Synthesis Of Transition Metal Single Atom And Oxygen Reduction Mechanism By Double Nitrogen Source Complexation

Single atom dispersion catalysts(SACs)are considered ideal for replacing conventional Pt-based catalysts due to their high atomic utilization and excellent oxygen reduction performance(ORR).In general,the central metal atom is considered to be the ORR active site and the most direct and effective way to modulate the intrinsic catalytic activity of SACs is to select the appropriate central metal atom.Secondly,by changing the type and structure of the coordination atom,the d-electron of the central metal atom can be adjusted to improve the ORR performance.However,SACs face problems such as easy aggregation during synthesis,low metal content and poor understanding of the catalytic mechanism.The dual nitrogen source ligand strategy not only introduces more N to produce a stronger M-N bond providing a higher active site density for the ORR,but also generates a large number of sites to anchor more SACs.Based on this,this thesis aims to develop efficient and stable oxygen-reacting electrocatalysts,using a dual nitrogen source coordination strategy to develop controllable preparations for a variety of SACs.In situ X-ray absorption spectroscopy(XAS)combined with density flooding theory(DFT)calculations are also used to investigate the ORR reaction mechanism in depth.The main studies are as follows:Cr-N-C catalysts supported on three-dimensional(3D)nitrogen-doped carbon support were synthesized by a dinitrogen source coordination strategy(aniline and dicyandiamide).The half-wave potential of Cr-N-C in the acidic electrolyte is 0.74 V and the limiting current density is 5.96 mA cm-2.The excellent ORR performance of Cr-N-C in acidic systems is due to the 3D structure which facilitates the adsorption and desorption of oxygen intermediates.The assembled fuel cells exhibit an open circuit voltage of 0.9 V and a maximum current density of 1060 mA cm-2 with Cr-N-C as the cathode catalyst for H2/O2 proton exchange membrane fuel cells.Theoretical calculations predict that the Pyridinic-N and Graphitic-N sites can synergistically adsorb O2 molecules and dissociate O-O bonds to enhance ORR activity.Based on the theoretical results,Fe-N-C/GC catalysts with abundant Pyridinic-N and Graphitic-N sites are synthesized by introducing formamide and urea as nitrogen sources.The ORR half-wave potential of 0.80 V has an ultimate current density of 6.05 mA cm-2 which is comparable to the performance of Pt/C.DFT calculations further show that the Pyridinic-N and Graphitic-N sites can be precisely optimized for the d-band center of Fe to obtain the appropriate adsorption energy,thus increasing the ORR activity.We have used formamide and aniline as raw materials to synthesis Mn-N-C catalysts with abundant Mn2+-Pyridinic-N and Mn3+-Pyrrolic-N sites by forming Pyridinic-N and Pyrrolic-N ligands to anchor the corresponding Mn2+ and Mn3+ ligands.The Mn-N-C material has good ORR properties.DFT calculations show that Mn2+-Pyridinic-N and Mn3+-Pyrrolic-N have higher carrier densities and stronger charge transfer capabilities for electrons,facilitating electron transfer.Synergistic interaction between Mn3+-Pyrrolic-N and Mn2+-Pyridinic-N reduces the energy barrier of the OOH*decisive step and promotes ORR activity.Cu-N-C/GC catalysts with a high density of Cu-N4 sites achieved by trapping and anchoring Cu single atoms through ligand groups formed by the condensation of formamide and glycine.A loading of 5.61 wt%Cu single atom was dispersed on an N-doped carbon carrier.In alkaline media,Cu-N-C/GC shows a high onset potential(Eonset)of 0.98 V,much higher than the corresponding single nitrogen source-derived catalysts.Cu-N-C/GC as alkaline H2/O2 fuel cell cathode catalyst for assembled fuel cells capable of reaching a peak power density of 324 mW cm-2.In siu XAS combined with DFT calculations show that successive structural changes not only optimize the d-band center of the central metal but also further reduce the potential barrier of the decisive step OOH*,thus increasing the ORR activity.