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.
