Thermochemical heat storage based on the Mn₂O₃/Mn₃O₄redox couple: influence of the initial particle size on the morphological evolution and cyclability

Abstract

Thermochemical energy storage (TCS) based on reduction-oxidation cycles of multivalent metal oxides is of great interest for concentrating solar facilities, as it can allow enhancing the global plant efficiency and improving the energy generation dispatchability. However, to guarantee the feasibility of the process, selected materials should present long term durability, which requires the evaluation of the redox couple cyclability. In this work we have demonstrated, for the first time, the suitability of the Mn 2 O 3 /Mn 3 O 4 pair for this application during 30 cycles, performed in thermobalance. Nevertheless, an appropriate design of such materials is crucial since it has been found that initial particle size influences the redox behaviour of these oxides. Results showed a 2-fold influence of particle size of the as-prepared materials on the redox reversibility. Firstly, this parameter affects to the thermodynamics and kinetics of the redox reactions. Namely a decrease of the particle size shifted the oxidation temperature to lower values and produced slower reduction and oxidation reactions. Secondly, depending on the particles size, samples followed different sintering mechanisms. This fact influenced dramatically the behaviour of the materials with lower particle size, which suffered from a higher degree of densification that eventually caused a total loss of cyclability. † Electronic supplementary information (ESI) available: XRD patterns; high resolution TEM micrographs; SEM micrographs, rst cooling step interval; DSC curves for M 1 sample. See

Improving the Thermochemical Energy Storage Performance of the Mn2O3/Mn3O4 Redox Couple by the Incorporation of Iron

“…In our previousw ork, [9] we concluded that reduction was limited by heat transfer,w hereas oxidation appeared to be controlled by diffusional phenomena and we attributed the decreaseo ft he reoxidation rate to sintering. In this respect, chemicalo rm icrostructural modificationso fr aw manganese sesquioxide are considered as the two main options to improve the redox properties of the Mn 2 O 3 /Mn 3 O 4 pair.D oping with other metals has been generally studied as af irst approach as the incorporation of cations of different oxidation states andr adii could create chargei mbalances, disorders, or vacancies that are beneficial for oxygen diffusion through the crystal lattice.H owever,i th as not alwaysb een efficient.…”

“…As commented before, we attributedt he cycleto-cycle decay on the oxidation rate to oxygen diffusion hindrance caused by sintering, [9] although this does not apply to the reduction, as reduction values were stable over cycling as this reaction was limited by heat transfer.The heat-transfer-limited nature of the reduction reaction of metal oxides has been reported previously in the case of ceria. [25,26] The reduction of undoped Mn 2 O 3 (0F) also followed such behavior ( Figure 8A): the faster the heating rate, the faster the reduction.…”

“…For instance, in the 25 th cycle, the oxidation reactionpresented aconversion of 87 %. However,wereported previously [7,9] the total reversibility of the Mn 2 O 3 /Mn 3 O 4 redox pair for a T max of 1000 8Ci nstead of 1050 8C, which evidences that an increase in T max has an egative effect on the cyclability of the pure Mn oxide.T he evolution of the oxidation rate for two runs of 30 redox cycles at T max = 1000 and 1050 8C, respectively,i ss hown in Figure 5. With a T max = 1000 8C, the sample exhibited complete reduction and oxidation reactions (Figure S3) and faster oxidation rates.…”

Thermodynamic study of CuO/Cu2O and Co3O4/CoO redox pairs for solar energy thermochemical storage

“…The general pathway of TCS based on metal oxides is: There are various concepts that could be used to implement the TCS in Concentrating Solar Power (CSP) plants. Solutions proposed by Agrafiotis 12 , or assumed by Carrillo 13 and Álvarez de Miguel 14 for their investigations are based on indirect storage systems. The solid metal oxide is retained into a storage tank or a particle reactor where the charging phase takes places by absorbing heat from a hot air stream coming from the solar receiver.…”

A solar receiver-storage modular cascade based on porous ceramic structures for hybrid sensible/thermochemical solar energy storage

“…Co 3 O 4 /CoO and Mn 2 O 3 /Mn 3 O 4 [1][2][3] with porous ceramic structures in order to effectively store solar heat in air-operated Solar Tower Power Plants has been set forth and tested in lab-scale in a series of previous publications [4][5][6][7]. If such oxides are coated on porous heat exchange modules, solar heat produced during on-sun operation can be used to power the endothermic reduction of the oxide (forward reactions of exemplary schemes 1, 2 below); the thermal energy can be entirely recovered by the reverse exothermic oxidation reaction during off-sun operation.…”