Theoretical And Experimental Research On The Electrocatalytic Performance Of Pyrochlore Structure

Due to the keyapplications in energy conversion and energy storage,oxygen evolution reaction（OER）has been the focus of research.Much effort has been devoted to finding a suitable catalyst to trigger the reaction.Recently,some reports have discovered that the pyrochlore structural materials showed excellent OER stability in acid environment.Since then,more attention has been paid to pyrochlore structure.Therefore,this paper combines theoretical and experimental studies to deeply investigate the catalytic mechanism of pyrochlore structure and the influence of doping on its properties.The electronic structural and electronic properties of pyrochlore Y₂Ru₂O_7-δand Y_2-xM_xRu₂O_7-δ（M=Mg,Ca,Sr,Ba,Zn,Cd and Hg）were investigated using first-principle calculation based on GGA+U and HSE06 hybrid functional.The schematic energy level diagram of Y₂Ru₂O_7-δwas drawn,and its band gap was found to locate inπ*antibonding orbital,dominated by Ru-4d electrons.[Ru O6]octahedrons in Y₂Ru₂O_7-δundergo a compressing trigonal distortion,with a local symmetry group of D_3d,which is quite different from Ru O₂ and other perovskite ruthenium oxide.When Y is substituted by divalent cation M（M_Y）,O vacancies can achieve low formation energy and introduce the M_Y-V_O and 2M_Y-V_O complex.Further calculation showed that M_Y-V_O complex in Y₂Ru₂O_7-δcan induce an impurity energy level into forbidden band,and narrow the band gap.As a result,high electrical conductivity of Y_2-xM_xRu₂O_7-δis expected,and M_Y-V_O complex may be the origin of electrocatalytic activity in Y_2-xM_xRu₂O_7-δ.Based on thefirst principle calculation result,pyrochlore structure Y_2-xMg_xRu₂O_7-δ（x=0.05,0.1,0.15）catalysts were prepared by sol-gel method and the effect of A-site doping on the performance of pyrochlore structure was studied.The X-ray photoelectron spectroscopy results showed that part of Ru⁴⁺was oxidized to Ru⁵⁺,accompanied by electron transfer to surface,which facilitates the reaction.Combined with thermogravimetric analysis,it was found that the Y_1.9Mg_0.1Ru₂O_7-δcontains the largest oxygen vacancy concentration.Meanwhile,the electrochemical results also indicated that the electrocatalytic activity of Y_1.9Mg_0.1Ru₂O_7-δwas higher than other prepared catalysts.Its overpotential is only 265 m V at 10 m A/cm²and Tafel slope is only 45 m V·dec^-1.In addition,first principle calculation results implied that the substitution atom Mg_Y formed a complex with oxygen vacancies,which favored the formation of oxygen vacancies.It also narrowed the band gap,and lowered the charge transfer energy,which boosted the electrocatalytic activity.In order to investigate the effect of B site doping on the catalytic performance of pyrochlore structure,Y₂Ru_1.9Mn_0.1O_7-δand Y₂Ru_1.9Fe_0.1O_7-δwere studied.On the basis of qualitative and quantitative analysis of oxygen vacancies by means of X-ray photoelectron spectroscopy and thermogravimetric,it was found that a mass of oxygen vacancies was introduced in the pyrochlore structure due to the dopping of Mn and Fe,which also could serve as active sites to promote catalytic performance.Meanwhile,X-ray photoelectron spectroscopy results suggested that Ru⁴⁺and Ru⁵⁺coexisted in structure.Polarization curves indicated that the overpotential of Y₂Ru_1.9Mn_0.1O_7-δand Y₂Ru_1.9Fe_0.1O_7-δat 10 m A/cm²was 256 and 273 m V,and Tafel slope was52 and 55 m V·Dec^-1,respectively.By cyclic voltammetry and chronopotential analysis,it was found that the stability of Y₂Ru_1.9Mn_0.1O_7-δwas poorest,while Y₂Ru_1.9Fe_0.1O_7-δshowed excellent stability.The first-principles calculation was conducted to further investigate the activity and stability of Y₂Ru_1.9Mn_0.1O_7-δand Y₂Ru_1.9Fe_0.1O_7-δ.The results implied that the lower the oxygen vacancy formation energy,the better the catalytic activity.In addition,with the increased dissolution energy and Ru-Ocovalence,the stability of the Y₂Ru_1.9Fe_0.1O_7-δcatalyst increased.The calculation successfully elucidated the high activity Mn/Fe doped YRO and stability origin in Y₂Ru_1.9Mn_0.1O_7-δ.