【作者】 黄浩；

【导师】 陈乾旺；

【作者基本信息】 中国科学技术大学 ， 材料物理与化学， 2023， 博士

【摘要】 在能源短缺加剧、环境污染和气候变化的背景下,电化学能源转化和存储因其环境友好性和可持续性等优点而受到研究人员的广泛关注。电解水制氢、质子交换膜燃料电池(PEMFC)以及使用电能将二氧化碳还原成有价值的工业产品等课题已得到大量研究。在这些技术中,阴极发生的析氢反应(HER)、氧还原反应(ORR)和二氧化碳还原反应(CO2RR)等电化学还原反应的高效进行是大规模应用的关键。然而,这些电化学还原反应的快速进行需要高效的电催化剂。目前,稀有的贵金属催化剂如Pt等常被用于催化这些电化学还原反应。因此发展高效且更加廉价的电催化剂是近年来研究的关键,如目前文献报道的非贵过渡金属基催化剂。然而,过渡金属基催化剂也面临许多问题。例如,常用的Fe、Co等过渡金属单原子催化剂(SACs)在电催化ORR时会发生芬顿反应,很难在PEMFC中大规模应用。大多数过渡金属基催化剂对HER的催化活性较高,这不利于与HER存在竞争的电催化还原反应,如电催化CO2RR等。为解决这些问题,地球上储量同样丰富和对环境更加友好的主族金属元素进入了我们的视野。但是相比于过渡金属元素,主族金属元素外电子层拥有更少的价电子,并且在形成固体材料时其s和p电子的分布更加宽化,这意味着主族金属元素更难被调控为催化活性中心。然而,结合主族金属元素的初始电子结构特征,依然有相应的策略能够改变其电子结构,使其成为电催化还原反应的催化活性中心。本论文采用N、O配位将主族金属基元镶嵌到石墨烯基底这一策略去改变主族金属原子的电子结构,通过理论计算预测了s区第一主族金属Li和Na的单原子结构用于电催化还原反应的活性,并实验合成了与N、O配位的主族金属单原子基元镶嵌到石墨烯的碳基催化剂,评估了其电催化还原反应的催化活性。具体内容如下:1.分析Li初始的电子结构,其空置的2p轨道能够为电子结构的调控提供很大的便利。在上述电子结构调控策略的指导下,一系列N和O配位的Li基元镶嵌的石墨烯模型被设计出来,本章通过密度泛函理论(DFT)模拟计算了所构建模型对于ORR、HER和CO2RR的起始电势,发现锯齿型Li-pyridinic-N1-C1和Li-O2构型分别表现出优异的ORR和CO2RR催化性能,表明经过电子结构调控后的Li能够成为电催化ORR和CO2RR的活性中心。进一步的电子结构分析表明Li原子的s轨道电子与配位的N、O和C的p轨道电子之间存在s-p轨道杂化,并且Li的2p轨道与石墨烯基底的大π键之间存在p-π共轭效应。DFT计算的结果证明了 N和O的配位将Li金属基元锚定在石墨烯基底中这一电子结构调控策略的有效性。从头计算分子动力学(AIMD)模拟也确定了这些构型中的Li在水溶液中的稳定性,为后续在实验上开发对应的Li催化剂提供了理论依据。2.在上一章的理论计算工作的指导下,本章通过类似于高温固相反应的合成方法,以ZIF-8这一沸石咪唑酯骨架为前驱体成功制备了 N和O配位的Li基元镶嵌于石墨烯基底的碳基纳米催化剂。相关表征证明了材料中的Li是被N和O原子所配位的。该催化剂表现出了优异的酸性ORR催化性能和催化CO2还原为CO的高活性。该催化剂催化ORR的半波电位为0.77 V,催化CO2还原产生CO的法拉第效率(FE)高达98.8%,产生CO的分电流密度也能够达到6.93 mA cm-2,其性能与文献报道的部分过渡金属SACs可比。同时,该催化剂也表现出良好的稳定性。最后,该催化剂也可作为实际PEMFC阴极反应的潜在催化剂。在氢气和氧气作为反应气的条件下,其峰值功率密度可达430 mW cm-2,低于文献报道的基准的Pt/C和Fe/Co的SACs,但高于部分文献报道的Mn和Ca的SACs。这一章的实验和上一章的理论计算结果显示主族金元素Li用于电催化还原反应是可行的。3.采用上两章所述的理论计算和实验相结合的方法,本章研究了 s区第一主族金属元素Na用于电催化CO2RR。首先,分析Na原子初始的电子结构,其外层的3s轨道上的电子有益于CO2向CO的转化,并且其2p轨道上的电子与石墨烯基底的π电子之间发生的p-π共轭可以作为电子结构调控的额外参量。基于以上分析,本章将与N、O配位Na单原子基元镶嵌到石墨烯基底中,成功调控了 Na金属中心的电子结构,使其能够具备高效的电催化CO2RR活性。DFT计算证明了 s-p轨道杂化和p-π共轭效应的存在,同时Na-O2原子基团在边缘位置的镶嵌能够有效地牵引整个石墨烯体系的电子向边缘镶嵌的Na金属中心富集,该效应有利于CO2向CO的转化。此外,N和O配位的Na基元镶嵌到石墨烯的碳基纳米材料以ZIF-8为前驱体通过高温煅烧制备出来,该催化剂表现出催化CO2RR的高选择性和高活性,产生CO的FE最高能达到99.9%,产生CO的分电流密度也能够达到6.68 mA cm-2,其性能与文献中报道的部分过渡金属SACs相当。更多还原

【Abstract】 Against the backdrop of worsening energy shortages,environmental pollution,and climate change,electrochemical energy conversion and storage have received extensive attention by researchers due to their advantages of environmental friendliness and sustainability.Topics such as electrolysis of water for hydrogen production,proton exchange membrane fuel cells(PEMFCs),and the reduction of carbon dioxide into valuable industrial products using renewable electricity have received considerable research.In these technologies,the efficient implementation of electrochemical reduction reactions such as hydrogen evolution reaction(HER),oxygen reduction reaction(ORR),and the carbon dioxide reduction reaction(CO2RR)occurring at the cathode is the key to large-scale applications.However,the rapid progress of these electrochemical reduction reactions requires efficient electrocatalysts.Currently,rare noble metal catalysts such as Pt are usually used to catalyze these electrochemical reduction reactions.Therefore,developing efficient and low-cost electrocatalysts for