(Fe,Co,Mn)Ox Mixed Metal Oxides

catalysts

Article Thermochemical Energy Storage Performance Analysis of (Fe,Co,Mn)Ox Mixed Metal Oxides

Yabibal Getahun Dessie 1, Qi Hong 1,* , Bachirou Guene Lougou 1,2,* , Juqi Zhang 1, Boshu Jiang 1, Junaid Anees 1 and Eyale Bayable Tegegne 1

1 School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; [email protected] (Y.G.D.); [email protected] (J.Z.); [email protected] (B.J.); [email protected] (J.A.); [email protected] (E.B.T.) 2 MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China * Correspondence: [email protected] (Q.H.); [email protected] (B.G.L.)

Abstract: Metal oxide materials are known for their ability to store thermochemical energy through reversible redox reactions. Metal oxides provide a new category of materials with exceptional performance in terms of thermochemical energy storage, reaction stability and oxygen-exchange and uptake capabilities. However, these characteristics are predicated on the right combination of the metal oxide candidates. In this study, metal oxide materials consisting of pure oxides, like cobalt(II)

oxide, manganese(II) oxide, and iron(II, III) oxide (Fe3O4), and mixed oxides, such as (100 wt.% CoO, 100 wt.% Fe3O4, 100 wt.% CoO, 25 wt.% MnO + 75 wt.% CoO, 75 wt.% MnO + 25 wt.% CoO) and 50 wt.% MnO + 50.wt.% CoO), which was subjected to a two-cycle redox reaction, was proposed. The various mixtures of metal oxide catalysts proposed were investigated through the thermogravimetric Citation: Dessie, Y.G.; Hong, Q.; analysis (TGA), differential scanning calorimetry (DSC), energy dispersive X-ray (EDS), and scanning Lougou, B.G.; Zhang, J.; Jiang, B.; electron microscopy (SEM) analyses. The effect of argon (Ar) and oxygen (O2) at different gas flow Anees, J.; Tegegne, E.B. rates (20, 30, and 50 mL/min) and temperature at thermal charging step and thermal discharging step Thermochemical Energy Storage ◦ Performance Analysis of (30–1400 C) during the redox reaction were investigated. It was revealed that on the overall, 50 wt.% (Fe,Co,Mn)Ox Mixed Metal Oxides. MnO + 50 wt.% CoO oxide had the most stable thermal stability and oxygen exchange to uptake Catalysts 2021, 11, 362. https:// ratio (0.83 and 0.99 at first and second redox reaction cycles, respectively). In addition, 30 mL/min doi.org/10.3390/catal11030362 Ar–20 mL/min O2 gas flow rate further increased the proposed (Fe,Co,Mn)Ox mixed oxide catalyst’s cyclic stability and oxygen uptake ratio. SEM revealed that the proposed (Fe,Co,Mn)Ox material had Academic Editor: Leonarda a smooth surface and consisted of polygonal-shaped structures. Thus, the proposed metallic oxide Francesca Liotta material can effectively be utilized for high-density thermochemical energy storage purposes. This study is of relevance to the power engineering industry and academia. Received: 22 January 2021 Accepted: 8 March 2021 Keywords: thermochemical energy storage; (Fe,Co,Mn)Ox mixed oxide catalyst; energy density; Published: 10 March 2021 redox kinetic; cyclic stability

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- 1. Introduction iations. “Clean energy” is a major area of focus in this century. A related aspect is efficient energy storage development and utilization, which are necessitated by an increase in global energy demand and the deleterious effects of the use of fossil fuels to the environment. There is an array of renewable energy sources such as wind, tidal wave, biogas, hydro, and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. solar, which aim at providing electricity, with hydroelectric energy taking the lead. On This article is an open access article the other hand, solar energy integration with thermal energy storage is a very important distributed under the terms and technology, particularly when the cost of electric supply and demand increases. Thus, conditions of the Creative Commons thermal energy storage provides a means of continuous and stable energy supply. There Attribution (CC BY) license (https:// are different solar energy concentrators developed to receive solar energy directly from creativecommons.org/licenses/by/ the sun, or as radiation received by the heliostats, which is then reflected back to the 4.0/). solar receiver. In the latter case, the collected energy is taken to the energy storage reactor

