrinsic merits.However,it still faces some critical problems,such as the large band gap and serious recombination of photo-excited electron-hole pairs,which lowers the photoelectrochemical conversion and solar energy utilization efficiencies.Aim at these deficiencies,the Ti₂O₃ nanoparticles and self-doped TiO₂ nanoparticles are first proposed to prepare the photoanode,which can broaden the light absorption range.Then the photoanode with TiO₂ nanotube array is developed to increase the specific surface area and enhance the electron transfer so as to inhibit the recombination of the electron-hole pairs.Finally,the self-doped TiO₂ nanotube array photoanode is developed,which can broaden the absorption spectrum and inhibit the recombination of electron-hole pairs and increase the specific surface area.The use of these photoanodes enables the improvements in the utilization of solar energy and the VPEC performance.Main outcomes of this thesis are summarized as follows.?1?VPEC with full-spectrum-responsive Ti₂O₃ photoanodeThe commercial Ti₂O₃ particles are employed to prepare the photoanode for the VPEC by wet-spray method.The prepared photoanode is characterized by the UV-vis and LSV,and compared with the P25 TiO₂ photoanode.The results show that the Ti₂O₃can respond almost the full spectrum of sunlight and generate more photo-excited electron-hole pairs.Therefore,the photoelectrochemical performance of the Ti₂O₃photoanode is better than P25 TiO₂ photoanode.The photoresponse and long-term operation of VPEC with the Ti₂O₃ photoanode are measured.It is shown that the developed cell has a good photo-response and stability.The effects of the light intensity,vanadium ion concentration and electrolyte flow rate are also studied.It is found that increasing the light intensity and vanadium ion concentration and reducing the electrolyte flow rate promote photoelectrochemical reactions and thus improve the cell performance.However,the large and irregular Ti₂O₃ particles lead to large impedance between catalyst and substrate,which results in a high electron transfer resistance and small specific surface area.?2?VPEC with self-doped TiO₂ photoanodeTo resolve the problems encountered in commercial Ti₂O₃ particles,the photoanode with self-doped TiO₂ nanoparticles was prepared by NaBH₄ treatment.This developed photoanode not only increases the specific surface area and provides more active sites,but also decreases the resistance of electron transfer and improves the photoanode electrical conductivity.The developed photoanode is characterized by the SEM,UV-vis,EIS and LSV.It is found that the self-doped TiO₂ nanoparticles are deposited uniformly on the substrate,which shows good photo-response to visible light and electrical conductivity and photoelectrochemical activity.The long-term operation of the developed photoanode confirms its good stability.It shows higher photocurrent density and conversion rate of VO²⁺than the Ti₂O₃ and P25 TiO₂ photoanodes.Finally,the influence of the light intensity and vanadium ion concentration is studied,and it is found that the photocurrent density and conversion rate of VO²⁺are increased with the increase of the light intensity and vanadium ion concentration.?3?VPEC with one-dimensional TiO₂ nanotube array photoanodeThe one-dimensional and highly ordered TiO₂ nanotube array?TNA?photoanode is prepared on titanium foil,which not only provides the large specific surface area but also intensifies the photo-charge transfer rate,inhibiting the recombination of photo-generated electron-hole pairs.Characterizations of SEM,EIS and LSV are performed.The results show that the TiO₂ nanotubes are uniformly distributed and compactly arranged on the titanium foil,which lower the electron transfer resistance and shows better photoelectrochemical activity.The photoresponse and long-term operation are carried out.It is shown that the TNA photoanode well responds to the illumination and exhibits good stability during the long-term operation.In addition,the effect of the light intensity,vanadium ion concentration and anodizing voltage on the cell performance are discussed.The results show that the increase of the light intensity,vanadium ion concentration and anodizing voltage results in the increased performance of the VPEC with one-dimensional TiO₂ nanotube array photoanode.?4?VPEC with self-doped TiO₂ nanotube array photoanodeThe self-doped one-dimensional TiO₂ nanotube array?B-TNA?is prepared by the anodizing method and NaBH₄ treatment.The physical and electrochemical features of B-TNA are characterized by the SEM,UV-vis,EIS and LSV.It is shown that the B-TNA not only retains the structure of TNA,but also broadens the light absorption spectrum and improves the solar energy utilization,which enhances the electron transfer and photoelectrochemical activity.The long-term performance measurement shows that the B-TNA photoanode has good stability,and its photocurrent density is higher than that of the TNA photoanode.The photocurrent density and vanadium ion conversion rate improve with the increase of both the light intensity and vanadium ion concentration.