Electrochemical CO2 reduction (CO2R) is a
promising avenue for the conversion of CO2 into fuels and beneficial chemicals.
Significant efforts have been devoted to the development of active
and selective electrocatalysts for CO2R. Atomically dispersed
transition metal electrocatalysts have recently attracted consideration
for CO2R due to their unique electronic and structural
properties that can impart good catalytic performance and high active
metal utilization. Among different precursors used for preparing these
catalysts, metal–organic frameworks (MOFs) have been utilized
as templates for generating atomically dispersed transition metal
active sites as they provide well-defined structures that contain
transition metals isolated from each other by organic ligands, along
with high surface areas. However, maintaining the isolation of the
transition metal species, achieving high surface concentrations of
active sites, and imparting porosity during the high-temperature heat
treatment of MOFs used during synthesis remain a challenge. In this
report, Mg2+ ions have been employed as spacers in a bimetallic
metal–organic framework (NiMg-MOF-74) to assist in preventing
the coalescence of the Ni atoms into Ni-based particles during heat
treatment, which combined with the use of urea as a nitrogen source
resulted in the formation of isolated Ni–N
x
/C active sites. Our findings demonstrated that Mg2+ ions play a crucial role in extending the distance between the adjacent
Ni sites in the precursor structure and generating atomically isolated
Ni sites. On the contrary, the utilization of Mg-free Ni-MOF-74 led
to the formation of metallic Ni particles embedded in a carbon nanotube-based
structure. Furthermore, we investigated the impact of pyrolysis temperature
on the produced catalyst morphology. The generation of isolated Ni–N
x
/C sites was promoted at higher pyrolysis
temperatures (900 °C), while Ni-based particles were predominantly
formed at a lower temperature (700 °C). The optimized atomically
dispersed Ni–N–C catalyst exhibited excellent selectivity
toward CO with a Faradaic efficiency of ∼90% and a current
density of −4.2 mA/cm2 at −0.76 V vs a reversible
hydrogen electrode.