Density Functional Study of Hydrogen Evolution on Cobalt-Embedded Carbon Nanotubes: Effects of Doping and Surface Curvature

Exploring low-cost, efficient, and stable nonprecious alternatives for Pt-based catalysts is of significance in the hydrogen evolution reaction (HER) in acidic environments. Previous experiments have found that 3d transition metals Fe, Co, and Ni incorporated with inert carbon templates or carbon–nitrogen materials exhibit long-term durability and high HER activity in acidic electrolytes. To clarify the underlying mechanism determining the HER activity, here we report a theoretical investigation of the HER on a series of defective carbon nanotubes (CNTs), doped with atomic Co (CoCNT(n,n), n = 3, 5, 7, and 9) and codoped with Co and double N (CoN2CNT(5,5)), based on the first-principle density functional calculations. Our calculations indicate that the HER on these Co- and Co, N-(co)doped CNTs occurs via the Volmer–Heyrovsky mechanism, and the primary active sites are the C atoms adjacent to the metal center. The enhancement of the HER activity is due to uplifting of the p-band center (εp) of the active C atoms induced by using a CNT with appropriate curvature, Co doping, and Co and N codoping. The HER activity of CoCNT(n,n)s follows a volcano dependence with surface curvature, showing nearly six orders of magnitude difference in exchange currents, peaked at CoCNT(5,5), with the activity comparable with Pt-catalysts. Doped with double N atoms in CoCNT(5,5), the exchange current could be further substantially enhanced (by 30 times), even one order of magnitude higher than that of Pt(111). The fact that CoN2CNT(5,5) has an εp (−4.16 eV) very close to the optimum value for the maximum exchange current (−4.14 eV) justifies the advance in improving the HER activity of CNTs.