Performance And Mechanism Of Electrospun Fe-N-C:O Nanofibers Electrocatalysts For Oxygen Reduction Reaction

With the depletion of traditional resources and promotion of conservation concept, people expect to find new clean energy. Fuel Cell, with high efficiency and environmental friendliness, has been considered to be a promising emerging energy conversion device in past decades. However, their commercialization is limited by high cost of platinum-based catalysts. Therefore, many researches have concentrated on low-cost non-precious metal electrocatalysts. In this dissertation, a new type of Fe-N-C:O nanofiber was synthesized by electrospinning and heating. Focusing on active sites, surface area and conductivity, we discussed the impacts of various experimental conditions on structure and component of electrocatalysts, and further investigated the performance and mechanism of electrocatalysis for oxygen reduction reaction.The cost of Fe-N-C:O catalysts was low, in which polyacrylonitrile and ferric nitrate were major precursors with cheap prices. This facile method could be compatible with carbon fiber industry. Moreover, compared with current impregnation method, we obtained homogeneous distribution of Fe-Nx active sites. Ultralow oxygen treatment can modify the surface area of the catalysts to investigate the influence of surface area on electrocatalysis performance, without changing active sites and conductivity.The linear relationship of half-wave potential and logarithm of loading was measured, and then explained by theoretical derivations. Under the optimized loading, the half-wave potentials of Fe-N-C:O catalysts were 0.76 V and 0.82 V in acidic and alkaline medium, respectively, reaching the same level of other types of non-precious metal electrocatalysts. Compared with commercial Pt /C catalysts, Fe-N-C:O catalysts had better durability, and could be applied in Proton Exchange Membrane Fuel Cell, Alkaline Fuel Cell and Metal-Air Battery.Meanwhile, we first discovered and confirmed that synergy of methanol and oxygen-containing groups induced electrocatalysis enhancement of Fe-N-C:O catalysts in alkaline medium by experiments and first-principle calculation. The synergy-induced enhancement was further improved experimentally to 2.94±0.03 %, and higher than performance of commercial Pt/C. This effect was continuable, reproducible and multipliable, and promising to solve the methanol crossover problem in Direct Methanol Fuel Cell.Furthermore, with increment of iron content, iron oxide and Fe3 C nanoparticles would appeare in the Fe-N-C:O catalysts, while the concentration of iron on the surface would have a limit value. The electrocatalysis performance reached the best half-wave potential of 0.82 V in acidic medium, when adding 6 % iron, and was closer to that of commercial Pt/C catalysts. Our experiments also supported that the active sites were Fe-Nx structure, while the conductivity of catalysts might be enhanced by the appearance of Fe3 C. At last, we fabricated a series of TM-N-C:O(TM = V, Cr, Mn, Fe, Co and Ni) nanofiber catalysts by altering transition metal. Nevertheless, combining with computing results of various active sites, there was a volcano-shaped dependence of half-wave potential on absorption energy of oxygen molecule. With increment of atomic number of transition metal, absorption energies of oxygen molecule on TM-N4 active site were increased, and d-band centers of transition metal were decreased.