Single-Electrode Tandem Electrocatalysis with Dual TiO₂@Cu and Cu Surfaces Enables Energy-Efficient Ammonia...

Electrochemical ammonia synthesis from nitrate can complement a hydrogen-based energy system with a liquid, shippable energy carrier. Ammonia (NH₃) is attractive because it serves both as an energy vector and as an essential feedstock. However, nitrate reduction is an eight-electron, proton-coupled pathway that forms multiple intermediates; the NO₃ ^- → NO₂ ^- step is widely viewed as rate-determining, followed by nitrite reduction and hydrogenation steps, until the formation of NH₃. Here, we implement a tandem strategy in a flow cell using a double-sided electrode with Cu and TiO₂@Cu surfaces that cooperatively boost the NO₃ ^- → NO₂ ^- and NO₂ ^- → NH₃ conversions. The system achieves a Faradaic efficiency toward NH₃ of 97%, with an NH₃ selectivity of 80%, and a productivity of 7.7 mg h^-1 cm^-2 (0.45 mmol h^-1 cm^-2), corresponding to a full-cell energy efficiency of 29%. This integrated electrode-reactor design expands the accessible performance space for the NO₃RR by combining nitrate adsorption with selective nitrite hydrogenation. These results establish a practical route for the efficient electrochemical synthesis of NH₃ from nitrate.