Earth Alkali Oxalates for Heat Storage: Linking Crystal Structure to (Ir)reversible (De)hydration

A promising candidate for thermochemical energy storage (TCES) is calcium oxalate. As calcium oxalate is part of the earth alkali oxalate group, other earth alkali oxalates such as magnesium oxalate, strontium oxalate, and barium oxalate are investigated for their applicability in thermochemical energy storage. It is observed that only calcium oxalate shows reversible (de)hydration behavior whereas as magnesium oxalate, strontium oxalate, and barium oxalate show irreversible behavior; rehydration is not possible. The irreversible behavior is linked to the crystal structures of the materials. Calcium oxalate remains in an open structure during dehydration whereas magnesium oxalate, strontium oxalate, and barium oxalate transition into dense network structures, hindering the access of water vapor into the material. This work opens the horizon to explore other metal oxalates and find a solution to the irreversibility challenges addressed in this work.