中文摘要：

向Fe/N/C非贵金属催化剂中再引入S掺杂是进一步提高其氧还原催化活性的有效方法。为了探究活性提高的原因,本文以三聚氰胺-甲醛树脂为前驱体,氯化钙为模板,氯化铁为铁源,通过添加硫氰化钾（KSCN）来控制热解催化剂的S掺杂量。通过对比分析催化剂的物化性质,结合密度泛函理论（DFT）计算,分析S掺杂促进Fe/N/C催化剂氧还原活性的原因。透射电子显微镜（TEM）和N_2吸脱附等温线测试结果表明,S元素可抑制含铁纳米粒子的形成,促使形成多孔碳结构,提高比表面积。X射线光电子能谱（XPS）结果表明,适量S前驱体可实现较高的S掺杂含量,得到最优的活性,过量的S反而会导致Fe和S的掺杂量同时降低,影响活性。DFT计算结果表明在Fe-N_4大环中引入S掺杂,可增强O_2分子和中间体OOH与Fe-N_4结构中的Fe的相互作用,促进形成Fe―O键,从而导致O―O键的键能显著降低,为后续反应O―O键的断裂提供可能,促进ORR反应的进行。

英文摘要：

S doping in Fe/N/C non-precious metal catalysts is an effective approach to further improve their catalytic activity for the oxygen reduction reaction （ORR）. However, the enhancement mechanism is not yet clear. Here, we synthesized an Fe/N/C catalyst using melamine-formaldehyde resin as the N and C precursors, CaCI2 as the template, and FeCI3 as the Fe precursor. The effects of S doping on the morphology, textural property, composition, and ORR catalytic activity were investigated by adding various amounts of KSCN as a precursor. Transmission electron microscopy （TEM） and N2 adsorption- desorption isotherm results revealed that S prevented the growth of Fe-containing nanoparticles, and facilitated the formation of a porous structure, which increased both the catalyst surface area and mass transfer rate. X-ray photoelectron spectroscopy （XPS） results indicated that a suitable amount of S precursor led to a high doping level of S and provided the highest ORR activity. However, too much S in the precursor decreased the doping levels of both Fe and S, due to the formation of FeS, which could be completely removed by acid leaching. Density functional theory （DFT） calculations showed that the addition of S in an Fe-N4 macrocycle could enhance the interaction strength of the Fe--O bond between the 02 molecule or the intermediate OOHspecies and Fe in the Fe-N4 structure, resulting in a significant decrease in the O--O bond energy, and may help in bond breaking in subsequent reactions, facilitating the ORR process.