Double Hydroxide Nanocatalysts for Urea Electrooxidation Engineered toward Environmentally Benign Products.

Abstract

Recent advancements in the electrochemical urea oxidation reaction (UOR) present promising avenues for wastewater remediation and energy recovery. Despite progress toward optimized efficiency, hurdles persist in steering oxidation products away from environmentally unfriendly products, mostly due to a lack of understanding of structure-selectivity relationships. In this study, the UOR performance of Ni and Cu double hydroxides, which show marked differences in their reactivity and selectivity is evaluated. CuCo hydroxides predominantly produce N₂, reaching a current density of 20 mA cm_geo ^-2 at 1.04 V - 250 mV less than NiCo hydroxides that generate nitrogen oxides. A collection of in-situ spectroscopies and scattering experiments reveal a unique in situ generated Cu^(2-x)+-OO^-• active sites in CuCo, which initiates nucleophilic substitution of NH₂ from the amide, leading to N-N coupling between ^*NH on Co and Cu. In contrast, the formation of nitrogen oxides on NiCo is primarily attributed to the presence of high-valence Ni³⁺ and Ni⁴⁺, which facilitates N-H activation. This process, in conjunction with the excessive accumulation of OH^- ions on Jahn-Teller (JT) distorted Co sites, leads to the generation of NO₂ ^- as the primary product. This work underscores the importance of catalyst composition and structural engineering in tailoring innocuous UOR products.