Scalable Electrocatalytic Urea Wastewater Treatment Coupled with Hydrogen Production by Regulating Adsorption Behavior of Urea Molecule

Abstract

Electrocatalytic urea wastewater treatment technology has emerged as a promising method for environmental remediation. However, the realization of highly efficient and scalable electrocatalytic urea wastewater treatment (SEUWT) is still an enormous challenge. Herein, through regulating the adsorption behavior of urea functional groups, the efficient SEUWT coupled hydrogen production is realized in anion exchange membrane water electrolyzer (AEMWE). Density functional theory calculations indicate that self-driven electron transfer at the heterogeneous interface (NiO/Co3O4) can induce charge redistribution, resulting in electron-rich NiO and electron-deficient Co3O4, which are superior to adsorbing C=O (electron-withdrawing group) and –NH2 (electron-donating group), respectively, regulating the adsorption behavior of urea molecule and accelerating the reaction kinetics of urea oxidation. This viewpoint is further verified by temperature-programmed desorption experiments. The SEUWT coupled hydrogen production in AEMWE assembled with NiO/Co3O4 (anode) and NiCoP (cathode) can continuously treat urea wastewater at an initial current density of 600 mA cm−2, with the average urea treatment efficiency about 53%. Compared with overall water splitting, the H2 production rate (8.33 mmol s−1) increases by approximately 3.5 times. This work provides a cost-effective strategy for scalable purifying urea-rich wastewater and energy-saving hydrogen production.

1. Introduction

Urea is a vital active nitrogen compound in the nitrogen cycle, playing a crucial role in water, energy and food domains [1, 2]. The large quantities of urea-rich domestic and industrial wastewater without treatment will produce harmful toxins, threatening water ecological balance [3, 4]. Traditional urea decomposition methods, such as enzymatic hydrolysis, biomass degradation and chemical oxidation, often have low economic benefits, complex technology and harsh working environments, making it difficult to meet industrial requirements [5]. It is important to note that electrochemical urea oxidation reaction (CO(NH2)2 + 6OH− → N2 + 5H2O + CO2 + 6e−, UOR) is considered to be a cost-effective method for treating urea-rich wastewater [6]. In particular, UOR has a wide range of applications in energy conversion and storage, including urea-assisted hydrogen production, direct urea fuel cells, photoelectrochemical urea decomposition and wastewater treatment [1, 7]. Ideally, urea-assisted hydrogen production, combined with urea wastewater treatment, can achieve purification of urea-rich wastewater and energy-saving hydrogen production [8, 9]. At present, research on this strategy was limited to laboratory scale, and the realization of highly efficient and scalable electrocatalytic urea wastewater treatment (SEUWT) is still an enormous challenge under industrial current densities in water electrolysis equipment such as anion exchange membrane water electrolyzer (AEMWE) [10, 11].

Urea molecule contains two electron-donating groups (–NH2) and one electron-withdrawing group (C=O), which tend to adsorb in the electron-deficient and electron-rich regions of the catalyst, respectively [12, 13]. Due to the influence of functional groups in urea on adsorption behavior, it is of great significance to reveal the regulation mechanism of urea adsorption behavior and search for suitable catalysts for UOR [13]. The investigation of semiconductor physics suggests that by utilizing two semiconductors with different energy structures, it is possible to construct a heterojunction, where an internal electric field and two opposing charge distribution regions can be formed at the heterojunction interface [14–16]. Transition metal-based (such as Ni, Co and Fe metals) sulfides, selenides and nitrides are highly efficient catalysts for UOR [17]. However, the strong polarization and fast reaction rate at high current density can [text truncated]