基于类普鲁士蓝前驱体制备电催化剂及其在碱性电解水中的应用

发布时间：2018-05-26 22:14

本文选题：金属有机框架化合物 + 类普鲁士蓝　；参考：《中国科学技术大学》2017年博士论文

【摘要】：目前,世界上面临的最严重的两大问题是环境恶化和能源危机。氢能是一种清洁且可持续的能源,具有高能量密度、零排放和储量丰富等优点。为了应对上述两大问题,氢能备受关注。在碱性电解池中电解水是目前最可能实现大规模产业化制氢的技术。然而电解水在动力学上需要越过较大的能垒,导致反应速率较慢,因此需要高活性的电催化剂来加速反应的进行。催化剂不仅仅改变反应动力学过程,还降低过电位从而减少电能消耗,进而降低制氢的成本。普鲁士蓝类似物早在1704年就已经被发现了,是一种最古老的且最简单的金属有机框架化合物。由于其在气体吸附、储能、催化及载药等领域的广泛应用,目前受到了广泛的关注和大量的研究。类普鲁士蓝化合物是由作为金属接头的过渡金属中心离子和作为有机连接器的氰根基团配体一起自组装连接所形成的一种超分子结构。在类普鲁士蓝的框架结构中过渡金属分别存在与碳原子相邻以及与氮原子相邻的两种不同位置,并且这两种位置上可以被不同的金属离子所占据。这些金属元素包含铁、钴、镍、锌和铜等过渡金属。此外,可以使用少量的贵金属离子取代框架结构中过渡金属离子位置的过渡金属,同时还能够确保类普鲁士蓝的框架结构保持不变。这类类普鲁士蓝化合物中的金属元素具有丰富的种类多样性,因此非常适合用作制备过渡金属、过渡金属合金以及过渡金属与贵金属合金与碳复合的前驱体。合金化可以改变金属原子间的键长从而改变表面的吸附能最终优化电催化活性。而氰根基团是由氮原子和碳原子构成,可以作为C源和N源在合金粒子表面包覆上一层N掺杂的石墨烯层。这些包覆的石墨烯壳层可以促进合金核与石墨烯壳层之间的电子转移,从而有利于提升电催化活性和循环稳定性。因此,我们合成了一系列类普鲁士蓝化合物,并且以此为前驱体通过模板法热解制备出了过渡金属基电催化剂。并对制备的纳米粒子在碱性电解液中析氢电催化性能和析氧电催化性能进行了研究。主要内容包含如下几个方面:1.目前,析氢反应中最大的瓶颈是缺乏便宜的高活性电催化剂来取代铂基电催化剂。我们使用钌掺杂的钴氰酸钴类普鲁士蓝粒子来作为前驱体制备了具有高活性和高稳定性的电催化剂。该电催化剂是由核壳复合结构(RuCo@NC)构成,内部是钌和钴的双金属合金纳米粒子,外部是在合金表面原位包覆的氮掺杂的多层石墨烯。该复合材料在对碱性电解液中表现出优异的析氢电催化活性。其析氢反应的电流密度达到10 mA cm-2和100 mA cm-2时,过电位分别只有28 mV和218 mV。并且循环反应10000次后活性无明显下降,表现出极好的循环稳定性。贵金属Ru是最便宜的铂族金属,而该电催化剂中的钌含量只占催化剂总质量的3.58%,因此还具有很好的价格成本优势。通过密度泛函理论计算表明,在电催化剂的钴核中引入钌可以增强电子从合金内核向外部石墨烯壳层的转移,可以大幅度降低石墨烯表面上N掺杂近邻处的C活性位点对氢的吸附自由能,因此有利于促进析氢反应的进行。2.析氧反应是电解水中另一个重要的半反应。然而,析氧反应在动力学上更加难以进行,需要高活性的析氧电催化剂来加速反应的进行。目前,Ru02和Ir02是性能最佳的析氧电催化剂。但是,它们在析氧反应中不够稳定且价格成本相对较高,无法在工业上大规模使用。过渡金属以及它们的合金在理论上具有非常高的活性并且还具有更加便宜的价格成本,因此有很好的潜力来取代这些贵金属基电催化剂在析氧反应中的应用。本章采用铁氰酸镍作为前驱体,简便地制备出了氮掺杂的多层石墨烯包覆铁镍合金的复合材料(FeNi@NC)。该催化剂在碱性电解液中析氧电流密度达到10 mA cm-2时过电位只有299 mV,并且还在循环5000次后表现出较高的稳定性。可以看出,该电催化剂在催化活性和循环稳定性方面都超出了 RuO2电催化剂。因此,我们合成出的电催化剂在析氧领域中具有很好的潜在应用前景。3.析氧反应是电解水反应中阴极半反应,也是电解水制氢中一个关键的半反应。此外,析氧反应和它的逆反应氧还原反应是可再生燃料电池和金属空气电池的核心电化学反应。然而,析氧反应在动力学上速率较慢,因而需要高活性的电催化剂来降低反应的过电位从而降低能量损耗。非贵金属基电催化剂被认为最有前景的材料之一,有潜力取代贵金属基电催化剂在析氧反应中的应用。然而,这些非贵金属电催化剂与贵金属电催化剂相比活性和稳定性还有不小的差距,因此需要研究加以提高。在此,我们通过一步法焙烧制备了高氮掺杂的多层石墨烯包覆碳化钴锌和金属钴组成的纳米异质结复合材料(ZnCo3C/Co@NC)。我们制备的电催化剂在电流密度达到10 mAcm-2时过电位只有366 mV,其活性超过了商用RuO2电催化剂。此外,该电催化剂在氧还原反应中的起始电位和峰电流电位分别为0.912V和0.814V,该性能远远超过相应的金属碳化物样品。在异质结中金属钴与碳化钴锌的界面处,金属Co作为电子施主具有更好的亲电性,有利于促进OH-和反应中间体活性物质发生亲核反应,从而加速析氧反应的进行;Co3ZnC作为电子受主具有更高的亲核性,有利于促进反应中间体活性物质发生亲电反应并将生成的OH-立即脱去,从而加速氧还原反应的进行。
[Abstract]:At present, the two most serious problems facing the world are environmental degradation and energy crisis. Hydrogen energy is a clean and sustainable energy, with the advantages of high energy density, zero emission and abundant reserves. In order to cope with the above two problems, hydrogen energy is paid much attention. In alkaline electrolysis pool, electrolysis water is the most likely to realize large-scale industry at present. The technology of hydrogen production. However, the electrolyzed water needs to cross a larger energy barrier in kinetics, resulting in a slower reaction rate. Therefore, a highly active electrocatalyst is needed to accelerate the reaction. The catalyst not only changes the reaction kinetics process, but also reduces the overpotential and reduces the energy dissipation, and then reduces the cost of hydrogen production. As early as 1704, it was discovered, the oldest and simplest metal organic frame compound. Due to its extensive application in the fields of gas adsorption, energy storage, catalysis and drug loading, the Prussian blue compound is a central