Coordination Environment Tuning Of Cobalt-based Catalyst For Water Splitting Reaction

Using renewable energy sources such as solar energy to electrocatalytically split water for hydrogen production is key to realizing a green hydrogen economy.However,the slow reaction kinetics and high overpotential of the anodic oxygen evolution reaction（OER）limit the rapid development of water splitting.It is widely believed that OER under alkaline conditions offers advantages for better reaction kinetics.Therefore,the development of alkaline-stable transition metal-based catalysts has garnered significant attention.Altering the electronic structure of the metal active center through doping,defect control,and ligand mediation can enhance the OER activity and stability.However,under anodic polarization conditions,these catalysts often undergo surface reconstruction,leading to irreversible changes in the coordination environment of the metal active center thus reducing activity and stability.Currently,the relationship between OER performance and coordination environment under electrochemical conditions remains unclear,particularly the association between coordination environment,valence state,and activity.Furthermore,the interaction mode of the ligand and the active center during the evolution of the catalyst’s metal active center coordination environment is not well understood,limiting the in-depth understanding of catalytic activity.This dissertation focuses on the regulation of the coordination environment of active metal cobalt（Co）sites and its impact on the anodic oxygen evolution reaction（OER）performance.The study primarily explores the variation patterns of interactions between different valence state Co active centers and ligands.Based on these findings,ligands were utilized to adaptively regulate the local electronic structure of metal ions during the OER process,thereby constructing an effective and stable coordination environment for metal active sites.This dissertation provides a new research paradigm for further designing and understanding the origins of activity and stability of electrocatalysts.The main research contents of this dissertation are as follows:（1）Under electrochemical conditions,the surface coordination environment of Co-based catalysts has been in-situ tuned by introducing 1,10-phenanthroline（phen）into the electrolyte.In contrast to traditional pre-catalyst studies involving bulk doping or surface modification,this study directly utilizes the phen ligand on the electrode/electrolyte interface to regulate the restructuring process and electronic structure of Co OOH catalysts during OER.The introduction of phen significantly inhibits the dissolution of Co OOH catalysts,facilitates the oxidation-reduction of Co metal ions,and promotes the in-situ generation of high-valence metal site Co⁴⁺.This in-situ tuning method of the coordination environment of metal sites enhances the OER performance of Co OOH catalysts and prolongs the operational stability of the catalyst.This research avoids the complication of the electronic structure resulting from the leaching of ligands during traditional catalyst reconfiguration and elucidates the role of inactive ligands at the electrode/electrolyte interface in catalyst reconfiguration,providing a research foundation for further designing methods to regulate the coordination environment of metal centers.（2）Through the investigation of the coordination between Co²⁺and phen in homogenous electrolytes,an alkaline-soluble metal-ligand complex has been prepared.Phen ligands could form complex with Co²⁺in homogeneous solutions and remain soluble in a 1.0 M sodium hydroxide（Na OH）alkaline environment.The study demonstrates the structural evolution of the metal-ligand complex in alkaline environments.This strongly alkaline-soluble metal-ligand complex highlights synergistic metal-ligand coordination ability,which could guide the in-situ deposition process of catalysts under alkaline conditions,laying the foundation for ligand-adaptive regulation of the local coordination environment of multivalent metal ions.（3）By utilizing alkaline-soluble metal-ligand complexes,the catalysts with in-situ adaptive growth under alkaline conditions are achieved.The chapter showcases that the Co（phen）₂（OH）₂ complex exhibits a deposition-dissolution equilibrium relationship with the Co metal valence state changes under applied potential,enabling the in-situ deposition of Co-PH catalysts.During the OER process,the ligand could adaptively tune the local coordination environment of multivalent metal ions.It could promote the in-situ generation and self-healing of Co⁴⁺sites,thereby facilitating the construction of effective and stable coordination environments for metal active centers.（4）By constructing model catalysts with different metal valence states,the interaction between Co sites in different valence states and phen is studied.It is found that the interaction between Co⁴⁺and phen is non-covalent.However,it remains unexplored in the existing literature whether this relatively weak non-covalent interaction influences the electrocatalytic OER performance.Through experimental and density functional theory（DFT）calculations,the non-covalent interaction between phen and Co⁴⁺was investigated,focusing on its influence on the electrocatalytic OER reaction performance.The non-covalent interaction promoted the generation of“polarized”Co-Co active centers,facilitating*OH deprotonation and accelerating the OER kinetics.Moreover,this non-covalent interaction ensures the stability of Co-PH catalyst over 2months（approximately 1600 hours,theoretically infinite stability）with an overpotential of～216 m V at 10 m A cm^-2.Additionally,its mass activity reaches 1.67 A mg^-1 at an overpotential of 0.35 V vs.RHE,surpassing Co OOH by～100 times.The turnover frequency（TOF）value was approximately 140 times higher than that of Co OOH.（5）To further broaden the applicability of ligands in modulating the coordination environment of metal active sites,the Co-bpy（2,2-bipyridine）complex in an alkaline solution is developed and applied to the OER under high current density.Since bpy exhibits a stronger electron-donating ability compared to phen,this alters the characteristics and reactivity of the metal-ligand complex,resulting in bpy existing in the form of Co-N covalent interaction in the Co-BH catalyst.This covalent interaction significantly enhances the OER stability at high current density,enabling the Co-BH catalyst to operate continuously over 500 hours at 200 m A cm^-2,surpassing classical Co Pi and commercial Co₃O₄ catalysts.