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-1cat. 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.