【作者】 罗艳；

【导师】 王瑞林；

【作者基本信息】 四川大学 ， 材料物理与化学， 2021， 博士

【摘要】 环境污染和能源短缺问题日益严重,大力发展清洁能源、提高能源的利用效率已迫在眉睫。近年来,聚合物膜燃料电池(PMFCs)和锌空气电池(ZABs)的研究和开发受到国内外的高度关注。PMFCs具有功率密度高、能量转换效率高和环境友好的优点;ZABs具备成本低和能量密度高的特点。这些新型电池从电动汽车到便携式设备均具有广泛的应用,然而其广泛的商业化仍需要进一步的突破。最亟待解决的问题是缓慢的氧电极反应,包括氧还原反应(ORR)和氧析出反应(OER)。解决问题的关键在于开发新型电催化剂。贵金属催化剂能有效催化反应进行,然而其价格昂贵和储量稀少的缺点不利于规模化使用。因此,开发具有高活性、高稳定性的非贵金属ORR催化剂和OER/ORR双功能催化剂对燃料电池和锌空气电池的发展具有重要意义。过渡金属-氮-碳(M-N-C,M=Fe、Co、Ni、Cu等)材料对氧电极反应表现出较好的电催化活性,是最有希望替代贵金属催化剂的材料。即便如此,M-NC电催化剂仍存在活性位点密度小、位点可利用率低、本征活性和稳定性差等缺点。针对上述的问题,本文通过杂原子掺杂、金属组分耦合、形貌调控和单原子构筑的方法来调节活性位的电子结构和协同作用进而优化催化剂的电化学性质,提高催化剂的催化活性和稳定性,并讨论催化剂的结构与电催化性能之间的关系以及催化反应机理,获得有意义的研究结果。本论文的主要研究内容和研究结果如下:(1)以功能碳黑为结构连接剂,MIL-101(Fe)为前驱体,通过热解过程制备纳米多孔M-N-C催化剂,研究其对ORR的催化性能。首先通过改变三聚氰胺的加入量,制备不同氮掺杂的金属有机框架化合物(MOFs)衍生碳材料。表征结果显示合成的催化剂中含有Fe和Fe3C纳米颗粒,包裹在富氮介孔碳中,并连接到功能碳黑上(Fe/Fe3C@NC)。这种复合结构设计可使得催化剂电导率提高,更多活性位点暴露,并且增强金属物种的稳定性。XPS结果表明,得到的催化剂中形成不同含量的Nx-C活性位,最佳氮掺杂的催化剂表现出优异的ORR活性,起始电位和半波电位分别为0.85 V和0.70 V,同时它的稳定性和耐甲醇性能优于商业Pt/C催化剂。通过进一步分析,认为Fe/Fe3C@NC催化活性的提高主要归因于氮掺杂在调节结构和电子变化中的作用,高效氮掺杂形成了更多的活性中心。同时,Fe/Fe3C纳米颗粒与Nx-C间协同效应的增强进一步促进ORR进程。(2)在(1)的基础上,探究制备条件对铁基催化剂性能的影响,获得优化的制备条件和具有Fe/Fe3O4@NC结构的催化剂。选用MIL-101(Fe)/C为前驱体,在不同热解温度(600℃、650℃和700℃)下获取含有不同金属物种组分的铁基催化剂,其实验结果表明:与其他热解温度相比,在650℃热解获得的Fe/Fe3O4@NC具有Fe/Fe3O4金属中心调控的氮掺杂碳结构,表现出更高的电催化活性和稳定性,起始电位和半波电位分别为0.90 V和0.77 V,呈现近4电子转移反应,同时它的ORR活性优于商业Pt/C。本工作的研究表明,热解温度能调节金属组分和碳结构,Fe/Fe3O4对表面碳层的电子结构表现出优异的调制效果,降低碳表面的局部功函数,进而增强表面活性中心的本征活性,提升ORR催化活性。总的来说,本工作研究了M-N-C催化剂中金属成分耦合对活性中心电子结构的调控作用,为高性能非贵金属催化剂的设计合成提供有益的参考。(3)采用双模板法制备铁氮共掺杂分级多孔碳(Fe-N-HPC-0.05)电催化剂,并研究其在酸性和碱性介质中对ORR的催化性能。首先以Si O2纳米微球作为硬模板,MIL-101(Fe)和ZIF-8作为自牺牲模板,通过控制MOFs的成分和含量得到形貌调控的M-N-C催化剂。表征结果显示Fe/Fe3C纳米晶、Fe-Nx和Nx-C构成催化剂的活性中心;分级多孔结构促进反应的传质过程,并且暴露更多的活性位。Fe-N-HPC-0.05在碱性和酸性条件下均表现出较高的ORR活性和稳定性。在碱性条件下,Fe-N-HPC-0.05的起始和半波电位分别为1.02 V和0.85 V,与Pt/C近似,但其稳定性较商业催化剂更好。在酸性条件下,Fe-N-HPC-0.05的半波电位为0.78 V,仅比Pt/C(0.84 V)低60 m V。分析表明层次孔结构提高了催化剂活性位的利用率,活性位点间的协同效应优化了ORR反应路径。Fe-NHPC-0.05作为燃料电池阴极催化剂具有潜在的应用前景,为设计具有暴露活性中心的分级多孔结构提供参考。(4)采用改性MOFs(Si O2@Fe-ZIF-8/67)作为前驱体,制备原子级分散的Fe和Co双金属位点修饰3D氮掺杂碳纳米片(A-Fe Co@NCNs),研究其对ORR和OER的双功能催化性能以及作为空气阴极的锌空气电池性能。首先以Si O2@Fe-ZIF-8/67作为前驱体,通过调节金属的含量获得A-Fe Co@NCNs催化剂。物相和形貌结构分析结果显示Fe或Co金属以单原子形式与N配位形成锚定在缺陷碳上的Fe N4、Co N4和N3Fe-Co N3。A-Fe Co@NCNs表现出优异的ORR/OER双功能活性,ΔE为0.80 V。与商用Pt/C+Ir O2电池相比,以AFe Co@NCNs组装的锌空气电池具有的比容量更高为736.2 m Ah g-1Zn,功率密度达到132 m Wcm-2,循环稳定性能更好。分析表明双金属单原子催化剂具有更高的活性位点密度,金属原子间的协同耦合效应能有效提高催化剂的双功能催化活性。密度泛函理论(DFT)研究表明Co的引入能提高Fe原子周围的电子密度,从而有利于活性位点对O2分子的吸附和活化,Fe N4-Co N4展示出更正的热力学限制电位(0.85 V)。进一步阐明A-Fe Co@NCNs中的Fe和Co之间的协同耦合作用能优化ORR反应路径,增强ORR性能。本研究结果为制备具有双金属单原子位点的M-N-C催化剂提供一种简便的方法,使其在双功能电催化反应中具有高活性和耐久性。更多还原

【Abstract】 As environmental pollution and energy shortages becoming increasingly severe globally,it is urgent to find renewable energy resources and improve the utilization efficiency of energy.A lot of attention has been paid to research and develop polymer membrane fuel cells(PMFCs)and Zn-air batteries(ZABs).PMFCs have the advantages of high-power density,high efficiency and environment-friendliness.ZABs possess the merits of low cost and high energy density.Those energy conversion devices offer a wide range of applications from electric vehicles to portable devices;however,more efforts should be paid for their extensive commercialization.The most urgent problems to be solved are sluggish kinetics oxygen electrode reactions such as oxygen reduction reaction(ORR)and oxygen evolution reaction(OER).The keys to those problems are the development of new electrocatalysts.Precious metal-based materials can effectively catalyze the electrocatalytic reactions.However,their high cost and scarce reserves are not conducive to large-scale use.Therefore,the development