Carbon dioxide and nitrate co-electroreduction to urea on CuOxZnOy

Abstract

Urea is a commonly used nitrogen fertiliser synthesised from ammonia and carbon dioxide using thermal catalysis. This process results in high carbon dioxide emissions associated with the required amounts of ammonia. Electrocatalysis provides an alternative method to urea production with reduced carbon emissions while utilising waste products like nitrate. This manuscript reports on urea synthesis from the electroreduction of nitrate and carbon dioxide using CuO_xZnO_y electrodes under mild conditions. Catalysts with different ratios of CuO and ZnO, synthesised via flame spray pyrolysis, were explored for the reaction. The results revealed that all the CuO_xZnO_y electrocatalyst compositions produce urea, but the efficiency strongly depends on the metal ratio composition of the catalysts. The CuO₅₀ZnO₅₀ composition had the best performance in terms of selectivity (41% at -0.8 V vs RHE) and activity (0.27 mA/cm² at -0.8 V vs RHE) towards urea production. Thus, this material is one of the most efficient electrocatalysts for urea production reported so far. This study systematically evaluates bimetallic catalysts with varying compositions for urea synthesis from carbon dioxide and nitrate.

Fig. 1. Structural and compositional characterisation of CuO_xZnO_y catalysts.

a TEM images and particle size distributions of CuO_xZnO_y catalysts, b WAXS patterns of CuO_xZnO_y catalysts and (c) XPS spectra of the CuO_xZnO_y catalysts (i) Cu 2p_3/2 and (ii) Zn 2p.

Fig. 2. Electron microscopy characterisation of the CuO_xZnO_y catalysts.

a SEM images of the CuO_xZnO_y catalysts, (b) SEM-EDX images of the CuO₉₀ZnO₁₀, CuO₇₀ZnO₃₀, and CuO₅₀ZnO₅₀ catalysts.

Fig. 3. Cyclic voltammograms of the CuO_xZnO_y catalysts.

a cyclic voltammograms of the CuO_xZnO_y catalysts in 0.1 M Na₂SO₄ (blank), (b) cyclic voltammograms of the CuO_xZnO_y catalysts in 0.1 M Na₂SO₄ and 0.1 M NaNO₃ and (c) cyclic voltammograms of the CuO_xZnO_y catalysts in 0.1 M Na₂SO₄ and 0.1 M NaNO₃ with CO_2. Scan rate: 10 mV/s.

Fig. 4. Electrocatalytic measurements for urea electrosynthesis.

a Faradaic efficiency between −0.6 and −0.9 V vs RHE in 0.1 M Na₂SO₄ and 0.1 M NaNO₃ with CO₂, (b) Partial current density to urea between −0.6 and −0.9 V vs RHE in 0.1 M Na₂SO₄ and 0.1 M NaNO₃ with CO₂. The error bars display the average error obtained between three measurements.