Cobalt nanoparticles on an amorphous Si3N4 matrix were synthesized by direct ball-milling of Co and Si3N4 powders for an improvement of their electrochemical performance. The microstructure, morphology and chemical state of the ball-milled Co–Si3N4 composites are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electrochemical performance of Co–Si3N4 composites was investigated by galvanostatic charge–discharge process and cyclic voltammetry (CV) technique. It is found that metallic Co nanoparticles of 10–20nm in size are highly dispersed on the amorphous inactive Si3N4 matrix after the ball-milling. The composite with a Co/Si molar ratio of 2/1 shows the optimized electrochemical performance, including discharge capacity and cycle stability. The formation of Co nanoparticles with a good reaction activity is responsible for the discharge capacity of the composites. The reversible faradic reaction between Co and β-Co(OH)2 is dominant for ball-milled Co–Si3N4 composite. The surface modification of the hydrogen storage PrMg12–Ni composites using Co–Si3N4 composites can enhance the initial discharge capacity based on the hydrogen electrochemical oxidation and Co redox reaction.