【作者】 闫伟；

【导师】 陈艳丽； 马长勤；

【作者基本信息】 中国石油大学(华东) ， 材料工程（专业学位）， 2020， 硕士

【摘要】 目前,新能源技术的开发越来越受到人们重视,其中,燃料电池作为一种清洁,便携的发电装置被广泛应用于新能源领域。然而,燃料电池的发电效率受到阴极氧还原反应（ORR）的制约,但目前常见的Pt/C催化剂又因为成本高昂、易被毒化等缺点不能被大规模商业应用,因此,开发一种新型的非贵金属ORR催化剂是当前急需解决的问题。近年来,卟啉类大环共轭分子在光电领域应用极为广泛。尤其在燃料电池领域,卟啉化合物作为天然生物酶催化活性中心,在氧还原领域也表现出卓越的电化学性质。然而,卟啉化合物本身相对于碳材料的低导电性和低稳定性限制了对其进一步的开发利用,因此将卟啉化合物进行设计并与碳材料进行复合,使其表现出较高的电化学催化活性,是ORR催化剂的开发的可行思路。具体研究内容如下:1)卟啉聚合物/生物质复合碳材料的制备及性能研究采用方法简单的一步原位合成策略,将卟啉前驱体与天然易得、绿色环保的生物质玉米芯一锅法制备了ORR电催化剂Fe₂O₃@NC@bio-C。分别探究了氧化铁纳米粒子（Fe₂O₃）,卟啉聚合物衍生的氮掺杂碳（NC）和生物质材料玉米芯衍生的生物质碳（bio-C）各组分的作用。通过XRD,XPS及SEM等测试结果发现:相同实验条件下,Fe₂O₃@NC@bio-C中铁含量（12.48%）远高于Fe₂O₃@bio-C中的铁含量（1.26%）,这是由于氮掺杂碳更利于锚定金属活性位点;而Fe₂O₃@NC@bio-C比NC@bio-C具有更高的石墨化程度,同时在碱性条件下的起始电位超出150 m V,半波电位超出100 m V,这说明Fe的存在不仅有助于提升材料石墨化程度,而且Fe作为活性物种降低了过电势,加快了氧还原反应进行。催化剂Fe₂O₃@NC@bio-C-800在碱性条件下的起始电位为0.96 V,半波电位为0.85 V、极限电流密度为6 m A cm^-2,在10000 s的稳定性测试中电流密度仅衰减9%,ORR测试各项指标均超越了商业Pt/C催化剂,同时表现出目前铁系生物质碳材料ORR催化剂最好的结果之一。该工作简化了催化剂材料的制备工艺、降低了制备成本,更加利于实际的应用,为生物质材料应用于燃料电池催化剂领域提供了一种思路。2)非金属异原子S卟啉聚合物的制备及ORR性能研究质子膜燃料电池需要酸性条件下稳定的非贵金属ORR催化剂。而目前报道的决大多数非贵金属ORR催化剂,在酸性条件下的催化活性和稳定性与其碱性条件下的性能差距很大。为此,我们设计合成了一种不含金属离子的新型的异原子S掺杂的卟啉聚合物（简称N,S-POP）,通过引入S原子替换卟啉中心的NH单元来改变卟啉环的周围环境,研究电荷密度对于氧气分子以及电子转移步骤的影响。通过红外光谱,紫外光谱和扫描电镜等测试证明了聚合物的成功制备。通过酸性条件下CV,LSV等电化学测试结果发现:相比于单体卟啉化合物,N,S-POP的起始电位（0.82 V）,半波电位（0.72 V）均高出50 m V以上;相比于没有S掺杂的卟啉聚合物（简称N,N-POP）,N,S-POP的塔菲尔（Tafel）斜率(89 m V dec^-1)比N,N-POP的(147 m V dec^-1)更低。表明在酸性条件下,卟啉聚合物具有优异的ORR性质,同时更低的Tafel斜率说明N,S-POP具有较快的反应动力学速度。通过DFT计算发现:卟啉环中的S原子增加了周围环境的电荷密度,氧气吸附在催化剂表面使得反应快速进行,同时,N,S-POP中的决速步骤的自由能（1.17 e V）比N,N-POP/C的更小（1.41 e V）。这意味着在氧气的脱附后得到电子转化为H₂O阶段时,N,S-POP所需克服的能量势垒更小,表现出了更高的得电子能力,从而加快氧还原的进行。N,S-POP在酸性条件下的起始电位（0.82 V）,半波电位（0.72 V）及电流密度(4.8 m A cm^-2)表现出媲美商业Pt/C的ORR催化性质,同时表现出酸性条件下非金属ORR催化剂最好的结果之一。该工作中催化剂有确定的分子结构,有助于更清晰的明确催化剂在氧还原过程中的反应机理,从分子水平研究探讨ORR的反应过程,这为下一步的非金属催化剂的设计以及酸性燃料电池电极材料的应用提供了良好的思路。更多还原

