Thermochemical Energy Storage: Calcium Doping Facilitates Water Dissociation in Magnesium Oxide (Adv. Sustainable Syst. 1/2018)

Aiming for highly reactive and reversible thermochemical energy storage materials based on magnesium hydroxide, the impact of Ca2+ as dopant in magnesium hydroxide/oxide is investigated in article number 1700096 by Danny Müller and co-workers. Calcium doped magnesium oxide reveals notably increased water dissociation rates, compared to pure MgO, enhancing both rate and reaction completeness of the hydration. For a series of mixed magnesium-calcium oxides (MgxCa1−xO) with varying Ca-contents between 0 and 40% the material of a composition Mg0.9Ca0.1O shows the fastest rehydration, transforming completely within 80 minutes to the mixed hydroxide. In consecutive dehydration/rehydration cycles reasonable cycle stability is found. A “regeneration” of the aged material (reactivity reduced by excessive cycling) in liquid water re-establishes the initial rehydration reactivity. Density functional theory (DFT) calculations support the experimental findings, confirming that calcium doping can reduce the energy of the rate limiting water dissociation reaction exploiting both electronic and steric (size) effects.