High-Rate and Sustainable Production of Urea and Ammonia via Electrochemical Reduction of CO2 and Nitrates
Authors: Zhao Qinglan , Shao Minhua 
Published: September 2023

Over the past century, nitrogen fertilization has fed approximately 27% of the world’s population.1 Urea and ammonia are two of the most important nitrogen fertilizers used world-widely. The current industrial production of urea is mainly realized by an energy-intensive reaction of carbon dioxide and ammonia under harsh conditions (150-200 °C, 150-250 bar) with large energy consumption, for which nearly 80% of synthesized ammonia is used.2 Furthermore, ammonia is also produced under extreme condition (350-550 °C, 150-350 bar) by an energy-consuming Haber-Bosch method.3 Therefore, it is of great significance to synthesize urea and ammonia under mild conditions to meet the demands of ever-increasing population in the world. The electrocatalytic co-reduction of CO2 and NO3
- emerges as a promising approach to realize the direct synthesis of urea via C-N coupling, with ammonia produced as a side product. However, the state-of-the-art yield rates of urea and ammonia through this electrosynthesis are typically below 1 mg h-1 mgcat
-1 and 3 mg h-1 mgcat
-1.4, 5

In this study, we realize high-rate production of urea on an economic Cu-based organic molecule catalyst through electrochemically coupling CO2 with NO3
-, with ammonia as another useful byproduct. The rationally design of the catalysts guarantees the accurate adsorption and activation of NO3
- and CO2, which further promotes the desired electrochemical C-N coupling in urea synthesis. Efficient urea synthesis was achieved with yield rates ranging from 2.7 to 3.6 mg h-1 mgcat
-1 in a potential window of -0.49~-0.67V vs. RHE, together with ammonia with high yield rates ranging from 0.15 to 9.7 mg h-1 mgcat
-1. This work proposes an appealing route of sustainable production of artificial nitrogen fertilizers with high efficiencies. Widespread adoption of this approach is promising to re-use the greenhouse gas CO2 and NO3
- from waste toward a full circulate of economy and sustainable energy consumption. The demonstrated production of nitrogen fertilizer in conjunction with renewable electricity is of great potential to meet the rising demand for global food security.

Acknowledgements

Research Grants Council (26206115, 16304821 and 16309418) and Innovation and Technology Commission (grant no. ITC-CNERC14EG03) of the Hong Kong Special Administrative Region. The work described in this paper was substantially supported by a fellowship award from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKUST PDFS2021-4S12).

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