Exploration of a novel electrochemical CN coupling process: Urea synthesis from direct air carbon capture with nitrate wastewater

Ying Chen; Yuan Liu; Shujie Hu; Di Wu; Mengyue Zhang; Zhiliang Cheng

doi:10.1016/j.scitotenv.2023.169722

Exploration of a novel electrochemical CN coupling process: Urea synthesis from direct air carbon capture with nitrate wastewater

Sci Total Environ. 2024 Feb 25:913:169722. doi: 10.1016/j.scitotenv.2023.169722. Epub 2023 Dec 30.

Authors

Ying Chen¹, Yuan Liu², Shujie Hu³, Di Wu³, Mengyue Zhang³, Zhiliang Cheng⁴

Affiliations

¹ Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China; School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China.
² Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China; Faculty of Geosciences and Environmental Engineering, Southwest Jiaotong University, Chengdu 611756, Sichuan, China. Electronic address: liuyuan@cigit.ac.cn.
³ Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China.
⁴ School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China. Electronic address: purper@cqut.edu.cn.

PMID: 38163593
DOI: 10.1016/j.scitotenv.2023.169722

Abstract

Direct air capture (DAC) can be used to decrease the CO₂ concentration in the atmosphere, but this requires substantial energy consumption. If residual waste carbon (in the form of bicarbonate solution) from DAC can be directly reused, it might present a novel method for overcoming the aforementioned challenges. Electrochemical CN coupling methods for synthesizing urea have garnered considerable attention for waste carbon utilization, but the carbon source is high-purity CO₂. No research has been conducted regarding the application of bicarbonate solution as the carbon source. This study proposes a proof-of-concept electrochemical CN coupling process for synthesizing urea using bicarbonate solution from DAC as the carbon source and nitrate from wastewater as the nitrogen source. These results confirmed the feasibility of synthesizing urea using a three-electrode system employing TF and CuInS₂/TF as the working electrodes via potentiostatic electrolysis. Under the optimal conditions (initial pH 5.0 and applied potential of -1.3 V vs. Ag/AgCl), the urea yield after 2 h of electrolysis reached 3017.2 μg h^-1 mg_cat.^-1 and an average Faradaic efficiency of 19.6 %. The in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy indicated a gradual increase in the intensity of the -CONH bond signal on the surface of the CuInS₂/TF electrode as the reaction progressed. This implied that this bond may be a key chemical group in this process. The density functional theory calculations demonstrated that *CONH was a pivotal intermediate during CN coupling, and a two-step CN coupling reaction path was proposed. *NH + *CO primarily transformed into *CONH, followed by the conversion reaction of *CONH + *NO to *NOCONH₂. This study offers a groundbreaking approach for waste carbon utilization from DAC and holds the potential to furnish technical underpinnings for advancing electrochemical CN coupling methods.

Keywords: Bicarbonate; CN coupling; Direct air capture; Electrocatalytic; Urea synthesis; Waste carbon.