Catalysts 2021, 11, 362. https://doi.org/10.3390/catal11030362 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 362 2 of 22

by means of heat transfer fluid (HTF). Energy storage can be achieved in different ways, such as sensible, latent, and phase change materials. Sensible energy storage depends on the internal energy of a material, which in turn depends on the material’s heat capacity. Latent heat energy storage is possible when the material undergoes a phase change such as freezing or melting [1,2], whereas phase change materials (PCM) are possible due to absorption or release of energy by the material. Thermal energy storage (TES) that is integrated with concentrated solar power (CSP) [3–6], can be used when variable energy or electricity demand is high. Thermochemical energy storage (TCES) is an attractive and alternative means of solar storage at higher temperatures (400–1200 ◦C). The high- temperature process associated with thermodynamic cycles facilitate the conversion of solar energy to electricity. TCES is advantageous in offering high energy density and a possibility of energy storage at room temperature using solid stable biomolecules. Pertaining to TCES application, there are oxides that were proven to show promising results for their energy storage capacity, such as metallic oxides [5,7–9], hydroxides, particularly calcium hydro oxides [10–16], and carbonate-made materials including calcium [17–19] and strontium carbonate [20,21]. However, these carbonates require structural stabilization to hinder sintering at high reaction temperature. Co3O4/CoO and Mn2O3/Mn3O4 are well known to be the most promising redox systems, while CuO/Cu2O also show good properties for TCES application [22–28]. Cobalt metallic oxides provide fast reaction activity, complete reversibility, and high energy density with cyclic stability and transition temperature in CSP application (892 ◦C). Studies have implemented cobalt metallic oxides with CSP integrated reactor as an energy storage unit. Sing et al. [29] developed a numerical model that simulated the asserting heat and mass transfer performance of a cobalt-based material as a thermochemical storage system. The aim of thermal energy storage application technology in solar thermal power system is to store solar energy as sensible, latent, and in PCM form or chemical forms, which can later be used for different energy demands particularly during low to non-sunshine hours. The dispatch ability [30] of TES and the nature of construction caused its demand to be increased. This study focused on the selection and characterization of materials with different proportion of metallic oxides that would be suitable for the development of thermochemical energy storage devices. The energy density capacity, cyclic stability, oxygen absorption, and release capacity of these were studied in the temperature range from 30 to 1400 ◦C and constant temperature supply was taken as the CSP power input. In the experimental analysis the endothermic and exothermic reaction with reversibility and cyclic stability were assessed. From Equations (1) and (2), heat was supplied to the compound to store solar energy in the form of chemical energy. When this system integrated with the CSP system, continuous energy production was possible in cloudy or dark skies [16–32]. The following Equations (1) and (2) can express the redox reactions mechanisms during charging–discharging processes. In the specific case of single metal oxides, air can be used as heat transfer fluid (Equations (1) and (2)), which allows operating with an open-loop system. The MO used in Equations (1) and (2) represents any metallic oxides.

MO(ox) + Heat → MO(red) + αO2(g) (1)

MO(red) + αO2(g) → MO(ox) + Heat (2) The use of metallic oxides as TES materials and air as heat transfer ﬂuid can be advantageous in an open-loop system of solar energy harassing technology. The development of thermochemical solar energy storage mechanism, which depends on pressurized air-based solar tower receivers has attracted the attention of researchers. CoO and Mn3O4 are the most used oxide materials for thermochemical energy storage. Existing studies show that Co3O4 undergoes fast kinetic reaction, complete reaction, cyclic stability and has a gravi- metric energy density of 576 kJ/kg [33] and a theoretical enthalpy of 844 kJ/kg [34–37]. However, the toxic nature cobalt oxides hamper their use. Studies have revealed that ◦ ◦ Mn2O3 occurs between 920–1000 C with notable slow re-oxidation between 850–500 C Catalysts 2021, 11, 362 3 of 22

