Preparation And Electrocatalytic Performance Of Transition Metal(Fe,Co,Ni)/N Modified Carbon-based Materials

In recent years,with the increasing consumption of fossil energy,clean and renewable energy is difficult to meet people’s daily needs due to the impact of time and space,coupled with the increasingly severe environmental problems,so that electrochemical energy storage and conversion technology has become a research hotspot.Oxygen reduction reaction（ORR）process and oxygen evolution reaction（OER）process are essential electrode reactions in these technologies.However,due to their slow kinetics and complex reaction process,the performance of related instruments and equipment is directly affected,and the development of electrochemical storage technology is also hindered.At present,precious metal-based（Pt,Ru）catalytic materials are still the main electrocatalysts in this field.However,due to the high price and cost,scarce reserves,and poor catalytic selectivity,it is not conducive to further large-scale commercial application and promotion.The effective way to solve these problems is to use low-cost raw materials and simple method to prepare highly efficient and stable non-noble metal-based catalyst.Carbon-based materials are considered to be one of the most promising electrocatalysts because of their extensive sources,abundant forms,harmonicity and good chemical stability.Based on the above analysis,a series of carbon-based catalysts modified with heteroatoms were designed and synthesized.Combined with the characterization of the morphology and structure of the materials and the electrochemical performance test,the catalytic activities of the prepared materials in ORR and OER were investigated.And discussion of its practical application potential in zinc-air batteries were made.The research work is as follows:（1）Using g-C₃N₄ as carbon source,nitrogen source,soft template agent and glucose as auxiliary carbon source,a series of N-C nanosheets were prepared by hydrothermal pyrolysis.By the characterization of the morphology and structure of the sample,it is proved that the catalyst has a larger specific surface area(627 m² g^-1)and a larger pyridine nitrogen content（23.3%）when the mass ratio of g-C₃N₄ to glucose is 1:4 and the calcination temperature is 900℃.In the electrochemical test,N-C（1:4）showed the best ORR performance,with a half-wave potential of 0.85 V（vs.RHE）,30 mV higher than that of commercial Pt/C electrode,and showed durable stability and good methanol tolerance.The open circuit voltage of N-C（1:4）was 1.40 V and the peak power density was 47 mW cm^-2 for 8 h.The voltage retention rate was 95.7%.The results showed that the catalyst has certain practical application.（2）The electrocatalytic performance of N-C materials was improved by introducing a single transition metal:using g-C₃N₄ as carbon carrier and nitrogen source precursors,and ferric nitrate,cobalt nitrate and nickel nitrate as transition metal precursors,the nanotube Fe-N-C,Co-N-C and Ni-N-C series catalysts were successfully prepared by simple two-step pyrolysis method,and the ORR and OER properties of the catalysts were investigated.In ORR test,the initial potential(E_onest)of Fe-N-C was 0.97 V and E_1/2 was0.87 V,which were significantly higher than that of Co-N-C,Ni-N-C and commercial Pt/C.At the same time,the ORR reaction process of Fe-N-C is 4e^-transfer,showing a long lasting stability and good methanol tolerance.In the OER test,the overpotential of Fe-N-C and Co-N-C at the current density of 10 mA cm^-2 is 380 mV and 340 mV,respectively,which is better than that of precious metal Ru O₂ materials.Fe-N-C was used as the cathode of the primary zinc-air battery in the assembly test.The results show that the open circuit voltage is 1.46 V and the peak power density is 78 mW cm^-2.There is almost no voltage attenuation in the constant current discharge test and the retention rate is 99.3%.（3）On the basis of the above work,double transition metals were introduced,with ferric nitrate and cobalt nitrate as transition metal precursors,g-C₃N₄ as carbon carrier and nitrogen source,and triblock copolymer F127 as auxiliary carbon source and surfactant,the 3D porous nano-sheet FeCo/NC catalyst was prepared.The ORR and OER properties of FeCo/NC catalyst in alkaline electrolyte were investigated.In ORR,the E_onestand E_1/2 of FeCo/NC were 0.98 V and 0.86 V,both of which were 40 mV higher than commercial Pt/C,and the limiting current density reached 6.02 mA cm^-2.In OER,when the current density of FeCo/NC is 10 mA cm^-2,the overpotential can reach 280 mV,which is lower than 140 mV of Ru O₂,indicating that FeCo/NC has better OER performance.After i-t test of 15000 s,the current density of the prepared material almost did not decrease,indicating its good cycling stability.Finally,the application potential of FeCo/NC in primary zinc-air battery was investigated.After 400 s test,the open circuit voltage remained at 1.53V,the polarization and power density test showed that the peak power density was 110 mW cm^-2,and the voltage retention rate was 99.52%after 8 h constant current discharge test.