基于杂质原子掺杂缺陷石墨烯的氧还原反应电催化剂的理论研究

发布时间：2018-02-13 20:46

本文关键词： 质子交换膜燃料电池氧还原反应机理密度泛函理论缺陷石墨烯自由能　出处：《内蒙古工业大学》2017年硕士论文　论文类型：学位论文

【摘要】：质子交换膜燃料电池(PEMFC)可以通过电化学反应将化学能直接转化成电能,并且因其转换效率高,无污染等特点,在过去的几十年里引起了广泛的关注。目前,贵金属铂(Pt)基催化剂为PEMFC的主要催化剂。但因其储量低,成本高,耐久性差等问题严重阻碍了PEMFC广泛的商业应用。因此,开发廉价高效的非贵金属或非金属电催化剂来催化氧还原反应(Oxygen Reduction Reaction:ORR)已成为PEMFC实现商业化的关键。近几年,由于掺杂石墨烯以及碳纳米管等sp~2纳米碳材料具有优异的导电性和高的比表面积等优点,已经成为了非贵金属或非金属ORR电催化剂的重要选项和研究热点。本论文结合量子化学计算和电化学热力学模型分析,系统地研究了不同非贵金属或非金属掺杂缺陷石墨烯构成的ORR电催化剂的催化性能,以及探讨了掺杂含缺陷石墨烯具有催化活性的内在机制。论文主要内容和研究结果如下:1.磷(P)掺杂缺陷石墨烯作为氧还原反应电催化剂的理论研究。通过形成能的计算,P掺杂双缺陷石墨烯要比P掺杂单缺陷和Stone Wales缺陷石墨烯更易形成。针对P掺杂双空位缺陷石墨烯,展开ORR反应机理和自由能曲线的研究。研究结果表明,O_2分子可以有效的吸附在电催化剂表面,满足了ORR发生的前提。随后,在ORR机理研究中发现活化O_2分子可以通过三个ORR竞争反应路径完成4e-过程且转化为最终产物H_2O分子。其中动力学最佳的路径是O_2分子氢化形成OOH,OOH氢化形成O+H_2O的过程。这个路径的决速步骤是最后一步,即OH氢化生成H_2O的过程。从ORR自由能曲线图中可以得出,每一步基元反应的吉布斯自由能都是负值,说明反应是自发进行的放热过程。结合外电势的影响,预测该体系的工作电压是0.27 V。2.硅(Si)掺杂缺陷石墨烯作为氧还原反应电催化剂的理论研究。研究结果表明,对于Si掺杂双缺陷石墨烯,O_2分解和OOH氢化成O+H_2O是反应的最佳路径,O原子氢化生成OH是最佳路径中的决速步骤。而对于Si掺杂单缺陷石墨烯,同一路径下,决速步骤为第二个H_2O分子生成且需要克服相当高的势垒。这说明相同非金属掺杂不同缺陷石墨烯构型会导致不同的ORR催化活性发生。对于双缺陷石墨烯,反应的主要活性位点是Si以及与Si成键的四个C原子,这一点从电荷转移和差分电荷密度可以得到证明。3.锰(Mn)和磷(P)共掺杂缺陷石墨烯作为氧还原反应电催化剂的理论研究。结果表明,对于MnP_x(x=1-4)共掺双缺陷石墨烯,MnP_2是最稳定的结构。针对MnP_2共掺双缺陷石墨烯,反应机理研究表明MnP_2和它相邻的六个C原子构成了催化活性中心,且在酸性介质中以4e-转移过程完成ORR催化反应。动力学最佳反应路径是O_2分子氢化形成OOH,然后OOH氢化形成O+H_2O的过程。决速步骤是第二个H_2O分子的形成。
[Abstract]:Proton exchange membrane fuel cell (PEMFC) can convert chemical energy directly into electric energy by electrochemical reaction, and has attracted wide attention in the past decades because of its high conversion efficiency and no pollution. Pt-based catalyst of noble metal platinum is the main catalyst of PEMFC. However, due to its low storage, high cost, poor durability and other problems, the wide commercial application of PEMFC is seriously hindered. The development of cheap and efficient non-precious metal or non-metallic electrocatalysts to catalyze oxygen Reduction reaction: orr has become the key to the commercialization of PEMFC. Because of the excellent electrical conductivity and high specific surface area of sp~2 nano-carbon materials, such as graphene and carbon nanotubes, It has become an important option and research hotspot in non-noble metal or non-metallic ORR electrocatalysts. The catalytic properties of ORR electrocatalysts with different non-noble metal or non-metal doped graphene defects were systematically studied. The internal mechanism of the catalytic activity of doped graphene with defects was also discussed. The main contents and results of this paper are as follows: 1. The theoretical study of doped graphene with defect as electrocatalyst for oxygen reduction reaction. P doped double defect graphene is easier to form than P doped single defect and Stone Wales defect graphene. The mechanism of ORR reaction and the free energy curve were studied. The results showed that the molecule of ORR could be effectively adsorbed on the surface of the electrocatalyst, which satisfied the premise of the occurrence of ORR. In the study of ORR mechanism, it was found that the activation of O _ s _ 2 could be completed through three ORR competitive reaction paths and transformed into the final product H _ 2O. The best path of kinetics was that O _ 2 molecules were hydrogenated to form O _ H _ 2O via hydrogenation of O _ (2) O _ (2) O _ (2) O _ (2) O _ (2) O _ (H _ 2O). The speed step of this path is the last step, From the curve of ORR free energy, the Gibbs free energy of each step of the elementary reaction is negative, which indicates that the reaction is a spontaneous exothermic process. It is predicted that the operating voltage of the system is 0.27 V. 2.The theoretical study of the doped graphene with silicon oxide as the electrocatalyst for oxygen reduction reaction shows that, The decomposition of Si-doped graphene O _ 2 and the hydrogenation of OOH to O _ H _ 2O are the best path for the reaction. The hydrogenation of O atom to OH is the critical step in the optimal path, while for Si doped graphene with single defect, the same path is obtained. The second H2O molecule is generated and the high potential barrier must be overcome. This indicates that the same nonmetallic doped with different defects graphene configuration will lead to different ORR catalytic activity. The main active sites of the reaction are Si and four C atoms bonded to Si. This point can be proved by charge transfer and differential charge density. The structure of MNP _ XX _ XX _ (1-4) is the most stable structure. For MnP_2 co-doped with double defect graphene, the reaction mechanism studies show that MnP_2 and its adjacent six C atoms form the catalytic active center. The ORR catalytic reaction was completed by 4e- transfer process in acid medium. The best reaction path of Kinetics is the hydrogenation of O _ (2) to O _ (H _ (2)) and the hydrogenation of OOH to O _ (H _ 2O). The final step is the formation of the second H _ 2O.
【学位授予单位】：内蒙古工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O643.36;TM911.4

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