Curvature effects regulate the catalytic activity of Co@N4-doped carbon nanotubes as bifunctional ORR/OER catalysts

The advancement of metal-air batteries relies significantly on the development of highly efficient bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we investigate the potential application of Co@N4-doped carbon nanotubes (Co@N4CNTs) as bifunctional catalysts using density functional theory calculations. We explore the stability and electronic properties of Co@N4CNTs by analyzing energies, bond lengths, conducting ab initio molecular dynamics simulations, and examining the density of states. Notably, the diameter of the nanotubes has a notable impact on the catalytic performance of Co@N4CNTs. A remarkable 54% improvement in catalytic activity when transitioning from (4, 4) to (24, 4) Co@N4CNTs, with ηBi from changing from 1.40 to 0.64 V. We have several exceptional catalysts with low overpotentials, including (18, 4), (22, 4), and (24, 4) Co@N4CNTs, which exhibit ηBi values of 0.68, 0.67, and 0.64 V, respectively. Moreover, we link the increased activity of Co@N4CNTs to the change of Co atom's partial d orbital energy, facilitated by adjustments in the diameter of Co@N4CNTs. This revelation offers valuable insights into the underlying factors driving the enhancement of catalytic activity through alterations in orbital energy levels. Our research uncovers several excellent catalysts and provides valuable insights for the design and development of efficient catalysts for metal-air batteries.