In-situ phase segregation in lanthanum-substituted Ni–Zn ferrites: Strain engineered biphasic electrocatalysts enabling efficient water splitting

The widespread deployment of green hydrogen technology is fundamentally bottlenecked by the sluggish four–electron kinetics of the oxygen evolution reaction (OER) and the highly expensive, routinely preferred noble–metal electrocatalysts. A structural phase–boundary engineering strategy was successfully executed by targeting La 3+ substitution into the Ni 0.3 Zn 0.7 Fe 2-x La x O 4 lattice via a scalable sol–gel auto–combustion route. Exceeding the solubility limit of La 3+ (x ≥ 0.15) triggered the in - situ evolution of a secondary LaFeO 3 perovskite phase alongside the primary cubic spinel. Interfacial coupling, evidenced through structural and surface characterizations, induces optimal lattice strain and promotes cation redistribution. This combined mechanism maximizes the density of oxygen vacancies, which act as active catalytic centers to boost electrocatalytic performance. The optimized biphasic electrocatalyst (x = 0.15) delivers exceptional OER performance in extreme alkaline media (1 M KOH), achieving a remarkably low overpotential of 214 mV along with a superficial Tafel slope of 78 mV dec -1 . Crucially, the optimized electrocatalyst demonstrates superior hydrogen evolution reaction (HER) activity, exhibiting an overpotential of 261 mV at 10 mA cm -2 with a minimum Tafel slope of 131 mV dec -1 . Collectively, these outstanding electrochemical properties underscore its excellent capability as a cost–effective bifunctional electrocatalyst for efficiently driving both the OER and HER pathways through electrocatalytic water splitting. Furthermore, the as-fabricated two-electrode asymmetric device, utilizing the optimized catalyst (x=0.15) as the anode and activated carbon (AC) as the cathode in an alkaline electrolyzer, achieves overall water splitting at a low onset potential of 1.64 V. The device sustains robust continuous operation for 50 h without significant degradation, demonstrating remarkable chemical stability and mechanical robustness. In a nutshell, this defect-driven approach highlights the potential of rare-earth-induced strain engineered and in-situ phase segregation to design cost-effective, earth–abundant biphasic electrocatalysts for scalable energy conversion.