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

Abstract 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 NO 3 − concentration, pH, and applied potential were maintained unchanged. Specifically, the faradaic efficiencies toward NH 3 (FE ) in Na 2 SO 4 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 NO 2 − (FE ), 77.0, 69.2, and 73.7 % in Na 2 SO 4 , significantly dropped to10.8, 7.4, and 4.4 % in PBS, evidencing an unexpected selectivity reversal of the nitrate reduction from NO 2 − to NH 3 . 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 NH 3 ( J ) and NH 3 formation rate ( r ) up to 133.5 mA cm −2 and 1.74×10 −7 mol s −1 cm −2 in 1.0 m KNO 3 at −1.25 V.