Study On The Electronic Structure Regulation And Performance Of Oxygen Evolving Electrocatalysts

The development of advanced and efficient electrocatalytic energy conversion system is of great practical significance for promoting the upgrading of human society’s energy utilization model and reducing the carbon dioxide emissions caused by the consumption of fossil fuels and the related greenhouse effect.Among them,the use of renewable energy to produce hydrogen by water splitting is considered to be the energy technology basis for the sustainable development of human society in the future.However,compared to hydrogen evolution reaction（HER）with two-electron transfer at the cathode,oxygen evolution reaction（OER）at the anode involves four electron transfer and is more complicated.The complex intermediate reaction and the adsorption and desorption of oxygen-containing species lead to the slow kinetics of OER,making the anode overpotential far higher than that of cathode,which becomes the core problem that hinders the large-scale application of hydrogen production by water splitting.In addition,OER is also an anode reaction for electrocatalytic conversion systems such as metal-air batteries,electrolytic reduction of small molecules（such as CO₂ and N₂）and organic electrosynthesis to produce high-value chemicals.Therefore,the development of advanced,efficient and inexpensive OER electrocatalysts has both theoretical and practical significance for breaking the bottleneck of the large-scale application of electrocatalytic energy conversion and material transformation systems.However,most of the high-performance electrocatalysts reported today have problems such as multiple chemical components,complex structures and cumbersome preparation,which not only increase the difficulty of studying the origin of electrocatalyst activity and understanding the catalytic mechanism,but also is not conducive to the practical application of electrocatalysts.Based on the understanding of the type of OER and its mechanism of adsorbate evolution mechanism（AEM）or lattice oxygen-mediated mechanism（LOM）during the OER process,this thesis puts forward strategies such as ligand engineering,defect engineering,and heteroatom doping to regulate the electronic structure of the catalyst to optimize the adsorption Gibbs free energy of reaction intermediates and lower the reaction energy barrier,studying the electrochemical performance of the corresponding electrocatalyst.Firstly,the interlayer ligand engineering ofβ-Ni（OH）₂was proposed to substitute the interlayer hydroxyl ligand by alkoxyl with different chain lengths and configurations,and studied their effects on the interlayer space and electronic structure modulation of electrocatalyst;Secondly,Ar plasma was used to etch the ligand of the fully coordinated Co-based metal organic compound cobalt trioxide（Co Gly）,realizing the ingenious combination of ligand engineering and defect engineering,and significantly enhancing the catalytic activity;Fe²⁺was used as the iron source to in-situ induce intercalation and cation desolvation to prepare vacancy-rich Fe doped Co Me（Co Fe Me）,realizing the synergistic effects of ligand engineering,defect engineering and heteroatom doping in one step;Construction of Ru-O-H bond within Ru O₂（P-Ru O₂）lattice was put forward,significantly improving its activity and stability in 0.5 mol dm^-3 H₂SO₄ and achieving long-term stable operation of solid polymer electrolyzer（SPE）to produce hydrogen.The specific research contents are as follows:（1）Because of its simple chemical composition and clear crystal structure,β-Ni（OH）₂ is an ideal model catalyst for studying OER reaction mechanism and identifying active sites.However,in practical applications,the OER activity ofβ-Ni（OH）₂ is much lower than most nickel-based catalysts due to the low conductivity caused by the wide band gap,strong hydrophobic interface and structural instability.In response to these inherent problems,this chapter proposes a strategy of interlayer ligand engineering.Through a one-step solvothermal reaction,a series of alkoxy-substitutedβ-Ni（OH）₂,namely Ni[（OH）_1-y（L）)_y]₂（L is an alkoxy group,0≤y≤1）are synthesized.Using the ligand effect of alkoxy groups with different carbon chain lengths and configurations,the electronic structure and reaction interface of the catalyst was modulated,the effects of the substitution of different alkoxy groups on the electrochemical performance ofβ-Ni（OH）₂ was systematically studied.It is found thatβ-Ni（OH）₂substituted by ethoxy group（Ni Et）owns the best OER apparent activity and intrinsic activity,its onset over-potential was only 240 m V,the required over-potential to reach current density of 10 m A cm^-2was 320 m V.The electrochemical polarization process of Ni Et was tracked by in-situ X-ray absorption spectroscopy（XAS）,and the changes of the valence state of Ni and the electrochemical behavior of oxygen-containing intermediate species during the polarization process of Ni Et were studied.Before the initial potential,the deprotonation of the Ni Et surface is closely related to the magnitude of the applied polarization potential;At the first stage of deprotonation,the formed Ni³⁺does not show OER activity;As the applied potential further increases,Ni³⁺species gradually transform into Ni^δ+（3<δ≤3.66）,thereby catalyzing the progress of OER.This chapter not only presents a new perspective for the design of OER electrocatalysts by interlayer ligand engineering,but also reveals the transformation process of active species in the reaction process and proposes the corresponding possible electrochemical mechanisms.