Study On The Protonation And Electrocatalytic Performance Of La_1-xSr_xCoO₃ Thin Films

The ever-increasing global energy demand and deteriorating environmental problems have forced mankind to constantly seek new energy sources that are recyclable,cheap,clean and environmentally friendly.Among various alternative energy sources,hydrogen energy is a high-calorie,non-polluting clean energy or the best carrier for energy storage.At present,it water splitting is a good way to obtain hydrogen.However,there is an overpotential problem in the process of water splitting to produce hydrogen,and a catalyst is needed to reduce the overpotential.La_1-xSr_xCoO₃ is well-known because of its good catalytic performance.Although the La_1-xSr_xCoO₃ material has been systematically studied electrochemically before,most of them have focused on the research on La_1-xSr_xCoO₃ powders.The dependence of powder materials on the surface structure complicates the study of electrochemical mechanism,because there may be different ends,structural defects,and even multiple phases in the same catalytic material,and the material can only be analyzed from the whole.Due to the two-dimensional nature of the surface of epitaxial film electrodes,their reaction field is limited,which provides favorable conditions for the study of the mechanism of oxygen evolution reaction,makes it possible to directly study the dependence of the reaction surface and electrochemical reaction conditions.This thesis starts from the fact that epitaxial single crystal films are beneficial to the study of catalytic mechanism.A high-quality epitaxial film was prepared by a pulsed laser deposition system to study the electrochemical performance and analyze the catalytic mechanism.The specific research content is as follows.（1）The valence state of transition metal in perovskite is essential for many physical properties of this material and its practical application.Specifically,the multivalent nature of transition metal oxides makes it form oxygen vacancies in the solid structure,which affects the redox reaction.In order to study the effect of different doping ratios on the catalytic activity of La_1-xSr_xCoO₃（x = 0,0.1,0.2,0.3,0.4,0.5）thin films,we use the solid-phase sintering method to prepare different doping ratios La_1-xSr_xCoO₃ ceramic target material.A better quality La_1-xSr_xCoO₃ film is prepared by pulsed laser deposition system,and the prepared film is packaged into an electrode structure.After cyclic voltammetry test,La_0.7Sr_0.3Co O₃ has the best catalytic activity,and then conduct detailed research and analysis on the thin film with this doping ratio.（2）Using electrolyte solutions with different p H values for testing,it is concluded that the catalytic activity of the membrane electrode increases with the p H value of the electrolyte solution.This phenomenon cannot be explained by the traditional perovskite oxygen evolution mechanism.Combined with cutting-edge research,it is believed that this experimental phenomenon may be caused by the protonation of the film lattice oxygen.By immersing the film with aqueous solutions of different p H values,it is found that different p H aqueous solutions have different effects on the electrocatalytic activity of the film.The catalytic activity of the electrode decreases significantly after the film is treated with an aqueous solution with a lower p H,while the p H is higher the catalytic activity of the electrode changes little after the film is treated with the aqueous solution.The results of X-ray photoelectron spectroscopy analysis showed that the content of lattice oxygen（M-O-M）on the surface of the film decreased after deionized water treatment,while protonated oxygen（M-OH）increased.This indicates that the degree of protonation of lattice oxygen is different after treatment with different p H aqueous solutions,which shows that the oxygen evolution activity is different.（3）Because some researchers use water to treat the perovskite film will cause the film resistance to increase significantly,considering that the change in resistance may affect the catalytic activity of the catalyst.Then the La_0.7Sr_0.3Co O₃ film was prepared on non-conductive Sr Ti O₃ single crystal substrate,and the in-situ test was performed with different p H aqueous solutions.It was obtained that the film resistance changed less after treatment with a larger p H aqueous solution,and the film resistance increased significantly after the aqueous solution with a lower p H was processed.This is because the protonation of the film causes the resistance to increase.However,when the film is treated with deionized water,the resistance of the film will increase by about three orders of magnitude within a few tens of minutes.And the film resistance changes slowly after treatment with an aqueous solution with a p H of 11,even if the resistance only rises after a few hours several times higher.In contrast,after using deionized water and an aqueous solution with a p H of 11 to treat the membrane electrode at the same time,the catalytic activity of the electrode is not much different,so as to eliminate the influence of the change of the membrane resistance on the catalytic activity.