【作者】 李志强；

【导师】 李箐；

【作者基本信息】 华中科技大学 ， 材料学， 2021， 硕士

【摘要】 质子交换膜燃料电池（PEMFC）是一种理想的能源转化技术,具有清洁无污染、能量密度和转化效率高等优点。开发低成本、高活性的非贵金属氧还原反应（Oxygen reduction reaction,ORR）催化剂来替代昂贵且资源有限的铂（Pt）基催化剂是实现PEMFC商业化的关键。在众多非贵金属ORR催化剂中,过渡金属-氮-碳催化剂（M-N-C）呈现出良好的发展前景。然而M-N-C催化剂在酸性条件下仍然存在活性不足和稳定性较差等问题,这主要与M-N_x/C位点的本征活性低、密度低和脱金属化,以及催化剂碳基体在高温条件下的氧化腐蚀有关。本文以金属-聚合物超分子结构设计配位策略为基础,再结合硅（Si）掺杂和引入丙烯酸（AA）到海藻酸钠（SA）中设计金属-聚合物双交联网络分别制备具有优异ORR性能的Si/Fe-N-C和AA/SA-Fe-N催化剂,通过一系列的物理和电化学表征对Fe-N-C催化剂ORR活性和稳定性增强的机理进行研究。主要研究内容与实验结果如下:（1）以海藻酸钠（Sodium alginate,SA）为前驱体,采用金属聚合物超分子设计配位-Si掺杂相结合的策略,制备出具有高密度和均匀分散Fe-N_x/C位点的Si/Fe-N-C催化剂。首先通过Fe³⁺与SA中含氧官能团之间的配位作用形成具有三维空间的“蛋-盒”结构,使Fe³⁺均匀的分散在前驱体中,在热解过程中含氧官能团与Fe³⁺之间的键合作用能够阻止Fe原子的团聚;然后利用硅酸四乙酯与SA之间的氢键作用在前驱体中引入Si,有利于ORR过程中O₂的活化和H₂O的解吸附,同时可以提升催化剂碳基体的石墨化程度。通过后续热解和酸洗获得具有高活性和稳定性的0.5-Si/Fe-N-C-800催化剂,在0.5 M H₂SO₄条件下半波电位(E_1/2)达到0.817 V（vs.Reversible hydrogen electrode,RHE）,比未进行Si掺杂的Fe-N-C-800催化剂高32 m V;在60°C、O₂饱和的0.5 M H₂SO₄溶液中,经过5000圈的电位循环（0.6-1.0 V）以后,0.5-Si/Fe-N-C-800催化剂的E_1/2只损失20 m V,Fe-N-C-800催化剂E_1/2的损失达到40 m V。研究表明,Si/Fe-N-C催化剂优异的ORR活性主要来源于碳基体中均匀分散和高密度的Fe-N_x/C活性位点;稳定性的提升主要来自于Si掺杂提升了催化剂碳基体的石墨化程度。（2）以SA、丙烯酸（Acrylic acid,AA）为前驱体,采用金属-聚合物双交联网络的策略,制备具有均匀分散的Fe-N_x/C活性位点的多孔碳材料（AA/SA-Fe-N）作为高效的ORR催化剂。首先AA单体聚合形成聚丙烯酸（Polyacrylic acid,PAA）网络,SA链物理穿插到PAA网络中,SA链的运动受到PAA网络的限制;随后将三氯化铁加入到PAA/SA溶液中,Fe³⁺与PAA/SA中的三个-COOH鳌合,形成“蛋-盒”结构,将Fe³⁺锚定的同时,还能将SA链紧紧的“钉”在PAA交联网络上;SA链物理穿插在PAA网络中,进而使得Fe³⁺在双交联网络中分散的更加均匀,增大了金属离子之间的距离,有效地缓解了热解过程中的团聚,Fe-N_x/C活性位点的密度得到有效地提升。同时为了研究前驱体干燥过程中颗粒的团聚对于催化剂活性的影响,分别采用蒸发溶剂干燥和冷冻干燥的方式去处理前驱体,热解以后获得的催化剂分别为e-AA/SA-Fe-N和f-AA/SA-Fe-N。在0.5M H₂SO₄溶液中,e-AA/SA-Fe-N-1的E_1/2达到0.796 V（vs.RHE）,比未引入AA的Fe-N-C-800高出11 m V;冷冻干燥获得的催化剂f-AA/SA-Fe-N-1的E_1/2达到0.83 V（vs.RHE）,高于同类的Fe-N-C催化剂。研究表明,f-AA/SA-Fe-N材料的高活性源自于催化剂中均匀分散的Fe-N_x/C位点和丰富的孔道结构。更多还原