（2）Utilizing the coordinatively saturated characteristics of Co Gly,Co Gly is used as a model catalyst to study the activity origin of the coordination unsaturation of transition metal atoms to enhance OER performance.Through the ingenious combination of ligand engineering and defect engineering,Ar plasma is used to bombard Co Gly to change the coordination environment between metal atoms and organic ligands and construct abundant Co coordination unsaturation sites.The electrochemical test results show that the anodic polarization overpotential required for Co Gly after plasma treatment（Co Gly-P）to reach current density of10 m A cm^-2 is only 290 m V,which is significantly lower than that of Co Gly（350 m V）,indicating the combination of defect engineering with ligand engineering effectively enhances the OER performance of the electrocatalyst.In short,this chapter not only expands the material range of metal-organic compounds for electrocatalysts,but also directly confirms the significant enhancement effect of coordinated unsaturated site on OER activity.（3）Taking the Co Me in which the hydroxyl group inβ-Co（OH）₂ is partially substituted by methoxy ligand as the research object,the aforementioned ligand engineering,defect engineering and heteroatom engineering are organically integrated,realizing synergistic catalysis of the three strategies towards OER.Making use of the fact that it is difficult for trivalent metal ions to thermodynamic stably exist in a large amount in the Co Me,the heteroatom Fe²⁺is introduced as an iron source during the material synthesis process to achieve in-situ intercalation and cations ex-solution during the synthesis of Fe-doped Co Me electrocatalyst（Co Fe Me）.Through the introduction of this strategy,element incorporation,intercalation and defect engineering can be organically integrated into a one-step solvothermal reaction.Compared with Co Me,the BET specific surface area of Co Fe Me is significantly increased,from 16.5 m² g^-1 to 101.6 m² g^-1.At the same time,with the help of XAS characterization and analysis,a large number of anion and cation vacancies were formed along with the exsolution of cations during the reaction.Combining with the test results of TGA,FTIR,ICP and EA,the exact chemical formula of Co Fe Me is determined as Co₈₀Fe₂₀[（OH）（OCH₃）]（Ac）_x·y H₂O.Due to the existence of a large number of vacancies and the electron withdrawing effect of alkoxy groups,the valence state of Co increased from+2.14（Co Me）to+2.3（Co Fe Me）,and Fe²⁺was almost completely oxidized and existed in a form of+2.94.Higher valence metals are widely considered to be conducive to the formation of high valence OER active species.Attributed to these advantages,the overpotential that drives the OER current density to 10 m A cm^-2 is drastically reduced,from 350 m V（Co Me）to 240 m V（Co Fe Me）.In short,this chapter not only proposes the use of Fe²⁺induction effect to realize the in-situ intercalation and vacancy formation of Co Me,but also provides a new method to realize the three-in-one including element incorporation,intercalation,and defect engineering,providing new ideas for the scale-up preparation and application of electrocatalysts.（4）Aiming at the bottleneck problem of Ru O₂ that is prone to form the high valence state Ru^δ+O_x（δ>4）,which is easily soluble in acid under anodic polarization potential,a strategy of co-doping P-Ru O₂ with protons and electrons to obtain hydrogen doped Ru O₂（H-Ru O₂）is proposed,and H-Ru O₂ with different hydrogen doping concentration is flexibly realized.The Ru-O-H bond constructed in Ru O₂ lattice could enhance the acidic OER activity of P-Ru O₂while inhibiting the rupture of the Ru-O bond.The XPS surface chemical state analysis of H-Ru O₂ with different hydrogen doping concentrations shows that hydrogen doping not only regulates the electronic structure of Ru O₂,thereby decreasing the average valence of Ru and reducing the proportion of lattice oxygen in the material,but also promotes the proportion of hydroxy-oxygen of the electrocatalyst and changes its hydrogen hydrophobicity.The electrochemical test results obtained in the three-electrode system show that 75-H-Ru O₂ owns the best electrochemical performance,and its polarization overpotential is reduced by 70 m V compared with P-Ru O₂,which is only 200 m V(@10 m A cm^-2).Under the chronopotentiometric measurement@10 m A cm^-2,its operating life is nearly 23 h,which is 2.7times that of P-Ru O₂,.75-H-Ru O₂ still shows excellent performance in the SPE test.As 75-H-Ru O₂ not only has much smaller charge transfer resistance,but also shows an improved hydrophility on the three-phase interface,75-H-Ru O₂ can run stably for more than 50 hours in the constant current test result of 0.5 A cm^-2under operating temperature of 80℃,which is close to 3.13 times that of P-Ru O₂ and better than that of the three-electrode system（2.7 times）.In a word,the hydrogen doping strategy proposed in this chapter not only achieves the regulation of the electronic structure of the catalyst,but also achieves the purpose of optimizing the three-phase interface environment of the catalyst.