Halloysite nanotube-templated carbon tubes confined copper‑nickel tandem sites: Interface synergy-boosted electrocatalytic nitrate-to-ammonia reduction.

The electrocatalytic nitrate reduction reaction (NO3RR) is a promising ammonia (NH3) synthesis route, addressing nitrate pollution and avoiding the high energy consumption of the Haber-Bosch process. However, low selectivity, slow kinetics, and hydrogen evolution reaction (HER) competition limit the practical application of NO3RR. Herein, a tandem electrocatalyst Cu0.2Ni0.2@CT-700 was constructed via in-situ growth of Cu0.2Ni0.2 bimetallic particles within carbon tubes (CTs), which were derived from low-cost halloysite nanotubes (HNTs). Under neutral conditions, Cu0.2Ni0.2@CT-700 achieved a Faradaic efficiency (FE) of 94.8% and an NH3 yield of 17.2 mg h-1 cm-2. The minimally changed NH3 yield and FE after 18 cycles as well as consistent current density after operation in a flow cell (at an industrial current density of 470 mA cm-2) for 163 h demonstrated the excellent stability of Cu0.2Ni0.2@CT-700. Based on the mechanism investigation, Cu sites adsorbed and reduced nitrate (NO3-) to nitrite (NO2-), while Ni sites generated adsorbed hydrogen (⁎H) via Volmer step to promote NO2- hydrogenation. Density functional theory (DFT) calculations confirmed that defects in CTs reduced NO3RR energy barriers and suppressed the HER. The abundant carbon defects, confinement effect, high hydrophilicity and conductivity of CTs derived from HNTs synergistically contributed to the enhanced NO3RR performance of Cu0.2Ni0.2@CT-700. Moreover, Cu0.2Ni0.2@CT-700 also exhibited excellent electrochemical performance as the cathode in the fabricated Zn-NO3- battery and achieved a maximum power density of 9.6 mW cm-2. This work not only offers a cost-effective approach for constructing high-efficiency NO3RR catalysts, but also paves a new path for the utilization of HNTs in the electrocatalysis field.