Preparation And Electrochemical Performance Of Iron-based Carbon Nanomaterials Derived From ZIF-8

Since the 21st century,the development and application of new energy vehicles has driven the research of a new generation of power battery systems.Fuel cell systems have been vigorously studied by major universities and scientific research institutions as an alternative to the existing power sources of future new energy vehicles.Among them,the cathode oxygen reduction reaction catalyst of the fuel cell system is a hot topic in basic research.As one of the alternatives that have the potential to replace high-performance platinum-based catalysts,the carbon-based catalysts with iron single atoms,iron clusters and iron nanoparticles as the main catalytic active sites exhibit excellent electrochemical performance and relatively For expensive platinum catalysts,the cost is greatly reduced.In this research,2-methylimidazole zinc salt,ZIF-8,is used as the carbon substrate,and the transition metal ions in the metal salt are adsorbed on the surface of the ZIF-8 molecular pores for doping modification.Based on this,two methods are designed to prepare the non-noble metal iron-based catalysts with different forms of active sites.The main contents and results of the research are as follows:1.By coating dopamine hydrochloride on the outer surface of the irondoped ZIF-8 molecule,a large number of carbon nanotubes surrounding the ZIF-8 are formed after carbonization,and the iron salt adsorbed in the pore structure of the ZIF-8 is doped at the same time.Fe3C and metallic Fe particles are formed on the surface as catalytically active sites.Among them,the 0.2-DPA-Fe/NC material exhibits the best ORR electrochemical activity in 0.1 M KOH medium,and its half-wave potential reaches 0.875 V,which is 20 mV higher than the commercial 20%Pt/C catalyst(0.855 V).At the same time,the material exhibits strong methanol tolerance and stability in acid and alkaline media(0.1 M KOH and 0.1 M HClO4).Its high activity can be attributed to the large number of Fe3C and metal Fe particles exposed on the outer surface of the material.At the same time,the carbon nanotubes formed by dopamine hydrochloride doping also improve the conductivity of the material to a certain extent,thereby jointly strengthening the catalytic reaction kinetics.2.The iron salt and copper salt are mixed and dissolved in a methanol solution containing dimethylimidazole and zinc nitrate by a one-step mixed impregnation method.The copper acetylacetonate in the solution is encapsulated in the molecule because its molecular size is smaller than the size of the ZIF-8 cavity.And ferric amine citrate,which has a larger molecular size,is adsorbed on the surface of ZIF-8 molecules in a large amount.Use this space restriction to prevent the aggregation between metals,and form a catalyst material containing iron and copper diatomic sites after carbonization.The size of the material is about 80 nm in diameter,and the content of iron and copper are 1.66 wt%and 1.58 wt%,respectively,which are evenly distributed on the surface of the material.It has the highest onset potential(0.96 V)in 0.1 M KOH,showing a half-wave potential(E1/2)of 0.882 V,which is nearly 40 mv higher than Pt/C,and is better than Fe-NC catalyst(0.858 V)and Cu-NC catalyst(0.825 V).Its kinetic current density is about 3 times that of Pt/C and nearly 2 times that of Fe-NC.In acidic medium,the E1/2 gap between FeCu-NC and Pt/C is only 40 mV,and the former has excellent stability and methanol resistance in acid and alkali solutions.The calculated electron transfer number of FeCu-NC is about 3.97,the hydrogen peroxide yield is less than 2%,and its ORR kinetics is a 4electron reaction process.In addition,FeCu-NC is used as an air electrode catalyst to assemble a zinc-air battery.The peak power density of the battery is 239 mW·cm-2,which is far beyond the Pt/C-IrO2 catalyst(183 mW·cm-2).A series of characterizations can prove that the high ORR activity of FeCuNC can be attributed to the synergistic effect of iron single atom and copper single atom.The ZIF-8 material with large specific surface area and rich microporous structure is beneficial to the large attachments for active sites of iron and copper atoms,while exposing more active centers on the surface of the material.