Design And Preparation Of Nanomaterials Based On Electronic Structure Modulation And Their Applications In The Field Of Inorganic Small Molecule Electrocatalysis

At present,people’s concerns about energy are mainly concentrated in two aspects,one is the depletion of fossil energy;the other is the pollution caused by fossil energy.The exhaustion of fossil energy has prompted people to look for renewable energy to maintain sustainable development.The pollution of fossil energy has caused people to start looking for energy without carbon element.Two small hydrogen-rich molecules,hydrogen and ammonia,can solve the two problems caused by fossil energy at the same time.First of all,the final products of these two gases after releasing energy are water and nitrogen,which are non-polluting.Secondly,water and nitrogen can be used as raw materials to synthesize hydrogen and ammonia,respectively.In this way,we can achieve the recycling of clean energy through converting the abundant inorganic small molecules(H2O,O2,N2).With the widespread use of electrical energy,electrocatalysis technology has been receiving more and more attention for the conversion of small inorganic molecules.In the electrochemical water splitting technology for hydrogen producing,Hydrogen evolution reaction(HER)and oxygen evolution reaction(OER)will be involved.The oxygen reduction reaction(ORR)occurs in the Proton exchange membrane fuel cell(PEMFC)technology which could efficiently use of hydrogen energy.In the filed of electrocatalytic nitrogen fixation for ammonia synthesis,the nitrogen reduction reaction(NRR)is getting more attraction.Seeking efficient electrochemical catalysts to reduce the kinetic energy barrier in these conversion reactions is the key to developing the electrocatalytic conversion technology in industry.The essence of the catalytic reaction is the electron transfer between the catalyst and the reaction molecule,so the electronic structure of the electrocatalyst determines its intrinsic activity,and regulating the electronic structure of the active site of the catalyst is an effective way to enhance the activity of the catalyst.Furthermore,a good conductivity of electrocatalst is important for the electrocatalytic reaction,which could effectively transport electrons during the reaction.Metal-organic framework(MOFs)are three-dimensional network composed of tunable metal center ions and organic ligands,with large specific surface area and pore volume.These characteristics make MOFs an ideal precursor for preparing catalysts and regulating the electronic structure of catalysts.During the calcination process in an inert atmosphere at high temperature,the carbon atoms and other heteroatoms(such as N and O atoms)in the organic ligand of MOFs form a heteroatom-doped graphene-like structure,which will serve as an effective substrate providing excellent conductivity for electrocatalysts.At the same time,the metal ions can be aggregated into metal or alloy nanoparticles.What’s more,we can converted them into metal single atom structures through further subsequent processing(such as acid pickling).By changing the metal centers of MOFs or modifying MOFs templates with other different metal ions,we can control the types of metal elements and their coordination environment in the final product.There are opportunities for us to regulate the electronic structure of the active site of the catalyst,thereby explore its impact on electrocatalytic performance.Based on abovementioned conclusion,according different electrocatalysis reaction,we designed and prepared a variety of metal nanoparticles and metal single-atom catalysts through pyrolysis or acid etching using MOFs precursors as templates.Through the combination of experiments and theoretical calculations,we investigated the catalytic performance and catalytic mechanism of these catalysts in OER,HER,ORR and NRR.The detail studies in dissertation are as the following aspects:1.Developing highly active electrocatalysts with superior durability for both the oxygen evolution reaction(OER)and hydrogen evolution reaction(HER)in the same electrolyte is a grand challenge to realize the practical application of electrolysis water for producing Hydrogen.In this work,we have synthesized ultra-small Ru/Cu-doped RuO2 complex embedded in amorphous carbon skeleton,through thermolysis of Ru-modified Cu-BTC,as highly efficient bifunctional catalyst for overall water splitting electrocatalysis.The ultra-small Ru nanoparticles in the complex expose more activity sites for hydrogen evolution and outperform than the commercial Pt/C.Meanwhile,the ultra-small RuO2 nanoparticles exhibit superior oxygen evolution performance over commercial RuO2,and the doping of Cu into the ultra-small RuO2 nanoparticles further enhances the oxygen evolution performance of the catalyst.Benefiting from the outstanding OER and decent