of ORR catalysts and bifunction OER/ORR no-precious catalysts with high activity and stability is of great significance for the commercial application of PMFCs and ZABs.Transitional metal-nitrogen-carbon(M-N-C,M=Fe,Co,Ni,Cu,etc.)materials exhibit fine electrocatalytic activity for the oxygen electrode reactions,and are the most promising materials to replace precious metal-based catalysts.However,M-N-C electrocatalysts still have the disadvantages of low active site density,low site availability and poor intrinsic activity ang stability.Therefore,in order to solve the above problems,in this paper,we used a series of strategies such as heteroatom doping,metal component coupling,morphology regulation and construction of single atom to adjust the electronic structure and synergies of the active sites,which could further optimize the electrochemical properties of catalysts,and improve the catalytic activity and stability of the catalysts.The relationship between structure and catalytic performance and the catalytic reaction mechanism were studied and some meaningful results were obtained.The main research contents and results of this paper are as follows:(1)Nanoporous M-N-C catalysts were prepared by using functional carbon black assembled with MIL-101(Fe)as a precursor,followed by pyrolysis,and their catalytic performances for ORR were studied.Firstly,by changing the amount of melamine,different nitrogen-doped carbon materials were derived by metal organic frameworks(MOF).The structure and morphology characterization results of derived materials showed that Fe and Fe3 C nanoparticles were encapsulated in nitrogen-doping mesoporous carbon and connected to functional carbon black(Fe/Fe3C@NC).The hierarchical structure improved the electronic conductivity of the catalyst,exposed more active sites and enhanced the stability of metal species.XPS results showed that different contents of Nx-C active sites were formed in the catalysts.The best catalyst exhibited excellent ORR activity with onset potentials(Eonste)and half-wave potentials(E1/2)of 0.85 and 0.70 V,respectively.Furthermore,it showed much higher stability and better methanol tolerance than those of the state-of-the-art Pt/C.Through further analysis of the effect of nitrogen doping on active site density and catalytic activity,we found that the improvement of catalytic activity of Fe/Fe3C@NC is mainly attributed to the role of nitrogen doping in adjusting the electronic structure,and highefficiency nitrogen dopants form more active centers.At the same time,the enhancement of the synergistic effect between Fe/Fe3 C nanoparticles and Nx-C further promotes the ORR process.(2)On the basis of(1),we explored the influence of preparation conditions on the performance of iron-based catalysts,and obtained the optimized conditions and the catalysts with Fe/Fe3O4@NC structure.Iron-based catalysts containing different metal species were obtained at different pyrolysis temperatures(600 ℃,650 ℃ and 700 ℃)by choosing MIL-101(Fe)/C as precursor.The experimental results showed that the Fe/Fe3O4@NC obtained by pyrolysis at 650 ℃ had a nitrogen-doped carbon structure which regulated by the Fe/Fe3O4,showing higher electrocatalytic activity and stability as compared with the other iron-based catalysts.Fe/Fe3O4@NC conducted a close to four-electron pathway with Eonste of 0.90 V and E1/2 of 0.77 V,and its ORR activity was better than commercial Pt/C.Studies have shown that the pyrolysis temperature has an important influence on the metal compositions and unique structures.Fe/Fe3O4 has an excellent modulation effect on the electronic structure of the carbon layer,which can reduce the local work function of the carbon layer,enhance the intrinsic activity of the surface active sites and improve the ORR catalytic activity.On the whole,the effect of metal components coupling on the electronic structure of active sites in M-N-C catalysts was studied in this work.It is important to provide study value for the design and synthesis of high-performance non-noble metal catalysts.