Exploiting underpotential deposited hydrogen enables energy-efficient nitrate electroreduction to ammonia

Electrochemical nitrate reduction in alkaline media offers a sustainable route for ammonia synthesis at rates rivaling those of the Haber–Bosch process. However, its energy efficiency is limited by sluggish nitrate deoxidation and hydrogenation, compounded by challenges in proton supply via H2O dissociation. Here, we develop an enzyme-like substrate transport channel through a hierarchical arrangement of metallic Ag and Ru nanophases, to enable cascade nitrate-to-ammonia conversion and optimize underpotential-deposited hydrogen utilization. Operando characterization and theoretical calculations reveal that Ag–Ru electronic synergy regulates underpotential-deposited hydrogen coverage at Ru-centered active sites by coupling facilitated H2O dissociation with *OH-mediated site regeneration, thereby promoting nitrite relay conversion. Our underpotential-deposited hydrogen-assisted nitrate electroreduction system delivers a half-cell ammonia energy efficiency of 53.7% at 0.2 V versus RHE with near-unity Faradaic efficiency across a wide nitrate concentration range, and an ammonia partial current density of 2.2 A cm−2 at 0 V versus RHE. Pairing cathodic nitrate reduction with anodic H2 oxidation enables ammonia production costs below $1.15 kg−1 while maintaining sustained energy efficiency over 100 h at 200 mA cm−2. Electrochemical nitrate reduction rivals Haber-Bosch ammonia synthesis rates but suffers from poor energy efficiency. Here, the authors show that underpotential deposited hydrogen can be unlocked to accelerate nitrate conversion, offering an pathway to energy-efficient ammonia production.