Structural Transformation Of Co-based Electrocatalysts And The Effects Of Fe Incorporation On The Water Oxidation Performance Of CoOOH

With the increase in energy consumption,it is critical to develop clean energy to replace traditional fossil fuels.Hydrogen,a clean and reproducible energy,has arisen much attention by researchers for could be as a substation to traditional energy.Electrocatalysis water splitting is one of the main ways to produce H₂.At present,noble metals such as platinum,ruthenium,and iridium can be used as high-efficiency water electrolysis catalysts.Such noble metal catalysts can greatly reduce the overpotential required for the electrolysis of water,but their high price extremely increases the cost of electrolysis water and limits their industrial production.The electrolysis of water can be divided into two-half reactions which are the hydrogen evolution reaction at cathode and the oxygen evolution reaction at anode.The sluggish kinetics of the 4-electrons anode oxygen evolution reaction determines the efficiency of water electrolysis.Therefore,it is very important to develop oxygen evolution catalysts with low cost,high activity and high stability.In recent years,many studies have shown that the catalytic reaction of water oxidation is a dynamic catalytic process,and the catalyst film will undergo different degrees of structural transformation.In alkaline electrolytes,non-noble metal-based catalysts are structurally transformed into metal oxyhydroxides,and the introduction of trace Fe can significantly improve the oxygen evolution activity of metal oxyhydroxides.The in-depth study of the catalyst structure transformation process and its influence on the performance of oxygen evolution reaction is crucial for the development of high activity oxygen evolution catalysts.Therefore,this thesis has carried out research on the structural transformation of Co-based metal catalysts,purifying the trace Fe,Co,Ni metals in the electrolyte that affect the activity study,and exploring the effect of trace Fe on the catalytic activity and stability of oxyhydroxide oxygen evolution.The studies are presented as below:1.Given the low activity of current electrocatalysts.In this work,a simple in situ self-assembly strategy was designed for the synthesis of catalysts with hierarchical 3D nanostructures（Co Mo O₄/Co₃O₄/NF,Co WO₄/Co₃O₄/NF,and Co₃V₂O₈/Co₃O₄/NF）.The3D array structure and interpenetrating mixed Co species provide a fast channel for charge transfer and mass exchange.Meanwhile,the inner Co₃O₄ nanowires serve as a framework to hold the abundant Co M_xO_y（M=Mo,W,V）,which facilitates the exposure of more OER active sites.The overpotential of Co Mo O₄/Co₃O₄/NF,Co WO₄/Co₃O₄/NF and Co₃V₂O₈/Co₃O₄/NF are 219 m V,259 m V and 268 m V,respectively when the current density is achieved at 10 m A/cm² which are prominently better than the current common Co-based catalysts.Besides,after 1,000 voltammetry cycles,only a slight increase in the overpotential verifies the excellent stability of these catalysts during the alkaline oxygen evolution reaction process.Based on postmortem characterizations,a dissolution–redeposition process is proposed to explain the formation of Co（oxy）hydroxides on the surface of the catalysts.This work plays a guiding role in further understanding the real active component of the catalyst and the relation between the structural stability and catalytic stability of the catalyst.2.It is critical to control Fe impurity concentrations in oxygen-evolution electrocatalysis experiments so that unambiguous assignments of activity and mechanistic details can be made.Currently,the most common purification method for the electrolyte is by using Ni（OH）₂ or Co（OH）₂ as adsorbents to remove Fe in neutral or alkaline electrolytes.However,the electrolyte purified by this method yields residues of Ni or Co which can be redeposited on the working electrode surface during electrocatalysis,complicating further studies on the catalytic activity and stability of electrocatalysts.According to ICP-MS results,it is found that the concentration of Ni/Co introduced in the electrolytes is much higher than the theoretically calculated saturated concentration of metal ions in nature.It is suggested that the residual Ni or Co exists mainly in the form of particles in KOH.Therefore,considering the residual metal impurities in the electrolyte,continuous electrolysis and nanomembrane filtration methods are used in this work to remove Ni or Co species from iron-free alkaline electrolytes.It is found that continuous electrolysis can only remove part of Ni or Co impurities.Using both hydrophilic and hydrophobic nanomembranes to filter the electrolyte can effectively remove metal residues in the electrolyte.Among them,0.1μm hydrophilic polyethersulfone filter provided the best filtration performance,and the concentration of Ni in 1 M KOH is reduced to below the detection limit of approximately 1.0 ppb.3.In an alkaline medium,the real active composition of electrocatalytic oxygen evolution catalysts such as metal chalcogenides and phosphorus compounds are metal hydroxides or oxyhydroxides.In Fe-free conditions,Co³⁺plays a vital role in the oxygen evolution catalytic process for Co³⁺ions are regarded as active sites.The development of Co³⁺-rich catalysts can significantly improve the efficiency of water oxidation.Guided by Pourbaix diagrams,surface Co³⁺-rich catalysts were synthesized by controlling p H value and applied potential.The as-prepared catalyst possesses enhanced electrode-electrolyte contact area and lower diffusion resistance,thus showing lower initial overpotential and high oxygen evolution activity.This low-cost,high-activity and easy to made catalyst is expected to achieve large-scale industrial application.4.The introduction of trace amounts of Fe can significantly improve the activity of Ni Fe and Co Fe（oxy）hydroxides towards oxygen evolution reaction.However,the actual catalytic mechanism of Fe in the catalytic process still needs further study.We find that the adsorption of Fe is controlled by diffusion.Increasing the diffusion rate of Fe in the electrolyte can promote the adsorption of Fe to the catalyst and thus improve the oxygen evolution activity of the catalyst.We identify and compare the surface TOF_Fe and bulk TOF_Feof CoFeO_xH_y films.We find surface Fe has higher intrinsic activity than bulk Fe which suggests the surface Fe likely is the real active site of Co/Fe（oxy）hydroxide.In addition,it is found that the adsorption amount of Fe on the electrode is influenced by the applied potentials,with the adsorption amount of Fe at higher potential significantly lower than that at lower potential.Microbalance measurements show that film appears chemically stable and no catalysts dissolve in 1 M KOH electrolyte over 4 h chronoamperometry test even though the activity decreased by 50%.Besides,X-ray photoelectron spectra and inductively coupled plasma mass spectrometry show that the values of the Fe/Co ratio decreased from 4.0±0.5%to 2.0±0.5%after the stability test which indicates the decayed activity results from Fe leaching.This result is further consistent with the hypothesis that the Fe species introduced during cycling in Fe³⁺-spiked KOH are mostly at surface/edge/defect sites and do not modulate the electrochemical response of the Co.