these reactions is a core research topic for the years to come,such as the recently reported non noble transition metal-based catalysts in the literature.However,transition metal-based catalysts also face many problems.For example,commonly used transition metal single atom catalysts(SACs)such as Fe and Co will undergo Fenton reactions during electrocatalytic ORR,causing the difficulty of largescale applications in PEMFCs.Majority of transition metal-based catalysts have high catalytic activity for HER,which is not conducive to some competitive electrocatalytic reduction reactions such as electrocatalytic CO2RR.In order to solve these problems,the main group metal elements have entered our visions due to their rich earth reserves and more environmentally friendliness.However,compared to transition metal elements,the outer electron layers of the main group metal elements have relatively fewer valence electrons.And they have a wide distribution of s and p electrons when forming solid materials,which means that the electronic structure of the main group metal elements is more difficult to regulate as a catalytic active center.However,combined with the initial electronic structure characteristics of the main group metal elements,there are still relevant strategies that can change their electronic structure and make them become the catalytic active centers for electrocatalytic reduction reactions.In this paper,a strategy of embedding the main group metal units coordinated with N,O into graphene substrates was adopted to change the electronic structure of the main group metal atoms.Theoretical calculations were conducted to predict the activity of the group IA metals Li and Na single atom configurations in the s block for electrocatalytic reduction reactions.Corresponding carbon-based catalysts with N,O coordinated main group metal single atom units embedded in graphene were synthesized experimentally,and their catalytic activity for electrocatalytic reduction reactions was evaluated.The specific content is as follows:1.It is found that the vacant 2p orbital of Li can provide great convenience for the electronic structure regulation.Under the guidance of the above electronic structure regulation strategy,a series of N and O coordinated Li embedded graphene models have been designed.In this chapter,the theoretical onset potentials of the constructed models for ORR,HER,and CO2RR have been calculated through density functional theory(DFT)simulation.It is found that the zigzag-type Li-pyridinic-N1-C1 and Li-O2 exhibit remarkable catalytic activity for ORR and CO2RR respectively,suggesting that Li atoms after the electronic structure regulation can become the active centers of electrocatalytic ORR and CO2RR.Further electronic structure analysis shows that there is an s-p orbital hybridization between the s-orbital electrons of Li atom and the porbital electrons of coordinated N,O,and C,and there is a p-π conjugation effect between the 2p orbital of Li atom and the π bond of the graphene substrate.The DFT calculations demonstrate the effectiveness of the electronic structure regulation strategy of anchoring Li metal into graphene