Polymer‐Mediated Confined Assembly of High‐Entropy Perovskite Hydroxide Nanocubes for Ammonia Production and Zinc–Nitrate Battery Energy Supply

High‐entropy perovskites, with their compositional diversity and structural tunability, have recently emerged as a versatile platform for electrocatalysis. Among them, high‐entropy perovskite hydroxides (HEPHs) offer abundant active sites and flexible electronic structures, yet their controllable synthesis with well‐defined morphology remains a major challenge. In this study, a polymer‐mediated confined assembly strategy is presented that enables the controllable synthesis of nanocubic Sn(CuMgCoZnNi)(OH) 6 (HEPH‐2). HEPH‐2, with highly accessible active sites and a large electrochemically active surface area, delivers outstanding electrocatalytic nitrate reduction to ammonia (eNRA) performance, achieving an ammonia (NH 3 ) yield rate of 9.28 mg h −1 mg −1 cat. and a high Faradaic efficiency (FE) of 90.06% in alkaline media, significantly outperforming irregular HEPH nanoparticles (HEPH‐0) synthesized without polyvinylpyrrolidone. Density functional theory (DFT) calculations reveal that Sn acts as the dominant active site with the strongest NO 3 − adsorption, while the hydrogenation of *NO to *NOH constitutes the rate‐determining step. Meanwhile, when used as the cathode material in a zinc‐nitrate battery, HEPH‐2 demonstrates a high NH 3 yield rate (3.70 mg h −1 cm −2 ) and a high FE value (94.21%). This work introduces a simple yet effective strategy for engineering well‐defined perovskite hydroxides, demonstrating their potential as next‐generation materials for sustainable ammonia production and energy conversion.