چكيده به لاتين
The oxygen reduction reaction (ORR), as one of the key reactions in renewable energy systems
such as fuel cells, metal-air batteries, and advanced chlor-alkali electrolysis, plays a vital role.
This reaction proceeds through two pathways: the favorable four-electron pathway and the
unfavorable two-electron pathway. The favorable four-electron ORR leads to the production
of hydroxide, while the two-electron pathway results in the formation of a peroxide byproduct.
Due to the slow kinetics of the ORR, significant challenges arise in the efficiency and
performance of electrochemical systems. Hence, the development of efficient electrocatalysts
to improve ORR reaction rates has attracted the attention of researchers. In recent years, singleatom electrocatalysts have become a hot research topic due to their high electrocatalytic activity
and optimal utilization of transition metals (both precious and non-precious). One of the
outstanding structures in terms of optimal activity and stability is the M-N-C structure, in which
transition metal atoms are stabilized in a carbon matrix with nitrogen heteroatoms. Therefore,
in the present study, a nickel single-atom electrocatalyst (Ni-N-C) was synthesized and
evaluated. The synthesis method involved nickel doping onto a ZIF-8 metal-organic framework
via a hydrothermal method. In the next step, the synthesized sample was pyrolyzed under
nitrogen ambient at 1000°C to form single atoms on a carbon support. Field emission scanning
electron microscopy revealed that the final product's morphology is rhombic polyhedral.
Fourier-transform infrared spectroscopy confirmed that imidazolate rings remained in the
carbon structure, and metal-nitrogen bonds were formed in the electrocatalyst structure.
Additionally, energy-dispersive X-ray spectroscopy demonstrated that nickel atoms were
uniformly distributed throughout the carbon structure. Subsequently, to enhance the
performance of Ni-N-C, heteroatom doping was employed to tune the coordination
environment of the active sites. Heteroatoms, due to their electronegativity difference with
carbon, improve charge and mass transfer and also play a key role in oxygen intermediate
adsorption. Among the elements added as heteroatoms to M-N-C are mentioned as sulfur (S),
phosphorus (P), and boron (B). Electrochemical tests on the Ni-B/N-C electrocatalyst,
compared to the Ni-N-C electrocatalyst, showed better activity based on the onset potential,
0.924 V and 0.885 V vs. RHE respectively, and the half-wave potential, 0.78 V and 0.75 V vs.
RHE, respectively. Additionally, the electron transfer number of Ni-B/N-C (3.96) was higher
than that of Ni-N-C (3.66), indicating its superior performance in the four-electron pathway.
Furthermore, according to chronoamperometry tests over a 10-hour period, the Ni-N-C
electrocatalyst retained about 80% of its initial current. The performance of Ni-B/N-C was
close to that of the commercial Pt/C 20% w/w electrocatalyst, with onset and half-wave
potentials of 0.95 V and 0.85 V vs. RHE, respectively, making it a promising candidate for
renewable energy systems in alkaline environments.
