Controllable Synthesis And Performance Investigation Of Nitrogen Doped Carbon-based Electrocatalysts Towards Oxygen Reduction And Oxygen Evolution Reactions

Hydrogen energy,one of the renewable energy,has been received widespread concerns by engineers and scientists due to its advantages such as a high caloric value,wide range of sources and environment-friendly feature,etc.In the current development of hydrogen energy,the key issues are how to produce hydrogen more inexpensively,store hydrogen more safely and use hydrogen more efficiently.Fuel cell,one of the most effective ways of hydrogen utilization,is high efficiently,green and quiet;yet,to produce hydrogen of high purity towards water splitting,is a green,environment-friendly and easy to operate method.Currently,the development bottlenecks of fuel cell and water splitting lie in the slow kinetics of oxygen electrode reactions,the high over-potential and the large use of precious metal oxygen electrode catalysts.So,it is of great significance to develop bifunctional electrocatalysts of ORR and OER with low cost,high electrocatalytic activity and outstanding durability.In this thesis,The study purposes are to prepare nitrogen-doped carbon-based bifunctional electrocatalysts with low cost and high-performance,and the mesoporous carbon and reduced graphene oxide(RGO)are used as the carbon support,and transition metal oxides,layered double hydroxides and spinels are used as the active components,and different nitrogen reagents are introduced to modify the carbon support.The synergistic effects and bifunctional electrocatalytic properties between nitrogen-doped carbon support and active components of different ORR and OER electrocatalysts are studied.It is of great significance to obtain several fabrication methods of non-noble metal a bifunctional electrocatalysts with excellent ORR and OER electrocatalytic performance,so as to promote the large scale application of fuel cell and water splitting systems.The main contents and conclusions are as follows:(1)Preparation of nitrogen-doped mesoporous carbon supported Co3O4 nanosheet catalystA series of nitrogen-doped ordered mesoporous carbons with different nitrogen content were prepared by hard template method and post-treated nitrogen doping method using SBA-15 as hard template,sucrose and melan mine as carbon and nitrogen reagents;nitrogen doped mesoporous carbon4 supported Co3O4 nanosheet composite was prepared by solvothermal metd hod using modified mesoporous carbon as the support,cobalt acetate and urea an scobalt and nitrogen reagents.The results show that when the mass ratio of melamine to modified mesoporous carbon is 5,the specific surface area(12548m2/g),pore volume(1.5m3/g)and nitrogen content(3.2 at %)of the prepared material are the largest,and the material shows the best ORR electrocatalytic performance(the onset potential,half-wave potential and limited current density at 1600 rpm and 0.5V of the material are 0.92 V,0.82 V and-4.33 mA cm-2,respectively).Compared with nitrogen-doped mesoporous carbon and the physical mixed sample,nitrogen doped mesoporous carbon supported Co3O4 nanosheet composite shows the best bifunctional electrocatalytic performance(the onset potential,half-wave potential and limited current density at 1600 rpm and 0.5 V of the ccomposite are 0.90 V,0.76 V and-4.31 mA cm-2,respectively;and the OER overpotential at 10.0 mA cm-2 is 365 mV),which reveals the strong synergistic effect between Co3O4 nanosheets and nitrogen-doped mesoporous carbon support.Compared with the ?E values of electrocatalysts in relevant references(?E value refers to the potential difference from ORR at-3.0 mA cm-2 to OER at 10.0 mA cm-2;the smaller AE value of catalyst,the better bifunctional electrocatalytic performance of the catalyst),the bifunctional electrocatalytic performance of the composite has advantages(?E=0.86 V).(2)Preparation of N-RGO supported Co3O4 nanoparticle catalystN1-RGO supported Co3O4 nanoparticles composite with RGO as support,NH4OH as precipitant and Co3O4 nanoparticles as active component was prepared by one-step solvothermal method in ethanol and deionized water mixed solvent.By adjusting the Co2+ molar concentration,the influence of the size effect of the active component and the strong synergistic effect between Co3O4 nanoparticles and nitrogen-doped RGO support on the electrocatalytic a ctivities of ORR and OER were investigated.The particle size of Co3O4 n anoparticles are reduced from 18.8 nm to 12.2 nm when the Co2+ molar concentration is from 0.025 M to 0.075 M;however,Co3O4 nanoparticles agglomerate when Co2+ molar concentration is greater than or equal to 0.1 M. The results show that the prepared material of 0.075 M shows the best ORR and OER electrocatalytic performace(the onset potential,half-wave potential and limited current density at 1600 rpm and 0.5 V of the composite are 0.92 V,0.82V and-5.24 mA cm-2,respectively;and the OER overpotential at 10.0 mA cm-2 is 331 mV),which is due to the size effect of Co3O4 nanoparticle and the strong synergistic effect between Co3O4 nanoparticles and nitrogen-doped mesoporous carbon supports.Up to date,the prepared electrocatalyst of 0.075 M is the best cobalt-based bifunctional electrocat(?E=0.75V).(3)Preparation of CoNiMn-LDH/PPy/RGO catalystCoNiMn-LDH/PPy/RGO composite was prepared by one-step coprecipitation method using RGO as carbon support and pyrrole as nitrogen source,and the polymerization of pyrrole was initiated by Mn4+,and pH of the solution was controlled by the addition of NaOH and Na2CO3 mixed base.The influence of the conductive polymer polypyrrole and the synergistic effect among different components of CoNiMn-LDH/PPy/RGO on the electrocatalytic performance of ORR and OER were investigated.Compared with CoNiMn-LDH/RGO and physical mixed sample CoNiMn-LDH+PPy,CoNiMn-LDH/PPy/RGO displayed the more excellent ORR and OER electrocatalytic performance(the onset potential,half-wave potential and limited current density at 1600 rpm and 0.5 V of CoNiMn-LDH/PPy/RGO are 0.88 V,0.77 V and-4.82 mA cm-2,respectively;and the OER overpotential at 10.0 mA cm-2 is 369 mV),which is due to the introduced PPy enhancing the conductivity of the composite and the strong synergistic effect of CoNiMn-LDH,PPy and RGO component.Compared with the electrocatalysts of relevant references,the bifunctional ectrocatalytic performance of CoNiMn-LDH/PPy/RGO has advantages(?E=0.85 V).(4)Preparation of N-RGO supported multicomponent compound catalystFirstly,NiFeMn-LDH/PPy/RGO composite was prepared by one-step coprecipitation method using RGO as carbon support and pyrrole as nitrogen source,and the polymerization of pyrrole was initiated by Fe3+ and Mn4+.Then NiFeMn-LDH/PPy/RGO was used as precursor,and the precursor was calcined in N2 atmosphene under different temperature,and a series of N-RGO supported multicomponent bifunctional electrocatalysts were obtained.The influence of calcination temperature on the components and electrocatalytic performance of ORR and OER were investigated.The results show that the calcined sample of 550? exhibits the best ORR activity(the onset potential,half-wave potential and limited current density at 1600 rpm and 0.5 V of 550? calcined sample are 0.90V,0.76 V and-4.40 mA cm-2,respectively),and the precursor NiFeMn-LDH/LPPy/RGO displays the best OER activity(the OER overpotential at 10.0mA cm-2 is 285 mV).The calcined sample of 550? shows the best bifunctional electrocatalytic performance (?E= 0.85 V),which is due to the strong synergistic effect among different components of the composite.In conclusion,four fabrication methods for the preparation of non-noble metal bifunctional electrocatalysts with excellent ORR and OER electrocatalytic performance were obtained by investigating the synthetic processes and the ORR and OER electrocatalytic performance of the four composites above.