and it has an energy storage density range 110–160 kJ/kg [38–40] and a theoretical enthalpy of 202 kJ/kg [39]. Generally, optimization of material reactivity particularly metallic oxides can be enhanced by adopting strategies with proper synthesis control techniques for tailored morphology or stabilization of inert materials to reduce sintering effect. Improvement of reaction kinetics, reaction turning, and flexibility of a material can be obtained by the addition dopants, particularly in the case of the oxides of manganese, which are known to slow down reaction cycles. Doping effects of transition metals are also an option to slow redox reaction [33,35]. The purpose of the present study is to investigate and identify the appropriate proportion of cobalt(II) oxide, manganese(II) oxide, and Fe3O4 that synthesizes a material with good cyclic stability, optimal redox reaction, and good oxygen absorption capacity, which are characteristics suitable for thermal energy storage purpose (TES). In this study, the best optimized metallic mixed oxides were tested and compared for their capability of energy density, the identification the optimized gas flow rates, the heat rates, cyclic stability and phase changing temperature were properly analyzed. These aforementioned parameters would assist in investigating how appropriate addition of transition metallic oxides im- proves the slow and the fast reaction to optimized [36] and sintering [41], loss-in-capacity over cycles [39] to decrease the reduction temperature lower so that at lower supply of energy can be effective for thermochemical energy storage capacity. Singh et al. [29] revealed that addition of iron on pure cobalt oxides or addition of cobalt oxides on iron slowed down chemical reaction kinetics of the mixed oxides and consequently lowered its enthalpy when compared to pure oxides. Pakoura et al. [40] that cobalt oxide with 10–20 wt.% of iron good thermo-mechanical stability over the temperature of redox cycles. Carrillow et al. [41] revealed that the iron does not prevent the sintering encountered by Mn2O3 but helps to increase the heat storage density of the material. Thus, the addition of iron helps to stabilize and enhance the oxidation rate of manganese oxide over repeating performance of redox cycles. According to their study, the fastest and most stable oxidation reaction was obtained by Mn2O3 doped 20 mol% Fe. In another study, the author’s also considered Fe-Cu Co-doping in manganese oxide and that the incorporation of Cu decreased reduction temperature. However, the doping of iron to copper found to increase the r-oxidation temperature. Recently models have been developed to provide thermodynamic descriptions of (Fe,Co,Mn)Ox [42–45] systems. Guene Lougou et al. [46–50] and Shuai [51,52] conducted studies on energy storage thermochemical reactor together with the synthesis of thermochemical energy storage material and Yabiabl et al. [53,54] studied about the impacts of thermochemical reactor designs for thermal energy storage and conversion of thermal efficiency. The researchers were able to produce a material that could resist the high-temperature thermal reduction super magnetic nanoparticles coated with aluminum (NiFe2O4@Alumina), (NiFe2O4@ZrO2), as well as support transits into new active phases including hercynite class materials (FeNiAlO4 and FeAlO4), Fe-oxide phases (Fe2O3, Fe3O4, and FeO) and NiO, (Ni,Fe), and AlNi phases. This present work is also aimed at oxygen storage and exchange capacity during mass loss and during redox reaction in the newly prepared (Fe,Co,Mn)Ox material. The uptake and release capability of oxygen in each redox reaction cycle has been studied and the phase, reaction enthalpy, phase change, or the transition temperature for each metallic oxides have been compared. In developing the newly synthesized material, six metallic oxides were subjected to thermogravimetric analysis (TGA), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and energy dispersive X-ray (EDS) analysis. In addition, different atmospheric argon gas flow rates (20, 30, and 50 mL/min), oxygen flow rates (20, 30, and 50 mL/min), and charging temperatures (30–1400 ◦C) were studied and their effects on the newly developed thermochemical energy storage material were elucidated. Catalysts 2021, 11, 362 4 of 22

