<p>Over the past decade, green strategy has emerged as a powerful route for the synthesis of various types of nanomaterials. The fine tuning of the synthesis parameters such as green source type and content, pH value, calcination temperature, etc. markedly influence the formed phases and their applications. In this article, we present, for the first time, the results of applying the green strategy for the synthesis of <i>NiO</i>_<i>x</i><i>-C</i> nanocomposites employing nickel nitrate as a source of Ni²⁺ ions and <i>Arugula</i> leaves extract as a green precursor. The calcination of the parent mixtures was affected over the 300–700&#xa0;°C temperature range in static air. Phase analysis of the obtained nanocomposites was performed using X–ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT–IR). Morphological inspections were conducted using scanning (SEM) and transmission (TEM) electron microscopies. Surface area and porosity were obtained from the analysis of N₂–adsorption isotherms. Surface existing elements and species were examined using X–ray photoelectron spectroscopy (XPS) and CHNS analysis. Due to the depletion of the ordinary fossil fuels, hydrogen source emerged as a clean and safe source of energy. The synthesized NiO_x-C nanocomposites catalyst was used for catalyzing the hydrolysis of NaBH₄ for generating hydrogen as a source of energy at 40–55&#xa0;°C temperature range.</p> Graphical Abstract <p></p>