| With the rapid increase in global demand for energy and intensified environmental degradation and other issues.The development of clean energy has become a major demand of the country,and Fuel cells as a very typical clean energy conversion device has attracted wide attention from researchers.The fuel cell can directly convert the chemical energy of the fuel into electrical energy,and has the advantages of high power density,cleanliness and environmental protection.and have great application potential in the fields of portable electronic products,vehicles and fixed devices.It has huge application potential in the fields of portable electronic products and vehicles,and is expected to replace traditional energy conversion devices in the future.However,fuel cells still face some challenges such as high cost,short life,and weak environmental adaptability.The main problem is the high cost,and the high cost mainly comes from the Pt-based catalyst used for cathode oxygen reduction(ORR).Although Pt-based catalysts have high catalytic activity,they have problems such as poor stability and high price,which have seriously hindered the large-scale application of fuel cells.Therefore,the development of low-cost catalysts with high activity and long life is of great significance for the commercialization of fuel cells.Among them,transition metal and nitrogen-doped carbon(M-N_x/C)catalysts have attracted wide attention because of their low price and high catalytic performance.In M-N_x/C materials,nitrogen atoms can stabilize the metal atoms and reduce the outer electron density of the metal atoms,thereby enhancing the binding energy between the metal atoms and the oxygen transition state to reduce the overpotential of the ORR.Therefore,it has high catalytic activity and is widely used in electrocatalytic reduction reactions.However,the existing M-N_x/C catalyst still faces the problem of low concentration of metal active sites and low catalytic activity.The main reason for the above problem is that the synthesized M-N_x/C precursor is easily decomposed at heat treatment,resulting the aggregation of metal atoms and losing a lot of metal active sites.In this paper,we import a new stable carbon skeleton and a new method of metal block copolymerization of M-N_x/C,which was effectively inhibited the metal atom aggregation and obtained the number of metal active sites of M-N_x/C.The specific research process is as follows:(1)Aiming to the problems of low active site concentration of existing transition metals and nitrogendoped carbon(M-N_x/C)catalysts easy aggregation in the process of metal pyrolysis.Fe-N_x/C materials with high density metal active sites were prepared by metal coordination block copolymer.Utiliting the rich NH2-groups of polymer to ligand iron ion,realized high density Fe-N4 in the precursor.The strong bonding force between the metal ions and coordination bond not only fixed and dispersed metal sites,but also can promote the electron cloud redistribute between the metal ions and the skeleton carrier(such as nitrogen atoms).Which is optimize the binding energy between the metal atoms and the oxygen transition state,to obtained high oxygen reduction activities at the same time reduce the ORR potential.Further,through block structure realized the stepped carbonization of the polymer,which is effectively improve the thermal stability of the skeleton during carbonization,thus reducing the destruction of coordination structure caused by carbon skeleton collapse and increasing the retention of Fe to achieve the purpose of constructing high concentration Fe-N metal active site.According to the above ideas,Fe-N_x/C-A catalyst was successfully prepared,and showing excellent oxygen reduction catalytic performance and long-term catalytic stability.Its exhibit the better oxygen reduction reaction activity with the initial potential and the half-wave potential of 0.86 V and 0.71 V respectively,and remained relatively high activity after 2000 cycles of CV.(2)In the first part of the study,although the metal coordination block copolymer catalyst we constructed obtained high iron retention,there was still has a certain gap with commercial Pt/C.The problem may be that the dispersion of Fe site is poor and a small number of exposed Fe-N active sites.To solve above problems,we carried out modification on the polymer precursor structure,,using the change of block ratio of precursor to improve its stability in the pyrolysis process.Meanwhile,optimizing the step carbonization process to implement the stability of coordination and the block structure,by enhancing Fe-N sites anchors to promote its confined growth within the block structure,successful achieved the high dispersed of Fe sites in the final.It was found that after proportional control of block units(MPD),Fe-N_x/C-A with block OPD: MPD was 2:6 had the highest oxygen reduction catalytic activity,and had the catalytic activity comparable to that of commercial Pt/C in both acidic and alkaline systems,and the long-term catalytic stability was much higher than that of Pt/C.Among them,the initial potential and half-wave potential in the HCl O4 electrolyte at p H = 1 reached 0.90 V and 0.81 V,respectively,and the initial potential and half-wave potential in the KOH electrolyte at p H = 13 reached 1.0 V and 0.90 V,respectively.The strategy adopted in this paper for the synthesis of highly dispersed Fe-N_x/C by metal-coordinated block copolymers is also applicable to the synthesis of other highly dispersed mono-metal sites of transition metal nitrogen and carbon materials. |
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