  Metal heteroatoms dispersed in nitrogen-doped graphene display promising
catalytic activity for fuel cell reactions such as the hydrogen evolution
reaction (HER). Here we explore the effects of dopant concentration on the
synergistic catalytic behaviour of a paired metal atom active site comprised of
Co and Pt atoms. The metals are coordinated to six atoms in a vacancy of
N-doped graphene. We find that HER activity is enhanced with increasing N
concentration, where the free energy of hydrogen atom adsorption ranges from
0.23 to -0.42 eV as the doping changes from a single N atom doped in the pore,
to fully doped coordination sites. The results indicated that the effect of N
is to make the Co atom more active towards H adsorption and presents a means
through which transition metals can be modified to make more effective and
sustainable fuel cell catalysts.
