Abstract: Electrochemical nitrate reduction reaction ($\mathrm{NO}_{3}^{-}$RR) is a highly attractive route for both ammonia (NH₃) synthesis and wastewater treatment. The $\mathrm{NO}_{3}^{-}$RR involves the reduction of $\mathrm{NO}_{3}^{-}$ to $\mathrm{NO}_{2}^{-}$, the conversion of $\mathrm{NO}_{2}^{-}$ to NH₃, and the dissociation of H₂O to *H. However, these three reactions depend on distinct catalyst properties that are difficult to achieve in a single-site catalyst. Here a tandem catalyst of Co/Co₂P heterostructures encapsulated by N-doped graphene shells on carbon nanotube (Co/Co₂P@NG/CNT) for $\mathrm{NO}_{3}^{-}$RR was developed, achieving an attractive NH₃ yield rate of 47.8 mg h^-1 mg^-1 with a corresponding NH₃ Faradaic efficiency of 99.2 % in 0.05 mol/L NO₃^- solution, exceeding most of the reported catalysts under the same $\mathrm{NO}_{3}^{-}$ concentration. Experimental and theoretical studies reveal that the Co₂P effectively reduces $\mathrm{NO}_{3}^{-}$ to $\mathrm{NO}_{2}^{-}$, while Co/Co₂P interface is responsible for the subsequent conversion of $\mathrm{NO}_{2}^{-}$ to NH₃. Meanwhile, the H₂O dissociation is promoted by the Co/Co₂P interface to generate *H for intermediates hydrogenation. Such a tandem catalysis process accelerates the conversion of $\mathrm{NO}_{3}^{-}$ into NH₃. The Co/Co₂P@NG/CNT also shows good stability due to the robust protection from N-doped graphene shell.

Key words: Nitrate reduction reaction, Ammonia synthesis, Tandem catalysis, Interface engineering