Preparation And Study Of Transition Metal Compound Counter Electrodes In Dye-sensitized Solar Cell

The performance of dye-sensitized solar cells?DSSCs?is strongly influenced by the catalytic performance of their counter electrode?CE?materials.Conventional Pt electrode exhibits excellent catalytic performance,but its high price,resource scarcity and instability in iodine electrolyte limited the practical application of DSSCs.Therefore,it has important scientific significance and broad application prospect to develop green,low cost,and high-performance electrode materials to replace the Pt to meet the demands in different types of DSSC devices.In this thesis,we aim to develop novel DSSC CE materials based on transition metal compounds?TMCs?.The relationships between preparation,structure and performance of the CE materials were systematically investigated,which leads to controllable synthesis of high performance CE materials for DSSCs.The main contents are as follows:1.Fabrications and performances of NiS₂ and NiS CEs by hydrothermal methodsNiS₂ cube was synthesized directly on fluorine-doped tin oxide?FTO?glass by a one-step hydrothermal method.The influence of growth conditions on microstructure and macroscopic properties was studied.The effects of different varieties and dosage of surfactants on the structure of the products were discussed.In addition,electrochemical impedance spectroscopy?EIS?technique was used to study the transport process of electrons and ions at the interface of electrolyte/electrode.EIS,cyclic voltammetry?CV?and Tafel polarization tests showed that the NiS₂ cube has excellent electrocatalytic activity towards the reduction of I3-.In addition,DSSC containing NiS₂ CE showed a photoelectric conversion efficiency?PCE?of 5.56%,which was close to that of Pt-based DSSC?7.05%?.The flower-like NiS structure has excellent catalytic activity for the reduction of I3-,and the PCE for DSSC based on flower-like NiS CE was 7.1%,which was significantly higher than that of Pt CE?7.05%?,NiS nanospheres CE?5.91%?and NiS cube CE?4.43%?.This is due to the low resistance of the ions in the electrolyte to diffuse and transfer in the structure.2.Fabrications and performances of FeS,NiCo₂O₄ and NiCo2S4 counter electrodes by electrospinning methodsFe₂O₃ and FeS nanorods were prepared by electrospinning and their valence states were studied using Mossbauer spectroscopy.Their catalytic performances as CE materials were evaluated by various electrochemical tests.It was found that the PCE increased from 3.79% to 6.47% when the n-type semiconductor ?-Fe₂O₃ was converted into p-type semiconductor FeS by the vulcanization process.This increase may be related to higher amounts of electron-hole pairs and higher conductivity and the mixed valence of Fe elements in FeS,which promotes the transfer of electrons to the electrolyte.In addition,due to the interconnection of the conductive path and excellent mechanical stability imparted by the one-dimensional morphology,the FeS exhibits better long-term stability than the Pt electrode.NiCo₂O₄ structure with different morphologies were prepared by spinning with a mixed solution of PAN and nickel and cobalt metal nitrates with different concentrations.The properties of these structures as CE were studied in DSSC.NiCo₂O₄ with a honeycomb morphology showed better performance than the nanotube,which exhibited a photoelectric conversion efficiency of 7.09% and is even higher than that of Pt CE?7.05%?.The enhanced photoelectric properties are attributed to their relatively large surface area and excellent electrical conductivity.In addition,the DSSC of honeycomb-like NiCo₂O₄ CEs has been tested for over 600 hours of photovoltaic performance,and multiple cycles of electrochemical tests have shown that it has better long-term stability than Pt-based CEs due to its 3D interconnection network.In addition,NiCo2S4 particles with porous 3D structure were prepared by vulcanization of NiCo₂O₄ electrospun nanofibers.The bimetallic sulfide showed an excellent PCE of 7.12%,which was higher than that of dye-sensitized solar cells using NiCo₂O₄?5.24%?and Pt CE?7.05%?.Electrochemical tests of the system showed that this extraordinary performance of NiCo2S4 may be related to improved electrocatalytic capacity and conductivity.It is also possible that the iodine electrolyte has a suitable adsorption energy at the electrode.In addition,long-term cyclic electrochemical tests showed that NiCo2S4 CE has better long-term stability comparable to that of Pt electrodes.