Preparation And Catalytic Mechanism Of (Ta,Co)-(N,C) Counter Electrode Catalyst For Dye-Sensitized Solar Cells

Developing dye-sensitized solar cells（DSSC）is benefit in both envioroment and energy.Counter electrode（CE）is an important component catalyzing redox couples in DSSC.Precious metal Pt is always the first choice as a counter electrode catalyst;however,high price and limited reserves of Pt have severely restricted large-scale commercialized application of DSSC.Therefore,it will be very meaningful to develop low-cost and high-efficiency new-type non-Pt CE.So far,no study has reported on（Ta,Co）-（N,C）as CE.Ta₄C₃was prepared by the carbothermic process using cotton as the template in this study,the catalytic properties,photovoltaic performance and electrochemical stability of Ta₄C₃ CE were investigated,d-band PDOSs of metal atoms on the surfaces of Ta₄C₃and Pt,P-band PDOS of C atoms on the Ta₄C₃surface and d-band centers of the surfaces of Ta₄C₃ and Pt were calculated.（Ta,Co）-N-C,a kind of binary transition metals（Ta,Co）and N co-doped carbon（N-C）,was fibricated by ZIF pyrolysis combined with ion-exchange,EXAFS was utilized to detect newly generated Ta-N bond in（Ta,Co）-N-C,catalytic properties,photovoltaic performances and electrochemical stabilities of（Ta,Co）-N-C and Co-N-C CEs were investigated,and fuether research on the synergistic effect between binary transition metals was carried out,TDOSs of coordination structures of Co N₄,Ta N₆ and Co N₄-Ta N₆,d-band PDOSs and surface centers of Co and Ta in Co N₄,Ta N₆ and Co N₄-Ta N₆ were calculated,adsorption energies of I₃^-on three coordination structures of Co N₄,Ta N₆ and Co N₄-Ta N₆were calculated.Ta₂Co O₆ and Ta₂Co₄O₉ were prepared by the one-pot method,and Ta₂Co O₆/BC and Ta₂Co₄O₉/BC were prepared by the recombination of bio-based carbon（BC）with them.The catalytic properties,photovoltaic performances and electrochemical stabilities of Ta₂Co O₆/BC and Ta₂Co₄O₉/BC were investigated,Mulliken charge of every element and d-band PDOS of metals and p-band PDOS of O in Ta₂Co O₆were calculated.The main achievements were as follows:（1）J_red,E_pp,R_s and R_ctof Ta₄C₃CE were-2.28 m A cm^-2、500 m V、19.35Ωcm²、6.60Ωcm²,respectively,while those of Pt CE were-3.72 m A cm^-2、250 m V、18.67Ωcm²、0.83Ωcm²,respectively,log J₀and log J_limof Ta₄C₃CE were lower than those of Pt CE,PCE of DSSC based on Ta₄C₃ was 6.37%and that on Pt was 7.19%,electrochemical stability of Ta₄C₃ was good.The calculations based on DFT indicated that transitional metallic carbide Ta₄C₃had an electron structure similar to Pt.Covalent bond formed between C atoms inserting into metallic lattic and metallic atoms,electrons on Ta-Ta metallic bond were distributed onto Ta-C bond,and local electrons on Ta-C bond pulled d-band center of Ta₄C₃to deep energy level,and thus the catalytic mechanism of Ta₄C₃ CE was explained from physical perspective.（2）The electrochemical properties of（Ta,Co）-N-C CE were comparable to those of Pt CE:J_red(-2.84 vs.-3.72 m A cm^-2)、E_pp（326 vs.250 m V）、R_s（16.87 vs.18.67Ωcm²）、R_ct（1.64 vs.0.83Ωcm²）、log J₀ and log J_lim were almost the same,the PCE of the DSSC based on（Ta,Co）-N-C far exceeded that on Pt:7.96%vs.7.19%,and the electrochemical stability of（Ta,Co）-N-C CE was superior to that of Pt CE.The excerllent catalytic performance of（Ta,Co）-N-C derived from N-coordinated meatallic activity sites,synergistic effect of binary transition metals,low resistance/high conductivity of N-C,along with the promoted catalytic activity of porous structure.（Ta,Co）-N-C was an ideal alternative to Pt.The calculation based on DFT manifested strong catalytic activities of metallic active centers in three coordination structures of Co N₄,Ta N₆ and Co N₄-Ta N₆ and revealed co-adsorptive characteristics of I₃^-on Co N₄-Ta N₆,and thus,synergistic effect between binary transition metals was expounded from principal level;carbon matrix and coordination structure improved the stability of（Ta,Co）-N-C.（3）PCEs of DSSCs based on Ta₂Co O₆/BC and Ta₂Co₄O₉/BC are higher than those of Ta₂Co O₆ and Ta₂Co₄O₉,from which the synergistic effect of the hybrid material is reflected.The calculation based on DFT declared that the electron transfer between Co²⁺and BC increased catalytic activity of BC and dioxides in complex systems,which enhanced the catalytic activity of the complex systems;the partly sp3 hybrided BC offered extraordinary electrochemical stability to Ta₂Co₄O₉/BC.The compulated Mulliken charge parameters as well as the d-band PDOS of metals and p-band PDOSof O expounded the catalytaic mechanism of Ta₂Co O₆/BC and Ta₂Co₄O₉/BC CEs further.