Ab Initio Study of the Doped Graphene for Oxygen Reduction Reaction

參雜石墨烯用於氧還原反應的第一性原理研究

Student thesis: Doctoral Thesis

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Award date7 Sep 2021

Abstract

Heteroatoms doped carbon materials has been reported to exhibit good catalytic activity for the oxygen reduction reaction (ORR) and to be a promising material to replace the use of platinum as the ORR catalyst in fuel cell application. Among them, nitrogen doped carbon (NC) and transition metal-nitrogen co-doped carbon (M-N-C) are the most studied ones. Although numerous experimental and theoretical studies have been reported, microscopic understanding of the ORR processes catalyzed on them is still limited. In particular, the ORR mechanism, the active sites, the roles of the different dopants played in catalyzing the ORR processes and the ORR activity of the various dopants remain elusive. In-depth knowledge of the details of the ORR processes on NC and M-N-C catalysts is essential to the development of efficient ORR catalysts.

In this thesis, systematic ab initio study was carried out to investigate the ORR mechanism on NC and M-N-C catalysts. I first studied the differences between the most reported nitrogen dopants, such as pyridinic-N and graphitic-N dopants, on NC catalysts. I found that the binding strength O2 on the graphene correlate closely with electron donation from the graphene to the O2. The presence of the graphitic-N dopants is found to facilitate the O2 binding, and thus the ORR reaction, as each graphitic-N dopant introduces one extra electron to the graphene. In addition, as the graphitic-N dopant percentage increases, the difference between the lowest-spin state energy and the second lowest spin state energy decreases, which favors the transfer of electrons to the O2 and thus the binding. Then I studied the ORR activity of 15 different nitrogen doping configurations. With the presence of the graphitic-N dopants, the Obinds to the carbon in the vicinity of the pyridinic-N dopants. Besides, I found that the armchair edge site pyridinic-N dopants favor O2 binding while the zigzag edge site pyridinic-N dopants weaken the O2 binding strength. Based on the thermodynamic activity volcano plot, the zigzag edge site carbon atoms, zigzag edge site pyridinic-N dopant, armchair edge site pyridinic-N dopant and armchair edge site graphitic-N dopant offer the appropriate binding strength of the key intermediates and exhibit high ORR activity. Finally, I studied the ORR activity of graphenes doped with 15 different transition metal atoms (including Mn, Fe, Co, Ni, Cu, Tc, Ru, Rh, Pd, Ag, Re, Os, Ir, Pt and Au) in the presence of the graphitic-N dopants. I found that the O2 binds to the Mn, Fe, Co, Tc, Ru, Rh, Re, Os and Ir centers and exhibits two configurations, end-on and side-on, which is correlated to the shape of the d orbitals of the metal atoms. Besides, the ORR activity of the metal atoms is mainly determined by the chemical activity of the metal atoms. Among all the transition metals, Fe, Co and Ir exhibit the highest ORR activity while Mn, Ni and Cu show poor performance. The good ORR performance for the graphene co-doped with Fe, Co or Ir is the joint effect of the metal and nitrogen dopants, while the ORR activity for the graphene co-doped with Mn, Ni or Cu is mainly contributed by the nitrogen dopants. In addition, the M-N4 motif is found to boost the ORR activity of the vicinity graphitic-N and pyridinic-N dopants.

Overall, this work contributes to the understanding of ORR processes on the NC and M-N-C catalysts and would help to the development of ORR catalysts with high activity.