Dynamically forming a Cu3Mo2O9/Cu heterojunction for efficient nitrate reduction in Zn–nitrate batteries

Abstract

The sustainable production of ammonia (NH₃) via the electrochemical nitrate reduction reaction (NO₃⁻RR) presents a dual solution for environmental remediation and renewable energy storage. However, this process is hindered by the sluggish kinetics of the sequential deoxygenation and hydrogenation steps, particularly under alkaline conditions, where proton scarcity exacerbates the competing hydrogen evolution reaction (HER). In this study, heterostructured Cu₃Mo₂O₉/Cu is purposely designed so that it can be reductively formed during the NO₃⁻RR to incorporate the advantages of dual-function active sites in the processes of water dissociation and nitrate reduction. Experimental and theoretical results indicate that the in situ generated Cu₃Mo₂O₉ is proposed to facilitate H₂O dissociation and likely contribute to proton (H⁺) supply, while metallic Cu enhances nitrate adsorption and facilitates subsequent deoxygenation. The Cu₃Mo₂O₉/Cu catalyst achieves an excellent NH₃ faradaic efficiency (FE) of 97.5% at −0.5 V vs. RHE with an NH₃ yield rate of 19.3 mg h⁻¹ mg_cat⁻¹ in 0.05 M KNO₃. This performance is among the highest under neutral/alkaline H-cell conditions. The Cu₃Mo₂O₉/Cu-based Zn–nitrate battery delivers a peak power density of 20.24 mW cm⁻² and maintains an FE_NH3 of 93.8% at 60 mA cm⁻². This study elucidates the dynamic synergy of heterostructured catalysts for multi-step reactions and establishes a general framework for coupling catalytic nitrate conversion with energy storage applications.