The global transition to sustainable energy sources and the evolution toward a hydrogen-based economy demand energy formats that enable prolonged storage and efficient transportation from remote, renewable energy-rich locations. Liquefied energy forms, notably ammonia (NH₃), are gaining favor as viable alternatives, serving not only as a prominent energy carrier but also playing a crucial role as feedstock for the chemical and fertilizer industries.

In this context, highly efficient nitrate electroreduction (NO₃^-RR) emerges as a pivotal process for sustainable NH3 production, promising to overcome limitations associated with the current Haber-Bosch process. However, existing electrocatalysts face significant drawbacks in productivity yield, energy efficiency, and stability, especially under industrial conditions.

The NO₃^-RR process involves an eight-electron transfer assisted by protons, generating multiple intermediates that can diminish overall efficiency, particularly when NH₃ is the desired end product. Several studies recognize the reduction of NO₃^- to nitrite (NO₂^-) as the rate-determining step (RDE), with subsequent chemical and electrochemical steps occurring after the formation of NO₂^-. Consequently, at low overpotentials with negligible competition from the Hydrogen Evolution Reaction (HER), the deoxygenation of adsorbed NO₂^- determines the overall NO₃^-RR, accentuating the importance of targeting this step to maximize NH₃ generation. The accumulation of NO₂^- intermediate byproducts in the electrolyte necessitates the simultaneous acceleration of NO₃^-RR and NO₂^-RR to NH₃, presenting a challenging yet promising approach for efficient ammonia generation.

This study introduces a tandem NO₃^-RR process, involving sequential electrochemical processes converting NO₃^- to NO₂^- and then NO₂^- to NH₃. An employed composite electrocatalyst of Titanium Dioxide with oxygen vacancies (TiO_2-x) deposited on a Copper oxide (I)-copper (Cu₂O-Cu) surface, coupled with an optimized flow-cell configuration, produces compelling results toward NH₃: a Faradaic Efficiency (FE_NH3) of 97%, selectivity (SE_NH3) of 80%, and a productivity yield of 0.45 mmol h^-1 cm^-2. The cooperative synergy of intrinsic properties of the electrode composition and cell configuration enables high Half-Cell (EE_NH3) and full-cell (EE_CELL) energy efficiencies (52% and 43%, respectively).

In summary, our tandem NO₃^-RR process represents a significant advancement in addressing the challenges of sustainable ammonia production, providing a promising and efficient approach for environmentally friendly energy applications.