Adsorption behavior of H₂O on the strontium bromide surface: first-principles and molecular dynamics calculations

Thermochemical adsorption heat storage based on gas-solid interaction is an energy storage technology for the effective recovery of industrial waste heat and renewable energy sources such as solar energy. Currently, hygroscopic salts and water are commonly used as the working pairs in thermochemical thermal storage systems. Strontium bromide (SrBr₂) is a highly potential material for low-grade thermal energy storage and building applications due to its high sorption capacity and reaction enthalpy. In order to better understand the microscopic mechanism of adsorption, the water adsorption behavior of strontium bromide surfaces on the atomic scale is investigated in this study by using density functional theory (DFT). The effects of doping Ca or Mg into strontium bromide on water adsorption are analyzed by the comparison of the energy barrier, density of states (DOS) and crystal orbital Hamilton population (COHP) obtained before and after metal atom doping. The energy barrier of the SrBr₂/H₂O reaction system is 5.916 kcal mol^-1, and the energy barrier of Ca-doped SrBr₂ reduces to 3.432 kcal mol^-1, whereas the energy barrier of Mg-doped SrBr₂ increases to 13.394 kcal mol^-1. The thermodynamic properties of strontium bromide are significantly improved by doping with calcium atoms. The absolute value of the COHP for Ca-doped SrBr₂ decreases, which indicates that Ca-doped SrBr₂ can adsorb the H₂O molecules more easily at the same temperature. The doping of Ca atoms has a positive effect on the adsorption heat storage process. This study provides insight into the adsorption mechanism of water molecules on strontium bromide and could facilitate the design of efficient composite adsorbents.