Enhanced mechanism of molten alkali nitrates on triple-phase interface during the carbonation of MgO

Abstract

Thermochemical energy storage has broad application prospect with the advantages of high energy density and trans-regional storage capability. For MgCO3/MgO reaction couple, conversion rate of MgO carbonation with molten alkali nitrates is significantly enhanced. However, its kinetic mechanism is still controversial. In this work, molecular dynamics research on MNO3-MgO-CO2 (M is Li/Na/K/Na0.n5K0.5) interface was carried out to reveal the mechanism and regulate its performance. Results indicate that MNO3 plays a dominant role in facilitating MgO(s) → Mg2+ + O2− after melting. Lattice distortion is observed on the surface of MgO due to oxygen ion exchange between NO3– and MgO, breaking MgO balance. Interestingly, generated oxygen vacancies become highly reactive sites. Overall, the participation of molten MNO3 gives rise to triple-phase boundary that works as a efficient pathway for CO2 to penetrate deeper into MgO. Subsequently, structure-activity relationships between microstructure and carbonation kinetics was proposed. Among all the dopants studied, Na0.5K0.5NO3 not only creates strong distortion of the MgO lattice with oxygen defect concentration of 0.0787 mol%, but also exhibits a small-contact angle of 67.5° on MgO at 320 °C and 0.1 MPa, boosting MgO carbonation. The ambient temperature plays opposite effects on interface wettability of molten MNO3 and thermodynamic driving force, causing a parabolic kinetics with a peak rate at 332 °C. Small and uniformly dispersed dopants are beneficial for energy carriers to adsorb CO2. The above regulation provides a guiding principle for the preparation of thermochemical energy carriers in subsequent applications.