Enhancing hydrogen evolution activity by doping and tuning the curvature of manganese-embedded carbon nanotubes

Carbon nanotubes (CNTs) incorporated with transition metals have been experimentally found to have great potential in the electrocatalytic hydrogen evolution reaction (HER) in acidic electrolytes. Further elucidating the underlying mechanism determining the HER activity will be helpful to design more highly efficient CNT-based catalysts for the HER. In this work, first-principles density functional theory calculations were performed to investigate the HER on a series of CNTs, substitutionally embedded with atomic Mn (MnCNT(n,n)s, n = 3, 4, 5, 6, 7, and 9) and co-embedded with Mn and double N (MnN2CNT(5,5)). The theoretical calculations suggest that the principal HER active sites on all studied CNT catalysts are the C atoms adjacent to the metal center, and the HER is dominated by the Volmer-Heyrovsky mechanism with the Heyrovsky reaction as the rate-determining step. Tuning the CNT curvatures and embedding heteroatoms (Mn and N) could elevate the C p-band center (epsilon(p)), weaken the absolute H adsorption free energy (|Delta G(H*)|), lower the absolute electrode potential (Uabs0), and thus enhance the HER performance. For MnCNT(n,n)s, the HER activity shows a volcano dependence on the surface curvature, peaking at n = 5. Substitutionally doping double N atoms into MnCNT(5,5) could further substantially enhance the HER activity, reflected by its thirtyfold current density relative to platinum. Furthermore, the Uabs0 descriptor, besides Delta G(H*) and the free energy barrier, could overcome the limitation of epsilon(p) applied in the same metal-embedded CNTs, extending to describe the HER activity of the transition metal-incorporated CNT system.