| Carbon-based catalysts have been regarded as a class of effecient materials for replacing noble metal catalysts due to their abundant resources,low cost,good toxicity and stability,while transition metal doped carbon-based materials with more efficient catalytic activity are received more and more attention.At the same time,with developing of portable energy devices,the requirements for the mechanical properties of the catalyst are also increased,such as toughness,compressibility.Therefore,the development of eatalyst materials with high catalytic activity and good mechanical toughness is one of the key directions of current research.Based on the above two research purposes,we prepared flexible composite carbon nanofiber membrane with high catalytic activity by using electrospinning technology.(1)In the first project,we prepared a flexible free-standing carbon nanofiber film with a specific surface area of 799 m2/g by using p-Phthalic acid(PTA)as pore-forming agent and KCl as salt template for regulating the pore structure,while using Fe(SCN)3 as a hetero atom doping source.Owing to its rich content of pyridinium nitrogen and graphitized nitrogen,high specific surface area,high density of metal active sites and special "core-shell structure",the catalyst represented a highly efficient four-electron process during the ORR reaction in 0.1 M KOH solution.The catalyst showed excellent electrocatalytic performance with a half-wave potential of 0.895 V and a kinetic density of 14.25 mA cm-2,which is superior to Pt/C.The catalyst also showed remarkable ORR performance with a half-wave potential of 0.725 V in 0.1 M HClO4 with four-electron process.In addition,its limiting current density can be compared to Pt/C.Though its ORR activity still had a certain gap with Pt/C,its resistance to methanol toxicity and stability in alkaline and acidic media are better than that of the Pt/C.More importantly,the flexible carbon-based nanofiber membrane could be directly used as a catalyst layer in a hydrogen-oxygen proton exchange membrane fuel cell with the maximum power density of236 mW cm-2,realizing the direct application of flexible materials in fuel cells.(2)In the second project,flexible polymer polyimide was used as precursor for preparing flexible carbon nanofiber.By controling the heteroatoms for the polyamic acid precursor,we finally prepared the cobalt-nitrogen composite flexible carbon nanofiber film and sulfur-doped cobalt-nitrogen composite flexible carbon nanofiber film.We further explored the influence of heteroatom S on the electrochemical properties of cobalt-nitrogen Co-doped composite system.While the results indicated that the S doping led to the formation of a new species of Cogss with OER activity,thus improving OER activity of the catalyst significantly.At the same time,we continued to investigate the reason of decreased ORR activity of the catalyst.We believed that the S doping could increase defects and became to be the active site for ORR.However,the ORR performance decreased due to the formation of large amount of Cogss species,the decreased specific surface area and reduced the possibility of active site exposure.At the same time,since the elemental part of Co with certain ORR activity was converted into Cogss,the corresponding amount of ORR active site would be reduced.Therefore,ORR activity of the catalyst doping with Co and S was decreased by above two factors.After optimization,the best catalyst exhibited good OER performance with a Tafel slope of 86 mV dec-1,which is superior to the RuO2 with a Tafel slope of 115 mV dec-1.Furthermore,the catalyst possessed an excellent resistance to methanol toxicity and stability for ORR and OER,which are much better than that of Pt/C and RuO2.As an bifunctional catalyst for ORR and OER,it also exhibited remarkable stability in the flexible All-solid Zn-Air battery. |
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