In‐Situ Exsolving Silver Nano‐islands on High‐Entropy Perovskites for Energy‐Efficient Coupled Nitrate Reduction and Sulfide Oxidation

Electrochemical nitrate reduction to ammonia (NO₃RR) is a promising pathway for nitrogen recycling but remains hindered by complex multistep kinetics and severe competition from the hydrogen evolution reaction. Coupling NO₃RR with the sulfide oxidation reaction (SOR) offers an energy-efficient alternative by simultaneously enabling dual-pollutant remediation with value-added products. Herein, we report an in situ exsolution strategy to construct a tandem electrocatalyst composed of exsolved Ag nano-islands (NIs) anchored on a high-entropy perovskite oxide matrix (Ag-LaSrAgFeCoO_x). The structural complexity and abundant oxygen vacancies (O_v) of the LaSrAgFeCoO_x synergistically interact with the exsolved Ag NIs, creating spatially and functionally distinct active sites. As a result, the Ag-LaSrAgFeCoO_x catalyst achieves high NH₄ ⁺ Faradaic efficiency of 97.6% and yield rate of 0.35 mmol h-¹ cm^-2. In situ characterization and theoretical calculations reveal a relay catalytic mechanism in which Ag sites of Ag NIs preferentially activate NO₃ ^-, while O_v-rich LaSrAgFeCoO_x promotes intermediates hydrogenation and NH₃ desorption, alongside efficient hydrogen supply. Moreover, the bifunctional Ag-LaSrAgFeCoO_x enables energy-efficient NO₃RR||SOR coupling, delivering a positive open-circuit potential of 557 mV and stable co-production of ammonia and sulfur. This work highlights high-entropy materials as a powerful platform for tandem electrocatalysis in complex coupled reactions.