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 NO₃ ^- concentration, pH, and applied potential were maintained unchanged. Specifically, the faradaic efficiencies toward NH₃ (FE <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>NH</mml:mi> <mml:msub><mml:mrow></mml:mrow> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>NO</mml:mi> <mml:msub><mml:mrow></mml:mrow> <mml:mn>2</mml:mn></mml:msub> <mml:msup><mml:mrow></mml:mrow> <mml:mo>-</mml:mo></mml:msup> </mml:mrow> </mml:msub> </mml:math> ), 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>NH</mml:mi> <mml:msub><mml:mrow></mml:mrow> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> ) and NH₃ formation rate (r <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow></mml:mrow> <mml:mrow><mml:mi>NH</mml:mi> <mml:msub><mml:mrow></mml:mrow> <mml:mn>3</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:math> ) 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.