Breathing Bimetallic MOF Confined Polyoxometalates for Hydration Layer Loosening and Electronic Redistribution Toward Efficient Nitrate Electroreduction and Zn−Nitrate Batteries

The traditional Haber-Bosch method suffers from harsh conditions and high energy consumption, while electrocatalytic nitrate reduction to ammonia (ENRA) is a green route for ammonia synthesis and can serve as a cathode reaction for Zn−nitrate batteries. Its development is limited by sluggish intermediate hydrogenation and severe hydrogen evolution reaction (HER). Herein, we develop topology-engineered isomeric polyoxometalate (POM)-confined bimetallic metal-organic framework (MOF) electrocatalysts (NH₂-MIL-53, -88, -101). Flexible NH₂-MIL-88(FeNi) enables tight encapsulation of [PW₁₂O₄₀]³⁻ (PW₁₂) clusters via the “breathing effect”, yielding PW₁₂@NH₂-MIL-88(FeNi) with synergistically modulated electronic distribution and proton transfer. Combined experimental and theoretical studies reveal that confined PW₁₂ induces electronic redistribution over Fe/Ni centers, concurrently strengthening NO₃⁻ adsorption on Fe and accelerating *NO₂ hydrogenation on Ni. Beyond electronic effects, PW₁₂ loosens the rigid hydration layer and forms conjugated acid-base pairs with MOF amino groups, promoting proton diffusion, boosting *NO₂ hydrogenation, and suppressing HER. Thus, PW₁₂@NH₂-MIL-88(FeNi) achieves an NH₃ yield rate of 20.1 mg h⁻¹ mg_cat.⁻¹ with a Faradaic efficiency of 98.6% under neutral electrolytes. When used as a cathode in rechargeable Zn−nitrate batteries, it delivers a peak power density of 13.2 mW cm⁻². This study establishes a generalizable paradigm for engineering interfacial proton transport and electronic properties via POM confinement in MOFs.