First-Principles Estimation of Thermodynamic Properties and Phase Stability of CaMnO3−δ for Chemical-Looping Combustion

Journal article, 2025

Chemical-looping is a technology that increases combustion efficiency and enables carbon capture with a low energy penalty. A central component in chemical-looping is the oxygen carrier particles, which transfer oxygen from the air reactor to the fuel reactor. It is, therefore, important to look for new materials that might perform better than those used today. How suitable a material is as an oxygen carrier is largely determined by its thermodynamic properties, as this governs the maximum extent of fuel conversion in the fuel reactor. Gathering data to estimate thermodynamic properties is sometimes difficult or even impossible using experimental methods. CaMnO3−δ is an example of a complex material that has shown promise as an oxygen carrier for chemical-looping combustion and chemical-looping with oxygen uncoupling. Still, the fundamentals of this system are not fully understood, and available thermodynamic data show significant variations. Computational modeling is a method for generating thermodynamic data of equivalent accuracy to experimental investigations for complex systems. In this study, first-principles calculations by using density functional theory (DFT) and three different functionals (GGA + U, SCAN, r2SCAN) have been used to investigate the perovskite CaMnO3−δ. Our unique approach allows for the detailed examination of individual oxygen vacancies and their impact on thermodynamic properties, which is critical for understanding oxygen transport in chemical-looping. This is important since vacancies largely govern the oxygen transport needed in chemical-looping. The generated data is used in multiphase thermodynamic calculations to achieve greater detail in the predicted Ca-Mn-O phase-diagrams. The calculated results using SCAN show good agreement with data found in commercial databases and experimental data when comparing heat capacity and entropy, with differences of −1.17 and −11.17 J/(molK), respectively, for δ = 0 and 298.15 K. However, a larger discrepancy is shown for the estimated formation enthalpy at the same conditions, +26 kJ/mol compared to data from a commercial database. The results using r2SCAN are within 0.5% of the results using SCAN. The functional GGA + U estimates larger differences than SCAN and r2SCAN, +0.77, −5.43 J/(molK), and +48 kJ/mol for heat capacity, entropy and formation enthalpy, respectively. This indicates that CaMnO3 is less stable than previously proposed in the literature.