(3)Iron-nitrogen co-doped hierarchically porous carbon(Fe-N-HPC-0.05)electrocatalytic catalyst was synthesized through a dual-template assisted method and its catalytic performances for ORR under acid and alkaline conditions were studied.By controlling the compositions and contents of MOFs,morphologically controlled M-N-C catalyst was obtained through applying silica as a hard template together with MOFs as a self-sacrificing template.The characterization results show that Fe/Fe3 C nanocrystals,Fe-Nx and nitrogen-doped carbon all exist as active sites in the catalyst;the hierarchical porous structure facilitates the exposure of active sites and increases the accessibility of reactants.Fe-N-HPC-0.05 showed enhanced ORR performance in both alkaline and acidic conditions.In alkaline conditions,Fe-N-HPC-0.05 exhibited onset and half-wave potentials of 1.02 and 0.85 V,which were similar to those of commercial Pt/C.In addition,the stability of Fe-N-HPC-0.05 is better than Pt/C.In acidic conditions,Fe-N-HPC-0.05 possessed half-wave potential of 0.78 V,only 60 m V lower than that of Pt/C(0.84 V).The analysis shows that the hierarchically porous structure and synergistic effect of active sites could optimize the ORR reaction pathway.Fe-N-HPC-0.05 has potential application prospects as a fuel cell cathode catalyst.At the same time,this work also provides the design principle for increasing the ORR catalytic activity of hierarchically porous structure with exposed active sites.(4)Atomically dispersed Fe and Co doping 3D nitrogen-doped carbon nanosheets(A-Fe Co@NCNs)was prepared by using modified MOFs(Si O2@Fe-ZIF-8/67)as precursor,and its bi-functional ORR/OER catalytic activity and performance of zincair batteries were studied.Firstly,A-Fe Co@NCNs catalyst was obtained by using Si O2@Fe-ZIF-8/67 as the precursor.The morphology and phase structures analysis results showed that Fe or Co single atoms were identified to be coordinated with N atoms and formed Fe N4,Co N4 and N3Fe-Co N3 anchoring on 3D defect carbon.AFe Co@NCNs exhibited excellent electrochemical performance with ORR/OER potential gap of 0.80 V.The performance of A-Fe Co@NCNs-based ZAB was better than that of a zinc-air battery assembled with a commercial Pt/C+Ir O2 catalyst.AFe Co@NCNs-based ZAB had a specific capacity of 736.2 m Ah g-1Zn,a power density of 132 m Wcm-2 and outstanding cycle stability.The results shown that bi-metal SACs were prepared to further increase the density of active sites,and the synergistic coupling effect between metal atoms can effectively improve the bifunctional activity of the catalyst.Density functional theory(DFT)study showed that the electron density of Fe atoms in A-Fe Co@NCNs apparently increases after the introduction of Co atoms,which is conducive to the adsorption and activation of O2 molecules.Fe N4-Co N4 shows a positive thermodynamic limiting potential of 0.85 V.It is further clarified that the synergetic coupling between Fe and Co can optimize the ORR reaction path and enhance the ORR performance.Results from this study may provide a facile method for precious control of dual metal doped carbon with highly activity and durability for bifunctional electrocatalysis.更多还原