Microstructural Regulation Of Cobalt-based Spinel Oxides Toward Electrocatalytic Oxygen Evolution Reaction

Electrolysis of water for hydrogen production is key to solving environmental pollution issues and developing clean,sustainable energy resources.The core challenge lies in developing efficient and stable electrocatalysts.The oxygen evolution reaction（OER）,which occurs at the anode during water electrolysis,involves a four-electron transfer process,thereby making the sluggish reaction kinetics.Spinel-type oxides,with their rich coordination structures and highly tunable component distributions,can catalyze OER efficiently.At the same time,compared to precious metal oxides,they have a significant cost advantage,making them one of the most promising candidate materials for large-scale electrolytic hydrogen production.However,due to limitations such as an insufficient number of active sites on the surface of spinel oxides,low intrinsic electrical conductivity,and weak metal-oxygen bond strength,there is still room for improvement in their catalytic activity.The local microstructure of the catalyst can significantly affect its geometric configuration and electronic structure,thereby impacting catalytic activity,selectivity,and stability.Cobalt,as a transition metal element with a rich 3d electron configuration,has advantages in microstructure regulation.Therefore,by adjusting the composition,spin state,and phase structure of cobalt-based spinel oxides,it is possible to achieve the purpose of controlling the microstructure and electronic structure of cobalt,optimizing its OER performance.Building upon the previous work on cobalt-based spinel oxides for electrocatalytic applications and a profound understanding of the spinel structure,this thesis aims to design and synthesize efficient and high-quality OER electrocatalysts.Focusing on several typical cobalt-based spinel materials,we embarked from the perspectives of synthetic chemistry and compositional regulation to construct a series of cobalt-based spinels.The structure-activity relationship between surface element concentration distribution,bulk phase compositional adjustment,phase transition,and spin state transition with the OER performance of the catalysts were investigated.The goal was to provide insights into enhancing the electrocatalytic performance of materials through reasonable adjustments of their composition and electronic structure.The main research findings of this thesis are as follows:1.Research on surface engineering of Mg Co₂O₄ spinel to enhance electrocatalytic OER performance:For the first time,a series of Mg Co₂O₄ spinels characterized by enrichment of Mg on the surface were synthesized through a two-step method involving solvothermal synthesis followed by high-temperature calcination and then applied to electrocatalytic OER reactions.Characterization of the samples treated at various temperatures showed that MCO-500（where 500 refers to the calcination temperature of 500°C）with an optimal surface Co/Mg ratio distribution（Co/Mg equals to 0.85）and the maximum Co³⁺/Co²⁺ratio(Co³⁺/Co²⁺equals to 1.735)required only 283 m V of overpotential to achieve a current density of 10 m A cm^-2,along with a Tafel slope of 66m V dec^-1,outperforming most of the reported cobalt-based spinel oxides.This study indicates that during high-temperature calcination,Mg²⁺can migrate from the bulk to the surface of the material,thus optimizing the surface composition of the catalyst.It also proves that the strategy of improving OER performance through surface engineering in Mg Co₂O₄ spinel is effective and changes the stereotype that Mg²⁺is detrimental to OER performance.2.Study of Co spin state regulation in Zn Co₂O₄ spinel and its OER performance:Cubic block-shaped Zn Co₂O₄ spinel with a nano-particle-modified surface was constructed through a two-step method.In this spinel,Co exists in the form of Co³⁺,which eliminates the effect of the valence state change of Co on its spin state.The heat treatment process and temperature ramping procedures are crucial for tuning the spin state of Co.Designing different calcination temperatures can cause lattice distortion in the spinel,thus achieving the spin state transition of cobalt ions.The results show that in ZCO-300,with 64.4%of Co in a high-spin state,has the highest high spin state proportion among all samples,and it exhibits the best OER performance.This is because increasing the spin state of cobalt cations in Zn Co₂O₄ provides more charge transfer channels for the catalyst during the OER,optimizing the adsorption process with the reaction intermediates and accelerating reaction kinetics.This work innovatively links temperature strategy with lattice distortion and spin states,providing ideas for designing catalysts with spin state transition effects.3.Enhancing electrocatalytic OER performance by expanding electronic channels in Zn_1-xMg_xCo₂O₄ spinel via Zn-Mg co-occupation:For the first time,different proportions of Mg²⁺doping in Zn Co₂O₄ spinel were sythesized by controlling the feed ratio of Mg（NO₃）₂·6 H₂O and Zn（NO₃）₂·6 H₂O during the solvothermal process and altering the heat treatment temperature.By comparing catalysts of Mg Co₂O₄,Zn Co₂O₄,Co₃O₄,and Zn_1-xMg_xCo₂O₄ with different morphologies and compositions,it was found that the Zn_0.8Mg_0.2Co₂O₄ catalyst had the lowest overpotential(258 m V at 10 m A cm^-2)and small Tafel slope(64 m V dec^-1),significantly outperforming most of the other reported cobalt-based spinel oxides in OER performance.FT-IR,Raman,XPS,XAS,and DFT theoretical calculations indicate that the introduction of Mg²⁺into the Zn Co₂O₄ lattice can cause lattice distortion due to the stress produced in the catalyst body and also change the coordination environment of Co³⁺at the octahedral sites and modulate the Co-O bond.This expands the transmission channels for electrons within the catalyst during the OER process,enabling more active sites to be utilized.Further research shows that the improvement in catalyst performance caused by this doping effect is generally applicable to other compositions of catalysts.This work,by constructing Zn/Mg co-occupation at the A-site of the spinel and thereby regulating the electronic structure and coordination state of B-site Co ions,can optimize the OER performance of single-component spinels（Mg Co₂O₄ and Zn Co₂O₄）.It provides ideas for designing multi-component spinel oxide catalysts based on doping/substitution strategies.4.Study of the structure-activity relationship between phase transition engineering of cobalt-based spinels and their OER performance:Building on previous work,this study investigated the phase transition behavior of Mg Co₂O₄ and Zn Co₂O₄ spinels at high temperatures and explored the relationship between phase transitions and changes in electrocatalytic OER performance.The results showed that although both spinels can undergo phase transitions at elevated heat treatment temperatures,the degree of transformation was different for each.XRD,XPS,and XAS analysis indicated that Mg Co₂O₄ could completely transition to a rock salt phase Mg_xCo_1-xO solid solution at900°C,which is the first such report in the literature.At the same temperature,Zn Co₂O₄spinel transitioned to a composite structure material of wurtzite and spinel phase,Zn O/(Zn_xCo_1-x)₃O₄.Interestingly,compared to the spinel materials before phase transition,both materials transformed by phase transition exhibited improved OER performance.In the case of the Mg_xCo_1-xO solid solution,the change in the surface elemental composition resulted in the exposure of more Co²⁺as active sites,and the electronic structure was also optimized,leading to faster electron transport kinetics.For the Zn O/(Zn_xCo_1-x)₃O₄ composite material,a heterojunction was constructed between Zn O and(Zn_xCo_1-x)₃O₄,forming a stable catalytic reaction interface that facilitated electron transport.Additionally,based on the excellent semiconductor properties of Zn O,abundant charge transfer channels were established both within the Zn O bulk and at the Zn O/(Zn_xCo_1-x)₃O₄ phase interface,accelerating the kinetics of the catalytic reaction.In summary,this work paves the way for research that seeks to regulate the electronic structure of catalysts and enhance electrocatalytic performance through phase transition behavior.