Preparation And Properties Of High-performance Electrocatalysts For Oxygen Reduction Based On Chitin

The oxygen reduction reaction（ORR）is a crucial process in fuel cells and metal-air batteries.The reaction rate and selectivity is heavily dependent on the electrocatalyst.There are drabacks for Pt which is the most widely used catalytst,including the high cost and limited resources.Therefore,it is highly desired to develop high-performance and inexpensive catalyst that can replace the Pt catalyst.M-N/C（M=Fe,Co,Mn and N i）are thought to be one of the most suitable candidates to replace Pt catalyst due to their excellent catalytic efficiency,stability and tolerance to other fuels such as methane.In this work,N-rich chitin is used as a precursor,which was converted to M-N/C（M=Fe,Co,Mn and N i）by high-temperature carbonization in the presence of transition metal source（Fe/Co/Mn/N i）.The Fe-N/C were further treated by plasma etching which resulted in open structures containing a large number of FeN₄ sites.The effects of plasma etching on the morphology,structure,chemical composition,catalytic activity and active sites were studied in detail.The major findings are as follows:（1）A range of M-N/C（M=Fe,Co,Mn and N i）were prepared by 800°C-treatment using chitin as the precursor,ZnC l₂ as the progen and FeC l₃,CoC l₂,MnC l₂ and N iC l₂ as the source of metallic active centers.It was found that the catalytic activity of Fe-N site is much better than that of Co-N,Mn-N and Ni-N sites,making Fe-N/C posess the highest catalytic performance.Co-N/C has a large amount of Co-N sites due to its largest specific surface area and high nitrogen content,and its performance is superior to that of Mn-N/C which possesses the lowest nitrogen content.N i-N/C has the lowest performance due to the presence of a large number of low-activity N i metal particles on the carbon surface and low specific surface area.Therefore,the order of catalytic activity of M-N/C catalysts is Fe-N/C>Co-N/C>Mn-N/C>Ni-N/C.In addition,the Fe-N/C catalyst prepared with 24h-acid leaching has the best performance in 0.1M KOH electrolyte.（2）The Fe-N/C was further treated by plasma etching.It was found that after 120 s plasma treatment,the catalytic performance of Fe-N/C catalyst(0.6 mg cm^-2)towards ORR is comparable to that of the commercial Pt/C catalyst(80μg cm^-2)in both acidic electrolytes and alkaline electrolytes:The onset potential reaches 1.04 V vs.RHE in 0.1M KOH alkaline medium,and the transfer electron number（n）is as high as 3.92³.98 in the potential range of 0.2 V⁰.8 V,and the corresponding₂^-yields is only 4.23%⁰.80%.After long-ter m durability test of 10000 s,the corresponding current of the Fe-N/C-120 catalyst retains 92%of its initial current.The onset potential reaches 0.88 V vs.RHE in 0.1M HC lO₄ acid medium.Within the potential range of 0.2 V⁰.8 V,n is as high as 3.90³.93,and the corresponding H₂O₂ yields is only 4.71%³.40%.After long-term durability test of 10000 s,the corresponding current of Fe-N/C-120 catalyst retains 81%of its initial current.At the same time,Fe-N/C-120 exhibits excellent tolerance to methanol in both acidic and alkaline media.（3）The effects of plasma etching on the composition and microstructure were investigated systematically.M?ssbauer spectroscopy results show that after 120 s plasma treatment of Fe-N/C catalyst,the low-spin state Fe^IIN₄ remains almost unchanged,and the high-spin state Fe^IIN₄ increases from 20%to 26%but the content of low-activity iron oxide nanoparticles decrease from 33%to 28%.Plasma-treated Fe-N/C catalysts can etch unstable sp³ and amorphous sp² hybrid carbons,create a large number of defects,increase the specific surface area of the material,and thus form an open structure that exposes a large number of FeN₄ active sites.At the same time,the sputtering of Fe atoms from the nanoparticles by plasma would migrate into the adjacent micropores to coordinate with the pyridinic N at the edges of graphitic domains to form highly active FeN₄,which is significantly improve electrocatalytic activity.