Revealing the Tandem Behavior of Iron‐Group/Copper Binary Catalysts in the Electroreduction of Nitrate to Ammonia

Abstract As a green strategy for both ammonia (NH ₃ ) production and wastewater purification, electrochemical reduction of nitrate (NO ₃ RR) faces challenges due to the nitrite (NO ₂ ⁻ ) accumulation and competitive hydrogen evolution reaction (HER). Tandem catalysis (NO ₃ ⁻ to NO ₂ ⁻ to NH ₃ ) offers great potential for enhancing NH ₃ production and selectivity. Herein, iron‐group (Fe, Co, or Ni) nanosheets are introduced onto Cu nanowires to construct Cu‐Fe, Cu‐Co, and Cu‐Ni tandem systems respectively. Specifically, Cu sites facilitate the conversion of NO ₃ ⁻ to NO ₂ ⁻ . Fe sites, similar to Cu, reduce NO ₃ ⁻ to NO ₂ ⁻ , exacerbating NO ₂ ⁻ accumulation rather than converting it to NH ₃ , due to their inability to precisely capture NO ₂ ⁻ . Co sites, exhibiting excellent NO ₂ ⁻ conversion and moderate HER activity, can seamlessly operate with Cu and realize a well‐ordered relay catalysis, which achieves a superior NH ₃ yield rate of 48.44 mg h ⁻¹ cm ⁻² . Ni sites demonstrate superior NO ₂ ⁻ removal capability at low overpotentials, leading to NH ₃ Faraday efficiency of 99.47%. However, its remarkable HER‐active property demonstrated via in situ polarization imaging makes it a challenge at ampere‐level current densities. This work identifies the relay behavior of iron‐group sites coupled with Cu, providing a reference for the design and further optimization of the tandem system.