Design,Preparation And Property Of Bimetal Iron-Cobalt And Nitrogen Co-Doped Carbon-Based Electrocatalysts

Electric energy plays an important role in the adjustment of the global energy structure to clean and renewable energy.Various types of related energy conversion and storage technologies,such as metal-air batteries,water splitting technology,etc.,have received extensive attention and development applications.And the design of efficient electrode materials is the key to efficient energy conversion and application.The precious metal-based catalysts are currently the benchmark materials,yet unsatisfactorily suffering from expensiveness and scarcity.In recent years,the M-N-C co-doped with transition metal such as iron or cobalt and nitrogen have become one of the most promising non-noble metal-based electrocatalysts due to their low price,high activity,and good stability.However,when it comes to practical applications,due to the limitations of synthesis methods and insufficient comprehension of structure and performance relationships,it is still challenging to develop a simple method to prepared M-N-C material with excellent bifunctional performance and interfacial effect.This thesis takes the highly dispersed bimetal iron-cobalt and nitrogen co-doped carbon-based materials as the research object,deeply explores the catalytic mechanism of the bimetal sites,and further clarifies the synergistic effect.Based on the understanding of the unique physical and chemical properties of the above-mentioned materials,a series of high-efficiency and low-cost bifunctional catalysts have been designed and prepared for the gas-involving electrocatalytic reaction involved in zinc-air batteries（ZAB）or water electrolysis devices.The research not only has guiding significance for the synthesis and development of such non-noble metal catalysts,but also has a positive role in promoting the wide application of related materials in electrochemical energy storage and conversion devices.The specific research results obtained are as follows:First,based on the understanding of metal/nitrogen co-doped carbon-based catalytic materials,an efficient preparation method of flexible carbon nanofiber（CNF）film materials modified by atomically dispersed Fe-Co sites is reported.The special metal/nitrogen co-doped carbon-based precursor is prepared by the solvothermal reaction of formamide and metal salt,and the Fe₁Co₁-CNF film with good mechanical properties and flexibility was finally obtained via electrospinning method.In the same way,Fe₁-CNF with single-atom Fe sites,Co₁-CNF with single-atom Co sites,and CNF without metal sites were prepared as comparative samples,the influence of the metal sites introduction on the physical and chemical properties of carbon nanofibers was explored in detail.And combined with electrochemical tests and simulation calculations,the catalytic mechanism of the bimetallic site was deeply explored.It was found that the Fe N₃-Co N₃ active sites promoted the redistribution of the local charge density,and the synergistic effect between Co and Fe is conducive to the reaction intermediates with the proper adsorption/desorption free energy at the active site during oxygen reduction reaction（ORR）/oxygen evolution reaction（OER）.ZAB assembled based on Fe₁Co₁-CNF exhibits a high discharge specific power of 201.7 m W cm^-2with good cycle stability.More importantly,due to its good mechanical strength and flexibility,Fe₁Co₁-CNF film can be used as a self-supporting electrode material for flexible solid ZAB,and can maintain charge and discharge stability under repeated bending.This material has good application prospects in the field of electrode materials for portable electronic devices.Then,based on the synthesis law of the iron-cobalt and nitrogen co-doped carbon-based material,the self-growth of carbon nanotubes during the pyrolysis process were realized by controlling the metal ratio in the precursor.The f-Fe Co-CNT catalyst with hierarchical structure composed of dispersed Fe Co-N_x active sites and highly conductive carbon nanotube network was in situ formed.Due to the abundant bimetallic active sites,proper pore structure,high conductivity network,excellent hydrophobicity,and high active specific surface area,the obtained material exhibits excellent ORR and OER bifunctional catalytic performance.ZAB assembled with f-Fe Co-CNT as the catalyst of the air cathode has demonstrated outstanding performance in specific power density,charge and discharge voltage,rate performance and stability,which can work stably for more than 180 hours at a current density of 20 m A cm^-2.It provides a reference for the hierarchical structured design of atomic-level metal-dispersed M-N-C materials used in electrochemical energy-related devices.Furthermore,it was found that the atomically dispersed M-N-C could be used as a special active substrate to prepare a bifunctional ORR/OER electrocatalyst with a specific morphology.Due to the strong coupling effect,the catalyst with Ni Fe-LDH nano-dots（Ni Fe-ND,d～5 nm）and three-dimensional flower-like atomically dispersed Fe Co-NC composite could be prepared.The three-dimensional flower-like structure and N/O doping in Fe Co-NC play a key role in limiting the growth of Ni Fe-ND,ensuring the maximum exposure and utilization of active sites.The prepared composite material（Ni Fe-ND/Fe Co-NC）exhibits excellent bifunctional ORR and OER performance,the potential gap between the ORR E_1/2（half-wave potential）and the OER E_{j=10 m A cm}-2 is only 0.81 V.In-situ Raman data showed that more Ni（Fe）OOH active component in Ni Fe-ND/Fe Co-NC was produce during the OER reaction compared to mechanical mixtures,confirming the positive effect of Fe Co-NC substrate to the OER performance of Ni Fe-ND.The Ni Fe-ND/Fe Co-NC assembled ZAB showed excellent power density and cycle life,which further proved that the composite material prepared with the atomically dispersed M-N-C as the substrate has high activity and stability during the reaction.Finally,based on the above-mentioned study,it was found that the high nitrogen-containing carbon materials with special morphology could be prepared with formamide and metal salts.Due to the special coordination between metal and nitrogen,the in-situ doping of N in the Fe Co phosphate（N-Fe Co P,about 5 nm in diameter）can be achieved.At the same time,due to the different strength of coordination effect between different metals and N,the morphology and properties of materials could be effectively controlled by adjusting the ratio of Fe and Co in the raw material.With the increasing Fe/Co ratio,the thickness of obtained sample becomes thinner.And the electrochemical test results show that when the Fe/Co molar ratio in the raw materials is 1/4,the final nitrogen-doped phosphide sample has the best bifunctional catalytic performance of hydrogen evolution and oxygen evolution reaction,exhibiting good electrochemical catalytic activity and can working stably for more than 10 hours when the current density is close to 10 m A cm^-2 in the water splitting test.