Reverse hydrogen spillover accelerates electrocatalytic nitrate reduction to ammonia on Ru/WO3-x in acidic media

The electrocatalytic nitrate reduction reaction (NO₃^-RR) offers a promising route to sustainable ammonia synthesis, potentially replacing the energy-intensive Haber-Bosch process. While often studied in neutral or alkaline media, NO₃^-RR in acidic conditions is particularly relevant due to widespread industrial acidic nitrate wastewater, yet it remains challenging due to corrosion and dominant hydrogen evolution. To address this, we designed a corrosion-resistant Ru/WO_3-x heterostructure that spatially separates proton and nitrate adsorption sites. Here, we show that a reverse hydrogen spillover effect, where the WO_3-x support stores and transports protons to surface Ru active sites, dramatically enhances hydrogenation kinetics and suppresses parasitic hydrogen evolution. This catalyst achieves an ammonia Faradaic efficiency of 94.09% at a high current density of 500 mA cm^-2 and a working potential of 0.026 V vs. reversible hydrogen electrode. Furthermore, we demonstrate a sulfide-nitrate "batterolyzer" with a discharge power density of 43.4 mW cm^-2. This work reveals an effective proton-management strategy for efficient acidic NO₃^-RR, advancing its potential for coupled ammonia synthesis and wastewater treatment.