<p>An advanced water electrolysis process that generates clean and sustainable hydrogen fuel offers a scalable solution for storing abundant but intermittent energy from renewable sources by converting water into hydrogen and oxygen using an electric current, facilitating the integration of renewable energy into practical applications. Moreover, synthesis of sustainable and environmentally friendly methods for synthesizing nanomaterials is correspondingly crucial for advancing water-splitting technology. This study introduces a green synthesis approach for Cu-doped CeO₂ nanoparticles using plant extracts as reducing and stabilizing agents. A 3D nanocage network of Cu-CeO₂ electrocatalyst exhibits featured electrochemical performances for HER and arduous OER significantly lowering the overpotential due to the reduced reaction barrier, lower resistance, and accelerated charge transfer process. The Cu-doped CeO₂ exhibits lower overpotentials of 142&#xa0;mV and 166&#xa0;mV at current densities of 50&#xa0;mA&#xa0;cm⁻² and 100&#xa0;mA&#xa0;cm⁻², respectively, and a Tafel slope of 58.8&#xa0;mV dec⁻¹, indicating superior catalytic activity. Density functional theory (DFT) calculations reveal that the Cu doping on the CeO₂ matrix increases the rate of H₂O adsorption during water-splitting reaction due to the introduction of Cu-3d orbitals near the Fermi level (<i>E</i>_F), which enhances charge carrier density. Overall, Cu-doped CeO₂ nanoparticles demonstrate enhanced performance for green hydrogen production as an energy vector, while the green synthesis method offers a sustainable, low-impact alternative for producing high-performance nanomaterials.</p> Graphical Abstract <p></p>