Efficient solar-to-H 2 production via electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remains a critical challenge. Herein, gadolinium-doped cobalt–nickel spinel nano-ferrite (CoNiSNF:Gd) was synthesized via auto-combustion followed by calcination and evaluated for overall water splitting. The optimized CoNiSNF:Gd 3 catalyst delivers cathodic and anodic current densities ( j c and j a ) of ±10 mA cm −2 at low overpotentials ( η ) of 148 mV (HER) and 250 mV (OER) in 1.0 M KOH. A solar-assisted alkaline electrolyzer using CoNiSNF:Gd 3 achieves 10 mA cm −2 at a low cell voltage of 1.60 V, sufficient for efficient H 2 and O 2 evolution. High intrinsic activity is confirmed by TOF values of 0.22 s −1 (HER) and 0.33 s −1 (OER), indicating extensive faradaic efficacy. The system exhibits excellent durability, maintaining stable performance for 250 h with minimal potential loss. The enhanced performance arises from the synergistic interaction between Gd and transition metals (Co, Ni, Fe), which optimizes the electronic structure and increases active sites. These findings establish rare-earth-modified spinel nano-ferrites as efficient and durable electrocatalysts for scalable hydrogen production.