First principles investigation of water adsorption, dopant segregation and surface thermodynamics of Mg and Sr substituted CaO(001) for Ca(OH)2/CaO thermochemical energy storage

The Ca(OH)$_{2}$/CaO cycle releases stored heat when CaO takes up water, an uptake that begins at the oxide surface. Isovalent Mg and Sr barely move the bulk equilibrium of the cycle, but their surface effect is unknown. We study water at 1/4 monolayer on CaO(001) with Mg, Sr or both in the top layer, or Mg in the second layer, by density functional theory. On every surface the most stable state we found is split: a hydroxyl bridges two cations and the proton sits on a neighbouring surface oxygen. Water binds by 0.84 eV on CaO, 0.98 eV with Sr, 1.17 eV with Mg, 1.18 eV with both and 0.81 eV with Mg below the surface. Mg acts through the oxygen that takes the proton, Sr through the site of the hydroxyl, and their gains do not add. The stretching frequency of the new surface hydroxyl tracks the binding energy (\textit{r} = $-$0.96). On the clean surface Sr prefers the top layer by 0.46 eV and Mg the second layer by 0.29 eV. Adsorbed water reverses the choice of Mg, but in equilibrium with the vapour only below 530 to 790 K at 1 bar. Relative to CaO, surface Sr raises the water desorption temperature by 65 to 105 K and surface Mg by 190 to 250 K, while the same ions move the bulk equilibrium temperature by only $-$5 to +25 K. Surface doping, most reliably with Sr, thus tunes water uptake almost independently of the storage temperature.