Crystal Structure Design And Active Sites Modulation Of Layered Iridates For Oxygen Evolution Reaction

In light of the high cost associated with anode catalysts（i.e.,iridium oxide）in proton exchange membrane water electrolysis,researchers are dedicated to exploring cost-reducing and efficiency-enhancing solutions.Over the past decade,iridate electrocatalysts for oxygen evolution reaction（OER）have attracted considerable attention due to their structural diversity and high activity.However,when these materials catalyze OER under acidic conditions,they often encounter significant challenges such as metal ion dissolution and surface amorphization,leading to notable structural stability issues and making it difficult to identify the actual active sites.Therefore,designing and synthesizing iridate OER electrocatalysts that possess both high activity and structural stability is of great significance.This thesis utilizes the structural directing effects of alkali and alkaline earth metals to precisely engineer a series of structurally stable layered iridate electrocatalysts,thereby stabilizing high-activity iridium sites,overcoming the activity-stability inverse relationship in acidic OER,and offering new insights for the design of efficient and stable OER electrocatalysts.The main research themes of this thesis include:1.A honeycomb-layered Sr Ir₂O₆ iridate OER electrocatalyst was designed and synthesized.Utilizing the structural directing role of Sr²⁺,the Sr Ir₂O₆ iridate features a stacking pattern where edge-sharing Ir O₆ octahedra honeycomb layers alternate with Sr atom layers.Both experimental and theoretical calculations demonstrate that the edges of the honeycomb-shaped layers host high-activity sites,displaying apparent and intrinsic activities higher than those of iridium oxide,with the iridium mass activity exceeding that of iridium oxide by more than tenfold.Moreover,owing to a more stable edge-sharing connection mode between Ir O₆ octahedra and stronger Sr-O interactions,the leaching of cations(Sr²⁺and Irⁿ⁺)in Sr Ir₂O₆ during the acidic OER process is significantly lower compared to other reported iridate catalysts（e.g.,Sr Ir O₃,Sr₂Ir O₄）,with iridium leaching even lower than that of iridium oxide,thus maintaining an excellent crystal structure post-catalysis and exhibiting outstanding structural stability.Sr Ir₂O₆ effectively addresses the common issue of poor structural stability faced by iridate OER electrocatalysts,making it an ideal model for understanding the relationship between crystal structure,catalytic activity,and structural stability.2.To further elucidate the synergistic optimization mechanism of activity and stability in layered iridate electrocatalysts,Na₂Ir O₃ layered iridate（Na-213）underwent protonation and chemical exfoliation treatments,yielding two-dimensional ultrathin nanosheets e-H-Na-213.Within e-H-Na-213,the Ir O₆ octahedra maintain an edge-sharing connection mode and a honeycomb-layer structure,with intrinsic Ir vacancies at the honeycomb centers.Experimental and theoretical calculations have proven that these intrinsic Ir vacancies contribute to the formation of a unique local environment around the edge Ir sites,effectively optimizing the oxygen adsorption properties of these sites,thereby enhancing their OER catalytic activity.Furthermore,thanks to the high dispersibility of the ultrathin nanosheets,e-H-Na-213 can form a uniform catalytic layer on the surface of the current collector at an ultra-low loading(0.13 mg/cm^-2),achieving an iridium mass activity 16.5 times greater than that of iridium oxide.In addition,e-H-Na-213 exhibits excellent catalytic stability,maintaining catalytic activity for over 1300 hours.Regarding structural stability,the inherently adsorbed hydroxyl groups at the edge Ir sites induce their participation in a non-traditional adsorption evolution mechanism,allowing the edge Ir sites to exhibit high activity while also being stabilized within the surface structure,with the post-catalysis crystal structure remaining intact.This work deepens the understanding of the surface structure and local environment of iridium active sites,providing a solution to the common issue of structural stability faced by traditional acidic OER electrocatalysts.3.To maximize the structural directing roles of alkali and alkaline earth metals at both the crystallographic and electronic levels,this work introduces a dual alkali metal regulation strategy,resulting in the synthesis of K_0.5Na_0.2Ir_0.8O₂ layered iridate（KNIO）with disordered intrinsic iridium vacancies.The K⁺ions,positioned in the middle of the layers,serve to increase the interlayer distance and enhance the exfoliation efficiency of the layered material.The disorderly introduced Na⁺ions construct defect-state iridium oxide layers.The protonated,exfoliated iridium oxide nanosheet electrocatalyst（e-H-KNIO）exhibits various defect structures.The introduction of these defect structures not only further optimizes the high activity of the edge Ir sites but also activates the inherently inert in-plane sites to some extent.These endow e-H-KNIO with higher catalytic activity,achieving a current density 15 times that of Ir O₂ at 1.55V versus the reversible hydrogen electrode.Furthermore,e-H-KNIO demonstrates exceptional stability,maintaining catalytic activity for over 600 hours.The dual alkali metal regulation strategy not only improves the exfoliation efficiency and thickness controllability of layered iridate but also provides an effective approach for designing efficient and stable electrocatalysts,promising to advance the application of two-dimensional layered iridates in catalysis.