Construction And Multifunctional Performance Of Metal-organic Frameworks Containing Nitrogen And Sulfur With Different Dimensions And Their Derivative Materials

BackgroundDue to the advantages of abundant raw materials,safeties and environmental friendlinesses,novel green energy devices in the 21st century（such as fuel cells,metal-air batteries and water electrolyzers）have good application and development prospects.However,the main problems to restrict the commercialization of these energy devices are the slow reaction kinetics of oxygen electrode（oxygen reduction reaction（ORR）and oxygen evolution reaction（OER））and hydrogen electrode（hydrogen evolution reaction（HER））.In order to speed up the reaction kinetics,noble metals and noble metal oxides catalysts,such as Pt/C,Ru O₂ or Ir O₂,are commonly used in the two electrodes.However,these noble-metal-based-catalysts can hardly catalyze ORR/OER/HER at the same time,and their limited resources and high costs cannot meet the needs of these energy devices.Therefore,the design and synthesis of multifunctional non-noble metal catalysts has become a research focus in this field.As an emerging porous material,MOFs usually use non-precious metals such as iron,cobalt and nickel as the metal coordination center,which greatly reduces the costs of such materials.Additionally,since the organic ligands are usually organic molecules containing carboxylic acids,nitrogen heterocycles and sulfur,it is easy to introduce heteroatoms such as N,P and S into the carbon skeleton after pyrolysis of MOFs,which can significantly change the properties of the materials.At the same time,ideal nanostructures can be obtained through design and preparation process control.The pyrolyzed materials often have secondary or multi-level nanostructures,which can expose more active sites and improve electrocatalytic efficiency.Objectives1.N,S-containing MOFs and MOFs-derived N and S codoped carbon materials were designed and synthesized with 2,5-thiophene dicarboxylic acid（Tdc）and 4,4’-bipyridine（4,4’-Bpy）as double organic ligands and transition metal salts（such as manganese salts and cobalt salts）as metal precursors.2.The ORR and/or OER and/or HER activities of the catalysts were evaluated by electrochemical performance measurements.Then,the catalysts with better OER and HER activities were assembled into water electrolyzers to evaluate their actual application prospects.Methods1.Synthetic methods:hydrothermal reaction and stirring at room temperature-pyrolysis at high temperature.2.Physical characterizations:The morphologies,compositions,layer thicknesses,defect degrees and specific surface areas of all the resultant materials were characterized by field emission scanning electron microscopy（FESEM）,transmission electron microscopy（TEM）,scanning transmission electron microscopy（STEM）,X-ray powder diffraction（XRD）,X-ray absorption near edge structure（XANES）,X-ray photoelectron spectroscopy（XPS）,atomic force microscopy（AFM）,Raman spectroscopy and Brunner-Emmet-Teller（BET）measurements.3.Electrochemical performances:Linear scanning voltammetry（LSV）mainly describes the onset potentials for ORR,OER and HER,half-wave potential for ORR,limiting current densities for ORR and OER and the overpotentials for OER and HER at10 m A cm^-2 of the catalyst.The electron transfer number and kinetic current density of the catalyst towards ORR are calculated by Koutecky-Levich（K-L）equation.The electrochemical conductivity and stability of the catalyst were evaluated by Nyquist plots,chronoamperometry（I-t）curves and chronopotentiometry（P-t）curves.Results1.A series of N,S-containing Mn-based MOFs（N-S-Mn-MOFs）with ORR activities and MOFs-derived N and S codoped carbon materials with OER and HER（Co O supported N and S codoped ultrathin porous carbon nanosheets（Co O/N-S-UPCNPs））were successfully prepared.2.Different hydrothermal reaction times,anions and pyrolysis temperatures can affect the morphologies,compositions and performances of N-S-Mn-MOFs and Co O/N-S-UPCNPs,respectively.3.The electrochemical performance tests show that the reaction time is 4 h and the anion is SO₄^2-,the resultant Mn^II[（Tdc）（4,4’-Bpy）]_n has the comparable ORR performance with that of Pt/C in alkaline medium.Therefore,Mn^II[（Tdc）（4,4’-Bpy）]_n can be expected to be used in fuel cells and metal-air batteries.Co O/N-S-UPCNPs-600 exhibit high OER and HER performances,and the water electrolyzer assembled with Co O/N-S-UPCNPS as cathode and anode also shows good activity and stability.Conclusion1.N-S-MOFs with different dimensions and their derived carbon materials can be easily synthesized by selecting design center metals and organic ligands.2.Nanostructured catalysts with high specific surface area,more defects and active sites will have better electrocatalytic activity.