Preparation And Investigation Of Nitrogen,Sulfur Doped Fe@C-Based Electrocatalysts For Oxygen Reduction Reaction

The fuel cell is a kind of power generation device which could turn chemical energy in fuel or oxidants into electricity, and is considered one of the best power generation device in the 21 st century. However, large-scale application of fuel cells has been influenced by many factors, one of the important reasons is that the excessive usage of platinum-based catalysts brought cost problems. Moreover, platinum-based catalysts are easy to be poisoned and have poor durability. Currently, nitrogen-doped carbon transition metal(TM/N/C)-based composite materials are considered one of the most potential materials to replace precious metal catalysts due to its good performance.In this paper, we used cheap and readily available compounds as raw materials, through the soft template pyrolysis, carbon black surface chemical modification and hetero-atom doping method, respectively, to prepare carbon-supported non-precious metal cathode catalysts with good oxygen reduction catalytic activity, stability and methanol resistance. Electrochemical research methods and characterization techniques such as EDS, XRD and XPS were employed to explore the relationships between the structure and compositions of the catalysts. The major works of this thesis are as follows:1-butyl-3-methyl imidazole ionic liquid was chosen as the template and iron salts as precursors, after dicyanodiamide coating, mixing rotary evaporation, pyrolysis and other procedures, the materials with surface areas up to 813 m2 g-1 were prepared. The catalyst has good ORR catalytic activity and stability, and the half-wave potential reaches up to 0.75 V in oxygen saturated 0.1 mol L-1 H2SO4 solution. After 4000 and 8000 test cycles, no significant change was arised in its electrochemically active area. The rotating ring disk electrode test shows that, ORR average number of electrons transferred of Fe-N/IL-C catalyst was more than 3.6, wherein the electron transfer number of 750 ℃synthesized sample was 3.89, very close to 4, indicating that the surface adsorped O2 reducing to H2 O mainly through more efficient 4 electronic path.The carbon black was conducted by surface chemical modification method, in which highly hydrophilic sulfonate(-Ph-SO3H) was introduced to its surface, to improve the inert carbon graphite layer and its hydrophility. The result shows that, after standing experiment, the hydrophilicity of carbon black was greatly enhanced. A bit of S element was found by EDS. Although the specific surface areas were deduced from 813.10 m2 g-1 to 567.75 m2 g-1, half-wave potential improved 30 mV in 0.1 mol L-1 H2SO4 solution with saturated oxygen, and its power density at current 0.7 V increased 2.87 folds.Using thiourea as sulfur source, we prepared sulfur-doped transition metal nitrogen-carbon material(Fe-N/TH-C). XRD and XPS characterization show S element was successfully doped into the system. The oxygen reduction tests indicate catalysts have excellent oxygen reduction activity in an acidic medium. The oxygen reduction onset potential have reached to 0.89 V and half-wave potential reached to 0.78 V(vs. RHE). The half-wave potential of as-prepared materials was only 51 mV lower than 20% of commercial platinum-carbon catalysts. The limiting current density was almost identical to commercial platinum-carbon catalyst. Methanol resistance experiments and chronoamperometry tests show that the catalysts have great stability and resistance to methanol permeation.Finally, combined with electrochemical methods and physical characterization techniques, the comparative experiments were performed by using nitrogen-diphenyl sulfide and thiourea as raw materials, to explore the impact of N and S elements, and the possible catalyst component active centers. Eventually we found that the presence of sulfur component was thiophene type sulfur and oxidized sulfur, while nitrogen was in three forms: pyridine type nitrogen, pyrrolic nitrogen and graphite-type structure. S doping makes ORR activity of the catalysts significantly improved. This upgrade has nothing to do with S-containing precursor species and configuration, but the synergistic effect of S and N, which changed the existing form of nitrogen, namely, reduced the content of pyridine type nitrogen and increased pyrrolic nitrogen.