Electrocatalytic nitrate reduction (e-NO₃^⁻RR) offers a sustainable route for nitrate (NO₃^⁻) removal from aqueous environments while concurrently producing ammonia (NH₃); however, its practical implementation is hindered by insufficient surface-adsorbed atomic hydrogen (*H) and the undesirable accumulation of nitrite (NO₂^⁻). For this purpose, we report a heterojunction tandem catalyst comprising cobalt (Co) nanosheets anchored on iron (Fe) phosphide (Co/FeP/CC) nanorods. This bifunctional electrocatalytic system achieves a Faradaic efficiency (FE) of 98.23% for NH₃ synthesis at -0.4 V vs. RHE, with a corresponding production rate of 31.6 mg·h^-1·cm^-2. Moreover, it exhibits exceptional electrochemical stability, retaining over 97% of its initial FE after 100 h of continuous operation. In situ studies and DFT calculations reveal that the Co/FeP interface promotes NO₃^- activation on Co sites and *H supply on FeP sites, while interfacial charge transfer from Co to FeP optimizes the energetics of hydrogenation, thereby enabling efficient tandem catalysis. Based on the above design, we integrated the catalyst into a Zn-NO₃^⁻ battery, enabling concurrent NH₃ synthesis and electricity generation, with a peak power density of 6.29 mW·cm^-2. A practical, scalable strategy for engineering efficient tandem electrocatalysts and advancing integrated energy-environment systems is provided by this work.