Enhancing The Electro-catalytic Activity And Durability Of Perovskite-based Electrode Materials By Controlling Surface Segregation Behavior Of Metal Cations

Solid oxide cells(SOCs)are promising devices for electrochemical energy conversion due to their wide advantages such as high energy conversion efficiency,excellent fuel flexibility and low pollutants.However,lack of efficient and stable eletrode materials inhibites the commercialization of SOCs.Perovskite-based oxides are widely used as electrode materials for SOCs due to their stable structures and tunable compositions.Under the high working temperature(>600?),nevertheless,cation segregation is widely observed on perovskite oxide surface,which critically impacts the physical and chemical surface properties of catalysts,and thus effects the performance of electrodes.Previous studies have shown that controlling the surface segregation behavior can effectively improve the electrocatalytic activity and stability of perovskite oxide electrodes,but the underlying mechanism is not clear.The objective of this thesis is to reveal the mechanism how cation segregation impacts the electrocatalytic activity and stability of perovskite oxides,and provide guideline for materials synthesis for highly efficient and stable electrodes.In this thesis,perovskite oxide thin films with controllable morphology,composition and crystal structure were fabricated by pulsed laser deposition(PLD)as the model system.The effect of cation segregation on the electronic structure,chemical environment and other surface properties of perovskite oxides were systematically studied by using advanced surface characterization technologies such as scanning tunneling microscopy,synchrotron-based near ambient pressure photoelectron spectroscopy.The relationship between cations segregation behaviors in different latiice sites and electrode performance was investigated.The main results of this thesis include the following three parts:(1)Effect of Pr_xCe_1-xO₂(x=0,0.2,1)modification layer on the oxygen reduction reaction(ORR)kinetics and stability of La_0.4Sr_0.6Co_0.2Fe_0.6O₃(LSCF)was systematically investigated.We found that suppressing A-site cation segregation helped electrodes to achieve improved activity and stability.Thin films with controllable composition and structure were prepared by PLD,and a modified layer of Pr_xCe_1-xO₂with different thicknesses was deposited on its surface.Electrochemical results showed that the modified LSCF electrodes displayed higher ORR activity and stability than the bare LSCF.Among the tested modification layers,Pr O₂/LSCF displayed the most pronounced enhancement in ORR activity,and Ce O₂ layer exhibited the best long-term stability.Using characterization technologies such as X-ray photoelectron spectroscopy,auger electron spectroscopy and transmission electron microscopy,we found that performance degradation in bare LSCF electrode was mainly due to surface Sr segregation at high temperature.Such Sr segregation was successfully suppressed by surface modification,leading to greatly improved stability.The effect of suppressing Sr segregation followed the trend of Ce O₂>Pr_0.2Ce_0.8O₂(PCO)>Pr O₂.Both the Sr solubility and oxygen vacancy concentration were lowest in Ce O₂ layer,which helped to suppress Sr segregation towards surface and greatly improve the long-term stability of electrodes.Oxygen vacancy formation energy and Sr substitution energy increased with the decreasing Pr content in Pr_xCe_1-xO₂.Therefore,Pr O₂/LSCF presented the most pronounced acitivity due to the highest oxygen vacancy concentration in Pr O₂layer.These results demonstrated that Sr segregation behaviors can be tuned by adjusting the oxygen defect concentration in the surface modification layer,leading to improved ORR activity and stability.(2)Using Pr_0.4Sr_0.6Co_0.2Fe_0.7Nb_0.1O₃(PSCFN)thin film as the model system,we systematically investigated the initial B-site cation exsolution(i.e.,B site caions segregate and form the particles on surface)under high-temperature and reductive condition.The evolutions of surface electronic structure and surface composition during the exsolution process were systematically investigated,and the corresponding effect mechanism on the catalytic activity was revealed.A prudent strategy was proposed to improve the electrode's activity by facilitating B-site cation exsolution.The results suggested that Co and Fe exsolved towards the electrode surface and subsequently formed Co-Fe enriched layer after reduced in hydrogen.After phase separation,a large number of(Co,Fe)O_xnanoparticles formed on PSCFN surface.Scanning tunneling spectroscopy results showed that the energy gap of the perovskite substrate and the nanoparticles continuously decreased under high-temperature and reductive condition.According to these electronic structure information and electrochemical measurement results,we found that the perovskite substrate was activated in reductive atmosphere.Both the activated perovskite matrix and exsolved nanoparticles provided active sites for hydrogen oxidation reaction.Based on the mechanism from thin film electrode,PSCFN powders were further prepared in order to clarify the effect of exsolution behaviors on performance of porous electrodes.The exsolved(Co,Fe)O_x nanoparticles were further reduced as Co-Fe alloy at higher reduction temperature.The obtained electrodes exhibited excellent activity and stability for HOR and methane reforming reaction,which is an ideal material as fuel electrode for SOCs.(3)Hydrogen oxidation reaction(HOR)activity and stability of Pr_0.4Sr_0.6(Co_xFe_0.9-xNb_0.1)O₃ thin film electrode were effectively enhanced by tuning the doping concentration of Co in B site(x=0,0.2,0.7).Pr_0.4Sr_0.6(Fe_0.9Nb_0.1)O₃(PSFN)had low concentration of oxygen vacancies,which is not conducive to the B site cation exsolution.Therefore,PSFN displayed good long-term stability but sluggish HOR kinetic process.Replacing Fe with more reducible B-site element Co was found to facilitate oxygen vacancy formation and promote cation surface exsolution.With Co doping ratio of 20%,Co-Fe oxide nanoparticles were dispersed on the surface of Pr_0.4Sr_0.6(Co_0.2Fe_0.7Nb_0.1)O₃(Co-20)after reduction,and the film showed good HOR activity and long-term stability.However,excessive Co doping leaded to immoderate oxygen vacancy concentration and cation exsolution,which resulted in partial decomposition of Pr_0.4Sr_0.6(Co_0.7Fe_0.2Nb_0.1)O₃(Co-70).Therefore,Co-70 had the fastest HOR reaction kinetics,but the performance deteriorated severely after long-term operation.These results indicated that the exsolution behavior of B-site cations can be modulated by bulk doping,which significantly impacted the performance of the perovskite electrode.In summary,the evolution of surface composition,electronic structure,and defect chemistry on perovskite during segregation behaviors of A-site and B-site cations were investigated,and the impact of cation segregation amd oxygen vacancies on electrode performance was clarified.We found that inhibiting A-site cation segregation behaviors or promoting B-site exsolution behaviors can effectively enhance the electro-catalytic performance.Relative results provide theoretical guidance for rational design of efficient and stable perovskite electrode materials via surface engineering.