Unveiling the Polar‐Solvent Stability of Heterovalent Metal Cation‐Substituted Perovskite Derivative for Promoting Photocatalytic Ethanol Dehydrogenation

The application of halide perovskites in photocatalysis is severely limited by structural instability in polar solvents, such as dissolution and lattice degradation. Herein, we investigate the intrinsic stability in ethanol polar solvent of heterovalent metal cation‐substituted perovskite derivative Cs 2 AgBiBr 6 via density functional theory (DFT) calculations, exhibiting the suppressed ethanol‐induced bond relaxation with a smaller Cs‐Br bond length variation compared to that of CsPbBr 3 . Substantial interfacial electron transfer in CsPbBr 3 ‐ethanol promotes Pb‐Br bond dissociation, whereas Cs 2 AgBiBr 6 exhibits negligible electronic perturbation. Moreover, experimental evidences demonstrate better stability of Cs 2 AgBiBr 6 after long‐term exposing to ethanol, light, and Ar/air atmospheres. Based on this, we construct Cs 2 AgBiBr 6 /CdS heterojunction for photocatalytic ethanol dehydrogenation reaction. Through modification with Rh cocatalyst, which achieves a hydrogen evolution rate of 49.15 mmol·g −1 ·h −1 , AQY of 22.5%, and TOF of 197.5 h −1 over 60 h. At 60 °C, the ethanol dehydrogenation rate further increase to 277.35 mmol·g −1 ·h −1 and TOF of 1114.5 h −1 . Mechanistically, the Rh sites and heterojunction interface synergistically govern hydrogen evolution and ethanol oxidation pathways, facilitating C‐H/O‐H bond activation and regulating product selectivity. Our work not only provides new insights into perovskite stability but also expands their applicability in polar solvent‐based photocatalysis.