Engineering Mo ratios in quaternary Zn0.5Ni0.5MoxCo2-xO4 spinel oxides and hydrothermal processing to investigate reconstruction-activity relationship for urea electrolysis

Quaternary spinel oxides (QSPOs) offer a versatile promising noble-metal-free electrocatalysts for alkali urea electrolysis owing to their compositional flexibility and structural stability. However, the coupled effects of multi-cation composition and synthesis conditions on electrocatalytic performance, reproducibility, and long-term durability are limited. Herein, Zn 0.5 Ni 0.5 Mo x Co 2-x O 4 (x = 0.02–0.10) QSPOs were synthesized through a hydrothermal-calcination method to systematically investigate the effect of Mo incorporation and hydrothermal processing on the hydrogen evolution (HER) performance. A Mo-free Zn − Ni − Co analogue was additionally examined to clarify the role of Mo within the spinel framework. Controlled Mo incorporation promoted phase-pure hierarchical urchin-like microspheres assembled from radially aligned nanoneedles, with Zn 0.5 Ni 0.5 Mo 0.08 Co 1.92 O 4 based on HER activity, reproducibility and operational stability. The optimized QSPOs exhibited HER potentials of −0.26 ± 0.01, −0.43 ± 0.01, and − 0.54 ± 0.01 V at 10, 50, and 100 mA cm −2 , respectively, together with a Tafel slope of 193 ± 2 mV dec −1 . The design of experiment (DOE) approach subsequently identified 120 °C and 10 h as the optimum hydrothermal condition, which enhanced HER performance and promoted post-stability UOR activity with pronounced reconstruction while preserving operational durability. The resulting composition-processing optimum demonstrated stable bifunctional performance in a symmetric urea electrolysis cell, while UOR potential decreased from 1.54 to 1.38 V at 10 mA cm −2 after 15 h operation. XPS and synchrotron XANES/EXAFS analyses further revealed that Ni served as the primary redox-responsive center together with Co which underwent the partial redox and local coordination changes, whereas Zn retained a comparatively stable coordination environment. These results indicate that Mo incorporation modulates the coupled Ni − Co − Zn local environment and electrochemical reconstruction of the spinel framework. This work establishes composition-processing-structure-performance relationships in quaternary spinel electrocatalysts and provides a practical strategy for developing durable noble-metal-free spinel electrocatalysts for alkaline urea electrolysis.