2. Result and Discussion 2.1. The TGA of Metallic Oxides during Redox Reactions From Figure1a, all the TGA tested results are explained by weight loss % with time of charging and discharging temperature. Figure1 shows the two cycles that all six materials were heated from room temperature (30 → 1400 ◦C → 800 → 1400 →800 ◦C) and different results were observed during the test process. The results were identified using broken lines and stages. In the first stage, as the applied temperature increased for about 30 min, all materials lost weight except Fe3O4. In this stage, the loss was mainly due to the dehydration of water. When the temperature reached 1400 ◦C in the second stage from 30 to 45 min, except Fe3O4 which gained weight, other materials lost weight due to decomposition. In the third stage from 45 to 75 min, all materials reacted with the oxygen molecules and gained weight. The process of weight gain continued even when the temperature decreased from 1400 to 800 ◦C during the process of chemical reactions. The weight gained for 100 wt.% CoO and 25 wt.% MnO + 75 wt.% CoO were higher than other materials. In the fourth stage, from 75 to 105 min, while the temperature remained at 800 ◦C, the weight of 100 wt.% CoO and 25 wt.% MnO + 75 wt.% CoO increased while weights of other materials were constant. In the fifth stage, from 105 to 110 min, the materials were re-heated from 800 to 1400 ◦C and all materials lost weight and the process was endothermic. In the sixth stage, at 110 to 130 min, the materials were again cooled to 800 ◦C and the result indicated that all reactions gained weight. However, the weight gained by 100 wt.% Fe3O4, 100 wt.% CoO and 25 wt.% MnO + 75 wt.% CoO were greater than other materials, which underwent similar chemical reaction under the same conditions. As it can be seen in Figure1b for individual oxides, the reduction onset temperature for Fe-O was higher than CoO and MoO (1061 ◦C > 465–1360 ◦C > 200–723 ◦C). However, the decomposition percentage for weight loss of CoO was far greater than MnO and Fe-O (6.27% >3.7% > 0.31%). The re-oxidation onset temperature for MnO was a little bit greater than Fe-O but higher than CoO, which was (1399 ◦C > 1393 > 913), respectively. From Figure1b, it is possible to conclude that CoO needs a high temperature for reduction followed by Fe3O4 as compared to MnO. From Figure1c the 50 wt.% MnO + 50 wt.% CoO underwent for the two step redox reaction and Figure1c shows the TGA results for the mixed metallic oxides of 50 wt.% MnO + 50 wt.% CoO and 25 wt.% MnO + 75 wt.% CoO. After for 23 min, 25 wt.% MnO + 75 wt.% CoO recorded a higher weight loss than 50 wt.% MnO + 50 wt.% CoO. From 23 to 40 min of, the weight loss of 50wt.% MnO + 50 wt.% CoO was slightly higher than that of 25 wt.% MnO + 75 wt.% CoO. From 40 to 180 min for both processes as it was observed in Figure1c, 50 wt.% MnO + 50 wt.% CoO was more stable than 25 wt.% MnO + 75 wt.% CoO. Thus, from the abovementioned results and findings, 50 wt.% MnO + 50 wt.% CoO was found the most stable compared to other metallic oxides. Figure1c also depicted the temperature at thermal charging step and thermal discharging step during the process. Figure1d shows the TGA for all computed metallic oxides. From that Figure1d, Fe3O4 was the least decomposed; CoO underwent a single stage of decomposition, whereas others underwent multi-decomposition stages during the charging process in the first cycle redox reaction. From Figure1e, CoO underwent an almost single visible stage of reduction and a single stage of dehydration, whereas Figure1d indicated that MnO underwent multi-stage decomposition. Figure1e shows the iron reduction reaction events during cycle 1. During the process of Fe3O4 reduction, the dehydration led to 0.31% weight loss, which was smaller than those of MnO and CoO. In ◦ ◦ the charging of Fe3O4 to 1400 C, the weight kept increasing until about 1200 C. Figure1h depicted the 25 wt.% MnO + 75 wt.% CoO underwent for first step of redox reaction and depicted the 2.52% of the initial weight probably lost due to evaporation and 5.47% of the weight was due to decomposition. Figure1h depicted multi-stage decomposition. Figure1i depicted the TGA with temperature and the percentage loss for 75 wt.% MnO + 25 wt.% CoO were 5.565 in the first stage and 3.775 in the second stage. Figure1i showed the first stage of decomposition was higher than the second stage. This was due to the content of MnO being higher than CoO, and CoO is more vulnerable for decomposition Catalysts 2021, 11, x FOR PEER REVIEW 5 of 22

