Metal oxides for thermochemical energy storage: A comparison of several metal oxide systems

The reversible redox reactions of metal oxides show high potential as thermochemical storage material. At high temperatures oxides of suitable transition metals will undergo a reduction reaction and by that thermal energy is absorbed (MxOy+z → MxOy + z/2 O2 (M = Metal)). Below specific equilibrium temperatures the reoxidation (MxOy+z → MxOy + z/2 O2) takes place and hence thermal energy will be delivered. Because of the gas–solid reaction and the fact that air can be used as heat transfer fluid thermochemical energy storage based on transition metal oxides are of special interest from the procedural point of view. Among pure metal oxides only cobalt oxide, iron oxide, copper oxide and manganese oxide show suitable reaction temperatures, reaction enthalpies, cycling stabilities and material costs. To broaden the range of suitable metal oxides binary oxide systems were analyzed in the present study. In the following several compositions of eight binary metal oxide systems as well as the listed pure metal oxides were tested in terms of their ability to store energy thermochemically. Cobalt oxide/iron oxide, copper oxide/cobalt oxide, copper oxide/manganese oxide and manganese oxide/iron oxide are found to show high potential as thermochemical storage material. It was shown, however, that none of the tested systems fulfill all the requirements of an ideal storage material regarding storage capacity, costs and cycling stability.