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Energies 2020, 13, 5859 14 of 23 addition of either 20 wt% TiO2, ZrO2 or CeO2 [94]. All the tested additives permitted an improvement against the manganese oxide particle agglomeration. However, the addition of TiO2 showed to have a negative effect on the chemical reactivity of the oxide, while the addition of ZrO2 brought the best enhancement towards increasing the attrition resistance of the particles. The effect of the addition of Al2O3, Fe2O3 and ZrO2 to manganese oxide spray-dried particles on their energy storage capacity, flowability and physical and chemical stability was studied [95]. The samples mixed with zirconia and alumina allowed the individual particles to better retain their structure; however, the sample containing iron performed better during redox cycles. A reaction enthalpy of 175.7 kJ/kg was measured for the mixture with Mn2O3 and 67 wt% Fe2O3 prepared by intensive mixing, which then performed better than the spray-dried sample with similar composition (contaminated with sodium). The formation of a spinel MnFe2O4 was obtained through the reduction of a mixture of 2:1 Fe2O3:Mn2O3. The reaction of re-oxidation was described through two reaction mechanisms, starting with a diffusion-controlled reaction mechanism with no phase change, and followed by a nucleation-growth reaction mechanism, with activation energies of 192 and 181 kJ/mol for each reaction, respectively. A similar mixed oxide with Fe/Mn (2:1) was studied in a packed-bed reactor using small particles (0.5–1.0 mm) of iron-manganese oxide [96]. This work also presents a model comparing heat transfer, mass transfer and the thermochemical reaction with experimental data. Tests in a lab-scale tube reactor, between 800 and 1040 ◦C in air, were conducted on a previously validated Fe/Mn 1/3 granular mixture which showed no degradation over the course of 17 cycles [97,98]. The experiment demonstrated the presence of characteristic temperature profiles along the bed height, which were shown to be dependent on the thermodynamic properties and kinetic behavior of the redox reaction. The tuning of the reaction temperatures of oxides is very important to optimize the system energy storage since the gap in temperature between both charge and discharge steps can be reduced. Co-doping, using Fe and Cu, on manganese oxide was used to reduce this gap in temperature [83]. Indeed, the incorporation of Fe to the system was used to increase oxidation temperature, and Cu addition was used to reduce the reduction temperature. The gap in temperature was decreased from 225 ◦C for pure manganese oxide, to 81 ◦C for a composition with 20 mol% Fe and 5 mol% Cu. However, the addition of Cu induced a decrease in the reduction rate and a gradual decrease in the oxidation rate, which was attributed to the formation of segregated mixed Mn–Cu spinel. Cobalt oxide based TCES systems have demonstrated the best performances among pure metal oxides, with high enthalpy and excellent cycling stability (Figure 7a), and are attracting attention for pilot scale tests [99]. However, this still leaves room for improvement and attempts to reduce the material cost and toxicity via the synthesis of mixed oxides [81,85–87,100]. In addition, the temperature gap between the reduction and oxidation step could be reduced with the same approach (Figure 7b). Storage material made from inert honeycomb supports (cordierite) and coated with cobalt oxide was studied at pilot-scale [99]. A large amount of redox material (88 kg) was cycled for 22 charging/discharging cycles with absence of degradation. The Co-Mn-O system demonstrated good reversibility for low amounts of manganese, and an increase in temperature compared to the pure oxides [81]. The reaction temperature of various Co-Mn mixed oxides, Co3−xMnxO4 (0 ≤ x ≤ 3), was investigated between 850 and 1700 ◦C [101,102]. The measured reaction temperatures for Co2.5Mn0.5O4, Co2MnO4, Co1.5Mn1.5O4, CoMn2O4 and Co0.5Mn2.5O4 were (red-ox) 980–910, 1129–1050, 1230–1162, 1320–1260 and 1428–1410◦C, respectively. The phase transition from the cubic-to-tetragonal phase within 1.2 < x < 1.9 was thoroughly examined. The mixed oxides presented higher enthalpies than the respective pure oxides, with the Co1.5Mn1.5O4 sample showing the highest enthalpy (1264 kJ/kg). The mixed oxides have higher reduction temperatures than pure Co3O4, reaching up to 1428 ◦C for Co0.5Mn2.5O4.PDF Image | Hi Temp Thermochemical Energy Storage via Solid Gas Reactions
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