Paired,
single-atom catalysts have been shown to demonstrate synergistic
effects computationally and experimentally which enable them to outperform
the benchmark catalyst, Pt/C, for electrochemical reactions. We explore
the limit of these catalysts by screening different transition metal
atoms (M = Co, Pt, Fe, Ni) in nitrogen-doped graphene for their ability
to catalyze the oxygen reduction reaction (ORR). We employ density
functional theory methods to explore the electronic factors affecting
catalytic activity in an effort to rationalize trends in the performance
of materials which are promising candidates for the next generation
of electrocatalysts. It is found that CoPt@N8V4, composed of paired
Co and Pt in a nitrogen-doped four-atom vacancy in graphene (N8V4),
performs ideally for the ORR with an overpotential (η) of 0.30
V, followed closely by Co and Ni (η = 0.35 V) and paired Co
(η = 0.37 V). The origin of activity is suggested to be the
changing reduction potential of the active Co atom via the local distortion
of the pore by the spectating metal partner. We utilize the ORR scaling
relations and plot catalytic activity on a volcano plot, which we
correlate with the degree of antibonding interactions with the O atom
in the OH intermediate of the ORR. We establish that the local tuning
of paired catalysts allows for the reactivity of metal atoms to be
specifically modified for desirable reactivity.
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Nitrogen-doped graphene (NG) has attracted increasing attention because its properties are significantly different to pristine graphene, making it useful for various applications in physics, chemistry, biology, and materials science. However, the NGs that can currently be fabricated using most experimental methods always have low N concentrations and a mixture of N dopants, which limits the desirable physical and chemical properties. In this work, first principles calculations combined with the local particle-swarm optimization algorithm method were applied to explore possible stable structures of 2D carbon nitrides (C1−xNx) with various C/N ratios. It is predicted that C1−xNx structures with low N-doping concentration contain both graphitic and pyridinic N based on their calculated formation energies, which explains the experimentally observed coexistence of graphitic and pyridinic N in NG. However, pyridinic N is predominant in C1−xNx when the N concentration is above 0.25. In addition, C1−xNx structures with low N-doping concentration were found to have considerably lower formation energies than those with a high N concentration, which means synthesized NGs with low N-doping concentration are favorable. Moreover, we found the restrictions of mixed doping and low N concentration can be circumvented by using different C and N feedstocks, and by growing NG at lower temperatures.
Multiple heteroatoms-doped carbons with 3D ordered macro/meso-microporous structures have not been realized by simple carbonization of metal-organic frameworks (MOFs). Herein, ordered macroporous phosphorus and nitrogen-doped carbon (M-PNC) is prepared successfully...
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