Thermodynamics of electrochemical urea oxidation reaction coupled with cathodic hydrogen evolution reaction in an alkaline solution: Effect of carbonate formation

Abstract

The development of efficient and energy-saving electrochemical processes for "green" hydrogen generation represents one of the most crucial challenges in modern chemistry and technology. An effective approach to realizing such processes involves alkaline water electrolysis with cathodic hydrogen evolution coupled with anodic urea oxidation reaction. As extensively reported in the literature, this configuration allows for a significant reduction in cell voltage. In this work, we present thermodynamic analysis of the electrochemical reaction CO(NH2)2 + 2OH− ±6e– = 3H2 + N2 + CO32−, for the first time accounting for the inevitable carbonate formation due to interaction of carbon dioxide with alkali. Standard changes in enthalpy, entropy, and Gibbs free energy, as well as the electromotive force were calculated over the temperature range of 298–338 K. The influence of pH on the electromotive force value was investigated. It was observed that the process under study is accompanied by heat release and an increase in entropy, becoming thermodynamically spontaneous at pH > 12.66. From a thermodynamic point of view, this suggests exceptionally favorable energy characteristics for the studied process, promoting the development of an energy-efficient "green" hydrogen generation technology. Additionally, we demonstrate that increasing the temperature further enhances the thermodynamic features of this reaction by reducing the energy consumption for its progression.