Preparation And Electrocatalytic Activity Of Nanosheet-like Carbon-based Composites

The consumption of fossil resources and environmental pollution are gradually threatening our live,so the development of new energy is very important,and electrochemical new energy is one of the important fields that all countries are trying to research and develop at present,because it has the advantages of energy storage technology and stable output.Metal-air battery is a new energy device that has attracted much attention.It has the advantages of high specific energy,safety and environmental protection,and it is convenient to carry.The cathode of the metal-air battery is continuously fueled by air,so its specific energy density is very high.However,in the process of cathodic oxygen reduction reaction（ORR）,intermediates are produced,which leads to slow oxygen conversion,and which seriously hinders the effective performance of metal-air battery.It is well known that platinum loaded on carbon（Pt/C）catalyst can effectively improve the rate of cathode oxygen reduction reaction,and it is called the best ORR electrocatalyst.However,metal platinum has such disadvantages as scarce resources and high price,which greatly increases the cost of metal-air battery.To solve this problem,the preparation of low-cost,stable and efficient cathodic oxygen reduction electrocatalysts is the most important task.Most electrocatalysts are mainly based on carbon-based materials,which are nanoscale graphite materials with stable conductive network structure and good electrical conductivity.Carbon nanosheet is a kind of carbon-based materials with rich mesoporous structure and high specific surface area,which can further improve the catalytic performance of materials by doping heteroatoms and metal particles.The main purpose of this work is to prepare low-cost,stable and efficient ORR electrocatalysts,and the main contents are as follows:（1）Firstly,according to the dicyandiamide self-template method,dicyandiamide was used as nitrogen source and water-soluble starch was used as carbon source.Nitrogen-doped nanosheet powder（CN-ns）was obtained by simple high-temperature pyrolysis of the dicyandiamide-starch solid mixture.Considering the synergistic effect of nitrogen-phosphorus diatomic co-doping,nitrogen-phosphorus co-doped nanosheet powder（CNP-ns）was obtained via further pyrolyzing the mixture of CN-ns and triphenylphosphine at high temperature by using triphenylphosphine as phosphorus source.From the SEM morphological image,we can see the nanosheets with abundant wrinkles on the material.The BET results show that the catalysts have high specific surface areas and abundant mesoporous structure.AFM detects the ultrathin nanosheet structure with a thickness of 1.29 nm,which is equivalent to four layers graphene.These characteristics of nanosheet can effectively increase the active sites and improve conductivity of the material,thereby improving the electrocatalytic performance of the catalyst.The electrochemical performance test shows that the catalyst has good ORR electrocatalytic performance and excellent methanol resistance.Further,the catalyst is applied to the alkaline metal-air battery,and the battery exhibits excellent discharge stability.（2）Three-dimensional porous N/P co-doped carbon nanosheet composites（CNP@PNS）were synthesized by mixing CNP-ns and NaCl,followed by heat treatment at high temperature.TEM images show that the material is rich in layered and porous structure,and BET test shows that the material is mesoporous structure between 3 and 4 nm.XPS test shows that the material is rich in pyridine nitrogen and graphite nitrogen.All these characteristics can effectively improve the ORR oxygen reduction efficiency.The DFT calculation results show that the porous structure and the synergistic effect of nitrogen and phosphorus effectively reduce the activation energy during the oxygen reduction process.In neutral solution,the limiting diffusion current curve（LSV）of CNP@PNS-4 catalyst has an onset potential of 0.98 V（reversible hydrogen electrode）and a half-wave potential of 0.91 V,which is comparable to the performance of commercial Pt/C（40%）and greatly surpasses those catalyst reported in the literature.The prepared catalysts were applied to Zn-air batteries,Al-air batteries and Fe-air batteries,and the maximum power densities of the batteries were 120.2,78.3 and 18.9 mW·cm^-2,respectively.Also,it has good discharge stability under different current densities.（3）Using cobalt phthalocyanine as cobalt source,ferrocene as iron source,and dicyandiamide as carbon and nitrogen source,iron-cobalt-loaded nitrogen-doped nanosheets（Fe-Co-C/N）were prepared by heat treatment of their mixture.The samples with different iron-cobalt ratios were obtained by changing the dosage of cobalt phthalocyanine and ferrocene.The SEM morphology and Mapping diagram show that the metal particles are evenly dispersed on the carbon material.The XRD test results show that the nanosheet materials are loaded with iron-cobalt alloys,which can improve the electrocatalytic activity and electrical conductivity of the materials.The electrochemical test results show that the sample with the iron/cobalt ratio of 1:1 presents the best electrocatalytic performance in both alkaline electrolyte and neutral electrolyte.It is observed from the RDE tests in alkaline and neutral electrolytes that the ORR onset potential reaches 0.95 and 0.90 V respectively,and the corresponding limiting current densities are 5.55 and 3.62 mA·cm^-2,respectively.In addition,when the Fe₁Co₁-C/N catalyst was applied to the alkaline and neutral Zn-air batteries,the batteries exhibit high discharge power and excellent durability.（4）On the basis of Fe₁Co₁-C/N catalyst,mixing it with Pt Cl₄ solution to deposit Pt atoms onto the carbon material.The mixture was pyrolyzed at high temperature to get FeCoPt-C/N catalyst with low amount of platinum content.ICP test results showed that the mass percentage of platinum atoms in carbon materials ranged from 2.36%to 3.58%.XRD results show that Pt-Co and Pt-Fe alloys are formed,which can effectively prevent the deposition of non-noble metal particles and improve the stability of the catalyst.Electrochemical tests show that the ORR onset potential of Fe₁Co₁-C/N catalyst in acidic solution is 0.82 V.After adding low amount of platinum,the onset potential of FeCoPt-C/N catalyst reaches 0.96 V,and the comprehensive performance is significantly improved.In addition,it is observed from the successive cycling LSV curves that the FeCoPt-C/N catalyst did not present any change on current and potential after 120 cycles at 1600 rpm,indicating the catalyst has excellent stability.The synergistic effect between non-precious metals and low-loaded precious metal（Pt）can effectively prevent the agglomeration and deposition of non-precious metals in acidic solutions,thereby improving the catalytic performance of the catalyst for ORR.