Brief introduction of the results
The development of metal-air batteries relies heavily on the development of efficient bifunctional catalysts for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). Here,Fan Jun, City University of Hong Kong, et alco@n4-doped carbon nanotubes (co@n4cnts) were studied as bifunctional catalysts, and density functional theory calculations were carried out. Calculation method:In this work, all calculations are performed in a de novo simulation package (VASP) based on density functional theory (DFT), and the projected enhanced wave (PAW) method is used to deal with the interaction of nuclear electrons and valence electrons. In addition, to describe the commutative association, the authors employed the Generalized Gradient Approximation (GGA) and the Perdew Burke Ernzerhof (PBE) functional, and set up an energy truncation of 520 EV. In the process of structural optimization, the convergence criterion for energy is 10-5 ev and the convergence criterion for force is 001ev/å。The authors used 1 1 4 k points for structural optimization and 1 1 8 k points for electronic property calculations. To avoid periodic interactions, the authors placed a vacuum layer of 20 in the X and Y directions and used DFT-D3 correction to account for dispersion.
Results & Discussion
Figure 1Model structure of graphene and carbon nanotubes
As shown in Figure 1a, the graphene monolayer has a size of 14 4 1 and a lattice constant of 2469, the C-C bond length is 1425å。co@n4 doping graphene is shown in Figure 1D, a process that involves removing two C atoms from the central region and replacing them with four N atoms, and then doping the Co atoms to the central position of the four N atoms. Figures 1b-c and 1e-f show the model structures of (14,4) CNT and (14,3) co@n4cnts.
Figure 2The relationship between strain energy, bond length, and m
As shown in Figure 2, on nanotubes with different m values, the strain energy decreases as m increases, eventually converging to zero. This trend shows that with the increase of tube diameter, the strain and tension in the nanotube gradually decrease, the interaction force between atoms in the nanotube is close to that in graphene, and the C-C bond length in the nanotube is also close to the C-C bond length in graphene.
Figure 3 pdos
As shown in Figure 3, all co@n4cnts exhibit metallic properties and have a significant DOS distribution on the Fermi level, which indicates the presence of freely moving carriers at the Fermi level, which can significantly enhance the conductivity of the material. The presence of such a carrier enhances the material's potential for efficient electrocatalytic applications.
Figure 4OER and ORR potential energy surfaces
As can be seen from Figure 4, as the diameter of the nanotubes increases, the free energy of OH on the surface of the catalyst also increases, resulting in an increase in the catalytic performance of the catalyst. An excellent ORR catalyst needs to maintain a balance in the bond strength between the catalytic intermediate and the catalyst, i.e., the binding should neither be too strong nor too weak, as moderate adsorption is essential to achieve optimal performance. As shown in Figure 4, (14, 4), (16, 4), (18, 4), (20, 4), (22, 4), and (24, 4) co@n4cntsThe catalyst has a very low catalytic overpotential (ORR) with a corresponding range from 023v to 027V, which means that these catalysts can provide no less than 0Discharge voltage of 96V.
Figure 5OER and ORR overpotentials and their proportional relationships
As shown in Figure 5a, the authors identified three classes of high-performance bifunctional co@n4cnts catalysts, each with a different diameter: (18,4) with an overpotential of 068V, (22,4) has an overpotential of 0The overpotential of 67V and (24,4) is 064v。As shown in Figure 5b, the relationship between δG*O and ΔG*Oh is: ΔG*O=128δg*oh+0.70, the correlation coefficient is 079, which indicates a clear linear relationship between oxidation intermediates. As shown in Figure 5c, as the tube diameter increases, the center of the D-band of the CO atom shifts towards the Fermi level, resulting in a decrease in the bond strength between the catalytic intermediate and the catalyst. As shown in Figures 5E and 5F, as the pipe diameter increases, the δG*OH increases, and so does the BI. This results suggest that ΔG*OH can be adjusted by adjusting the diameter of the nanotubes, which in turn can adjust the activity of the co@n4cnts catalyst.
Summarize the outlook
It was found that when the diameter of the co@n4cnts transitioned from (4,4) to (24,4), the catalytic activity was significantly increased by 54%, and the bi was increased from 140 to 064v。For catalysts co@n4cnts (18,4), (22,4), and (24,4), Bi is, respectively. 67 and 064 v。In addition, by adjusting the diameter of the co@n4cntsco, the d-orbital energy can be adjusted, thereby increasing the catalytic activity. This study provides important insights into the design and development of efficient catalysts for metal-air batteries.
Bibliographic information
ninggui ma et.al curvature effects regulate the catalytic activity of co@n4-doped carbon nanotubes as bifunctional orr/oer catalysts jcis 2023