A form of manganese oxide that incorporates crystals of water between its layers could make an ideal heat-storage material. The newly-synthesized oxide is similar to the common mineral birnessite in its composition, and the Japan-based researchers who created it say that its volumetric energy density exceeds 1000 MJ/m3 – close to the theoretical maximum and the highest among all minerals that “lock” other molecules within their structure.
Led by Tetsu Ichitsubo and Norihiko Okamoto of Tohoku University’s Institute for Materials Research and Rigaku Corporation, the researchers fabricated their layered manganese oxide with crystalline water, forming a so-called intercalated compound designated as δ-type K0.33MnO2 ⋅ nH2O. The water molecules between the layers are easily lost through heating, and the team used X-ray diffractometry and transmission electron microscopy to study how the material’s structure changed as it heated or cooled.
Fast and reversible transformation
The team found that the water disappeared when the material was heated to 200°C. The resulting dehydrated material then released this absorbed heat energy when cooled to 120°C and exposed to moisture in the ambient surroundings. According to Ichitsubo, this mechanism is very advantageous for heat storage. It is also very fast, reversible and the material’s structure remains stable.
“The water is almost frozen in the material to start with and changes to vapour form once it is released,” he explains. “We can exploit the large entropy gap between these solid and gas phases for heat storage.”
A host of heat-storage applications
The researchers analysed the material’s heat storage properties using humidity-controlled thermogravimetry and differential thermal analysis together with differential scanning calorimetry. According to Ichitsubo, the calorimetric energy density they measured is high enough to be practical for heat-storage applications where space is limited.
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One possibility, he suggests, would be a rooftop system in which the energy collected from solar heat in the daytime gets released at night by exposing the dehydrated manganese oxide compound to moisture in its surroundings. Another option could involve automobiles. “Here, the waste heat can be stored while driving the vehicle and then released on demand for warming up the engine or the battery at a later time,” he tells Physics World.
The researchers, who detail their work in Nature Communications, say that their layered manganese oxide is easy to synthesize by thermally decomposing potassium permanganate (KMnO4). They now plan to increase the amount of water they can accommodate in the material. “We will do this by optimizing the concentration of the interspersed ion species in our compound and hence further increase its volumetric energy density,” Okamoto reveals.The reactive stability and energy density of magnesium-manganese oxides for high-temperature thermochemical energy storage have been investigated. Three variations of material with molar ratios of manganese to magnesium of 2/3, 1/1, and 2/1 were prepared using solid-state reaction synthesis and were tested for thermo-chemical reactive stability and energy storage capability. Results show that oxygen released and absorbed (standard cm 3 g −1) by the materials remains unchanged over 20 cycles when cycled between 1200 and 1500 °C under an oxygen partial pressure (P O2) of 0.2 atm, indicating excellent reactive stability at high temperatures. Additional confirmation of reactive stability was obtained through testing over 10 energy storage cycles between 1000 and 1500 °C. The energy density of the material between 1000 and 1500 °C was determined through a combination of acid-solution calorimetry and drop calorimetry. The total volumetric energy densities (chemical, phase change, and sensible) obtained for samples of Mn/Mg of 2/3, 1/1, and 2/1 cycled between 1000 and 1500 °C are 1596, 1626 and 1654 MJ m-3 , respectively.
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