Theory-assisted Active Site Engineering And Mechanism Analysis In Iridium-based Oxygen Evolution Catalysts

The oxygen evolution reaction（OER）is an anodic half-reaction,participating in many energy conversion reactions such as water splitting,and CO₂ reduction.However,OER is a sluggish kinetics reaction and extremely limits the overall reaction efficiency,requiring highly active catalysts.IrO₂ is the representive OER catalyst operating in acidic condition.However,iridium is one of the scarcest elements in the Earth’s crust.The OER performance of IrO₂ is still unsatisfactory.Guided by theoretical calculations,we systematically adjusted the crystal phase and composition of the iridium-based catalysts and studied the relationship between the OER performance and the electronic structure of the catalytic sites.Our results provide a guideline for design and optimization of the catalyst.The main research contents of this paper are as follows:1.A series of SrTi（Ir）O₃ solid solution models（STIO）were constructed to investgate the effects of components,concentration and other factors on the OER activity of catalytic sites.It is found that Ir atom reduces the e_g-filling of Ti site.In STIO with Ir content from 25%to 75%,iridium atoms activate the intrinsically inert Ti sites,enhance the adsorption of oxygen-containing intermediates on Ti sites.As a result,non-noble metal active Ti sites shows comparable activity with IrO₂.In addition,surface Ir atoms have a significant effect on the activation of the surface Ti sites,while the inner Ir atoms in STIO show a negligible effect.Based on the above findings,a novel core/shell catalyst model（i.e.,SrTiO₃ covered with a single layer of SrTi（Ir）O₃ solid solution）was proposed to achieve high oxygen evolution activity at very low Ir content.2.A series of TiO₂-IrO₂ solid solution models are contructed.The results show that the surface Ir sites in anatase Ti（Ir）O₂ exhibit extremely high activity compared with pure rutile IrO₂,while surface Ir sites on the rutile Ti（Ir）O₂ have a generally low activity.The high active anatase TiO₂-IrO₂ solid solution can be produced in situ on the perovskite SrTiO₃-SrIrO₃ surface during the acidic OER process as surface Sratoms can be easily etched.The bimetallic synergistic effect between Ti and Ir atom is demonstrated to weaken the adsorption strength of oxygen-containing-intermediates on Ir site.Inspired by the theoretical results,SrTiO₃-SrIrO₃ with hollow nanotube structure is successfully synthesized by electrospinning method.Optimized by crystal phase,composition,and morphology,the obtained low-iridium catalyst exhibit ten times higher OER activity yet 56 wt%less iridium compared with IrO₂.3.The layered H₄IrO₄ model containing only corner-shared IrO₆ octahedra is constructed.Both of the corner-shared IrO₆ octahedra and layered structure are crucial for weakening the adsorption of the surface Ir site.As a result,the iridium site in H₄IrO₄shows a higher activity than that in IrO₂.Based on above result,a special structure,layered Ruddlesden-Popper perovskite Sr₂IrO₄ was chosen.After protonation,intercalation and exfoliation,the H₄IrO₄ nanosheets are obtained.H₄IrO₄ nanosheets have a fully protonated layered perovskite structure,forming stable colloid when dispersing in water.Experimental results confirmed the unique advantages of the layered H₄IrO₄ nanosheets to achieve high catalytic activity and structural stability under ultra-low iridium loading conditions.4.The open-framework SrIrO₃ electrocatalyst is reported.This unusual catalyst reconstrusted into ultra-small,surface-hydroxylated,（200）plane oriented rutile IrO₂nanocatalyst during OER process.Theoretical results showed that compared with the most thermodynamically stable（110）surface,the（200）surface of rutile IrO₂ has more Ir active sites with higher intrinsic activity.Benefiting from the large density of highly active Ir sites on the unusual（200）facet,coupled with ultrasmall nanocrystallite size,the catalyst achieves excellent activity and retains the activity for more than 1000 h.