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.