Double-shell structured mixed-spinel oxides for highly efficient oxygen evolution

• Unique double-shell structure boosts mass transfer and ensures long-term stability. • Synergy in double spinel composites optimizes electron distribution. • Double-shell P-(NCO/FCO) achieves 252 mV overpotential at 10 mA cm −2 for OER. Spinel oxides with both spinel and inverse spinel structures are considered highly promising electrocatalysts for the oxygen evolution reaction (OER). However, simultaneously activating two types of spinel-structured composites is essential to unlock their catalytic potential, yet this remains challenging. Herein, we demonstrate a co-doping strategy to introduce both Ni and Fe elements into phosphorus-modified Co 3 O 4 microsphere composites to optimize electron distribution and generate abundant oxygen defects. Additionally, the resulting double-shell structure enables rapid mass transfer and enhanced electrolyte enrichment. As a result, the phosphorus-doped ternary Ni-Fe-Co oxide exhibits a low overpotential of 252 mV to achieve a current density of 10 mA cm −2 , outperforming commercial RuO 2 . This work provides an effective approach to enhancing the OER activity of spinel oxides by fabricating dual-type spinel complexes.