due to decomposition. Figure 1h depicted multi-stage decomposition. Figure 1i depicted the TGA with temperature and the percentage loss for 75 wt.% MnO + 25 wt.% CoO were Catalysts 2021, 11, 362 5 of 22 5.565 in the first stage and 3.775 in the second stage. Figure 1i showed the first stage of decomposition was higher than the second stage. This was due to the content of MnO being higher than CoO, and CoO is more vulnerable for decomposition than MnO. Figure 1j depictedthan MnO. the Figureredox 1forj depicted 50 wt.% theMnO redox + 50 forwt.% 50 wt.%CoO. MnOAs it has + 50 been wt.% seen CoO. from As Figure it has been1j the seenpercentage from Figure loss were1j the 3.91 percentage and 3.67 loss in the were first- 3.91 and and second-stage 3.67 in the first- decomposition, and second-stage respectively.decomposition, Figure 1k respectively. shows the TGA Figure for1k 25 shows wt.% theMnO TGA + 75 for wt.% 25 wt.% CoO MnOand 50 + wt.% 75 wt.% MnO CoO + 50and wt.% 50 CoO. wt.% From MnO +Figure 50 wt.% 1k, CoO.it was From noted Figure that 1chargingk, it was temperature noted that charging (applied temperature temperature)(applied from 30 temperature) to 1400 °C fromwith 30 to°C 1400/min ◦heatingC with 30rate◦C/min led to a heating significant rate reduction led to a significant of 30 mL/minreduction Ar and of/ 30min mL/min O2, 25 wt.% Ar and/min MnO + 75 O2 ,wt.% 25 wt.% CoO, MnO and + all 75 developed wt.% CoO, significant and all developed plat- eauingsignificant in reduction plateauing values in of reduction 50 wt.% valuesMnO + of 50 50 wt.% wt.% CoO MnO around + 50 wt.% 1000 CoO °C. around 1000 ◦C.

104 105 25MnO-75CoO 102 Stage-1 1061°C 50MnO-50CoO CoO 100 75MnO-25CoO 1393°C Stage-4 100 1.64% Fe O 98 Stage-2Stage-3 CoO 723°C 3 4 Ar Fe3O4 MnO 96 Mno 95 94 Ar Stage-5 6.27% O O2 92 2 °

Weight (%) 465 C ° 90 (%) Weight 90 660°C 890 C 88 3.7% 1360°C 86 Stage-6 85 84 Evaporation 1399°C Decomposion 82 Decomposion 80 0 20 40 60 80 100 120 140 160 180 0 25 50 75 100 125 150 175 Time (minutes) time (min) (a) (b) 100 102 50 wt. % MnO+50 wt. % CoO 1600 98

Temperature d i 1400 96 g s 99 g c n 25 wt. % MnO+75 wt.% CoO i n h i 94 g a r d g r r 1200 a i g s a i h c n 92 h 96 C h g a C r g 1000 90 in 93 g 800 88 Weight ( %) 86 w CoO Weight (%) Weight n e in i i a a 600 90 g w g 84 50 wt. % MnO+ 50 wt. % CoO t g t t e h lo h i g 75 wt.% MnO+ 25 wt. % CoO s ig g i 82 t e 400 s e Fe O 87 w l w 3 4 o 80 s s 200 25 wt. % MnO+ 75 wt.CoO 78 MnO 84 76 0 20 40 60 80 100120140160180 200 400 600 800 1000 1200 1400 Time (min) Temperature (°C) (c) (d) 102 100 CoO 100 3.05% MnO 1.38% 98 98 96 96 94 92 Weight (%) Weight

Weight (%) 94 6.27% 90 88 9.18% 92 86 3.26% 84 90 82 200 400 600 800 1000 1200 1400 ° ) 200 400 600 800 1000 1200 1400 Temperature ( C Temperature (°C) (e) (f)

Figure 1. Cont.

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101.0 100.8 100 25 wt.% MnO+ 75 wt.% CoO 100.6 98 100.4 Fe3O4 100.2 96 2.52% 100.0 0.31% Weight (%) 94

99.8 1.64% (%) weihgt 99.6 92 5.47% 99.4 90 99.2 99.0 88 200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400 ° ° Temperature ( C) Temperature ( C) (g) (h) 102 100 75 wt. % MnO+25 wt. % CoO 50 wt. % MnO+50 wt. % CoO 100 98 98 96 96 94 94 5.565% 3.91% 92 92 90 Weight (%)

Weight (%) 90 88 3.775% 3.67% 86 88 84 86 82 84 200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 1400 Temperature (°C) Temperature (°C) (i) (j) 102 50 wt. MnO+50 wt. % CoO 99 25 wt. MnO+75 wt. % CoO

