Theoretical Calculation Study Of Graphene-based Electrocatalysts For Oxygen Reduction Reaction

Low/non-platinum oxygen reduction reaction (ORR) electrocatalysts are the key that makes fuel cell technology practical. In recent years, various doped carbon nanomaterials as ORR electrocatalysts or catalyst supports have been extensively studied. In particular, sp2nanocarbon materials such as doped graphene and nanotubes have become an important candidate and hotspot of nonprecious metal ORR electrocatalysts because of their excellent electrical conductivity and high surface area. At present, the nature of ORR active sites in these materials is still lack of a unified understanding. This thesis systematically studied different types of nitrogen(N) doped graphene, N and transition metal (Fe, Co, Ni) co-doped graphene and these metals support graphene three kinds materials formation and electrocatalytic activity for oxygen reaction within a unified electrochemical thermodynamic framework and quantum chemistry calculation, in order to understand the possible oxygen reduction activity sites in doped nanocarbon materials. The main contents and results are summarized as follows:1. Research strategies and methods in this thesis(1) Systematically calculated adsorption characteristics of various possible ORR oxygen adsorbed intermediates (*O2,*OOH,*HOOH, and*OH) on various doped or supported graphene surface, obtained adsorption configuration and adsorption free energy data, and the quantitative relationship between adsorption free energy of various oxygenated intermediates. Using adsorption free energy of oxygenated intermediates, calculated reaction free energy of possible ORR steps and activation energy barrier of some steps (such as*O2dissociation), so as to determine the most likely reaction pathway on various graphene structure surface.(2) Calculated standard equilibrium electrode potential (0i) of each reaction step in the pathway, compared with standard equilibrium electrode potential (1.23V) of ORR total reaction, and determined the activity-determining step (the step of maximum value1.23-0i).(3) Constructed activity volcano relation plot by0i of the activity-determining step and adsorption free energy of oxygenated intermediates (such as*OH). According to the position of various graphene structure in the volcano plot, to analyze its possibility as ORR active center.(4) Calculated the electronic structure to investigate the essential influence on adsorption capacity of oxygenated species on various graphene structure. 2. Different types of N-doped graphene as electrocatalysts for oxygen reductionWe constructed twelve different types of N-doped graphene structures and study their ORR electocatalytic properties. The results are as follows:The initial step of ORR on N-doped graphene surface is the proton-electron transfer coupled O2adsorption to form*OOH, except for in-plane graphitic N (GN), in-plane meta-position graphitic N (m-GN), GN at zigzag edge (zig-GN), and hydrogenated PyN at armchair edge (arm-PyN-H). The electrocatalytic activity of various doping structures for the ORR is determined by the reaction steps of the proton-electron transfer coupled O2adsorption and/or the reduction of adsorbed*OH, except for GN, m-GN, and zig-GN. At surface sites in different types of N-doped graphene, the adsorption free energy values of key oxygenated intermediates in ORR, such as*OOH, and*OH, approximately linearly scale with each other, namely,△G*OOH≈1.01△G*OH+3.21eV. This makes the ORR activity of various graphene doping structures can be described with a single thermodynamic descriptor (*OH adsorption free energy) to construct the activity volcano plot. According to the model volcano plot of ORR activity as a function of the adsorption free energy of*OH, surface sites in a few edge N-doping structures, such as armchair graphitic N (arm-GN), zigzag pyridinic N (zig-PyN), and zigzag pyridinic N oxide (zig-PyN-O), may offer optimized strength of oxygenated species for catalyzing ORR. Electronic structure calculations suggest that the trends in the binding strength of oxygenated species can be correlated with the density of pz states near the Fermi level of doping structures. Those have a higher density of pz states bind oxygenated species stronger.3. Transition metal (Fe, Co, Ni) and nitrogen co-doped graphene as electrocatalysts for oxygen reductionWe constructed eighteen types of transition metal (Fe, Co, Ni) and nitrogen co-doped graphene and study their ORR electocatalytic properties. The results are as follows:The initial step of ORR on all co-doped graphene surface is the proton-electron transfer coupled O2adsorption to form*OOH. The ORR activity of co-doped graphene is determined by the reaction steps of the proton-electron transfer coupled O2adsorption and/or the reduction of adsorbed*OH, except for Ni embedded at edge (Ni-G-edge). Compared with N-doped graphene structures, the linear correlation of*OOH,*O, and*OH adsorption free energy is better, and*O or*OH adsorption free energy can be used as a descriptor of metal (Fe, Co, Ni) and N co-doped graphene surface ORR activity to construct the activity volcano plot. The ORR activity of in plane and at edge Metal-N4co-doped structures (Metal-N4-G and Metal-N4-G-edge) is more efficient.4. Fe, Co, and Ni support graphene as electrocatalysts for oxygen reductionWe constructed nine types of Fe (111), Co (111), and Ni (111) support graphene structures and study their ORR electocatalytic properties. The results are as follows: The electrocatalytic activity of metal (Fe, Co, Ni) support structures for the ORR is determined by the reaction steps of the proton-electron transfer coupled O2adsorption and/or the reduction of adsorbed*OH, except for Ni (111) support N-graphene and Fe (111) support N-graphene.*O or*OH adsorption free energy can be used as a descriptor of metal (Fe, Co, Ni) support graphene surface ORR activity to construct the activity volcano plot. The ORR activity of Ni (111) support pure and N-doped graphene and Fe (111) support pure graphene is more efficient. Similar to N-doped graphene structures, electronic structure calculations suggest that the trends in the binding strength of oxygenated species can be correlated with the density of pz states near the Fermi level of metal support doped graphene structures. Those have a higher density of pz states bind oxygenated species stronger.