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碳负载非贵金属电化学催化氧还原/析氧反应材料的合成与应用
沈行加
Subtype博士
Thesis Advisor胡广志
2018-05-25
Degree Grantor中国科学院大学
Place of Conferral北京
Degree Discipline材料物理与化学
Keyword电化学催化 氧气还原反应 氧气析出反应 非贵金属 碳载体
Abstract

在可持续能量转换或储存技术(如燃料电池、电解水、金属-空气电池等)中,氧气电极的催化剂决定了能量转化效率及成本。目前,传统氧气电极材料多以稀贵金属为主,如催化氧气还原反应(ORR)所采用的高负载量Pt/C,以Ir或Ru为主的氧气析出反应(OER)催化剂。贵金属储量稀少、价格高昂,促使人们积极寻找开发性能优异且成本低廉的非贵金属催化剂。以Fe,Co,Ni为代表的非贵过渡金属,因其具有未饱和的3d电子轨道,通过对其结构、组成等的合理设计调控,有望取代贵金属在氧气催化反应中的应用。本论文致力于碳以及杂原子掺杂碳材料负载非贵金属材料的合成和调控,研究材料性质与催化氧气反应活性间的关系,开发高效能、低成本的新型非贵金属催化材料。一、利用模板反向复制法制备了表面负载Fe-N-C催化活性中心的新型ORR催化剂,并通过Fe前驱体含量调控优化材料表面活性位的含量,实现在碱性条件下优于商业铂碳的ORR催化性能。球差电镜面扫描结果显示元素Fe、N、C均匀分布在材料表面;通过对57Fe的M??bauer谱解析,证明Fe只与N配位形成Fe-N结构;EXAFS进一步证明Fe周围N的配位数为2,因此合成的材料活性中心主要以FeN2存在;采用X光电子能谱有效定量了合成材料表面FeN2含量。密度泛函计算表明,在碱性条件下,由于对中间产物O*和OH*吸附作用较弱,以及较强的电子传输能力,FeN2结构催化ORR活性高于常规FeN4结构。结合对材料活性中心的定量分析,推导、建立了材料催化活性与活性位数量间的数学关系,为预测此类材料的催化性能提供了理论支持与实验证明。二、为进一步提高Fe-N-C材料催化氧气还原活性及拓展其在燃料电池中的应用,在利用硬模板法自下而上合成Fe-N-C材料过程中,通过引入硫制备了相应的硫掺杂催化剂。表征结果证明掺杂的硫只形成C-S-C结构,不影响材料中Fe-N-C的结构与性质。由于C-S-C结构促进了活性中心Fe-N-C催化表面吸附的H2O2进一步被还原,极大的提高了酸性条件下催化ORR活性。相应的反应半波电位达到了0.77V,优异的抗甲醇能力使得材料在直接甲醇燃料电池中具有广阔应用前景。三、通过微波辅助法,利用弧光反应释放的高热量,高效快捷的合成了炭黑负载的合金纳米颗粒催化剂。Mo的掺杂提高了FeNi3纳米合金分散性,从而促进了材料催化OER活性。在1.0 M氢氧化钾溶液中,电流密度为10 mA cm-2时反应过电位仅为280 mV,明显小于贵金属参比样RuO2。同时,合金纳米颗粒外包覆的石墨烯层以及均匀分布的Mo原子可以占据合金纳米颗粒的边缘缺陷,抑制了颗粒的团聚和离析,从而提高了材料稳定性。四、结合前两部分工作,我们以具有ORR活性的材料作为基质,采用微波法,在其表面修饰具有OER高活性的FeNiMo纳米合金颗粒。通过对材料结构和性能的表征,发现同理于嫁接技术,微波处理对于载体基质的结构和性能几乎无影响,合成的材料保持了基质原有的ORR活性,同时OER活性物质的引入极大的提高了材料析氧活性,实现了双功能催化剂的有效构成。 本论文针对不同应用中氧气电极发生的不同反应,采用不同策略合成活性中心不同的碳负载非贵金属催化剂。有以下五个创新点:(1)首次合成了氮平均配位数为2的Fe-N-C材料。(2)证明了材料催化ORR活性与活性位Fe-N-C结构浓度正相关。(3)揭示了硫掺杂材料中C-S-C结构对活性中心Fe-N-C的作用机理。(4)报道了微波法快速制备高性能OER催化剂。(5)发展了一种兼具ORR和OER活性的双功能催化剂合成策略。因此,本论文从基础理论研究的角度加深了对材料表面结构、组成等性质对电化学催化反应活性的作用和规律的认识,为高效能、低成本的新型非贵金属催化材料的研究和开发提供了理论指导和技术支持。

