Electrochemical Production of Ammonia and Urea from Coreduction of Nitrite and Carbon Dioxide at Iron Phthalocyanine Electrodes and Comparison of Analytical Methods

Abstract

There is rapidly growing interest in the electrochemical reduction of both nitrite and carbon dioxide to mitigate environmental concerns and sustainably produce fuels, chemicals, and fertilizers. Ammonia and urea are key targets in this area, and have been produced simultaneously by coreduction of NO 2 ⁻ and CO 2 at a variety of electrocatalysts. The salicylate spectrophotometric method is commonly employed to quantify both products, with urea determined by difference following decomposition with urease. However, this method can be influenced by numerous interferences and matrix effects. Here the electrochemical coreduction of NO 2 ⁻ and CO 2 at iron phthalocyanine electrodes has been investigated in order to accurately determine the ammonia and urea production. Urea concentrations were determined by salicylate, diacetyl monoxime and liquid-chromatography-mass spectrometry methods, while the salicylate method and nuclear magnetic resonance spectroscopy were used to determine ammonia. Urea was produced at low overpotentials, with a maximum faradaic yield of 5.8% at −0.047 V vs RHE, while a maximum 85% yield of ammonia occurred at −0.347 V. It is shown that the salicylate method can provide accurate ammonia and urea analyses when a standard addition method is employed, reaction conditions are carefully controlled and the accuracy of calibration slopes are verified.