Influence Of The Electro-catalytic Properties Of Multinary Metal Chalcogenides On The Performance Of Dye-sensitized Solar Cells

Multinary metal chalcogenides（MMC）have been widely used in the fields of energy conversion,electrochemical energy storage and electro-catalysis,due to their excellent electronic conductivity,variable chemical composition,and rich redox active sites.Numerous efforts have been devoted to the designation and modification of MMC materials,towards higher electrochemical activity and stability.MMC materials are a kind of promising candidates to replace expensive Pt electrodes in a dye-sensitized solar cell（DSSC）,in respects of their low cost and high photoelectric conversion efficiency（PCE）.However,the complex preparation process and its poor stability greatly hindered its further development.In addition,the various compositions and categories of MMC materials make it extremely hard to make a precise selection through the classic“trial and error”method,which is costly,inefficient and time-consuming.Theoretical calculations can be an efficient approach to analyze the electrocatalytic process and thus guide the design for efficient MMC materials.In this dissertation,multidimensional Cobalt（Co）based MMC materials were investigated to obtain highly efficient and durable counter electrode（CE）materials for DSSC.The key factors affecting the electro-catalytic efficiency and DSSC performance were further revealed by the theoretical calculation.Several efficient strategies have been involved to achieve a series of high-efficiency,long-term MMC materials,including structural engineering,facet control,element doping and so on.The main contents of this dissertation are as follows:（1）By taking advantages of the facet control and element doping,we rationally proposed a facile and controllable method for growing Cu-doped Co S₂ polyhedron directly on the surface of fluorine-doped tin oxide（FTO）.The influence of composition of mixed solvents,reaction temperature,reaction time and doping concentration on the growth process and electro-catalytic performance of Co S₂ thin film electrodes were investigated.The morphology and particle size of Co S₂ can be effectively controlled by adjusting the amount of Cu dopant.The growth mechanism of the Cu-doped Co S₂ polyhedron,which contains 8{111},12{110}and 6 equivalent{100}planes,were proposed by monitoring the growth process and the Wulff construction method.The DSSCs based on the optimized Cu doped-Co S₂ CEs（2.8 at.%）achieved a high PCE of 7.34%,which is very close to Pt（7.37%）.The state density calculation results show that both the Cu-doped Co S₂ electrode and the pure Co S₂ electrode exhibit metallic nature,while the Cu-doped Co S₂ electrode has stronger conductivity.In addition,we calculated the change in the adsorption energy of the I atoms on the（111）,（100）,and（110）planes before and after Cu doping by Density Functional Theory（DFT）.The results show that Cu doping can effectively regulate the adsorption energy of each crystal plane.Before doping,the adsorption energies of-3.49,-4.25 and 0.68 e V were adjusted to-2.00,-1.52 and-3.13 e V（closer to the-1.26 e V of the conventional Pt electrode）,which facilitated the rapid desorption of I atoms.（2）To improve the electro-catalytic activity and long-term stability of Co-based MMC,Cu was replaced with Fe,which is more active and possess variable valences for redox reactions.The one-dimensional（1D）FeCo₂S₄ hollow nanowire arrays and mirror-like FeCo₂S₄ films were successfully fabricated.The 1D FeCo₂S₄ hollow nanowire arrays showed a desirable electro-catalytic ability for the reduction of I₃^ˉ.However,due to the large contact resistance（R_s）and charge transfer resistance(R_ct),FeCo₂S₄ nanowire CE based DSSC only obtained a PCE of 6.07%.In addition,the mirror-like FeCo₂S₄ electrode exhibited a strong adhesion to the FTO substrate.No obvious change or fragment of FeCo₂S₄ was observed after 50 times of detachment.Electrochemical tests showed that FeCo₂S₄has smaller