Other Abstract

Oxygen reaction is of outmost significance for renewable and sustainable energy systems, such as fuel cells, water splitting and metal-air batteries. Traditionally, precious metals and their alloy have been regarded as the best catalysts. Pt based materials and Ru or Ir composites are still considered as state-of-the-art catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), respectively. However, owning to their low abundance, extensive efforts have been made to identify cost effective alternatives comprising earth-abundant transition metals and carbonaceous materials. Earth-abundant first-row (3d) transition metal based catalysts have been widely developed as promising electrocatalysts for the ORR/OER due to their high activity, durability and cost-efficiency. This thesis focuses on preparation of carbon or heteroatom doped carbon supported non-precious metals as efficient catalysts for ORR and OER, and exploring the correlation between the property of materials and catalytical activity. The main contents of this thesis are summarized as follow: 1. An atomically dispersed FeN2 moieties was introduced onto the surface of N-doped ordered mesoporous carbon via an intriguing template casting strategy. One of unique parts herein is that the use of FeCl3/SBA-15 as template for catalyst synthesis facilitates the removal of excessive Fe during silica elimination by HF etching, resulting in a desirable model structure comprising only atomic dispersion of FeN2 sites as confirmed by HAADF-STEM, EXAFS and M??bauer spectroscopy analysis. In this way we can reflect on the nature of the active sites, and demonstrate that the activity of catalyst is linearly proportional to the concentration of FeN2 sites. The obtained electrocatalyst exhibits superior performance with a more positive half-wave potential for ORR compared to Pt/C catalyst. We further establish a kinetic model to predict the ORR activity of these single-atom ORR catalysts for the first time. The present work elaborates on a profound understanding for designing low-cost, highly efficient Fe-N-C based ORR electrocatalysts.2. A novel S-doped Fe-N-C catalyst was synthesized via a template casting procedure. Sublimed sulfur was introduced as S precursor during the synthesis and finally doped into the carbon skeleton to form thiophene-like structures (C-S-C). The new strategy of pre-anchoring Fe precursor on the template surface can only produce the acid-resistant Fe-N-C catalyst after acid treatment. In acidic medium, although the site C-S-C is less-active for ORR, it plays an important role in reducing the electron localization around the Fe center, improves the interaction with oxygenated species, and thereby lowering the ORR activation barriers on Fe-N-C. The observed synergistic effects between C-S-C and Fe-N-C sites boost the ORR activity manifested by a positive shift of the half-wave potential by 30 mV when compared with the sulfur-free Fe/NC counterpart.3. A facile and efficient synthesis of multimetal supported on commercial carbon black was carried out via an instantaneous microwave synthesis process. A small size of FeNi3 nanoparticles is achieved by doping Mo, giving rise to high activity as well as the enhanced the stability for the OER. The OER overpotential at 10 mA cm-2 in alkaline electrolyte is only 280 mV for FeNiMo/C, which is 40 mV better than a noble metal catalyst RuO2. Additionally, the catalyst FeNiMo nanoparticles are encapsulated by a uniform graphene shell preventing the active nanoparticles from aggregation and peeling off during the electrochemical reactions.4. Combined the advantages of template casted Fe/SNC and microwave treated FeNiMo nanoparticles, a general approach was developed to construct efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions. The Fe/SNC, serving as a matrix for graft, maintains its single-atom dispersed structure and high ORR activity. The coupled FeNiMo nanocrystals, homogeneously dispersed on external surface of the matrix, were responsible for OER activity. A potential of 1.54 V and 0.80 V has been achieved at the current density of 10 mA cm-2 for OER and 3 mA cm-2 for ORR, respectively. In summary, different strategy was employed to synthesize several catalysts for oxygen electrochemical reaction. Meanwhile, innovation points were as follows: (1) It’s the first time to prepare a Fe-N-C material with the average coordination number of N is 2.0. (2) Elaborate the striking linear relationship between the catalytical activity and the concentration of FeN2. (3) Reveal the synergistic effects between C-S-C and Fe-N-C sites to boost the ORR activity. (4) A time-saving microwave process was adopted to synthesize multimetal catalysts for OER. (5) A general approach was developed to construct efficient bifunctional electrocatalysts for oxygen reduction and evolution reactions. This thesis enhance the comprehesion on the correlation between the property of materials and electrocatalytical activity, and therefore provide an important guideline and strategy for the development of non-precious metal supported catalysts.

Pages108
Document Type学位论文
Identifierhttp://ir.xjipc.cas.cn/handle/365002/5417
Collection资源化学研究室
Recommended Citation
GB/T 7714
沈行加. 碳负载非贵金属电化学催化氧还原/析氧反应材料的合成与应用[D]. 北京. 中国科学院大学,2018.
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