Preparations And Properties Of Carbon-Based Materials As Pt-Free Counter Electrode For Dye-Sensitized Solar Cells

As the energy demand rises up and environment polution aggravates,the development of sustainable green energy has been an urgent issue.Since the breakthrough made by Prof.M.Gr?tzel's research group in 1991,dye-sensitized solar cells?DSSCs?have attractted considerable industrial and scientific attention owing to their low-cost,facile fabrication,non-pollution and relatively high conversion efficiency.In general,the counter electrode,as one of key componets in DSSCs,plays a critical role in collecting the electron from the external circuit and catalyzing the reduction of I₃^-diffused from the TiO₂ photoanode at the interface of electrolyte and counter electrode.Platinum as a preferred material was usually used as counter electrode material in DSSC for the I₃^-reduction,because of its excellent electrocatalytic performance and high conductivity.However,its cost and the scarcity make it unaccommodated in terms of large-scale manufacture.Moreover,the resistance to corrosion in iodine electrolyte is proved to be worth discussing.Therefore,the development of the new counter electrode materials with low-cost,high catalytic activity and excellent stability has been a crucial issue for the practical application.Based on different types of carbon materials,such as carbon nanotubes and graphene,this thesis has achieved some innovative results in low-cost counter electrode,which is summarized as follows:?1?The composites of hollow CoFe₂O₄ and carbon nanotubes?h-CoFe₂O₄@CNTs?are successfully prepared by using a simple hydrothermal process coupling with the in-situ chemical vapor deposition?CVD?method.Moreover,the addition of conductive polypyrrole nanoparticles into composites could further improve the conductivity and catalytic performance of h-CoFe₂O₄@CNTs,thus yielding the optimum photovoltaic conversion efficiency of 6.55%and obtaining a good long-term electrochemical stability in I₃^-/I^-electrolyte.?2?A novel sandwich-like hierarchical structure composed of RGO and uniform CoS₂octahedrons?CoS₂/RGO?is successfully prepared by a simple one-step solvothermal process.Due to the unique structure as well as the presence of conductive RGO and electrocatalytical CoS₂,CoS₂/RGO composite shows excellent electrocatalytic performance for the reduction of triiodide,yielding 7.69%of photoelectric conversion efficiency and displaying outstanding electrochemical stability in I₃^-/I^-redox electrolyte.?3?A novel sandwich-like CoS_1.097 nanoflakes/RGO hierarchical materials is successfully prepared by a simple one-step solvothermal process.Due to the synergistic effect of RGO and CoS_1.097 as well as unique sandwich-like structure,the DSSC assembled with this CoS_1.097/RGO CE exhibits excellent photoelectric conversion efficiency?6.83%?.In addition,CoS_1.097/RGO CE also exhibits a good long-term electrochemical stability.?4?A ultrathin-walled Co₉S₈ nanotube/reduced graphene oxide?Co₉S₈/RGO?electrocatalyst is successfully prepared based on a simple hydrothermal process coupling with an ion exchange process.The DSSC with Co₉S₈/RGO CE yields the optimum photovoltaic conversion efficiency of 7.58%.In addition,this DSSC with Co₉S₈/RGO electrode exhibits a good repeatability and long-term electrochemical stability.?5?The Co_0.85Se nanotubes/RGO nanocomposite is synthesized through a facile two-step method.Fristly,a novel Co?CO₃?_0.35Cl_0.20?OH?_1.10 nanorods/graphene precursor is prepared by a simple hydrothermal method.Then,Co_0.85Se nanotubes/RGO composite is prepared by in situ selenization of precursor in NaHSe solution.Electrochemical analyses show that Co_0.85Se nanotubes/RGO shows excellent electrocatalytic performance for the reduction of triiodide.The DSSC fabricated with the Co_0.85Se/RGO electrode presents a high photovoltaic conversion efficiency of 7.81%,exceeding the cell based on a Pt electrode?7.55%?.Moreover,a considerable electrochemical stability is also achieved,indicating a a great potential to replace Pt in DSSC.?6?g-C₃N₄ quantum dots decorated three-dimension graphene as an efficient metal-free electrocatalyst for the reduction of triiodide is successfully prepared by a facile solvothermal process.Because of the additional photocatalytic capability provided by the visible light absorption of g-C₃N₄ QDs,the g-C₃N₄ QDs@3DG exhibits improved electrocatalytic performance for the rduction of triiodide,yielding a high photoelectric conversion efficiency of 7.46%,which is comparable to that of the cell with the conventional noble Pt?7.59%?.Moreover,a considerable electrochemical stability is also achieved in I₃^-/I^-electrolyte.This work will encourage new developments in metal-free counter electrode materials.