| The extensive use of fossil fuels is an important cause of environmental pollution and energy crises.The development of clean and renewable energy is an effective way to solve the above problems.Hydrogen-oxygen fuel cell,as a typical and highly efficient device for converting chemical energy into electrical energy,has become a hot spot in new energy research.However,the slow kinetics and high overpotential of the oxygen reduction reaction(ORR)at the cathode of fuel cells have severely restricted its development.Therefore,the development of high efficiency and low-cost ORR catalysts is the key to the industrialization of fuel cells.Pt/C catalyst is currently the most excellent oxygen reduction catalyst.However,Pt/C is not only expensive and has poor durability,which seriously hinders its commercial application.Based on this,we have developed an oxygen reduction catalyst based on metal/covalent organic framework materials(MOFs/COFs).Through the use of high gravity process enhancement technology,the micro-mixing transfer efficiency in the synthesis process of MOFs/COFs is enhanced,the composition of MOFs/COFs is precisely controlled,the reaction efficiency is effectively improved,and the rapid,uniform and macro-quantified preparation of nanomaterials is achieved.Through structural characterization and electrochemical testing,the relationship between the composition,structure and performance of the catalyst is analyzed,and the macro-preparation method of the electrochemical oxygen reduction high efficiency catalyst is optimized and established.The main innovations and research contents of this paper are summarized as follows:1.Develop a high gravity synthesis method of COFBTC-M material with a conjugate structure,and prepare electrocatalysts with uniform performance in a large scale.The micro-mixing and mass transfer process of the reaction are strengthened by the super gravity reactor,which effectively improves the efficiency of the COFBTC-M material synthesis reaction.Compared with the traditional oil bath reaction process,the reaction time is shortened from 12 h to 5-30 min,which realizes the rapid preparation of COFBTC-Fe material,and its space-time yield is as high as 357 kg/(m3·day).By introducing different metal sources,a series of materials such as COFBTC-Co,COFBTC-Mn,COFBTc-Ni,COFBTC-Pd,etc are prepared to achieve precise control of the composition of COFs catalysts.The characterization results of COFBTC material proved that it has a two-dimensional layered structure,and strong polar and alkaline solvents can be inserted into the interlayer structure,resulting in COFBTC having good solubility in polar solvents and can be used as a homogeneous catalyst in the ORR catalytic process It provides possibilities for the design of new fuel cells.In addition,the conjugated structure of COFBTC allows it to have a certain degree of conductivity,which can be directly applied to the ORR catalytic process,avoiding the unclear active center caused by high temperature calcination,and laying the foundation for the subsequent analysis of the catalyst active center.Electrochemical test results prove that COFBTC-Fe has the best oxygen reduction performance,and E1/2 can reach 800 mV(vs RHE)in alkaline electrolyte.2.The synthesis process of nano-ZIF-8 particles is adjusted through the combination of high gravity process intensification technology and surfactants,and the size of nano-particles is adjusted to prepare high activity and high stability ORR catalysts.By adjusting the super gravity intensity(the super gravity factor β varies from 50-210),and introducing CTAB,F127,PVP and other surfactants of different composition and content,the particle size in the range of 20-200 nm can be controlled.ZIF-8 nanoparticles are further pyrolyzed and carbonized at high temperature to obtain a single-atom catalyst with porous structure and rich in Fe-Nx active sites.The structure analysis of the material by XRD,Raman,XPS and other characterization methods proves that our method can well control the catalyst particle size and exhibits excellent uniformity.At the same time,the super gravity process strengthening technology greatly strengthens the reaction.In the microscopic process,the addition of surfactant not only controls the particle size of the precursor,but also introduces a richer nitrogen source,forming more and more dispersed Fe-N4 active sites during the high temperature carbonization process.The obtained HG-Fe-NC@F127 catalyst shows excellent ORR activity under alkaline conditions,with E1/2 reaching 881 mV and limiting current density j close to 5.0 mA/cm2.The above data are significantly better than commercial Pt/C catalyst.At the same time,our catalyst is a 4e’ process in the ORR catalytic process,the electron transfer number is above 3.95,the hydrogen peroxide yield is below 5%,and it has good stability.After aging for 40000 s,it still maintains 80%activity,E1/2 only dropped by 30 mV.In addition,the catalyst also has good ORR performance under acidic conditions. |