96 Weight (%) 93

87 200 400 600 800 1000 1200 1400 Temperaure (°C) (k) ◦ FigureFigure 1. 1.TheThe TGA TGA on on mixed mixed and and pure pure metallic metallic oxides oxides at at 30 30 mL mL Ar/min Ar/min and and 20 20 mL mL O O2/min2/min gas gas flow flow rates rates and and 30 30 °C/minC/min heatingheating rate rate with with applying applying temperature temperature from from room room to to1400 1400 °C◦ Cand and cooling cooling to to800 800 °C◦ Cfor for two two step step redox redox reaction: reaction: (a) ( aTGA) TGA forfor two- two- step step cyclic cyclic redox redox reacti reactionon for for all all oxides oxides with with time; (b) TGATGA forfor purepure oxides oxides with with time; time; (c ()c TGA) TGA for for mixed mixed oxides oxides that that shows redox with time; (d) TGA for all oxides in the test with temperature in the first step of redox; (e) TGA for CoO shows redox with time; (d) TGA for all oxides in the test with temperature in the first step of redox; (e) TGA for CoO with with temperature in the first step of redox; (f) TGA for MnO with temperature in the first step of redox; (g) TGA for Fe3O4 temperature in the first step of redox; (f) TGA for MnO with temperature in the first step of redox; (g) TGA for Fe O with with temperature in the first step of redox; (h) TGA for 25 wt.% MnO + 75 wt.% CoO with temperature in the first 3step4 of redox;temperature (i) TGA infor the 75 firstwt.% step MnO of redox;+ 25 wt.% (h) TGACoO with for 25 temperature wt.% MnO +in 75 the wt.% first CoO step withof redox; temperature (j) TGA infor the 50wt.% first step MnO of + redox; 50 wt.%(i) TGACoO forwith 75 temperature wt.% MnO + in 25 the wt.% first CoO step with of redox; temperature (k) TGA in for the 50 first wt.% step MnO of redox; + 50 wt.% (j) TGA CoO for and 50 wt.%25 wt.% MnO MnO + 50 + wt.%75 wt.%CoO CoO with with temperature tempertaure. in the first step of redox; (k) TGA for 50 wt.% MnO + 50 wt.% CoO and 25 wt.% MnO + 75 wt.% CoO with tempertaure. 2.2. The Phase Change Conditions Using Weight Derivative Graphs (WDG) on Metallic Oxides during2.2. Thethe PhaseRedox ChangeReaction Conditions Using Weight Derivative Graphs (WDG) on Metallic Oxides during the Redox Reaction Figure 2a shows the derivative weight for all metallic oxides with charging tempera- Figure2a shows the derivative weight for all metallic oxides with charging temperature in the first cycle redox reaction. From that figure, it was noted that unlike iron oxide, ture in the first cycle redox reaction. From that figure, it was noted that unlike iron oxide, which had a phase change temperature occurring at >1200 °C, for other metallic oxides, which had a phase change temperature occurring at >1200 ◦C, for other metallic oxides, which were considered in this study, phase change temperature gaps were observed at