ion of transition metal as a metal joint. A supramolecular structure formed by a self-assembly connection with the ligands of the cyanogen group as the organic connector. In the framework of Prussian blue, the transition metals are adjacent to the carbon atoms and two different positions adjacent to the nitrogen atom, and these two positions can be occupied by different metal ions. The elements contain transition metals such as iron, cobalt, nickel, zinc and copper. In addition, a small amount of precious metal ions can be used to replace transition metals in the transition metal ions in the frame structure, while the frame structure of the Prussian blue can be kept constant. The metal elements in the Prussian blue complex are rich in variety, So it is very suitable for the preparation of transition metal, transition metal alloy and precursor of transition metal and metal alloy and carbon composite. Alloying can change the bond length between metal atoms and change the surface adsorption energy to optimize the electrocatalytic activity. The cyanogen group is made up of nitrogen source and carbon atom, which can be used as the source of C and the source of N. A layer of N doped graphene is coated on the surface of the alloy particles. These coated graphene shells can promote electron transfer between the alloy core and the graphene shell. Thus, the electrocatalytic activity and the cyclic stability are promoted. Therefore, a series of Prussian blue compounds have been synthesized and used as precursors through the template. Transition metal based electrocatalysts were prepared by pyrolysis. The electrocatalytic properties of hydrogen evolution and oxygen evolution in alkaline electrolyte were studied. The main contents include the following aspects: 1. the biggest bottleneck in the process of hydrogen evolution is the lack of cheap and highly active electrocatalysts to replace platinum based electrocatalysis. We use ruthenium doped cobalt cyanate Prussian blue particles as precursors to prepare an electrocatalyst with high activity and high stability. The electrocatalyst is composed of a nuclear shell composite structure (RuCo@NC), a bimetallic alloy nanoparticle with ruthenium and cobalt, and a nitrogen doped multilayer stone coated on the surface of the alloy. The composites exhibit excellent hydrogen evolution electrocatalytic activity in the alkaline electrolyte. When the current density of the hydrogen evolution reaction reaches 10 mA cm-2 and 100 mA cm-2, the overpotential is only 28 mV and 218 mV., respectively, and the activity has no obvious decrease after 10000 cycles, and the noble metal Ru is the cheapest. The content of ruthenium in the electrocatalyst is only 3.58% of the total mass of the catalyst, so it has a good price cost advantage. The density functional theory shows that the introduction of ruthenium in the cobalt core of the electrocatalyst can enhance the transfer of electrons from the alloy core to the external graphene shell, which can greatly reduce the surface of the graphene surface. The N doped C active site in the near neighbour is free energy for hydrogen adsorption. Therefore, it is beneficial to promote the hydrogen evolution reaction to promote the hydrogen evolution reaction is another important half reaction in the electrolysis water. However, the