Research On The Design And Performance Control Of Doped Porous Carbon-based Non-precious Metal Electroctalyst

Oxygen reduction reaction（ORR）,hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）are the core reactions of green electrochemical energy storage and conversion technologies such as regenerative fuel cells,rechargeable metal-air batteries,and electrochemical water splitting devices.However,ORR and OER reactions are relatively complex,slow in kinetics,and cannot proceed spontaneously.Both rely on the catalysis of precious metal catalysts,which severely restricts the generation and conversion of green energy.Based on this,reasonable design and development of non-noble metal oxygen electrocatalysts is the most important way to solve the current dilemma.Although the latest research advances are constantly being made,the rational design and synthesis of non-noble metal electrocatalysts for ORR and OER in alkaline and acidic media is still a key issue that researchers need to continue to explore.Heteroatom doped carbon based materials have attracted extensive attention because of their excellent conductivity,porous structure and good chemical stability.Therefore,in this project,heteroatom doped porous carbon based catalyst materials with unique composition and structure were synthesized with 2,6-diacetylpyridine as precursor through component regulation,and have excellent electrocatalytic activity.In addition,the source of active sites of the catalyst was discussed by theoretical calculation,and the relationship between microstructure and catalytic activity was analyzed.The main contents are as follows:（1）Using 1,8-diaminonaphthalene and 2,6-diacetylpyridine as reactant precursors,a new bis（imino）-pyridine ligand based polymer was synthesized.Carbon spherical monoatom dispersed Fe-N-C catalyst was prepared by pyrolysis with Iron（III）chloride hexahydrate as metal precursor.A series of physical characterization and electrochemical activity tests were carried out on the catalyst.The results show that the Fe-N-C catalyst prepared with the precursor without templates or supports possess the advantages of ultrahigh specific surface area up to 1796.0 m²·g^-1 and high graphitization degree.The highly dispersed monoatomic Fe in the interior and surface of the Fe-N-C material forms a unique Fe-N₅ coordination structure.At the same time,the catalyst material showed not only excellent ORR catalytic activity and selectivity,but also excellent OER catalytic performance in both 0.5 M H₂SO₄and 0.1 M KOH electrolyte.The total oxygen electrocatalytic activity is 0.70 V in alkaline medium and 0.86 V in acid medium.The total oxygen electrocatalytic bifunctional activity was 0.70 V in alkaline medium and 0.86 V in acidic medium.（2）Using 2,6-diacetylpyridine and 3,3′-diaminobenzidine as the reactant precursors,this work explored the relationship between the three cobalt precursors（Cobalt chloride hexahydrate,cobalt nitrate hexahydrate,and cobalt acetate tetrahydrate）and the structure and performance of the catalyst.Co-N-C multifunctional electrocatalyst was prepared by pyrolysis and post-treatment of the above complexes for high-efficiency water-splitting and oxygen reduction.The results show that the Co-N structure and the metal cobalt nanoparticles encapsulated in the N-doped carbon matrix have a strong synergistic effect.The cobalt ion source directly affects the performance of the final carbon material.The Co-N-C catalyst prepared with cobalt acetate as the precursor possess good overall performance.This excellent activity is attributed to the synergy between the Co-N groups and the metallic cobalt.The study also shows that Co-N_x is a very effective active site for ORR,while the metallic cobalt has a great positive contribution to the water splitting reaction.（3）Based on the above two works,organometallic complexes containing iron-N and cobalt-N structures were prepared with 2,6-diacetylpyridine and 3,3′-diaminobenzidine as reactants according to different molar ratios of Fe and Co.Fe_xCo_yNC catalyst is obtained by pyrolyzing and post-processing the complex powder.The influence of the ratio of Fe and Co on the structure and performance of the catalyst was investigated.The catalysts prepared in this work possess Fe single atoms in the form of Fe-N_x structure,while Co exists in two states:Co-N_x（a small amount）and metallic Co.Among them,the catalyst with Fe:Co molar ratio of 1:5（Fe₁Co₅NC）has the best ORR performance in both acidic and alkaline electrolytes.The Zn-air battery assembled by Fe₁Co₅NC has a peak energy density of 284 mW·cm^-2 during discharge.The test results show that a small amount of single atoms Fe can significantly improve the ORR catalytic activity of the Co-N-C catalyst.DFT calculations also help to prove that the Fe-N-C layer（contains a small amount of Co-N_x）wrapped Co metal nanoparticle structure possess a better adsorption energy barrier for oxygen intermediates than that of Fe-N-C and metal Co is used alone.That is,Fe-N-C layer encapsulates Co metal nanoparticle structure possess a higher ORR catalytic activity.（4）N,S co-doped carbon encapsulating Co₉S₈ nanoparticles（Co₉S₈@N,S-C）catalyst was successfully synthesized by Using 2,6-diacetylpyridine and 4,4’-dithiodianiline as reaction precursors,and cobalt chloride hexahydrate as cobalt salt.The as-prepared Co₉S₈@N,S-C has rich macroporous and mesoporous structures,as a high-efficiency bifunctional oxygen electrocatalytic activity in 0.1 M KOH,and its ORR half-wave potential reaches 0.89 V,and a low overpotential of 304 mV for OER.The bifunctional potential difference of ORR/OER is only 0.647 V vs RHE,which is much better than the commercial precious metal Pt/C+IrO₂.The test of the rechargeable Zn-Air battery assembled with Co₉S₈@N,S-C shows that its maximum output power density is 259mW·cm^-2,and the specific capacity is 862 m Ah·g _Zn^-1.In addition,it has a higher stability than Pt/C+IrO₂.After 110 hours of continuous operation,the charge-discharge voltage gap increased only slightly by about 90 mV,and the round-trip efficiency decreased by only 4.83%（while Pt/C+IrO₂ decreased by14.48%）.Theoretical calculation studies shown that the surface N,S-C layers and Co₉S₈ can adjust each other’s Fermi levels,greatly improving the electrical conductivity of the material and rapidly transferring electrons to the active site,so that the adsorption energy of Co₉S₈@N,S-C on O intermediate is more favorable than using Co₉S₈ alone.（5）Because the metal Fe and Co are easy to fall off in the catalytic process,therefore,this work using 2,6-diacetylpyridine,1,5-naphthalenediamine and 2,5-dithiobiurea as nitrogen,carbon and sulfur sources,metal-free NS/C oxygen bifunctional electrocatalyst was prepared by adjusting the molar ratio of the three monomers.When the molar ratio of the three organics is 1:0.5:0.5,the obtained NS/C catalyst possess the highest ORR and OER catalytic activity in0.1 M KOH,ORR/OER bifunctional potential difference is only 0.72 V vs RHE,better than commercial Pt/C+IrO₂.The result shows that the rechargeable Zn-Air battery assembled with NS/C catalyst can provides a maximum output power density of 149 mW·cm^-2 and a specific capacity of up to 769 m Ah·g_zn^-1.And it can be charged and discharged stably for 100 hours with a decay of only 28.36%,and its activity and stability far exceed the same mass load of Pt/C and IrO₂（1:1wt.%）mixed catalyst assembled Zn-air battery（attenuation 37.18%）.The theoretical calculation results of this work show that the high activity of NS/C is mainly derived from C atoms with positive charge and spin density induced by specific N and S doping.