Unraveling Cation Effects in Spinel Aluminates: Comparative Photocatalytic H ₂ Evolution Over MnAl ₂ O ₄ , NiAl ₂ O ₄ and CuAl ₂ O ₄

ABSTRACT Developing efficient and earth‐abundant photocatalysts for solar hydrogen production remains a key challenge for sustainable energy conversion. In this work, spinel aluminates MAl 2 O 4 (M = Mn, Ni, and Cu) were systematically investigated to understand the influence of the divalent cation on structural, optical, and photocatalytic properties. All samples exhibited phase‐pure cubic spinel structures with cation‐dependent morphology and band‐gaps (NiAl 2 O 4 > MnAl 2 O 4 > CuAl 2 O 4 ). Photocatalytic tests under visible light revealed that MnAl 2 O 4 exhibited the highest hydrogen‐evolution activity (∼250 µmol in 25 min), significantly outperforming NiAl 2 O 4 and CuAl 2 O 4 . This enhanced performance is attributed to its favorable band alignment and improved charge separation. In contrast, NiAl 2 O 4 is limited by poor visible‐light absorption, while CuAl 2 O 4 exhibits an unsuitable conduction band position for proton reduction. These findings highlight the critical role of cation electronic structure in tuning photocatalytic performance and provide insights for designing efficient aluminate‐based photocatalysts.