Emulsion-template synthesis of mesoporous nickel oxide nanoflowers composed of crossed nanosheets for effective nitrogen reduction

Abstract

A novel emulsion-template synthesis approach was developed for the preparation of nickel oxide nanoflowers (NiO-NFs) composed of crossed mesoporous nanosheets. The interface assembly process was regulated by tuning the dosage of NH₃·H₂O, resulting in the tunability of thickness and size of mesoporous NiO nanosheets. The as-prepared NiO-NFs were characterized by field emission scanning electron microscopy, transmission electron microscopy, Brunauer–Emmett–Teller analysis, X-ray diffraction, and X-ray photoelectron spectroscopy (XPS). The results indicate that NiO-NFs have a mesopore size of about 9.5–15 nm and a crossed nanosheet thickness of about 12.4–50 nm. XPS results demonstrated that all NiO-NF samples consisted of Ni²⁺ and Ni³⁺. Electrochemical nitrogen reduction reaction (NRR) measurements revealed that NiO-NF-3.0 showed an optimal NRR performance of NH₃ yield and faradaic efficiency (16.16 μg h⁻¹ mg⁻¹_cat. and 9.17% at −0.4 V vs. RHE) in 0.1 M Na₂SO₄. Interestingly, NiO-NF-3.0 also displayed the highest Ni³⁺ content, which correlates with the order of electrochemical NRR performance. This can be attributed to the fact that Ni³⁺ promotes the electropositivity of NiO-NFs, resulting in more facile adsorption of N₂ gas than Ni²⁺, and leading to enhanced electrocatalytic properties. These enhanced NRR performances are comparable or superior to those of reported noble–metal catalysts. This study provides a novel method for the fabrication of low-cost metal oxide nanomaterials that allows the construction of electrochemical NRR catalysts to meet the needs of industrial production. Also, it provides a new approach to improve the electrochemical properties by increasing the content of high-valent metal ions in a metal oxide.