Enhancing Compatibility of Two-Step Tandem Catalytic Nitrate Reduction to Ammonia Over P-Cu/Co(OH)₂.

Abstract

Electrochemical nitrate reduction reaction (NO₃RR) is a promising approach to realize ammonia generation and wastewater treatment. However, the transformation from NO₃ ^- to NH₃ involves multiple proton-coupled electron transfer processes and by-products (NO₂ ^-, H₂, etc.), making high ammonia selectivity a challenge. Herein, a two-phase nanoflower P-Cu/Co(OH)₂ electrocatalyst consisting of P-Cu clusters and P-Co(OH)₂ nanosheets is designed to match the two-step tandem process (NO₃ ^- to NO₂ ^- and NO₂ ^- to NH₃) more compatible, avoiding excessive NO₂ ^- accumulation and optimizing the whole tandem reaction. Focusing on the initial 2e^- process, the inhibited ^*NO₂ desorption on Cu sites in P-Cu gives rise to the more appropriate NO₂ ^- released in electrolyte. Subsequently, P-Co(OH)₂ exhibits a superior capacity for trapping and transforming the desorbed NO₂ ^- during the latter 6e^- process due to the thermodynamic advantage and contributions of active hydrogen. In 1 m KOH + 0.1 m NO₃ ^-, P-Cu/Co(OH)₂ leads to superior NH₃ yield rate of 42.63 mg h^- ¹ cm^- ² and NH₃ Faradaic efficiency of 97.04% at -0.4 V versus the reversible hydrogen electrode. Such a well-matched two-step process achieves remarkable NH₃ synthesis performance from the perspective of optimizing the tandem catalytic reaction, offering a novel guideline for the design of NO₃RR electrocatalysts.