Paired‐Cell Ammonia over Black CoWO ₄ via Electrocatalysis–Acidification Cascade from Wastewater at Ampere‐Level Current Density

Abstract

Abstract Electrochemical nitrate reduction to ammonia (NO 3 RR) offers a sustainable alternative to the energy‐intensive Haber–Bosch process. However, its practical implementation is limited by the sluggish and energy‐demanding oxygen evolution reaction at the anode. Herein, we report a strategically designed paired‐electrolysis system coupling NO 3 RR with the urea oxidation reaction (UOR), followed by chemical acidification, to establish an economical route of bipolar ammonia (NH 3 ) production, using a black CoWO 4 (B‐CoWO 4 ) with abundant oxygen vacancies (OVs) as the electrocatalyst. B‐CoWO 4 shows a record‐breaking performance with a current density of ∼1.25 A cm −2 at an ultra‐low potential of 0 V versus reversible hydrogen electrode. Combined spectroscopic and electrochemical analyses reveal a “fill‐restore” cycle of OVs during NO 3 RR: oxygen from NO 3 − incorporates into the OVs, which are subsequently restored after the formation of NH 3 . Theoretical calculations demonstrate that the OVs modify the electronic structure of the catalyst and facilitate the formation of key intermediate (NO 3 H*). Importantly, in the coupled NO 3 RR||UOR flow‐cell system, B‐CoWO 4 delivers an apparent bipolar NH 3 Faradaic efficiency of 173.12% and a production rate of 9.43 mmol h −1 cm −2 . This integrated strategy boosts overall energy efficiency and enables simultaneous valorization of nitrate‐contaminated water and urea‐rich wastewater streams.