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Calorimetric method for determining the thermochemical energy storage capacities of redox metal oxides

King, Keith, Randhir, Kelvin, Klausner, James
Thermochimica acta 2019 v.673 pp. 105-118
calorimeters, calorimetry, energy, enthalpy, manganese, oxides, oxygen, reducing agents, temperature, tin
Transition metal oxide compounds have been demonstrated to be promising candidates for thermochemical processes, particularly energy storage. A calorimetric method for measuring enthalpy of these materials is developed in this work. A combination of drop calorimetry and acid-solution calorimetry is used to measure the total enthalpy and standard enthalpy of formation of these materials for compounds that form at high temperatures (≥ 1000 °C) after undergoing thermal reduction. These measurements are used to compute the energy storage potential of these materials for a specified set of redox cycle operating temperatures. The construction and calibration of appropriate calorimeter devices are described. A novel approach to performing acid-solution calorimetry is introduced by including tin (II) chloride as a reducing agent for enhancing the dissolution rate of these compounds sufficiently so that acid-solution calorimetry can be implemented accurately. The general procedure is presented and applied to three variations of magnesium-manganese oxide materials for a redox cycle operating under an oxygen partial pressure of 0.2 atm between 1000 and 1500 °C. Using this method, the total energies stored by magnesium-manganese oxides of molar ratios Mn/Mg of 2/1, 1/1, and 2/3 were found to be 924 ± 56.7, 1029 ± 57.0, and 1070 ± 64.2 kJ kg−1, respectively.