High‐Spin‐State Engineering in High‐Entropy Perovskite Oxides via Crystal Phase Modulation for Paired Electrochemical Nitrate Reduction and Sulfur Ion Oxidation

Abstract High‐entropy materials offer unique advantages in catalysis due to lattice distortion, compositional complexity, and entropy‐driven stability. In this study, we report a crystal phase‐engineered high‐entropy perovskite oxide, LaB 5 O 3 (B═Fe, Cu, Co, Cr, and Ni), which enables efficient bifunctional electrocatalysis. Structural transformation from orthorhombic to cubic symmetry modulates the spin states of B‐site metals, particularly stabilizing the high‐spin state of Fe 2 ⁺/Fe 3 ⁺, enhancing electron density and nitrate adsorption. This leads to a NH 3 Faradaic efficiency (FE) of 95.83% at −0.7 V vs. RHE for nitrate reduction (NO 3 RR), with minimal NO 2 ;− byproduct (FE < 2%). Concurrently, LaB 5 O 3 exhibits excellent sulfide oxidation reaction (SOR) activity, requiring only 0.484 V vs. RHE at 10 mA cm −2 —considerably lower than oxygen evolution reaction (OER) overpotential. Integrated into a paired electrolyzer, the system achieves simultaneous nitrate‐to‐ammonia conversion (max FE: 81.27%) and sulfide‐to‐sulfur transformation at a cell voltage of just 1.079 V. The superior performance arises from synergistic effects of high‐entropy design, including enhanced oxygen vacancy formation, lattice strain, and ferroelectricity. This work demonstrates the potential of crystal phase engineering in high‐entropy oxides for sustainable nitrogen fixation and sulfur recovery in water treatment applications.