The Catalytic Properties And Growth Of Two-dimensional Carbon-based Materials:a First Principles Calculation Study

For low-temperature fuel cell, Oxygen reduction reaction (ORR) at the cathode is the rate-limiting and key reaction step, cause its reactivity is much lower than the anode hydrogen oxidation reaction. Nowadays, the most efficient and stable cathode catalysts is Platinum, but its expensive price and rareness drastically hinder the commercialization of Platinum-containing fuel cell technology. Therefore low-Platinum loading cathode catalysis and non-Platinum cathode catalysis are receiving more and more research attention currently. N-doped carbon alloys have shown catalytic activity for cathode ORR, especially the recently reported N doped carbon nanotube and N doped graphene both possessed surprisingly high catalytic activity. However the mechanisms of ORR at N doped carbon alloy cathode catalysts are still mysterious, which hinder the further improvement of N doped carbon alloy cathode catalysts. First principles method palys a more and more important role in uncovering the properties of materiels and desighing new functional materials. First, in this paper, we investigate the catalytic properties of N doped gaphene for ORR.Besides, Heat-treated metal (Fe or Co)-N/C as another kind of non-Platinum cathode catalyst have been under development for several decades, starting with the works by Jasinski and by Yeager and co-workers. Recently some work in this area is very striking too, reporting the catalytic efficiency of trasition metal (Fe or Co)-N/C ORR electro catalysts has approached and even exceeded the commercial Pt-C catalysts. Actually the transition metal-N/C system can be regarded as another kind of carbon alloy codoped with N and transition metal atoms, similar to the N-B codoped carbon alloy. At present the nature of catalytic site and the role of transition metal and N atom also need a deep understanding.We study Ⅰ:the formation energy of substitutional clusters (as adsorption site) on N-doped graphene layer and Ⅱ:those on N-B, N-Fe and N-Co codoped graphene layer and ⅡⅡ:the molecular oxygen bonding mechanism on these substrate from the first principle method. And we find that N clusters with3or4N atoms are the most efficient catalytic sites in N doped graphene, and codoping with B, Fe or Co atoms can greatly stabilize the high energy N clustersVery recently, two dimensional carbon nitride (2D-CN) materials have entered researchers’attention as a new kind of cathode ORR catalyst candidates for ORR, due to their much larger nitrogen concentration comparing with N-doped carbon-based materials. But the catalytic mechanisms of2D-CNs are still unclear. Through first principles method based PSO algorithm, we find that the structures of2D-CNs are sensitive to C:N ratio. The larger the C:N ratio, the energetically more favorable the structure, and the more likely to exist the graphene-like structure.02adsorption energies on these2D-CNs show a volcano-like shape along with the variation of C:N ratios; and the largest O2adsorption energy reaches1.66eV, even superior to that on Pt (111) surface. We suggest that at the C:N ratio range of2.0-3.0,2D-CNs may possess high catalytic efficiency. Through a in detail analysis, we also find out that at2D-CN sheets, graphite-like structures are necessary for stable02bonding, thus experimentally forming of graphite-like N atoms is essential for ORR catalysis. Surprisingly it is revealed that the catalytic active sites are those C atoms with N neighbors rather than N atoms. The Fermi level and the density of states at the Fermi level of the2D-CN sheet can be tuned by varying the C:N ratio of the grown structure. All these conclusions can be taken as guidelines for experimental researchers.At present, the qualities of CVD-grown graphene layers are still to be improved. The poor electronic properties and too restrictive processing conditions are major obstacles to better utilize CVD-grown graphene. Our investigations indicate that the step edge at Ni (111) surface is a better adsorption site and segregation site for C atoms than metal terrace. Thus the graphene growthprocess at Ni (111) is a non-uniform mode, and thereby high temperature is needed to growuniform large-area graphene sheets experimentally. Then we find that, when alloying with metal Au, Au atoms prefer to substitute Ni atoms at step edges. And Au-alloyed step edge can no longer adsorb C atoms so strongly as non-alloyed step edge. The segregation barrier of C atoms at step edge is also increased a lot. Consequently alloying with Au can effectively deactivate the chemically active step edge, and lead to a more uniform growth mode of graphene. Through a further investigation, the underlying mechanisms of deactivating Ni step edgeare revealed. The bonding strength between C atom and alloying TM atom can be tuned based on the d-shell filling factor and d-shell electron state location of TM atoms.In conclusion, based on first principle methods, we investigate the catalytic properties of N doped graphene and two dimensional carbon nitrides for ORR. We also investigate the CVD growth of graphene on Ni(111) surface and the effect of Au-alloying.