substrates through the coordination of N and O.Ab initio molecular dynamics(AIMD)simulation also confirmed the stability of Li in these configurations in aqueous solutions,providing a theoretical basis for the subsequent development of corresponding Li catalysts in experiment.2.Under the guidance of the theoretical calculations in the previous chapter,in this chapter,a carbon based nano catalyst with N,O coordinated Li embedded in graphene substrates was successfully prepared experimentally through a synthesis method similar to high-temperature solid-phase reactions using ZIF-8,a zeolite imidazolate backbone,as a precursor.The relative characterizations prove that Li in the catalyst is coordinated by N and O atoms.The catalyst exhibits excellent acidic ORR catalytic performance and high activity in catalytic reduction of CO2 to CO.The catalyst has a half wave potential of 0.77 V for ORR,a Faradaic efficiency of 98.8%for catalytic CO2 reduction to CO,and a partial current density of 6.93 mA cm-2 for CO production,comparable to some reported transition metal SACs.Meanwhile,the catalyst also showed good stability.Finally,the catalyst can also be used as a potential electrocatalyst for cathode reactions of actual PEMFC.With hydrogen and oxygen as reaction gases,the peak power density can reach 430 mW cm-2,lower than that of the reported benchmark Pt/C and Fe/Co SACs,but higher than that of some reported Mn and Ca SACs.The experimental results in this chapter and the theoretical calculations in the previous chapter show that the use of main group Li elements in electrocatalytic reduction reactions is feasible.3.Using the method of combining theoretical calculations and experiments described in the previous two chapters,this chapter studied the use of the group IA metal element Na in the s block for electrocatalytic CO2RR.Firstly,the initial electronic structure of Na atom is analyzed.The outer 3 s electrons of Na is considered beneficial for the conversion of CO2 to CO,and the p-π conjugation between the 2p electrons of Na and the π electrons of the graphene substrate can be used as an additional parameter for electronic structure regulation.Based on the above analysis,this chapter successfully regulates the electronic structure of the Na metal center by embedding Na single atoms with N,O coordination into the graphene frameworks,enabling it to have efficient electrocatalytic CO2RR activity.DFT calculations have demonstrated the existence of s-p orbital hybridization and p-π conjugation effect,while the embedding of Na-O2 groups at the edge positions can effectively draw electrons from the entire graphene system to the edge doped Na metal center,which is beneficial to the conversion of CO2 to CO.In addition,carbon-based nanomaterials embedded with N and O coordinated Na elements into graphene were prepared experimentally by hightemperature calcination using ZIF-8 as precursor.The catalyst exhibits high selectivity and activity in catalyzing CO2RR,with a Faradaic efficiency of up to 99.9%for CO generation,and a partial current density of 6.68 mA cm-2 for CO production,which is comparable to some reported transition metal SACs.更多还原