Harnessing electrochemical CO₂ reduction and assisted water electrolysis via constrained thermodynamic modeling

Electrochemical CO₂ reduction reaction (CO₂RR) and assisted water electrolysis (AWE) using organic compounds offer promising pathways for sustainable energy conversion. However, the thermodynamic feasibility and efficiency of these processes are strongly influenced by CO₂ phase transitions (both gaseous and aqueous) and operating conditions, such as temperature and pH. This study systematically examines the thermodynamic behavior of CO₂RR and AWE by calculating Gibbs free energy (ΔG), enthalpy (ΔH), and theoretical potentials (E_TN and E_RE) over a broad temperature range (0-1000 °C) and varying pH conditions. Pourbaix diagrams for key CO₂-derived products, including CO, hydrocarbons, organic acids, and alcohols, are constructed to assess their stability across different electrochemical environments. The analysis reveals that in aqueous-phase CO₂ systems, equilibrium potentials shift due to the effects of CO₂ speciation. In alkaline conditions, dissolved CO₂ undergoes sequential conversion into HCO₃^- and CO₃^2-, resulting in increased overpotentials in CO₂RR. Conversely, gaseous CO₂ maintains a stable equilibrium potential, mitigating pH-induced fluctuations that could hinder reaction selectivity and efficiency. In AWE, the phase transition during reaction conditions lowers oxidation potentials, resulting in enhanced energy efficiency. The calculated V_TN and V_RE values demonstrate that organic oxidation reactions in AWE require substantially lower energy inputs than conventional oxygen evolution reactions, providing a thermodynamic advantage for energy-efficient hydrogen production. This study establishes a comprehensive thermodynamic framework for CO₂ electrochemical conversion, integrating Pourbaix diagrams and temperature-dependent electrochemical modeling to optimize reaction conditions and energy efficiency. These insights contribute to the rational design of electrocatalytic systems and the development of scalable CO₂ conversion technologies for industrial applications.