Paired-Cell Ammonia over Black CoWO₄ via Electrocatalysis-Acidification Cascade from Wastewater at Ampere-Level...

Electrochemical nitrate reduction to ammonia (NO₃RR) 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 NO₃RR with the urea oxidation reaction (UOR), followed by chemical acidification, to establish an economical route of bipolar ammonia (NH₃) production, using a black CoWO₄ (B-CoWO₄) with abundant oxygen vacancies (OVs) as the electrocatalyst. B-CoWO₄ 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 NO₃RR: oxygen from NO₃ ^- incorporates into the OVs, which are subsequently restored after the formation of NH₃. Theoretical calculations demonstrate that the OVs modify the electronic structure of the catalyst and facilitate the formation of key intermediate (NO₃H*). Importantly, in the coupled NO₃RR||UOR flow-cell system, B-CoWO₄ delivers an apparent bipolar NH₃ 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.