Coupled Pd^δ--Cu^δ<b>+</b> Dipole for Enhanced Aqueous Nitrate Valorization at Ultralow Potentials.

Abstract

Nitrate <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>(</mo><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>-</mo></mrow></msubsup><mo>)</mo></math> pollution poses significant threats to water quality and the global nitrogen cycle. The electrochemical <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> reduction reaction (NO₃RR) emerges as a promising solution for <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> removal and sustainable ammonia (NH₃) production. However, it suffers from an insufficient atomic hydrogen (*H) supply and poor nitrite <math xmlns="http://www.w3.org/1998/Math/MathML"><mo>(</mo><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>-</mo></mrow></msubsup><mo>)</mo></math> adsorption at low potentials, which results in restrained <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math>-to-NH₃ conversion and notorious <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> accumulation. Herein, we propose a dipole strategy that utilizes coupled divergent dual centers (Pd^δ--Cu^δ+) in binder-free monolithic single-atom alloy electrodes (Pd₁Cu) to overcome these challenges at ultralow potentials. In-situ experiments and theoretical simulations reveal that the polarized atomic Pd^δ- dramatically enhances *H supply by facilitating water dissociation into *H, which then readily spills over to the adsorption-strengthened <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> on adjacent Cu^δ+, thus promoting rapid deep hydrodeoxygenation at ultralow potentials. Furthermore, the upshifted <i>d</i>-band center inhibits *H self-coupling and reduces the thermodynamic energy barrier for the *NO intermediate hydrogenation. Leveraging these advantages, the coupled Pd^δ--Cu^δ+ dipole achieved 100% <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> removal, 100% NH₃ selectivity, near-zero <math xmlns="http://www.w3.org/1998/Math/MathML"><msubsup><mrow><mi>NO</mi></mrow><mrow><mn>2</mn></mrow><mrow><mo>-</mo></mrow></msubsup></math> accumulation, 94.4% NH₃ Faradaic efficiency, and an NH₃ yield rate of 1.98 mM h^-1 cm^-2 at just -0.2 V vs RHE, outperforming the monometallic counterparts and reported advanced catalysts. The proposed metal-dipole strategy can provide a universal principle for the rational design of electrocatalysts to valorize pollutants into valuable ammonia products.