Pulse-Driven Active-Site Switching on Bromine-Modified Cu Enables Sequential 2e^- + 6e^- Nitrate-to-Ammonia Conversion

Steady active-site electronic states commonly lead to product decentralization and energy consumption increase when overcoming the kinetic mismatch within 8-electron transfer inherent in the electrocatalytic nitrate reduction reaction (NO₃RR) to NH₃. Here, we first report a reversible and self-adaptive regulation of Cu active sites by Br modification in pulsed electroreduction electrolysis (PE) to match distinct reaction kinetics steps of the (2 + 6)-electron pathway for the NO₃RR on trace-Br-modified Cu/Cu₂O catalysts (Br-Cu). Mechanistic investigations integrating in situ spectroscopy and DFT calculations reveal that Br-mediated electronic modulation and d-band center downshift strengthen Cu active sites to possess a reversibly tunable Cu^δ+ (0 < δ < 1) state under PE, which selectively favor the first 2-electron (*NO₃ → *NO₂, δ close to 1) and the latter 6-electron (*NO₂ → *NH₃, δ close to 0) steps, delivering 94% Faradaic efficiency. The continuous-flow electrochemical cell employing Br-Cu maintains high NO₃^- conversion, NH₃ Faradaic efficiency, and NH₃ selectivity in approximately 700-h electrolysis while enabling efficient NH₃ recovery from real aquaculture tailwater. This work presents a robust strategy for programmable self-adaptive electronic modulation toward sustainable NH₃ electrosynthesis.