Nanoscale Ru unlocking nanoneedles assembled into hierarchical CoO microspheres for efficient nitrate-to-ammonia electroconversion

Abstract

Because of the strong ability of cobalt oxide (CoO) to render active hydrogen (*H) for selective hydrogenation of NO₃⁻, it is well established as a promising catalyst for the electrocatalytic reduction of NO₃⁻ to ammonia (eRNO₃⁻-to-NH₃) for green NH₃ synthesis along with NO₃⁻ pollutant treatment. Unfortunately, its practical application is severely limited by easy agglomeration, weak NO₃⁻ adsorption, and the side reaction of H* coupling, causing the blockage of active sites and low selectivity of NH₃. For this, we purposefully designed a hetero hybrid electrocatalyst (denoted as CoO–Ru/CC), where nano-dimensional metallic Ru is anchored onto nanoneedles assembled into hierarchical CoO microspheres that are self-supported on carbon cloth (CC). Because of the vertical order open architecture, intrinsic oxygen vacancies, and local electric field in the self-standing CoO–Ru heterostructure, there are abundant active sites, efficient adsorption of NO₃⁻, weakened N–O bonds, and a reduced reaction energy barrier, as well as high reduction selectivity. CoO–Ru/CC efficiently catalyzes eRNO₃⁻ to NH₃ with a high NH₃ yield rate of 2.502 mg h⁻¹ cm_cat.⁻² and faradaic efficiency of 93.41%. In situ characterization provided evidence for the *NO₃–*NO₂–*NO–*N–*NH–*NH₃ pathway involving the reaction mechanism for CoO–Ru/CC. Additionally, constructed CoO–Ru/CC-based Zn–NO₃⁻ batteries exhibited remarkable power density. More significantly, the in-depth insights we achieved offer meaningful guidance for rationally designing advanced CoO-based catalysts for high-efficiency eRNO₃⁻-to-NH₃ electroconversion.