(Photo)Electrochemical Conversion Of Redox Couples In Solar Rechargeable Flow Cell

Solar energy conversion,storage and utilization is considered as one of the most promising strategy to solve the energy and environmental crisis.Developing reasonable and efficient solar energy utilization technology is therefore indispensable.Based on the（photo）electrochemical conversion of reversible redox couples,Solar rechargeable flow cell（SRFC）can harvest,convert and store solar energy into chemical energy in single system,and further convert it to electricity when needed.Hence,SRFCs are thought to be one of the effective ways to attain efficient and sustainable solar energy utilization.We dedicate our effort investiging the redox reaction mechanisms,designing and developing the efficient SRFC system.The main results of this dissertation are summarized as follows.（1）The electrochemical behavior of vanadium redox couples has been systematically investigated on the basis of the typical carbon-based material,Vulcan XC72.By varying the starting electrolyte and potential window,it is found that both the redox reactions of V³⁺/V²⁺and V⁵⁺/V⁴⁺are quasi-reversible processes controlled by diffusion,while V⁴⁺/V³⁺shows an irreversible electrochemical behavior.Moreover,the conversion of V³⁺/V²⁺follows an outer-sphere electron transfer mechanism while V⁵⁺/V⁴⁺is an inner-sphere electron transfer redox reaction coupling with a preceding adsorption process.Based on the above mechanism,we studied the kinetic parameters of V⁵⁺/V⁴⁺and V³⁺/V²⁺on Vulcan XC72.The redox reaction of V³⁺/V²⁺reveals fast kinetics on Vulcan XC72(1.5×10^-3 cm s^-1)without hydrogen evolution side reaction.Hence,Vulcan XC72 is an ideal cathode material for vanadium-based SRFC.（2）The electrochemical behavior of vanadium redox couples including V⁴⁺/V³⁺has been investigated on the nitrogen-doped ordered mesoporous carbon（NOMC）which is synthesized by nano-casting method.It is found that both the redox reactions of V⁴⁺/V³⁺and V⁵⁺/V⁴⁺are quasi-reversible processes.And V⁵⁺/V⁴⁺demonstrates a 4.7 times higher charge transfer rate constant of 7.0×10^-3 cm s^-1 comparing to Vulcan XC72.The improved kinetics on NOMC should originate from both the enriched electrochemically active function groups on the surface and electronic effect conferred by the nitrogen doping.In addition,different from the diffusion-controlled behavior of V⁵⁺/V⁴⁺,the redox reaction of V⁴⁺/V³⁺shows a kinetics-controlled behavior,which is determined by the preceding adsorption of the symmetrical ions(V³⁺).Based on the above findings,NOMC is the ideal electrode material of the vanadium-based SRFC,which can reduce the energy loss by minimizing the over potential in the charging/discharging process of the V⁴⁺/V³⁺-V⁵⁺/V⁴⁺energy strorage system.（3）The essential elements of SRFC construction has been proposed by analyzing the energy loss and device design in SRFCs.Based on that,a SRFC system consisting of an atmosphere silicon（a-Si）photoanode,Fe³⁺/Fe²⁺and AQDS/AQDSH₂ as energy conversion and storage media is designed and fabricated.By analyzing the（photo）electrochemical behavior of the reactions involved,the photoelectrochemical cell part and the redox flow battery part were specifically designed and optimized.Among them,the solar-to-chemical conversion efficiency in the photoelectrochemical cell part reaches 6.5%,while the energy efficiency in the redox flow battery part is 80%under an operating current density of 50 mA cm^-2.Finally,for reducing the coupling energy loss including polarization loss and side reactions,a deeply integrated“all-in-one”SRFC is designed and reveals an overall solar-chemical-electricity conversion efficiency of 4.9%,which is 1.5 times higher than the system of a vanadium redox flow battery directly charged by the a-Si solar cell.Furthermore,the photoelectrode utilization ratio of this Fe-AQDS based SRFC is 70%,which is the best in the published SRFCs,indicating the energy loss in the system can be reduced by the outstanding energy-level matching between the photoelectrode and redox couples.