Design Of Nanostructures Based On Cobalt/Iron-based Transition Metal Catalysts And Their Electrocatalytic Performance

Replacing traditional fossil fuels with new renewable energy-related equipment has gradually become the focus of scientific research.In this regard,electrochemical energy storage and conversion technologies provide great opportunities for the development of next-generation clean energy systems.For example,an oxygen reduction reaction?ORR?fuel cell with oxygen can directly convert chemical energy into electrical energy.Similarly,electrocatalytic water splitting devices driven by hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?play an important role in clean hydrogen energy supply.Noble metal electrocatalysts generally have good catalytic activity,but their high cost and scarcity limit the application prospects of noble metal electrocatalysts.Therefore,it is essential to develop non-noble metal electrocatalysts with high activity to replace precious metal electrocatalysts.For electrocatalysts,the number of active sites and inherent conductivity are the key factors that limit the catalytic efficiency of electrocatalysts.Generally,more active sites can be obtained by adjusting the morphology and surface defects,which helps the catalyst completely contact with the electrolyte and further improve the catalytic activity.On the other hand,metal doping or heteroatom doping can adjust the internal electronic structure of the catalyst to improve the electrical conductivity,which is beneficial to the transfer of electrons during the catalytic process.How to realize the collaborative optimization of these key parameters of electrocatalyst is very urgent for sustainable energy installations.Therefore,the work of this thesis starts with the nanostructure design of cobalt-based/iron-based transition metal catalysts,and improves the HER/OER/ORR catalytic performance of these materials through morphology adjustment,composition adjustment,electronic adjustment,and construction of surface defects.The research work of this thesis is as follows.1.The combination of transition metal sulfide and transition metal phosphide has the advantages of excellent stability and catalytic activity in electrocatalytic OER,and this study first adopted F127?addition polymer of polypropylene glycol and ethylene oxide?as a self-sacrificing template A nano-core-shell structure nitrogen-doped cobalt sulfide?Co₉S₈-F127?was synthesized;then,it was doped with phosphorus to obtain phosphorus-doped Co₉S₈-F127?P-doped-Co₉S₈-F127?catalyst.X-ray photoelectron spectroscopy,electron microscopy,Raman and other characterization confirmed the doping effect of P on Co₉S₈-F127 and the formation of defective cobalt sulfide.Studies have shown that P-doped-Co9S8-F127(over-potential at 10 mA cm^-2 is 290.7 mV)is better than Co₉S₈-F127(over-potential at 10 mA cm^-2 is 309.7 mV)and is not synthesized using F127 as template The Co₉S₈(over potential at 10 mA cm^-2 is 323.7 mV)exhibits excellent OER catalytic performance,and is far superior to the catalytic performance of the precious metal RuO₂(over potential at 10 mA cm^-2 is 347.9 mV).In addition,the stability test found that the OER overpotential of P-doped-Co₉S₈-F127 at 10 mA cm^-2 after 40,000 s test basically did not rise,and the excellent stability of transition metal sulfide made P-doped-Co₉S₈-F127 perform Delightful catalytic stability.The P-doped-Co₉S₈-F127 designed and synthesized in this study provides new ideas for the efficient OER catalyst co-doped with non-noble metals S and P.2.The iron-nitrogen co-doped carbon catalyst has made significant progress in the application of electrocatalytic ORR,but its electrocatalytic activity still has great potential for improvement.In this study,1,2,4-triazole was used as the organic ligand,and zinc nitrate hexahydrate was used as the zinc source to synthesize a three-dimensional metal-organic framework material?Zn-MOF?;followed by the wet chemical method to Zn-MOF Introduce iron?Fe?source,g-C₃N₄?carbon nitride?in sequence to obtain Fe/Zn-MOF-C₃N₄,and then pyrolyze it at high temperature to obtain defective nitrogen-doped iron-based carbon nanotube catalyst?Fe@NC-C₃N₄?.Structural characterization shows that the introduction of Fe does not destroy the main frame of Zn-MOF itself;Fe@NC-C₃N₄ exhibits the characteristics of iron nanoparticles and oxygen defects embedded at the top of carbon nanotubes.Electrochemical studies have shown that the Fe@NC catalyst?half wave potential 0.85 V?free of oxygen defects and the NC catalyst?half wave potential 0.76 V?obtained by direct pyrolysis of Zn-MOF are obtained in phase In comparison,Fe@NC-C₃N₄ catalyst showed excellent ORR catalytic performance?half wave potential 0.88 V?.This defect construction method greatly improves the ORR catalytic performance of the iron-nitrogen co-doped carbon catalyst,and provides ideas for constructing a defect nano catalyst.3.Exploring the pyrolysis process of the carbon source is of great significance to realize the morphology control and catalytic performance optimization of the carbon material electrocatalyst.This research work combined with thermogravimetry-mass spectrometry online technology?TG-MS?to track the pyrolysis process of Co/Zn-MOF-146 obtained by cobalt-doped Zn-MOF?obtained in the above work?to obtain CH₄,CO,CO₂ and HC=N and other small molecule intermediate material information,deducing the decomposition,polymerization,redox and other reaction steps involved in the pyrolysis process.The study found that the reducing atmosphere formed by the stage pyrolysis can promote the formation of a uniform carbon-intercalated nitrogen-doped carbon nanotube catalyst?Co@NC-146?with the carbon intermediate produced by the pyrolysis.Structural studies show that Co@NC-146prepared under optimized conditions has uniformly distributed carbon nanotubes,appropriate carbon defects,high specific surface area,and nitrogen content,and it exhibits excellent ORR,OER,and HER multifunctional catalysis.As an ORR catalyst,Co@NC-146(0.875 V,75.8 mV dec^-1)exhibits a higher half-wave potential and faster electrochemical power than the commercial Pt/C catalyst(0.852 V,86.3 mV dec^-1).Learning rate.After phosphating Co@NC-146,the obtained P-doped-Co@NC-146 showed higher OER and HER catalytic performance than Co@NC-146.As an OER catalyst,P-doped-Co@NC-146 has an overpotential of 330 mV at 10 mA cm^-2),which is much lower than the catalytic performance of commercial RuO₂ catalyst(overpotential at 336 mV at 10 mA cm^-2).In addition,P-doped-Co@NC-146 also showed considerable HER catalytic performance,with an overpotential of 172.7 mV at 10 mA cm^-2.This method of preparing a more uniform and highly active catalyst by studying the pyrolysis mechanism of materials provides valuable ideas for the multifunctional application of transition metal catalysts.