Study On The Mechanism And Law Of Pyrochlore RU-O Octahedral Structure Regulation For Acidic Oxygen Evolution Reaction

Recently,proton exchange membrane electrolyzers（PEMWEs）electrolyze water to produce hydrogen,owing the advantages of environmental protection,high purity of hydrogen production,fast start-up response and strong power adaptability,which could couple intermittent renewable energy sources to achieve carbon neutrality or even negative carbon,seen as one of the best paths to the"hydrogen economy".However,the kinetic process of anodic oxygen evolution reaction（OER）is slow under acidic conditions,and the catalyst is prone to corrosion under strong acid and high potential conditions.Therefore,the development of acidic OER catalysts has become one of the most challenging difficulties and hotspots in water electrolysis technology.At present,researches at home and abroad mainly focus on noble metal iridium and ruthenium-based oxides,especially ruthenium-based oxides,which have higher catalytic activity,large reserves and low cost,but unsatisfactory stability.Focusing on ruthenium-based pyrochlore oxide（A₂Ru₂O₇）,this paper analyzed the structure-effect relationship between the structure and catalytic performance of Ru-O octahedron from the perspective of the effect of inactive A-site atom regulation on the active site,and studied the internal mechanism of its structure distortion and compressive stress on the regulation of catalytic activity and stability,respectively.On this basis,the influence of lanthanide elements（A-site）was systematically studied,and the electronegativity and radius of A-site were proposed as the descriptors for the activity and stability of Ruthenium-based pyrochlore oxides,respectively.The main research contents are as follows:（1）Great challenges still remain on how to balance the activity and stability of Ru-based catalysts for acidic oxygen evolution reaction.Stress engineering has been gradually developed for Ru alloy by atoms doping,but difficult for oxides due to the inevitable Ru-O_x lattice distortion or defects.Herein,the Y_2-xM_xRu₂O₇(M=Co³⁺or Fe³⁺)catalysts are well-designed because the replacing Y³⁺by trivalent cations do not result in the change of Ru oxidation state or oxygen vacancy.As the Co³⁺substituent,the geometric structure distortion of YO₈ gradually alleviate the distortion of the Ru O₆octahedron,and decreases the Ru-O bond length（proportion:0～2.3%）to produce compressive strain on Ru O₆ octahedra.Interestingly,the Co³⁺and Fe³⁺with almost the same ionic radius causes the same octahedra distortion.Combined with theoretical calculations,thus,it is demonstrated that the exclusive compressive-stress on Ru O₆enlarged the gap between the 4d band center and the Fermi level（E_f）of Ru,allowing optimized free energy of oxygen species(?G_O*-?G _OOH*)for enhanced catalytic activity.In addition,it also reinforced the Ru-O bond strength for better resistance to dissolution.This work provides a novel approach for introducing lattice stress into metal oxides to study the relationship between the lattice structure of active site and the activity-stability.（2）Exploring the relationship between the A-site cation and the structure of the active site（Ru）is highly desirable toward for designing efficient electrocatalysts.Herein,we rationally manipulate the A-site atom substitution in Y₂Ru₂O₇（YRO）by Ho³⁺,which has the identical ionic radius with Y³⁺,but higher electronegativity due to the 4f electron effect.It demonstrated that the higher electronegativity could cause the Ru-O-Ru bond angle enlarged and the Ru-O bond length reduced,mitigating the Ru O₆ octahedral distortion in Ho₂Ru₂O₇（HRO）for enhancing the intrinsic OER activity.Compared with other pyrochlore oxides,HRO displayed an ultralow overpotential of 215 m V@10m A/cm~2 with lower Ru content and higher mass activities,showing long-term（>60 h）stability in acid media.Density functional theory（DFT）calculation revealed that the higher electronegativity of Ho could strengthen the Ru-O covalency,thereby optimizing the free energy of oxygen species(?G_OOH*-?G_O2)for better catalytic activity.In addition,the higher electronegativity could reduce the oxygen vacancies and improve the formation energy of oxygen vacancies for better resistance to the Ru dissolution.This work reveals the inherent relationship of the A-site atom electronegativity,the lattice structure of the active site,and the activity-stability of the catalysts.（3）The inactive A-site atoms of metal oxides have a certain influence on the catalytic performance.It is of great significance to explore the properties and influence rules of the A-site atoms for the reasonable design of catalysts with high stability and high activity.In this paper,a series of Ru-based lanthanide pyrochlore oxides RE₂Ru₂O₇（RE=Sm,Eu,Gd,Ho,and so on）were prepared by electrospinning.The influence of lanthanide atomic electronegativity and ionic radius on catalytic activity and stability was systematically studied,and the volcanic relationships between catalytic activity and A-site ionic electronegativity（AIE）,stability and A-site ion radius（AIR）were successfully constructed.On the basis of this guidance,activity and stability of the catalysts were improved by adjusting the electronegativity and radius of the A-site ions.The(Sm_0.5Ho_0.5)₂Ru₂O₇ sample has both optimal AIE and AIR values,showing the best acidic OER activity and stability.Therefore,the atomic electronegativity and radius of the A-site are proposed as descriptors for the activity and stability of Ru-based pyrochlore oxides,respectively,which provides a new theoretical support for the design of Ru-based oxide acidic OER catalysts.