oxygen evolution reaction is more difficult to carry out in kinetics, and the high active oxygen evolution electrocatalyst is needed to accelerate the reaction. At present, Ru02 and Ir02 are the nature of the reaction. The best oxygen evolution electrocatalysts are available. However, they are not stable in the oxygen evolution reaction and have relatively high price costs and can not be widely used in industry. Transition metals and their alloys have very high activity in theory and have cheaper price costs, so there is a great potential to replace these precious metals. The application of electrocatalyst in the oxygen evolution reaction. This chapter uses nickel ferricyanate as a precursor to prepare a nitrogen doped multilayer graphite coated iron nickel alloy composite (FeNi@NC). The catalyst is only 299 mV when the oxygen evolution current density reaches 10 mA cm-2 in the alkaline electrolyte, and is also performed after 5000 cycles. It can be seen that the electrocatalyst has exceeded the RuO2 electrocatalyst in both catalytic activity and cyclic stability. Therefore, our synthesized electrocatalyst has a good potential application prospect in the field of oxygen evolution. The.3. oxygen evolution reaction is the cathode semi reaction in the electrolysis water reaction, and is also a key to the hydrogen production of the electrolysis water. In addition, the oxygen evolution reaction and its reverse redox reaction are the core electrochemical reactions of the renewable fuel cell and the metal air battery. However, the kinetic rate of the oxygen evolution reaction is slower, so the highly active electrocatalyst is needed to reduce the overpotential of the reaction and reduce the energy loss. One of the most promising materials has the potential to replace the application of the noble metal based electrocatalysts in the oxygen evolution reaction. However, these non noble metal electrocatalysts have no small gap between the activity and stability of the noble metal electrocatalysts. Therefore, it is necessary to study it. In this case, we have prepared high nitrogen doping by one-step roasting. A nano heterojunction composite (ZnCo3C/Co@NC) consisting of cobalt zinc and metal cobalt (ZnCo3C/Co@NC) is coated with multilayer graphene. The electrocatalyst prepared by us has a over potential of only 366 mV when the current density reaches 10 mAcm-2, and its activity exceeds the commercial RuO2 electrocatalyst. In addition, the starting potential and peak current potential of the electrocatalyst in the oxygen reduction reaction are in addition. Unlike 0.912V and 0.814V, the performance is far more than the corresponding metal carbide samples. In the heterojunction, the metal cobalt and cobalt carbide at the interface, metal Co as the electron donor has better electrophilic property, is beneficial to promote the nucleophilic reaction of the active substances of the OH- and the reaction intermediates, and accelerate the oxygen evolution reaction; Co3ZnC is the electron acceptor. The host has a higher nucleophilic activity, which helps to promote the reaction of reactive intermediates and remove the generated OH- immediately, thus accelerating the oxygen reduction reaction.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36;TQ116.2

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