Synthesis and eva‎luation of Nickel Single Atom Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium

9/25/2025 12:00:00 AM

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.

واكنش احياي اكسيژن , الكتروكاتاليست منفرد اتم , پيروليز , نيكل , هترواتم ها

Oxygen reduction reaction , Single atom electrocatalyst , Pyrolysis , Nickel , Heteroatoms

Shaghayegh Ghanbari

Susan Rowshanzamir