| 3d transition metal solids have broad application prospects in the field of electrochemical energy storage and conversion due to their abundant reserves,low price and diverse electronic structures.In particular,nickel-based and cobalt-based derivatives are often used as excellent precursors for electrocatalysts in the oxygen evolution reaction and urea oxidation reaction.Pyrolysis is a common control method,and processing precursors with specific structures is an effective strategy to construct efficient electrocatalysts.Different from the carbon layerwrapped nano core-shell catalyst with stable properties and clear crystalline structure formed under high-temperature pyrolysis,during the low-temperature pyrolysis process,the M-O bonds of the transition metal-based complexes can be decomposed and disturbed to generate electrocatalytic active sites.This provides us with a good opportunity to realize micropyrolysis perturbation-directed construction of electrocatalysts.At the same time,coordination molecular clusters provide a new idea for the exploration of precursors.Compared with other types of precursors,coordination molecular clusters,as a zerodimensional system,have no periodic extensions in different dimensions and are composed of countless identical coordination units.After low-temperature pyrolysis,the uniformity of the structural units is beneficial to the study and realization of the transition from ordinary crystals to high-performance material structures.In addition,the guests and weakly interacting molecules in molecular clusters are lost in the crystal along with decomposition and recombination reactions,and the unique coordination bridges in the structure are destroyed during pyrolysis,and small organic molecules escape,thus exposing abundant active sites.All of these provide a good material platform for us to study the transformation of material structure and the improvement of catalytic activity in low-temperature pyrolysis.Based on the above description,this thesis designed molecular clusters based on singlemetallic nickel(Ni4C46H68N4O18)and bimetallic nickel-cobalt(Ni2Co2C46H68N4O18)doped molecules,and prepared amorphous electrocatalysts with unique structures by lowtemperature pyrolysis.Among them,the oxygen atom and nitrogen atom provided by the Schiff base ligand coordinate with the metal to fix the metal atom,and the metal atom and the bridging oxygen atom form a stable four-membered ring structure,so that the main frame of the molecular cluster can still be maintained under low-temperature pyrolysis.Analyze the thermal decomposition process through the small organic molecule fragments captured by TG-MS,and study the material composition,crystallization degree and metal valence state changes of the product under low temperature pyrolysis by combining XRD,XAS,TEM,IR,Raman and XPS and other characterization methods.To explore the effect of molecular cluster structure evolution on catalytic activity after low-temperature pyrolysis.This thesis is divided into the following three chapters:In the introduction part of the first chapter,the application of 3d transition metal Schiff base complexes in the fields of electrocatalysis,photocatalysis and biomedicine is summarized;the difference between high-efficiency electrocatalysts constructed by high-temperature pyrolysis and low-temperature pyrolysis is illustrated and compared by examples;briefly described the reaction mechanism and research status of oxygen evolution reaction and urea oxidation reaction;at the same time,combined with the research foundation of our research group in the design of inorganic functional nanomaterials,exploration of pyrolysis mechanism and electrochemical energy storage catalysis,this paper introduced meaning of the topic.In the second chapter,1,3-diamino-2-propanol and o-vanillin were selected to form the Schiff base ligand HL(N,N’-bis(3-methoxy-salicylidene)-1,3-Diamino-2-propanol),synthesized tetranuclear nickel-based molecular cluster Ni4 with symmetrical structure under solvothermal reaction with nickel chloride hexahydrate,the molecular cluster was prepared by low-temperature pyrolysis(250℃-350℃)under helium atmosphere to obtain amorphous state catalyst.Using TG-MS to investigate the fragmentation of small organic molecules of molecular clusters under low-temperature pyrolysis and analyze the internal mechanism of the pyrolysis process,the test found that the organic ligand framework was destroyed,but the four-membered ring formed by the bridging of metal nickel atoms and oxygen atoms was obtained retained,the metal active center is exposed,which provides a theoretical basis for its excellent catalytic activity.At the same time,combined with XAS and XPS to analyze the bonding mode of metal atoms,it was found that the existing Ni2+/Ni3+species served as the catalytic active center,which confirmed the recombination phenomenon in the dynamic change of the pyrolysis process.In addition,electrochemical tests show that in the oxygen evolution reaction,the catalyst Ni4-350 needs an overpotential of 247 mV to reach a current density of 10 mA·cm-2;in the urea oxidation reaction,the overpotential of Ni4-350 reaches 1.366 V.In the third chapter,the Schiff base ligand HL is also selected,and the tetranuclear nickelcobalt-based molecular cluster Ni2Co2 is synthesized under the solvothermal reaction with nickel chloride hexahydrate and cobalt chloride hexahydrate.The ratio of the amount of the two metal salts is 1:1,which is consistent with the ratio of the number of metal atoms in the synthesized Ni2Co2,and the molecular clusters are prepared by low-temperature pyrolysis in a helium atmosphere to obtain a bimetallic amorphous catalyst.Due to the similar electronic configuration and coordination mode of metal nickel atom and cobalt atom,benefiting from the synergistic effect of the two metals,the inherent polarity of the metal atom is enhanced and the charge transfer rate is improved.Using TG-MS to explore the low-temperature thermal decomposition process of molecular clusters,the detected organic fragments indicate that the organic ligand framework collapses under 250℃-350℃ pyrolysis,and the metal main frame is retained.With the increase of solution temperature,the stretching vibration of the benzene ring skeleton and the stretching vibration of the C=N bond indicate that the ligand framework is gradually broken and the degree of graphitization is gradually intensified,which is consistent with the analysis results of TG-MS.Electrochemical tests show that in the oxygen evolution reaction,the catalyst Ni2Co2-350 needs an overpotential of 232 mV to reach a current density of 10 mA·cm-2;in the urea oxidation reaction,the potential of Ni2Co2-350 reaches 1.367 V at the same current density. |
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