Fabrication Of Fe-N-C Type Catalysts And Their Reearch On Electrocatalytic Oxygen Reduction

Fuel cells and metal-air batteries are considered as the next-generation,green batteries owing to their advantages of affordable price,safety,environmental friendliness and high energy density,etc.However,the sluggish oxygen reduction reaction in cathode limits the overall conversion efficiency of the batteries(such as power density and energy density).Precious metals Pt and its alloys are regardes as the best ORR catalysts,but the scarcity,high cost and poor durability have hindered the large-scale applications.Therefore,rational design and develop the low-cost,high-performance noble-metal-free catalysts to replace Pt-based electrocatalysts is the current research hots.Nanocarbon materials have attracted the widespread concerns and attention in the field of nano-energy due to the high specific surface area,porosity,good conductivity,and good corrosion resistance and so on.Recent researches demonstrate that surface modification,heteroatom element doping and the combination of the electrochemically active inorganic metal nanoparticles can significantly boost the performance of the nanocarbon material.In particularly,transition metal-nitrogen-carbon type compounds(M/N/C;M = Co,Fe)are considered to be the most promising candidate due to their facile synthesis,remarkable ORR activity,low cost,excellent lifetime and environmental friendliness property,displaying the great potential in replace the Pt-based ORR catalyst.Currently,there is no consensus on the actual catalytic mechanism of this type of catalyst.Taking the above-mentioned problems into consideration,we did the relevant research on the design and preparation of the highly efficient Fe-N-C materials and identification of catalytic sites.The main conclusions are as follows:1.Using carbon aerogel as raw material,we used alkaline etching(KOH)and heat treatment to increase the specific surface area and optimize the pore structure.Simultaneously,Fe,N co-doping was also achieved.The influence of heat treatment temperature on catalytic ORR performance was fully investigated.The onset potential and half-wave potential of the optimal sample(Fe-N-CA-800)are 0.918 V and 0.790 V,respectively,and the ORR is catalyzed by a 4e-process.Importantly,Fe-N-CA-800 exhibits better methanol poisoning resistance and stability than that of commercial Pt/C.The SCN-poisoning experiment shows that the introduction of trace Fe(0.60 wt.%)plays an important role in boosting activity.Combined with HRTEM,XRD,XPS characterization and electrochemical analysis,we conclude that Fe species exist mainly in the form of Fe-Nx active sites.2.We have developed a "structure-active site" bifunctional catalyst.We simply constructed Fe/Fe3C@C(Fe@C)nanoparticles encapsulated in 3D N-doped graphene and bamboo-like CNTs composites(Fe@C-NG/NCNTs)via a simple solid-state-pyrolysis.SEM,TEM,EELS and EXAFS characterizations indicated that the presence of Fe-Nx bonds,Fe@C nanoparticles and NG/NCNTs in Fe@C-NG/NCNTs sample.On the one hand,Fe-Nx act as active site,accepted electrons injected from Fe@C,which can greatly enhance ORR activity.On the other hand,NG/NCNTs hybrid structure is favorable for electron transfer and mass diffusion perporty.Fe@C-NG/NCNTs exhibited excellent the bifunctional ORR/OER performance in alkaline with a potential difference of 0.84 V.Most importantly,Zn-air batteries using Fe@C-NG/NCNTs catalyst deliver a peak power density of 101.2 mW cm-2,a specific capacity of 682.6 mAh g-1 and energy density of 764.5 Wh kg-1 at 10 mA cm-2.After 297 continuous cycle tests,the rechargeable Zn-air batteries show a voltage gap increase of only 0.13 V,better than of noble-metal-based Pt/C +Ir/C catalysts under the same conditions.