R_s and R_ct and a higher electro-catalytic performance towards the reduction of I₃^ˉthan that of Fe S and Co S.The PCE of mirror-like FeCo₂S₄ CE based DSSC reached up to 7.35%,exceeding the performance of Pt CE（6.80%）.Moreover,the FeCo₂S₄ thin film exhibited extremely strong cycle stability and thermal stability.After successive CV cycle test for 300 cycles and boiling water treatment for 2 h,its electrochemical performance remains almost unchanged.Furthermore,we employed density functional theory（DFT）calculation to simulate the adsorption of I atoms on the Fe,Co and S sites on the（311）,（422）and（440）planes of FeCo₂S₄.The results showed that the adsorption energy of I atoms on the（311）,（422）and（440）crystal planes were between-1.16 to-2.78e V,-1.77 to-3.47 e V and-1.57 to-2.55 e V,respectively.Almost all planes showed higher adsorption energies than Pt（-1.26 e V）.Especially,the Fe-2 site（-2.78 e V）on the（311）plane and the Co-1（-3.68 e V）and Fe-1（-3.34 e V）sites on the（422）plane exhibited the largest adsorption energies,which are relatively easy to be the catalytic active sites,thereby increasing the electro-catalytic activity of FeCo₂S₄.（3）To optimize the preparation process and increase the active sites of Co-based MMC,a general and facile protocol to directly fabricate one-dimensional（2D）Ni Co₂S₄ nanosheets onto FTO substrate was demonstrated by a rapid cyclic voltammetry electrodeposition method without any post-treatment process.The reaction process and underlying mechanism of the electrodeposition were carefully investigated and revealed.The effects of the number of deposition cycles and film thickness on the electro-catalytic performance were mainly studied.After two cycles of deposition involved,the film thickness of Ni Co₂S₄ counter electrode reached to 650 nm and the corresponding PCE achieved to 7.13%.When the reaction time and the film thickness further increased,the PCE remains almost unchanged.This can be ascribed to the increased R_ct and diffusion resistance（Z_w）,which severely impeded the reduction and diffusion of I₃^ˉ.Compared with binary Ni S and Co S,multinary Ni Co₂S₄ CE was endowed with more active sites and thus a smaller R_ct.The Ni Co₂S₄ based DSSC showed a PCE of 7.44%,which is higher than that of Pt（7.09%）.Meanwhile,Ni Co₂S₄ nanosheets can also be constructed onto flexible Ti electrodes,using the same fabrication approach,obtaining a PCE of 5.28%.（4）Cu_xCo_3-xO₄（x=0,0.3,0.6,0.9 and 1.2）nanowires were constructed on FTO,and then sulphurated to Cu_xCo_3-xS₄.The doping of Cu ions provided more active sites for the catalytic reaction,exhibiting an increased short circuit current from 12.6 to 14.4 m A cm^-1.Surprisingly,Cu_0.9Co_2.1S₄sample was observed to be a stable and free-standing 3D network,which is supposed to favor a fast electron transmission and withstand structural damage during the ion exchange process.In this case,Cu_0.9Co_2.1S₄ CE was endowed with the best electrocatalytic performance,achieving a high PCE of6.95%which is competitive to Pt CE（7.34%）.A one-step hydrothermal preparation of Cu Co₂S₄electrode materials was studied,and the effects of different reaction solvents and sulfur sources on the purity of Cu Co₂S₄ were investigated.Subsequently,a method for one-step hydrothermal growth of Cu Co₂S₄ on the surface of a flexible carbon cloth substrate was proposed.The results showed that when deionized water/glycol（volume ratio 1:3）is used as the solvent,the carbon cloth surface uniform Cu Co₂S₄ single-crystalline nanosheet electrode can be readily prepared.Cu Co₂S₄@CC-36h electrode contacts well with the substrate and thus facilitates a rapid transmission of electrons on the electrode,with a very small R_s of 2.56Ωcm².In addition,numerous reactive sites are supposed to emerge on the vertically aligned Cu Co₂S₄ nanosheets,endowing a small R_ct of 0.18Ωcm².The excellent electrocatalytic ability facilitates a high PCE of 7.68%,which is higher than 7.39%and3.21%of Pt and carbon cloth counter electrodes,respectively.