Urea is the most common synthetic nitrogen fertilizer and in the last decades its synthesis has played a key role in increasing crop production and feeding a growing population. Urea conventional synthesis occurs via the Haber-Bosh and Bosh-Meiser processes, energy-intensive and fossil fuel-based reactions, accounting for more than 2% of global energy consumption. In contrast, the electrochemical coupling between CO₂RR and NO₃RR can provide novel and sustainable routes to synthesize urea.

The investigation of such a complex co-reduction process requires multiple efforts. Firstly, combining the extensive knowledge on CO₂RR in flow cells and the lab-scale level knowledge on NO₃RR, we have developed gas diffusion electrodes (GDEs)-based setup to maximize urea production and thus simplify its detection.

Secondly, the electrocatalytic performance of the basic catalytic platforms (i.e., zerovalent metals) was assessed across various aqueous electrolytes and pH.

Thirdly, a comprehensive examination of the four primary analytical methods reported in the literature has been carried out. Colorimetric methods, either direct (DAMO-TSC) and indirect (urease + indophenol blue reaction) are easily affected by both the environmental conditions (e.g., electrolyte pH) and the presence of interferers (i.e., CO₂RR and NO₃RR individual reaction products). ¹H-NMR is instead hampered by the currently low concentration of urea in post-electrolysis samples. Methods based on liquid chromatographic separation and detection emerge as the most reliable for precise and unbiased urea quantification.

Our research compares the electrochemical synthesis of urea results obtained in limited and non-scalable systems (e.g., H-cells) with those collected in an industrially relevant configuration and environment, providing guidelines for optimizing the process. Additionally, it proposes an ad-hoc analytical methodology for quantifying urea aimed to avoiding false positives and negatives, currently the major threats in this field.

Marzia Ferrera

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