The electrochemical oxygen evolution reaction (OER) plays
a fundamental
role in several energy technologies, which performance and cost-effectiveness
are in large part related to the used OER electrocatalyst. Herein,
we detail the synthesis of cobalt-iron oxide nanosheets containing
controlled amounts of well-anchored SO42– anionic groups (CoFexOy-SO4). We use a cobalt-based zeolitic imidazolate
framework (ZIF-67) as the structural template and a cobalt source
and Mohr’s salt ((NH4)2Fe(SO4)2·6H2O) as the source of iron and sulfate.
When combining the ZIF-67 with ammonium iron sulfate, the protons
produced by the ammonium ion hydrolysis (NH4+ + H2O = NH3·H2O + H+) etch the ZIF-67, dissociating its polyhedron structure, and form
porous assemblies of two-dimensional nanostructures through a diffusion-controlled
process. At the same time, iron ions partially replace cobalt within
the structure, and SO42– ions are anchored
on the material surface by exchange with organic ligands. As a result,
ultrathin CoFexOy-SO4 nanosheets are obtained. The proposed synthetic
procedure enables controlling the amount of Fe and SO4 ions
and analyzing the effect of each element on the electrocatalytic activity.
The optimized CoFexOy-SO4 material displays outstanding OER activity
with a 10 mA cm–2 overpotential of 268 mV, a Tafel
slope of 46.5 mV dec–1, and excellent stability
during 62 h. This excellent performance is correlated to the material’s
structural and chemical parameters. The assembled nanosheet structure
is characterized by a large electrochemically active surface area,
a high density of reaction sites, and fast electron transportation.
Meanwhile, the introduction of iron increases the electrical conductivity
of the catalysts and provides fast reaction sites with optimum bond
energy and spin state for the adsorption of OER intermediates. The
presence of sulfate ions at the catalyst surface modifies the electronic
energy level of active sites, regulates the adsorption of intermediates
to reduce the OER overpotential, and promotes the surface charge transfer,
which accelerates the formation of oxygenated intermediates. Overall,
the present work details the synthesis of a high-efficiency OER electrocatalyst
and demonstrates the introduction of nonmetallic anionic groups as
an excellent strategy to promote electrocatalytic activity in energy
conversion technologies.