【Abstract】 Proton exchange membrane fuel cell（PEMFC）is an ideal energy conversion technology,which has the advantages of cleanliness,high energy density and high conversion efficiency.The development of low-cost,high-activity platinum group metal（PGM）-free oxygen reduction reaction catalysts to replace expensive and resource-limited platinum-based catalysts is the key to the commercialization of PEMFC.Among the PGM-free catalysts,metal-nitrogen-carbon catalysts（M-N-C）show good prospects for development.However,the M-N-C catalysts still suffer from insufficient activity and unsatisfactory stability under acidic conditions,which are mainly related to the low intrinsic activity,low density and demetallization of the M-N_x/C sites,as well as the oxidative corrosion of the catalyst carbon matrix under high temperature conditions.In this paper,the metal-polymer supramolecular structure design coordination strategy is used as the basis for the preparation of Si/Fe-N-C and AA/SA-Fe-N catalysts with high ORR performance by combining silicon doping and the introduction of acrylic acid（AA）to design metal-polymer double cross-linking network,respectively.The mechanism of the enhanced ORR activity and stability of Fe-N-C catalysts is investigated by a series of physical and electrochemical characterizations.The main research contents and experimental results are as follows:（1）The Si/Fe-N-C catalysts with high density and uniformly dispersed Fe-N_x/C sites are prepared using sodium alginate（SA）as the precursor,combined with metal-polymer supramolecular design coordination and Si doping strategy.Firstly,the coordination between Fe³⁺and the oxygen-containing functional group in SA forms an"egg-box"structure with three-dimensional space,which makes Fe³⁺uniformly dispersed in the precursor and prevents the agglomeration of Fe atoms during the pyrolysis process;then the hydrogen bonding between tetraethyl silicate and SA is used to introduce Si,which is conducive to the activation of O₂ and the desorption of H₂O in the catalytic process,and can enhance the graphitization of the carbon matrix of the catalyst.The 0.5-Si/Fe-NC-800 catalyst with high activity and stability is obtained by subsequent pyrolysis and acid washing,and the half-wave potential(E_1/2)reached 0.817V（vs.Reversible hydrogen electrode,RHE）in 0.5 M H₂SO₄,which is 32 m V higher than the Fe-NC-800 catalyst without Si doping;the 0.5-Si/Fe-N-C-800 catalyst exhibited excellent stability,as evidenced by a loss of only 20 m V in E_1/2 after 5000potential cycles from 0.6 to 1.0 V in O₂-saturated 0.5 M H₂SO₄ solution at 60°C.The loss in E_1/2 of Fe-N-C-800 catalyst reaches 40 m V.The excellent ORR activity of the Si/Fe-N-C catalyst is mainly due to the homogeneous dispersion and high density of Fe-N_x/C active sites in the carbon matrix;the stability enhancement is mainly due to the Si doping which enhances the graphitization of the carbon matrix of the catalyst.（2）The porous carbon materials（AA/SA-Fe-N）with uniformly dispersed Fe-N_x/C active sites are prepared as efficient ORR catalysts by using SA and AA as precursors in combination with a metal-polymer double cross-linking network strategy.Firstly,the AA monomer polymerizes to form a polyacrylic acid（PAA）network,SA chain is physically interspersed into the PAA network.Subsequently,Fe Cl₃ is added to the PAA/SA solution,and Fe³⁺chelates with three carboxyl groups in PAA/SA to form an"egg-box"structure.SA chain is physically interspersed in the PAA network,which in turn makes Fe³⁺more uniformly dispersed in the double cross-linked network and increases the distance between metal ions,effectively alleviating agglomeration during the pyrolysis,and the density of Fe-N_x/C active sites is effectively enhanced.To study the effect of particle agglomeration on the catalyst activity during precursor drying,we use evaporative solvent drying and freeze-drying to treat the precursors,and the catalysts obtained after pyrolysis are e-AA/SA-Fe-N and f-AA/SA-Fe-N,respectively.In 0.5 M H₂SO₄,the E_1/2 of e-AA/SA-Fe-N-1 is 0.796 V（vs.RHE）,which is 11 m V higher than Fe-N-C-800 without AA;the E_1/2 of f-AA/SA-Fe-N-1 reached 0.83 V（vs.RHE）.It is shown that the high activity of the f-AA/SA-Fe-N originated from the uniformly dispersed Fe-N_x/C sites and the extensive pore structure in the catalyst.更多还原