Design And Study Of Electrodes For Flexible Fibrous Dye-Sensitized Solar Cells

Dye-sensitized solar cells(DSSCs)have attracted much attention in recent 30years because of their environmental friendliness,simple preparation process and low cost.However,the rapid development of intelligent,portable and wearable electronic products has made the rigid design of traditional DSSCs unable to meet the needs of their wearable power supply.Thus,aiming at this goal,fibrous dye-sensitized solar cells(FDSSCs)have been considered as one of the most promising potential energy devices for wearable electronic devices due to their high flexibility,light weight and easiness to integration that effectively meet the application requirements of wearable devices.At present,most researchers are focusing on how to improve FDSSCs'deficiencies in photoelectric conversion efficiency(PCE).However,as a wearable power supply,the FDSSCs should be also required to have excellent dynamic flexible bending performances that suitable for the dynamic energy collection and energy output.Therefore,fully considering both flexibility and electrochemical properties of photoanodes and counter electrodes,this dissertation has proposed the following designs:i)to introduce carbon nanomaterials(including carbon nanotube(CNT)and graphene)with high mechanical properties and high electrical conductivity into photoanodes to improve their flexible bending performance;ii)to design highly catalytic Ti N nanorod and Ni₃Te₂microsheet array to replace the traditional nobel metal Pt counter electrode to improve the bending performances of the devices;iii)using commercial carbon fibers(CFs,?100?m)with high mechanical properties as the substrate for the counter electrode can reduce the device loss caused by electrode bending.At the same time,in order to improve the transport of electrons in the electrodes,we introduced SnO₂,CNT and graphene in the photoanodes,as well as the carbon nanofiber(CF)skeleton in the counter electrodes to increase the electron collection efficiency,reduce the recombination of photo-generated charges,and thus improve the PCE of FDSSCs.The main work of this paper is summarized as follows:(1)Preparation of SnO₂@TiO₂heterojunction based fiber photoanodes and their structure optimization.First,one-dimensional(1D)SnO₂nanofibers(NWs)were prepared by electrospinning technique;then coat a layer of SnO₂NWs on conductive stainless steel wire(SSW)substrate,followed by modification of TiO₂nanothorns on the surface of SnO₂NW layer via sol-gel method,and finally a type of SnO₂@TiO₂heterojunction based fiber photoanodes.Electrochemical tests showed that after TiO₂modification,the assembled FDSSCs output a PCE of 5.11%,which are 4.26 times and 30%those of the un-modified and the pure P25 ones.According to the analysis on the obtained data,it can be seen that the significant increase in short circuit current and photo-generated electrons'life in SnO₂@TiO₂electrodes mainly contribute to the increase of PCE,and is mainly attributed to the heterojunction formed between SnO₂and TiO₂.Furthermore,when the coating thickness is ca.10?m,the assembled FDSSCs has the highest PCE value.Unfortunately,the TiO₂modification of this design does not improve the flexibility performances of the fiber devices.(2)Preparation of P25-rGO based fiber photoanodes and their structure optimizationTo enhance the flexibility of the membrane materials in phtoanodes,we mixed certain appropriate amount of reduced graphene oxide(rGO)materials into the P25TiO₂(P25)electrodes.The addition of rGO not only enhances the mechanical properties,but also accelerates the transport of electrons in the photoanodes.Then,commercial P25 powder was selected as the photoanode materials,and the aqueous GO solution was quantitatively added to the powder.After sufficiently grinding,the obtained slurry was coated on the SSW substrate,and finally after annealing,the P25-rGO based fiber photoanodes were obtained.By regulating the doping amounts of rGO and the thicknesses of the P25-rGO membranes,the assembled FDSSCs showed the highest PCE of 5.364%,which is improved by 36%compared to that of pure P25 NP based ones.Related analysis shows that the high conductivity of rGO can greatly accelerate the electrons'transport in the photoanodes,and thereby improve the PCE of the devices.Besides,the incorporation of rGO also improves the mechanical properties of FDSSCs.In the flexible bending test,the FDSSCs device showed good flexibility and bending after hundreds of times of bending,which should be inherited from the introduction of high mechanical rGO,which improved the mechanical stability of the device.(3)Preparation of P25/CNT based fiber photoanodes and their structure optimizationSimilar as Work(2),here,highly conductive CNTs were incorporated into the commercial P25 photoanodes.To further improve the electrochemical properties of the P25-TiO₂/CNT composite membranes,a dense layer of TiO₂was designed on their surface to inhibit the recombination of the photogenerated electrons and I₃^-ions in the electrolyte,and thus to improve the PCE of FDSSCs.By regulating the thicknesses of the P25/CNT membrane layer and the doping amounts of CNTs,the assembled FDSSCs showed the average PCE value of 10.13%with a Ni₃Te₂microsheet array as the counter electrode.This PCE value is 25%higher than that of the P25-based devices.Furthermore,series of analyses showed that the introduction of CNTs enhances the separation of photo-generated carriers and the transport of electrons in the photoanodes.Besides,the incorporation of CNTs into the photoanode can effectively improve the mechanical properties of FDSSCs.In the flexible bending test,the PCE of P25-TiO₂/CNT-based FDSSCs has a slight attenuation of 27.6%after8500 bending times(500 times interval)of the FDSSC device,and the attenuation rate per bending is only 0.033‰.This indicates that our designed FDSSCs incorporating highly mechanical CNTs into the fiber photoanode exhibit excellent bending mechanical stability,which should be attributed to the improvement brought by the introduced CNTs.