Controllable Preparation Of Cathode Oxygen Reduction Catalyst And Construction Of High Efficiency Membrane Electrode For Fuel Cell

The development of hydrogen energy is of great strategic significance for China to realize the goal of“carbon peaking and carbon neutrality”.Fuel cells can directly convert the chemical energy of hydrogen into electricity and can realize zero pollution and zero carbon emission during operation.However,the slow kinetics of the cathodic oxygen reduction reaction（ORR）process in fuel cells limits the performance of these devices.At present,the precious metal-based platinum materials and their alloys exhibit the best performance in this catalytic process,but their high cost,scarcity,and poor durability seriously hinder their large-scale application.Therefore,the development of non-precious metal catalytic materials is an important goal for sustainable development.Additionally,the membrane electrode of the fuel cell also needs to meet the requirements of continuous fuel transmission during the electrochemical reaction,timely discharge of generated water,and efficient transfer of protons and electrons.A good porous structure can provide an ideal channel for reactants and electrolytes to enter the active center,thus effectively improving the performance of the fuel cell.Therefore,how to design and synthesize ORR catalysts with high performance active centers and overcome the problem of limited mass transfer rate during membrane electrode reaction is the key bottleneck to realize high performance fuel cell devices.Aiming at the above key bottlenecks,this thesis investigates the catalyst intrinsic activity and the mass transfer at the interface of the three-phase reaction of membrane electrodes,so as to realize the controllable preparation of cathodic ORR catalysts for fuel cells and the construction of high-efficiency membrane electrodes.The specific research contents are as follows:（1）The adsorption structure was altered by constructing Fe-Ce atom pairs to break the linear relationship based on monometallic sites,thus enhancing the ORR performance.In the synthesized Fe Ce-single-atom dispersed hierarchical porous nitrogen-doped carbon（Fe Ce-SAD/HPNC）catalysts,the 4f cruise electrons of elemental cerium lowered the d-orbital centers of the iron,changing the rate-determing step and changed the rate-determing step,which made the Fe Ce-SAD/HPNC catalyst with excellent ORR performance.Additionally,by constructing a three-phase reaction interface with a hierarchical porous structure,the power density was significantly increased by a factor of 1.6 under the same conditions as compared with the Fe single-point catalysts in proton exchange membrane fuel cells（PEMFCs）.（2）Highly loaded single-atom catalyst was prepared using a solution impregnation strategy,enabling the assembly of fuel cell energy device and electrocatalytic performance studies.A solution-processable covalent organic polymer（COP）with a well-defined structure of Co-N₄ was introduced into honeycomb porous carbon,and the loading of single atoms after pyrolysis reached 3.19%.The ORR half-wave potentials of the obtained catalysts were as high as 0.827 V in 0.1 M HCl O₄ electrolyte,and their maximum power densities reached 963 m W cm^-2 in PEMFCs.The excellent electrochemical performance was mainly attributed to the fact that the COP exhibited good solubility,which effectively prevented the agglomeration of metals.In addition,the honeycomb3D porous network structure can promote electron transport and charge transfer at the interface,thus accelerating the reaction process.（3）A novel ORR catalyst with Co-N₅ active sites was synthesized using an axial coordination strategy,which significantly increased the maximum power density by a factor of 1.63 in alkaline polymer electrolyte fuel cells（APEFCs）.A novel ORR catalyst with Co-N₅ active site was prepared by anchoring Co-porphyrin molecules to nitrogen-functionalized reduced graphene oxide and providing axial ligands for the cobalt center.Density-functional theory（DFT）calculations showed that the electronic and geometrical structures of the Co 3d orbitals were significantly changed after rehybridization with the axial coordination ligand orbitals,which greatly improved the ORR rate.Additionally,in-situ Raman tests can systematically explain the conversion process of the oxygen reduction reaction intermediates Co-OH and Co-O,further demonstrating that Co-N₅ is the active site of the reaction.（4）Cross-linked nanofiber electrode with hierarchical porous structure was synthesized by electrostatic spinning,and the atomic-level Co active sites were fully exposed,enabling the application of pyrolysis-free catalysts in energy devices for PEMFCs.The focused ion beam field emission scanning electron microscopy and computational fluid dynamics（CFD）experiments show that the relative diffusion coefficient of cross-linked nanofiber electrode was increased by 3.5 times,which provides an effective way for the reactant to enter the active site and provides a channel for the efficient discharge of water.The results show that the peak power density of the integrated Co-COP nanofiber electrode is increased by 1.72 times compared with the traditional spraying method,and the durability was significantly improved.In addition,this nanomanufacturing technology also maintains good scalability and uniformity.（5）Highly efficient all-in-one membrane electrode with well-defined structures was synthesized by in-situ growth of nanoarrays,and the low ionization barriers and unobstructed mass transfer pathways endowed the catalyst with good device performances for PEMFCs.Co-COP with a well-defined Co-N₄-C structure was synthesized by a pyrolysis-free strategy and anchored on the surface of Co₃O₄ nano-arrays to obtain efficient nanoarrays electrocatalyst with fully exposed active sites（Co-COP@Co₃O₄/CP）.As a result,the Co-COP@Co₃O₄/CP catalyst provided an unprecedented peak power density(0.413 W cm^-2)at real H₂-Air PEMFCs,which is a new record for pyrolysis-free Co-N-C based electrocatalysts and comparable to current pyrolyzed Co-N-C.Remarkably,the cell voltage remained 82%of the original after a 100 hours stability test,which is more stable than most reported pyrolyzed Co-N-C.