Compact Thermochemical Heat Storage Based on Hydrothermal Dehydration of Cobalt Hydroxide

Heat storage for waste heat sources at 200–250 C, where most power is available, remains challenging due to the lack of suitable storage materials. Here, we explore thermochemical heat storage at these temperatures based on cobalt oxide/hydroxide chemistry under hydrothermal conditions that is uniquely closed to mass flows. The closed system removes the need to separate and store individual products and is therefore, expected to be simpler and more compact than existing chemistries that require separation and storage of the product gas. The hydrothermal dehydration of cobalt hydroxide is attractive among other metal hydroxides due to its relatively modest pressure requirement and a temperature that is well-matched to low-temperature waste heat recovery. We find the theoretical round-trip energetic and exergetic efficiencies to be ~60% and ~50% respectively. In cycling experiments of hydrothermal dehydration and hydration, characterized using TGA, XRD, and XPS, we find the dehydration kinetics to be reasonably fast but the hydration to be limited to 40% on repeated conversion. TGA and TEM analysis of the product further suggest that this limit arises from the diffusion resistance of water through the cobalt hydroxide layer on the surface of the reacting oxide. Doping with Mg can yield a higher conversion limit. This work yields fundamental insights into a chemistry for novel thermochemical heat storage useful for low-to-mid temperature waste heat recovery.