【Abstract】 At present,the development of new energy technologies is receiving more and more attention.Among them,fuel cells are widely used in the field of new energy as a clean and portable power generation device.However,the power generation efficiency of fuel cells is restricted by the cathodic oxygen reduction reaction（ORR）,while current common Pt/C catalysts cannot be used for large-scale commercial applications due to the disadvantages of high cost and easy poisoning.Therefore,developing the new types of non-precious metal（NPM）oxygen reduction catalysts is highly desired-.In recent years,macrocyclic conjugated molecules,such as porphyrins,have been widely used in the field of optoelectronics.Especially in the field of fuel cells,porphyrin compounds,as natural biological enzyme catalytic active centers,also exhibit excellent electrochemical properties in the field of oxygen reduction.However,the low conductivity and low stability of porphyrin compounds have limited their further development and utilization.Therefore,it is a feasible idea for the development of ORR catalysts by designing and synthesizing new porphyrin compounds with novel molecular structures,and then recombing with carbon materials,endowing the materials efficiently electrochemical catalytic activity.The specific research contents are as follows:1)Preparation and properties of porphyrin polymer/biomass composite carbon materialsWe used a simple one-step in-situ synthetic strategy to prepare the ORR electrocatalyst,named as Fe₂O₃@NC@bio-C,from the porphyrin precursor and the naturally available biomass corn cob in one pot.In addition,due to the complexity of the composition,the effect of the different components,the iron oxide nanoparticles,nitrogen-doped carbon（NC）and the biomass carbon（bio-C）,on the ORR,is assessed.The XRD,XPS and SEM measurments indicate that under the same experimental conditions,the iron content in Fe₂O₃@NC@bio-C（12.48%）is much higher than the iron content in Fe₂O₃@bio-C（1.26%）,due to nitrogen doped-carbon is more conducive to anchoring active sites of metals,while Fe₂O₃@NC@bio-C has a higher degree of graphitization than NC@bio-C.The onset potential under alkaline conditions exceeds 150 m V,the half-wave potential exceeds 100 m V,which indicates that the presence of Fe not only boosts the degree of graphitization of the material,but also greatly reduces the overpotential as an active site species and thus accelerates the oxygen reduction reaction.The Fe₂O₃@NC@bio-800 catalyst has an onset potential of 0.96 V,a half-wave potential of 0.85 V and a limiting current density of 6 m A cm^-2under alkaline conditions,meanwhile a current of 10,000 s for stability density is only attenuated by 9%.All of these in the ORR surpass the performance of commercial Pt/C catalysts,among the best results for iron-based biomass carbon ORR catalysts materials.This work simplifies the preparation process of the catalyst material,reduces the preparation cost,is more conducive to practical application.2)Nonmetallic heteroatom S porphyrin polymer and its ORR PerformanceIn order to get the highly active ORR catalyst having a good durability under the acidic medium,we designed and synthesized a new type of non-metal heteroatom S-doped porphyrin polymer（named as N,S-POP）.The S atom was introduced into the porphyrin core by replacing the NH units in porphyrin center,then studying the effect of charge density of porphyrin macrocycle on oxygen molecules and electron transfer steps.The infrared spectroscopy,ultraviolet spectroscopy,and scanning electron microscopy tests confirmed the successful preparation of the polymer.The electrochemical test for N,S-POP focuses on the ORR properties under acidic conditions.According to the results of electrochemical tests such as CV and LSV,it is found that compared with monomer porphyrin compounds,the onset potential of N,S-POP（0.82 V）and half-wave potential（0.72 V）are all higher than 50 m V.Furthermore,tafel slope of the N,S-POP(89 m V dec^-1)is lower than non-S-doped porphyrin polymers(named as N,N-POP,147 m V dec^-1),indicating that a lower overpotential for N,S-POP relative to N,N-POP.DFT calculations reveals that the S atoms in the porphyrin ring increased the charge density of the surrounding environment that favors adsorption of oxygen on the catalyst surface.A smaller free-energy of speed-determining step in N,S-POP（1.17 e V）than in N,N-POP（1.41e V）makes the reaction proceed rapidly.The onset potential（0.82 V）,half-wave potential（0.72 V）,and current density(4.8 m A cm^-2)of N,S-POP exhibits excellent ORR catalytic properties comparable to commercial Pt/C under acidic conditions,achieving one of the best results for non-metallic ORR catalysts under acidic conditions.In this work,a definite molecular structure of the catalyst helps us to clarify the reaction mechanism of the ORR catalyst at the molecular level.This is very useful in the design of non-metal catalysts for the application of fuel cells.更多还原