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

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.