Controlled Interruption of Electrochemical Nitrite Reduction for Switchable NH₂OH and Formamide Synthesis

Electrochemical nitrite reduction has the potential to yield a wide range of nitrogen-containing products, yet it typically converges to fully reduced NH₃. Here, we introduce a reduction-interruption strategy that programs the reaction pathway on a Bi@C catalyst through the cooperative regulation of pH and CO, enabling precise control over product distribution. Depending on the coordinated pH-CO environment, nitrite can be selectively intercepted at NH₂OH or diverted toward C─N coupling. Under optimized alkaline conditions with CO, formamide is produced with a Faradaic efficiency of 80.2% and a yield rate of 204.8 mmol·g_cat ^-1·h^-1, while at near-neutral conditions, the same strategy enhances NH₂OH Faradaic efficiency to 79.1%. Mechanistic studies reveal that pH governs the reorientation and hydrogen-bond structure of interfacial water, which dictates active hydrogen (*H) generation kinetics and thereby defines the attainable reduction depth, whether it stops at NH₂OH or proceeds to deeper deoxygenation to *NH₂. Only when *H is sufficiently available, *NH₂ then selectively captures CO, redirecting it away from complete hydrogenation. Collectively, we show that multi-electron electrocatalysis can be programmed by coupling interfacial structural control with targeted molecular trapping, offering a generalizable route to accessing metastable intermediates and expanding nitrogen electrosynthesis beyond ammonia.