Construction Of Carbon-based Multiple Transition Metal Compound Electrocatalysts And Their Oxygen Reduction Performances

Nowadays,with the continuous upgrading of technology,the extensive use of fossil fuels has brought a series of environmental and climate problems.This makes the contradiction between energy supply and environmental protection is particularly prominent,so it is particularly important to develop new energy supply equipment and optimize the proportion of new energy in the energy supply chain.Fuel cells are favored by people due to their advantages of high energy conversion efficiency and no pollution and noise during operation.Now they have been applied in some scenes in daily life.However,the reaction rate of oxygen reduction reaction（ORR）of its cathode is slow,and the cost of commercial precious metal catalyst（Pt/C）used today is relatively high,which seriously affects the further large-scale application of fuel cells in practical scenarios.In recent years,a series of carbon-based transition metal ORR electrocatalysts have been prepared by combining carbon materials with transition metal materials.Their high catalytic activity and low synthesis cost make them gradually become a research hotspot.However,the catalytic activity and stability of these catalysts still have a lot of room for improvement.Many studies have confirmed that the combination of two or more metals can give full play to the synergistic effect between metals and further improve the ORR activity of the catalyst.In this paper,supported transition metal alloy particle/carbon nanotube catalysts and encapsulated carbon-based multi-transition metal catalysts were constructed and their ORR properties and structure-activity relationships were studied.Specific research contents are as follows:（1）The transition metal anchored on the surface of carbon nanotubes（CNTs）is an effective strategy to improve the ORR performance of catalysts.In this paper,a simple sol-gel method was used to anchor three metal particles Fe Co Ni directly on the surface of CNTs to give full play to their synergistic effects and to investigate the contribution of metal elements in ORR reaction path selectivity.TEM,XRD and XPS results show that the three metal particles are successfully modified on the surface of CNTs,and due to natural oxidation in the air,a metastable oxide layer is formed on the surface of the alloy particles,which is favorable to ORR.In 0.1 M KOH solution,the onset potential of Fe Co Ni/N-CNTs is 0.81 V/RHE,the half-wave potential is 0.75 V/RHE,and the limiting current density is 5.28 m A cm^–2.The good ORR catalytic activity of Fe Co Ni/N-CNTs is close to that of commercial Pt/C catalysts.In addition,the stability and methanol resistance of Fe Co Ni/N-CNTs are obviously better than that of Pt/C catalysts in long-term operation.（2）Heteroatom doping strategy is also one of the main strategies to improve the activity of the catalyst.Usually,researchers use N,P,S and other elements to doping the material to further improve its electrochemical activity.In this paper,Fe Co metal particles were encapsulated into carbon materials by a simple preparation method to avoid the influence of the external solution on the internal core,and the surface P doping was carried out by a simple method.The characterization indicates that P is successfully anchored on the surface of the material and exists in the form of P–C.The ORR performance of P-Fe Co@NC/CNTs doped with P was significantly improved compared with that of Na Cl/CNTs.The onset potential,half-wave potential and limiting current density of P-Fe Co@NC/CNTs in 0.1 M KOH solution are 0.88 V/RHE,0.81 V/RHE and 5.29 m A cm^–2.Subsequently,through comparative tests,we guessed that element P was successfully doped at the defect position of carbon material.Finally,P-Fe Co@NC/CNTs were used as the cathode to assemble the zinc-air fuel cell.The maximum power density of the cell was 73 m W cm^–2,and the specific capacity was763 m Ah g^–1,both of which had better performance than the cell assembled by Pt/C catalyst as the cathode.（3）The ternary metal particles Fe Co Ni were encapsulated into carbon materials by simple stirring and calcination,and the morphology of Fe Co Ni was controlled and studied by adjusting the amount of carbon（nitrogen）source melamine.The characterization indicated that with the increase of melamine,the outer layer of the metal particles gradually changed from being exposed to the air to being wrapped by carbon layer and finally encapsulated into the bamboo-like carbon material.The ORR performance of the catalysts prepared with different amounts of melamine were significantly different.The ORR activity of Fe Co Ni@NC/CNTs-6 with 6 g of melamine was better than that of other catalysts,with onset potential of 0.89 V/RHE,half-wave potential of 0.83 V/RHE and limiting current density of 5.16 m A cm^–2.It was found that the high content of Fe（Co/Ni）N_x,the main active substance,may be the main reason for its excellent ORR activity.Finally,Fe Co Ni@NC/CNTs-6 was also used as the cathode to assemble the Zinc-air cell,which showed better battery performance than Pt/C catalyst.It provides theoretical and practical support for the further application of such catalysts.