Study On Synthesis Of Metal Oxide And Its Water Splitting By High Temperature Molten Salt Method

With the increase of global energy consumption and environmental pollution,the development of clean energy has become one of the important topics for scientists.Hydrogen energy is considered as a potential energy carrier due to its high combustion value and no pollution to the environment.At present,electrolysis of water is one of the cleanest and efficient method to product hydrogen.However,direct electrolysis of water needs to overcome higher energy barriers,so it is necessary to develop a suitable catalyst to lower the overpotential.Precious metal catalysts are still the most efficient catalysts,but their high cost and limited reserves limit their wide application.Therefore,it is mandatory to develop alternative electrocatalysts that are inexpensive,sufficiently active,and stable upon prolonged exposure to oxidizing conditions in alkaline solutions.The high-temperature molten salt method is a new synthetic method for the synthesis of inorganic materials.It uses one or several low-melting salts as the reaction medium to conduct the synthesis reaction in the high-temperature molten salt from 100°C to1000°C.After the reaction,cooled and washed the salt away,finally the product was obtained.The synthesis process combined the advantages of solid and liquid phase synthesis,which makes it possible to synthesize different kinds of catalysts.In this paper,we synthesized variety of oxide catalytic materials by molten salt method,and performed their electrocatalytic hydrogen production performance.By studying the effects of different precursors and molten salt,we have developed a new method for synthesizing high-efficiency hydrogen production catalytic materials.X-ray absorption spectroscopy can obtain the electronic structure and local structure information of the catalyst,which can provide new insight information for the catalyst.The operando XAS method can study the active of the catalytic activity during the reaction process,which is a hot spot in the research field of electrolytic hydrogen production catalytic materials.The research contents of this paper are as follows:1.We first explored the regulatory factors of Co₃O₄ morphology during the molten salt method,including the types of precursors and molten salt.We found that the precursor has a direct effect on the morphology of Co₃O₄,while the type of molten salt has no effect on the morphology.The electrochemical properties found that Co₃O₄ with flocculent structure had the best electrochemical performance.Then,we prepared the layered CoOOH by molten salt assisted method,and investigated the effect of reaction temperature on the properties of the product.It was found that CoOOH had the best symmetry and crystallinity at 750°C,indicating that Co is closer to+3 valence in the system.Since Co³⁺is recognized as catalytic activity site,we speculate that at 750°C has the best catalytic activity,and the electrochemical experimental also confirms this.2.We further synthesized the perovskite material La_0.6Sr_0.4Co_0.2Fe_0.8O_3-?by the molten salt method.Compared with the solid state method,the molten salt method not only reduces the synthesis temperature and reaction time,but also suitable for mass production,and the product had a unique porous structure and better catalytic performance.Then we synthesized La_0.6Sr_0.4Co_1-xFe_xO_3-?with different Fe content,and found that the unique porous structure is derived from the molten salt synthesis method and the content of Fe.Also the surface area of the system increases with the increase of Fe content.The electrochemical test results show that the system has the best catalytic activity when the Fe content reaches 80%,and the optimal valence state and suitable oxygen vacancy content is the source of the activity.3.Combining the molten salt method with the in-situ exsolution,we developed a new catalyst consisting of homo-geneously dispersed Co/Fe metal nanoparticles that are socketed into the La₂O₃ surface,from the pristine LaCo_0.8Fe_0.2O_3-?perovskite.The optimized catalyst exhibits a low overpotential of 293 mV to reach the current density of 10 mA cm^-2 in 0.1 M KOH.Most importantly,operando X-ray absorption spectroscopy?XAS?measurements demonstrate that the CoFe species in the catalyst are nearly transformed into unique?Co/Fe?O?OH?with a definite coordination-unsaturated structure under electrochemical conditions,which actually contributes to this superior performance.Moreover,the presence of the lanthana support promotes this transformation.Our work not only suggests a facile reconstructive strategy to dramatically enhance the OER activity of perovskite oxides in alkaline media,but also unravels the fine structure of true active sites through operando X-ray spectroscopic tracking.4.On the basis of the previous chapter,in order to further study the formation process of the unsaturated structure in the catalytic process,we studied the changes of surface structure of La_1-xSr_xCo_0.8Fe_0.2O_3-?with different Sr contents before and after the reaction.Preliminary results show that the surface of after the reaction produces an amorphous layer,and the thickness increases as the Sr content increases.The increase in Sr content means the content of hole increase,while the Co L_2,3-edge and O K-edge absorption spectrum experiments show that the increased hole are mainly distributed around the oxygen atoms,forming an unstable O 2p hole.Combined with other related research results,we speculate that this unstable O 2p hole contributes to the formation of an amorphous layer on the catalyst surface.Further conclusions need more experimental results to confirm.