Fabrication Of Electrospun Carbon Nanofibers-based Composites And Their Application In The Counter Electrodes Of Dye-sensitized Solar Cells

Since the 21st century,it is of urgent hunt for clean,renewable energy and to develop efficient energy conversion as well as storage devices in response to the contradiction between the growing world population and the limited fossil energy.Dye-sensitized solar cells(DSSCs)are a new type of thin-film solar cells with easy preparation,low-cost,and free-environmental pollution.The counter electrode(CE)is an important part of DSSC,which is used to collect electrons from the external circuit and catalyze the redox reaction of the iodine-based electrolyte.The traditional CE is Pt CE.Due to the high price of Pt,and corrosion by electrolytes,it is of emergency to develop an effective,stable,low-cost,abundant CE material to replace Pt CE in order to realize the large-scale commercial application of DSSCs.Electrospun carbon nanofibers(CNFs)have great potential as DSSCs CE because of their large specific surface area,porous pores that facilitate ion diffusion and electron transport.In addition,conductive polymers,metal compounds,and metal alloys are frequently-used CE materials for DSSCs.However,the single-component CE materials usually suffer from a poor conductivity,easy agglomeration,and small specific surface area,leading to their performance is far inferior to that of Pt CE.Therefore,researchers have combined two or more materials and achieved good results with the synergistic effect between the components.In this thesis,we will design and synthesize a series of CNFs-based composite materials as Pt alternative CE materials,which show excellent electrocatalytic activities and low cost.The electrocatalytic activities of these prepared materials were significantly enhanced by regulating their composition,morphology and constructing Mott-Schottky heterojunction.The specific content is as follows:1.Conductive CNFs were prepared by an electrospinning and calcination process,which were used as the substrates to construct metal sulfide-based nanofiber composites that were applied to DSSCs CEs.The details are as follows:(1)High catalytic and ultrafine Co₉S₈ nanoparticles were grown on the surface of CNFs via one-step hydrothermal method to obtain Co₉S₈/CNFs composite.Power conversion efficiency(PCE)of DSSC based on Co₉S₈/CNFs CE(8.37%)is comparable to that based on Pt CE.This result is due to the one-dimensional(1D)nanostructure with CNFs as core and Co₉S₈ nanoparticles as shell can not only provide an effective electron transfer path,but also increase the effective catalytic area of active components.(2)Compared with monometallic sulfide Co₉S₈,bimetallic sulfide NiCo₂S₄ with multivalent elements shows more redox active sites and stronger electrical conductivity.The NiCo₂S₄ shell was also constructed via a hydrothermal strategy,and the NiCo₂S₄/CNFs composite materials with two different morphologies(nanorods(NRs)and nanoparticles(NPs))were prepared by changing the type of reaction solvents.Compared with Pt CE,the PCE of NiCo₂S₄ NRs/CNFs composite increased significantly(from 8.18%to 9.47%).This result demonstrates that the construction of hierarchical 1D core-shell structure can lead to a synergistic effect between active components and conductive substrates,which provides a direction for finding alternative Pt CE materials.2.In order to avoid the highly toxic H₂S gas generated during the preparation of sulfides in the first part and simplify the synthesis route of CE materials,the metal-metal carbide/CNFs composites were prepared by one-step calcination in the second part of the experiment.The details are as follows:(1)Co and Mo₂C nanoparticles were successfully dispersed within CNFs via a simple electrospinning and carbonization process,and the influence of different mass ratios of Co and Mo salts on the catalytic performance of the composites was investigated.The results showed that Co-Mo₂C(2:1)/CNFs composite possessed better PCE of 8.60%than Pt and other control samples as well as a good chemical stability,which stemmed from the heterojunction formed between Co with good conductivity and Mo₂C with high catalytic activity(similar d-band electronic structure with Pt).(2)In order to further enrich the active sites of CE materials,nickel with greater electronegativity was selected.Using similar synthetic method with Co-Mo₂C/CNFs,Ni-Mo₂C/CNFs composites were also prepared.Owing to the synergistic effects among Ni,Mo₂C and CNFs,as well as the prevention of the agglomeration of active components,the PCE of Ni-Mo₂C(2:1)/CNFs CE achieved8.90%,and the composite material shows considerable application prospects.3.To further enrich the electrocatalytic activity sites of CE materials,reduce the interfacial resistance of CE,shorten the electron transfer path,and enhance the electrocatalytic activity of CE,the metal oxide(sulfide)-metal alloy/CNFs composite materials with Mott-Schottky were prepared.The details are as follows:(1)NiCo/CNFs were prepared through an electrospinning and calcination process at a high temperature.Then NiCo₂O₄ nanosheets were loaded on NiCo/CNFs under a hydrothermal reaction.The effects of different shell thicknesses of NiCo₂O₄ nanosheets on the properties of the composites were investigated.The NiCo₂O₄-NiCo/CNFs-3 composite possessed a PCE of 9.18%,which is better than Pt and other control samples because of the rectifying effect of Mott-Schottky formed by NiCo₂O₄ and NiCo as well as the synergistic effect between the active components and CNFs.(2)To further improve the conductivity and catalytic properties of CE materials,MoS₂/CNFs were prepared via an electrospinning and followed with a hydrothermal reaction.Then,Pt Ni nanoparticles were deposited on MoS₂/CNFs through a chemical bath deposition.The PCE of MoS₂-Pt Ni/CNFs composites was 9.31%due to the Mott-Schottky formed between MoS₂ and Pt Ni as well as the synergistic effect between active components and CNFs,and the composite material shows a great potential for practical application.In summary,in this thesis,metal sulfide,metal/metal carbide and metal oxide(sulfide)-metal alloy-based CNFs composites were synthesized and used as DSSCs CEs.It was found that the synergistic effect between the different active components and the superior electron transport of conducting CNFs in the reaction process were important factors affecting the electrocatalytic activity of CE materials.By constructing 1D nanofiber-based composites with multilevel structure,the electrocatalytic activity of CE materials can be significantly improved,which provides a new idea for the development of efficient Pt-free CE materials.