燃料电池中石墨烯基非金属催化剂及其氧还原催化机理研究

Since the reaction rate at the cathode is much lower than that at the anode, the oxygen reduction reaction (ORR) at the cathode plays a key role in controlling the performance of a fuel cell. Platinum has conventionally been employed as the cathode catalyst due to its high activity for the ORR. However, the large-scale application of fuel cells has been hampered by the high cost and inadequacy, poor stability and deactivation in the presence of CO (a by-product from the use of methanol) of this metal. Thus, developing inexpensive electrocatalysts with high catalytic activity will accelerate the process of fuel cell commercialization. The aim of this project is to design metal-free catalyst with stability and high catalytic efficiency, especially in acid solution. In this project, not only the catalytic pathways of ORR on nitrogen doped grapheme but also on other non-metal elements doped ones are investigated using density functional theory. Combined with the study of the change of their electronic structures, we will propose their catalytic mechanism and establish the correlation between ORR activities and material microstructure. Based on these results, some new graphene based metal-free catalysts will be designed by co-doping or multi-doping to further improve their catalytic efficiency. This study will provide new ways to prepare graphene-based metal free catalysts with high ORR performace and enrich the basic catalytic mechanisms of carbon nanomaterials.

氧气在燃料电池中的还原反应是一个动力学慢反应，这导致了燃料电池的效率大大降低。开发可替代铂基催化剂的非金属燃料电池催化剂对燃料电池的广泛应用及发展具有重要的意义。本项目以设计高活性非金属石墨烯基催化剂为目标，利用密度泛函理论对多种非金属元素掺杂修饰的石墨烯非金属催化剂的形成与稳定性以及掺杂元素对石墨烯结构电子性质和化学吸附性质的影响进行系统深入的研究，并通过寻找酸性及碱性条件下石墨烯基非金属催化剂氧还原过程的最优反应路径，提出不同非金属元素掺杂下石墨烯结构的催化活性点及催化机理，建立"结构-催化活性"相互关系，并开发设计酸性条件下具有高活性高稳定性的二元及多元掺杂非金属石墨烯材料作为氧还原催化剂。通过本项目，将为高活性非金属催化剂的制备研究提供新的途径和思路，丰富和完善碳基非金属催化剂的研究。

本项目对一元、二元及多元非金属掺杂石墨烯的氧还原催化性能及机理进行了详细的研究。研究发现，在单原子掺杂下，N及B掺杂的氧还原催化性能最好，而多元掺杂能够进一步改变体系的电子结构分布，其协同作用可以提高氧气分子在催化剂表面的吸附能力，从而弱化O-O键，促进氧还原过程的进行，因此在多元掺杂情况下，石墨烯的催化性能均有不同程度的提高。并对一系列多元掺杂石墨烯的催化能力进行了比较，热力学研究发现非金属石墨烯催化活性与金属单质类似，氧气还原反应中间产物的自由能线性相关，因此催化活性可由OH的自由能来描述。通过比较发现B、N共掺杂，或B、N、S共掺杂的石墨烯催化活性最强。同时也对各类石墨烯的催化反应机理进行了研究，发现在非金属石墨烯中，氧还原反应更倾向于直接4电子反应，且多以associative reaction路径进行。速率控制步骤多集中在OOH的形成或者OH的解吸附形成H2O分子的步骤上。这些研究结果为拓展功能化石墨烯在燃料电池及空气电池催化剂方面的应用，设计新型催化剂提供有益的参考。