Preparation Of Gelatin-based Carbon Nanofibers By Electro Spinning And Their Properties For Oxygen Reduction

Oxygen reduction reaction(ORR),as the heart reaction in energy conversion devices such as fuel cell and Zn-air battery,requires highly efficient electrocatalyst to promote its kinetic process.At present,the common precious metal platinum catalyst has limited its commercial application because of the high cost and scarce reserves.Therefore,it is of great significance to develop a high catalytic activity and low cost non-platinum catalyst for ORR catalysis.In this thesis,the electro spinning technology was used to prepare the gelatin fiber materials by using the water-soluble and N-rich biomass gelatin as the electrospun precursor and Fe(?)complexes as iron source.A series of onedimensional(1-D)Fe/N co-doped carbon nanofibers catalysts were then prepared by pre-oxidation and carbonization treatment,which could effectively catalyze ORR.The formation mechanism of gelatin-based carbon nanofibers was elucidated,and the effect of catalyst composition and structure on the electrocatalytic performance was revealed.Firstly,the formation mechanism of gelatin-based carbon nanofibers materials was elucidated,and results show that the selection of Fe(?)complexes with a high stability constant(e.g.,EDTA-FeNa and K3[Fe(CN)6])as iron source is the key factor for the formation of 1-D gelatin-based carbon nano fibers materials.The presence of Fe(?)complexes avoids the crosslinking between Fe(?)and gelatin molecules in aqueous solution that guarantees the spinning successfully.Furthermore,the presence of Fe(?)improves the thermal stability of the pre-oxidated fiber by promoting the pre-oxidation process,and thus the 1-D nanostructure of gelatin fiber can be retained,which overcomes the problem that the gelatin fiber is easy to be destroyed at high temperature because of the low softening temperature of gelatin.In addition,the selection of suitable pre-oxidation temperature(?270 ?)also affects the successful preparation of gelatin-based nanofibers.Low temperature leads to an insufficient pre-oxidation reaction,while high temperature reduces the char yield of the product.Secondly,the Fe/Fe3C modified Fe and N co-doped porous carbon nanofibers catalyst was prepared by using EDTA-FeNa as iron source.Results show that the low content of ferric salt is not enough to maintain the 1-D morphology,while excess ferric salt leads to the severely aggregations of ironbased particles after heat treatment that is not conducive to the dispersion of active sites.When the mass ratio of gelatin to EDTA-FeNa is 50/9,the specific surface area of the catalyst is as high as 462.5 m2 g-1,and the microporous percentage and the N doping amount in the catalyst are 64%and 2.65 at.%,respectively.In the alkaline electrolyte,the catalyst exhibits an outstanding ORR performance with the half-wave potential(0.861 V),which is comparable to that of commercial Pt/C(0.857 V).In order to further improve the N doping amount and specific surface area of gelatin-based carbon nanofibers,and thus enhance their catalytic activity,the Fe/Fe3C modified Fe and N co-doped gelatin-based carbon nanofibers catalyst is prepared by using K3[Fe(CN)6]as iron source that possesses a high nitrogen content and oxygen-free ligand.Results show that the catalyst has an obviously improved specific surface area(869.9 m2 g-1),microporous percentage(87%)and N doping amount(5.22 at.%)than those of other catalysts prepared by using EDTA-FeNa as iron source or without spinning.Electrochemical tests show that the ORR catalytic performance of the catalyst is significantly improved in alkaline electrolyte,showing a half-wave potential of 0.885 V that exceeds the commercial Pt/C by 28 mV,and an excellent electrochemical stability and methanol resistance.X-ray absorption near-edge structure and 57Fe Mossbauer spectra both indicate the existence of Fe-Nx moieties and Fe/Fe3C nanoparticles in carbon nanofibers,contributing for the improvement of ORR performance.Acid pickling treatment analysis indicates that Fe/Fe3C particles play a positive role for ORR catalysis.In addition,a homemade Zn-air battery using the catalyst as air cathode presents a high power density(144.7mW cm-2)and good long-term stability.