The cobalt-oxide/iron-oxide binary system for use as high temperature thermochemical energy storage material

• Binary cobalt-oxide/iron-oxide system tested as thermochemical storage material.
• Redox-reactions tested for different compositions.
• Fe addition lowers reaction enthalpy of the Co-spinel to Co-wustite transformation.
• Fe addition improves microstructural stability of cobalt-oxide phases.
• For thermochemical storage applications 13% Fe to 87% Co3O4 is promising.

The use of thermochemical reactions is a promising approach for heat storage applications. Redox-reactions involving multivalent cations are recently envisaged for high temperature applications. In temperature range of 900–1000 °C, however, where heat storage required for concentrated solar power (CSP) processes only few metal oxides with sufficient heat storage capabilities do exist. Binary systems, on the other hand, could provide a wider range of suitable materials. In the present experimental study the cobalt-oxide/iron-oxide binary system is investigated. For pure iron-oxide the transformation of Fe2O3/Fe3O4 occurs at 1392 °C with a reaction enthalpy of 599 J/g. The reaction temperature, however, is far too high for CSP applications. Cobalt-oxide, on the other hand, reacts from Co3O4/CoO at 915 °C with an enthalpy of 576 J/g. Iron-doped cobalt-oxides transform at similar temperature as pure cobalt-oxide but the reaction enthalpy gradually decreases with increasing iron content. Microstructural stability and related long-term reversibility of the chemical reaction, however, is higher with respect to pure cobalt-oxide. Compositions of around 10% iron-oxide were identified having appropriate enthalpies and being beneficial in terms of microstructural stability.