HER catalytic activity in an alkaline electrolyte,the overall water splitting performance of the complex is superior to that of the state-of-the-art electrocatalysts,which just require 1.47 V to achieve a current density of 10 mA cm-2.Furthermore,the porous structure of carbon skeleton endows the catalysts with 3D electrode performance at high current density of 100 mA cm-2 with overpotehtial of 1.67 V.The DFT calculations revealed that Cu dopant could effectively tailor the d-band center,thereby tuning electronic structure of Ru activity sites on the RuO2(110)plane and ultimately improve the OER performance of RuO2.2.Designing and exploring catalysts with high activity and durability for the cathodic oxygen reduction reaction(ORR)in acidic environment are imperative for industrialization of the proton exchange membrane fuel cells(PEMFCs).Theoretical calculations and experiments have demonstrated alloying Pt with a transition metal not only reduces the usage of scarce Pt metal but also improves performance as compared with that of pure Pt on mass activity because the surface electronic structure of Pt can be tuned after introducing a transition metal into Pt,consequently decreasing the blocking species coverage and exposing more active sites.In this work,we report the preparation of nanoporous PtFe nanoparticles(np-PtFe NPs)supported on N-doped nanoporous carbon sheets(NPCSs)via facile in-situ carburization of Pt modified Fe-based metal-organic framework.The np-PtFe/NPCSs exhibit a more positive half-wave potential(0.92 V)compared with commercial Pt/C catalyst(0.883V).The nanoporous structure allows our catalyst to possess a great mass activity,which is found to be 0.533 A mgpt-1 and 3.04 times greater than that of Pt/C(0.175 A mgPt-1).Moreover,the structure conversion from porous to hollow enables excellent durability over 2000 cycles in acidic electrolytes.Our strategy provides a facile design and synthesis process of noble-transition metal alloy electrocatalysts via noble metal modified MOFs as precursors.3.The ammonia has been the second largest chemical in the world since the Haber-Bosch(HB)process successfully converted nitrogen(N2)into ammonia(NH3).While the huge energy consumption and pollution of the HB process does not meet the requirements of sustainable development.The high activity of nitrogenase in nature encourages people to explore electrocatalytic nitrogen fixation technology at normal temperature and pressure.However,as an emerging research field,uniform standards for the detection and test methods of ammonia yield haven’t been established.Espeacilly,the neglected ammonia pollution in the experimental environment in most research work will result in exaggerated or wrong performance of electrocatalyst.So,developing a universal method that can ensure the reliability of the test performance for ordinary laboratories is very helpful to promote the research process of electrocatalytic nitrogen fixation.For the first time,we propose a method to concentrate all environment ammonia pollution which cannot be avoided into one parameter.Repeat the sampling experiment on this parameter to obtain the average value of the total content of unavoidable ammonia pollution.The significance of this parameter is that the performance value of this parameter can be directly compared with the specific catalyst to avoid unnecessary repetitive experiments,thereby accelerating the development of new catalysts.4.Regardless of the type of nitrogen fixation technology,the adsorption and activation of N2 molecules is the first step in the ammonia synthesis reaction.However,very few catalysts can activate the N2 molecules due to its unique molecules structure.Transition metal elements with abundant d electrons have always been the first choice for catalytic reactions,and the main group metal elements can form a combination of unoccupied and occupied orbitals through a suitable coordination structure,which is has the potential to activate N2 molecule through "accept-reverse electron donation"machnism,thereby catalyzing the reduction of nitrogen to ammonia.Here,for the first time,we have demonstrated the Mg-C/O structure possess the potential in activating nitrogen molecules through DFT theoretical calculations.At the same time,the Mg-O4 structure is thermodynamically inclined to catalyse nitrogen reduction to ammonia through an alternating combination mechanism.With the support of theory result,we further attempted to prepare catalyst containing this structure through experiments synthesis and explored the impact of synthesis temperature for the performance of ammonia yield.The experiment proves that the Mg-C/O structure has the performance of electrocatalytic nitrogen reduction to ammonia in an acidic environment.The ammonia yield rate at 900℃ can be as high as 18 μg·h-1· mgcat and a maximum Faraday efficiency of 14.5%.