Insight into Hydrogenation Selectivity of the Electrocatalytic Nitrate‐to‐Ammonia Reduction Reaction via Enhancing the Proton Transport

The electrocatalytic nitrate-to-ammonia reduction reaction route (NARR) is one of the emerging routes toward green ammonia synthesis, and its conversion efficiency is controlled mainly by the hydrogenation selectivity. This study proposed a likely NARR route feasible and effective even in a neutral condition. Its high catalytic selectivity and efficiency were achieved by a switch of the sulfate solution to the phosphate buffer solution (PBS), while conditions of NO3- concentration, pH, and applied potential were maintained unchanged. Specifically, the faradaic efficiencies toward NH3 (FE NH3 ) in Na2 SO4 were as low as 9.8, 19.8, and 11.4 % versus remarkably jumping to 82.8, 90.5, and 89.5 % in PBS under -0.75, -1.0, and -1.25 V, respectively. The corresponding faradaic efficiencies toward NO2- (FE NO2- ), 77.0, 69.2, and 73.7 % in Na2 SO4 , significantly dropped to10.8, 7.4, and 4.4 % in PBS, evidencing an unexpected selectivity reversal of the nitrate reduction from NO2- to NH3 . This insight was further revealed by the visualization of the pH gradient near the electrode surface during NARR and confirmed by density functional theory calculations; PBS notably facilitated the proton transport and active mitigation over the proton transfer barrier. The use of PBS resulted in a maximal partial current density toward NH3 (J NH3 ) and NH3 formation rate (r NH3 ) up to 133.5 mA cm-2 and 1.74×10-7 mol s-1 cm-2 in 1.0 m KNO3 at -1.25 V.