We examine the performance of a number of single-atom
M–N/C
electrocatalysts with a common structure in order to deconvolute the
activity of the framework N/C support from the metal M–N4sites in M–N/Cs. The formation of the N/C framework
with coordinating nitrogen sites is performed using zinc as a templating
agent. After the formation of the electrically conducting carbon–nitrogen
metal-coordinating network, we (trans)metalate with different metals
producing a range of different catalysts (Fe–N/C, Co–N/C,
Ni–N/C, Sn–N/C, Sb–N/C, and Bi–N/C) without
the formation of any metal particles. In these materials, the structure
of the carbon/nitrogen framework remains unchanged—only the
coordinated metal is substituted. We assess the performance of the
subsequent catalysts in acid, near-neutral, and alkaline environments
toward the oxygen reduction reaction (ORR) and ascribe and quantify
the performance to a combination of metal site activity and activity
of the carbon/nitrogen framework. The ORR activity of the carbon/nitrogen
framework is about 1000-fold higher in alkaline than it is in acid,
suggesting a change in mechanism. At 0.80 VRHE, only Fe
and Co contribute ORR activity significantly beyond that provided
by the carbon/nitrogen framework at all pH values studied. In acid
and near-neutral pH values (pH 0.3 and 5.2, respectively), Fe shows
a 30-fold improvement and Co shows a 5-fold improvement, whereas in
alkaline pH (pH 13), both Fe and Co show a 7-fold improvement beyond
the baseline framework activity. The site density of the single metal
atom sites is estimated using the nitrite adsorption and stripping
method. This method allows us to deconvolute the framework sites and
metal-based active sites. The framework site density of catalysts
is estimated as 7.8 × 1018sites g–1. The metal M−N4site densities in Fe−N/C
and Co–N/C are 9.4 × 1018sites–1and 4.8 × 1018sites g–1, respectively.