(4)Preparation of Ti N-CF based fiber counter electrodes and their structure optimization.It has been demonstated that conventional Pt electrodes are not only expensive,but also have insufficient electrochemical stability.Thus,to search a new,stable and inexpensive counter electrode material is one of research hotspots in the field of DSSCs.In this work,we chose the Ti N materials to replace the conventional Pt pair electrode due to their excellent catalytic performances.First,we prepared the TiO₂nanowire array on the surfaces of commercial carbon fibers by hydrothermal method,and then a type of Ti N-CF-based fiber counter electrodes was obtained via amination treatment.By regulating the morphologies of the Ti N NRAs(including NRs'height and diameter,and array density),the Ti N-CF base FDSSCs showed the maximum PCE of 5.690%,which is slightly higher than that of the Pt based ones.A series of analyses showed that the enhancement of PCEs is mainly attributed to the high electrochemical catalytic activity of Ti N NRs for the reduction reaction of I₃^-ions.Besides,Ti N-CF-based FDSSCs show high electrochemical stability,rapid optical response and good integration.And the array structure design of Ti N NRAs enhances the dynamic bending stability of the devices.In the flexible bending test,after 5500times of bending(500 times interval),the Ti N-10min-based FDSSCs only suffered a small attenuation of 15.3%,and the attenuation rate per bending was only 0.028‰.This indicates that the introduction of our designed high-strength carbon fibers and Ti N nanorod arrays can minimize the device loss caused by the strain force generated during bending,providing stronger mechanical properties.(5)Preparation of Ni₃Te₂based fiber counter electrodes and their structure optimizationIn the above work,the design of the Ti N NRA structure requires a secondary amination treatment,which greatly increases the difficulty and instability of the device's preparation and design.Here,in this work,we directly in-situ grew the Ni₃Te₂microsheet arrays on the nickel wire substrates using an atmospheric pressure chemical vapor deposition method,and then directly used as fiber counter electrode to replace Pt one.Results showed that the Ni₃Te₂based FDSSCs show the highest PCE up to 11.81%,which is 2.10 times the largest PCE(5.62%)of the Pt based ones.A series of analyses(including EIS,CV and Tafel tests)show that the diameter of the microsheet determines the photochemical properties of FDSSCs when not exceeding3.0 m;when the diameter exceeds 40 m,the thickness imposes a large effect on the devices'performances.And appropriate thickness and diameter favor the diffusion of I₃^-ions between the Ni₃Te₂layers.In addition,the Ni₃Te₂-based FDSSCs also shows high electrochemical stability and excellent dynamic bending stability,with a decay rate of 0.12‰per bending cycle(2,000 cycles),as well as the device's rapid photo response,good integration and physical environment adaptability.The designed topographical structure of the Ni₃Te₂microplate array provides a better strain space for the flexible bending of our fiber electrodes,which contributes to the mechanical stability of FDSSCs during bending.(6)Preparation of Ni₃Te₂/NiTe based fiber counter electrodes and their structure optimizationThe toughness of the nickel wire substrate of Ni₃Te₂fiber electrode decreases after annealing at high temperature,and the substrate will fracture after certain bending,which reduces the mechanical stability of the device.In this work,we use carbon fiber with high mechanical and electrical conductivity as the conductive substrate,and use the same simple atmospheric pressure chemical vapor deposition method to directly grow with high catalytic activity of Ni₃Te₂/NiTe square microplates on the carbon fiber substrate,and then directly replace Pt as the fiber counterelectrode.The mean PCE of Ni₃Te₂/NiTe FDSSC is 9.91%,which is 35%of the maximum PCE of Pt group(7.53%).A series of analyses(including EIS,CV and Tafel tests)showed that the reaction time directly determines the crystallinity of Ni₃Te₂/NiTe,especially the crystallinity of NiTe,which in turn determines the photoelectric chemical properties of the battery.With the increase of reaction time,the crystallinity of NiTe also increases.When the reaction time is 45min,Ni₃Te₂/NiTe has the best crystallinity,which is conducive to the reduction of I3-ions and shows the best catalytic activity.In addition,Ni₃Te₂/NiTe based FDSSCs also exhibit high electrochemical stability and excellent dynamic bending stability,with a decay rate of0.1‰per bending cycle(3000 cycles),as well as fast optical response,good integration and adaptability to physical environments.Ni₃Te₂/NiTe FCE,a conductive substrate designed with high mechanical properties,provides better mechanical stability for flexible bending of our fiber electrodes and contributes to the mechanical stability of FDSSCs during bending.