| The realization of the"dual carbon"goal has put forward new era requirements for Chinese green development,technological innovation and responsibility as a major country;under the combined influence of the global climate governance process and the domestic modernization high-quality development goals,it is more and more urgent for China to build a clean,low-carbon,safe and efficient energy system.Hydrogen energy is a secondary energy source with a wide range of sources,green and low-carbon,and rich application scenarios.In the hydrogen energy industry chain,the upstream electrolysis of water for hydrogen production is considered to be the most potential green process for obtaining hydrogen energy.The fuel cell and its application in the middle and lower reaches are an important energy conversion technology using hydrogen energy.However,the slow kinetics of hydrogen evolution reaction,oxygen evolution reaction and oxygen reduction reaction are the key factors restricting the development of the above energy systems.The development of efficient catalysts to accelerate the chemical reaction rate,energy conversion efficiency and product selectivity is the key to promote the rapid development of hydrogen energy.Precious metal（platinum,iridium,ruthenium）-based catalysts are highly efficient electrocatalysts,but their high cost and rare reserves limit their large-scale development.Transition metal-based catalysts,especially iron,cobalt,and nickel-based catalysts,have the advantages of abundant active sites and low cost,and are the most potential catalysts to replace precious metals in the future.However,these catalysts often also exhibit disadvantages such as poor electrical conductivity,low catalytic activity,and instability.Based on this,this thesis aims to improve the catalytic activity and stability of Fe,Co and Ni-based catalysts,and designs highly active Fe,Co,and Ni-based catalysts from the electronic structure and atomic scales to achieve high-efficiency HER,OER,and ORR.The main research contents and conclusions are as follows:（1）Using thermodynamically stable Co O nanoarrays as precursors,they were converted into Co-MOFs with a"nanowall"structure by a liquid-phase impregnation method.MOF-derived hierarchical porous three-dimensional nitrogen-doped binary transition metal phosphide nanocatalysts（N-Co Px/Ni2P）were then prepared by a simple topochemical transformation between TMPs and Co-MOF.Benefiting from the unique catalyst morphology,optimized electronic structure,and fully exposed active sites,N-Co Px/Ni2P exhibits excellent hydrogen evolution activity at high current densities.At a current density of 650 m A cm-2,the overpotentials of N-Co Px/Ni2P are only 201 m V（0.5 M H2SO4）,409 m V（1 M PBS）,and 152 m V（1 M KOH）,surpassing that of commercial Pt/C.Meanwhile,the N-Co Px/Ni2P electrode has a small Tafel slope of 40.2m V dec-1（0.5 M H2SO4）,92.7 m V dec-1（1 M PBS）,30 m V dec-1（1 M KOH）and excellent cycling stability;the hydrogen evolution activity of continuous electrolysis for 24 h did not decrease significantly.（2）Using an electronic structure modulation strategy,a self-supporting V-Co P/Ni2P/NF with a three-dimensional cross-linked network structure was synthesized by a hydrothermal and simple phosphating process with nickel foam as the substrate and source.Benefiting from the improved electronic structure,electron transfer ability,and electron density of active sites,V-Co P/Ni2P/NF exhibits excellent HER electrochemical activity at all p H conditions,especially under alkaline conditions.When the current density was 10 m A cm-2,the overpotentials of V-Co P/Ni2P/NF were20 m V（1 M KOH）,58 m V（1 M PBS）,79 m V（0.5 M H2SO4）.When the current density was 300 m A cm-2（1 M KOH）,the overpotential was only 130 m V.Meanwhile,V-Co P/Ni2P/NF also exhibited small Tafel slopes of 54.2 m V dec-1（1 M KOH）,88.7 m V dec-1（1 M PBS）,58.3 m V dec-1（0.5 M H2SO4）and excellent cycle stability;When the electrolysis was continuous for 35 h,the hydrogen evolution activity did not decrease significantly.（3）Two-dimensional porous Ni Co P nanosheets were prepared by ion exchange and a simple phosphating process using metal-organic framework ZIF-L as a template.Oxygen vacancy-rich Ce O2 was introduced by an interfacial engineering strategy to obtain a novel heterostructured Ni Co P/Ce O2 nanocatalyst.The strong synergistic effect between Ni Co P and Ce O2 can effectively adjust the electronic structure of Ni Co P,which is beneficial to reduce the catalytic reaction energy barrier of OER.When the current density was 10 m A cm-2,the Ni Co P/Ce O2 overpotential was 260 m V.When the current density was 300 m A cm-2,its overpotential was only 368 m V,surpassing that of commercial Ru O2.Meanwhile,the Ni Co P/Ce O2 catalyst exhibits a low Tafel slope(68.1 m V dec-1)and a large electrochemically active area(24.2 m F cm-2).In addition,the electrocatalyst can also show excellent cycle stability,and the oxygen evolution activity can still maintain 87%after continuous electrolysis for 30 h.（4）Using metal-organic framework Fe-MOF（MIL-88A）as template and iron source,aniline monomer as nitrogen source,polymerized onto the surface of Fe-MOF to form PANI-wrapped metal-organic framework composite,which was finally calcined and acid washed at high temperature,and obtained iron single-atom catalysts（Fe-SAs/NC）dispersed on hollow carbon nanorods.Benefiting from the precise control of the coordinated environment of iron single atoms at the atomic level,the catalytic activity and selectivity of Fe-SAs/NCs are greatly improved.By EXAFS analysis,the isolated Fe atom is coordinated by four N atoms and it is the central active sites of ORR.Under the condition of 0.1 M KOH for ORR electrochemical tests,Fe-SAs/NCs exhibit excellent ORR activity:the onset potential(Eonset)is 0.96 V,and the half-wave potential(E1/2)is 0.86 V.In addition,Fe-SAs/NC also exhibited excellent methanol resistance and long-term stability,maintaining 92%oxygen reduction activity for 10 h of continuous operation. |
PDF Full Text Request