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which were considered in this study, phase change temperature gaps were observed at 400–600 °C and 750–1000 °C. Figure 2b shows the weight derivative of 75 wt.% MnO + 25 400–600 ◦C and 750–1000 ◦C. Figure2b shows the weight derivative of 75 wt.% MnO + wt.% CoO with room to 1400 °C in the first cycle, which revealed that phase changes oc- 25 wt.% CoO with room to 1400 ◦C in the first cycle, which revealed that phase changes curred at 528 °C and 801 °C, respectively. However, it was difficult to show the changing occurred at 528 ◦C and 801 ◦C, respectively. However, it was difficult to show the changing phase reaction products in the TGA analysis because it needs further investigation, like phase reaction products in the TGA analysis because it needs further investigation, like XRD tests for checking whether the phase changing temperatures, which were observed XRD tests for checking whether the phase changing temperatures, which were observed in TGA, belong to reactants products, and what types of products are observed. From in TGA, belong to reactants products, and what types of products are observed. From Figure 2c, the phase change of 50 wt.% MnO + 50 wt.% CoO was observed at 527 °C and Figure2c, the phase change of 50 wt.% MnO + 50 wt.% CoO was observed at 527 ◦C and 893 °C, but the reduction of the compound started at 400 °C and Figure 2c clearly shows 893 ◦C, but the reduction of the compound started at 400 ◦C and Figure2c clearly shows thethe stability stability of of the the redox redox reaction reaction in in both both the the second second and and first first steps steps of of the the process. process. From From FigureFigure 2d,2d, the the derivative derivative weight weight graph graph with with temperature temperature depicted depicted that that the the phase phase change change ofof 25 25 wt.% wt.% MnO MnO + +75 75 wt.% wt.% CoO CoO occurred occurred at at 902 902 °C◦C and and reduction reduction of of the the oxide oxide started started at at 688688 °C.◦C. The The phase phase change change indicating indicating the the critical critical curve curve for for 25 25 wt.% wt.% MnO MnO + +75 75 wt.% wt.% CoO CoO waswas sharper sharper than than that that of of 50 50 wt.% wt.% MnO MnO + +50 50 wt wt.%.% CoO, CoO, specifically specifically at at the the second second turning turning curve,curve, which which indicated indicated that that higher higher weight weight losses losses occurred occurred at at this this stage. stage. From From Figure Figure 2e,2e, thethe first first stage stage of of the the derivative derivative weight weight loss loss curve curve of of MnO MnO depicted depicted that that the the weight weight loss loss waswas due due to to evaporation evaporation of of water, water, the the second second events events were were observed observed at at 529 529 °C◦C and and 830 830 °C,◦C, respectively,respectively, which which were were the the phase phase change change temp temperatureseratures of of MnO. MnO. From From Figure Figure 2f,2f, the the first first stagestage of of the the reduction reduction step step of of CoO CoO curve curve re revealedvealed that that dehydration dehydration occurred. occurred. The The onset onset temperaturetemperature for for the the reduction reduction was was observed observed at at 723 723 °C◦C and and the the phase phase change change occurred occurred at at ◦ ◦ 845845 °C.C. From From Figure Figure 22g,g, inin the the first first stages stages from from 200 200 to 450 to 450C, Fe°C,3 OFe4 3lostO4 lost its weight its weight by 0.31%, by 0.31%,which which was also was seen also inseen Figure in Figure1g, and 1g, decomposition and decomposition of the ironof the oxide iron started oxide started around around1061 ◦ C1061 and °C changed and changed its phase its phase around around 1245 ◦ 1245C. From °C. From Figure Figure2b–d, the2b–d, derivative the derivative mixed mixedoxides oxides analysis analysis revealed revealed that the that reduction the reduction of 50 of wt.% 50 wt.% MnO MnO + 50 wt.%+ 50 wt.% CoO, CoO, 75 wt.% 75 wt.% MnO MnO+ 25 wt.%+ 25 wt.% CoO CoO and 25and wt.% 25 wt.% MnO MnO + 75 wt.%+ 75 wt.% CoO tookCoO placetook atplace 400 at◦C, 400 410 °C,◦C, 410 and °C, 688 and◦C, 688respectively. °C, respectively. From Figure From1 b,Figures under 1b, Section under 3.1 Section, it was seen3.1, it that was the seen reduction that the temperatures reduction temperaturesfor MnO and for CoO MnO occurred and CoO at the occurred same heating at the same and gases heating flow and rates gases (400 flow◦C andrates 723 (400◦C, °Crespectively), and 723 °C, respectively), which were similarwhich were to those simila ofr mixedto those oxides of mixed of 50 oxides wt.% of MnO 50 wt.% + 50 MnO wt.% + CoO.50 wt.% However, CoO. However, the reduction the onsetreduction temperature onset temperature for 50 wt.% for MnO 50 +wt.% 50 wt.% MnO CoO + 50 was wt.% less CoOthan was the less parent than oxides the parent (MnO oxides and CoO), (MnO which and CoO) was ,which about 420 was◦ Cabout as observed 420 °C as in observed Figure2c. inHowever, Figure 2c. other However, mixed other oxides mixed revealed oxides higher revealed onset higher reduction onset temperatures reduction temperatures particularly particularlyoxides with oxides more with proportion more proportion weight of CoO.weight Figure of CoO.2h showsFigure the2h shows derivative the derivative weight for weight50 wt.% for MnO 50 wt.% + 50 MnO wt.% + CoO50 wt.% and CoO 25 wt.% and MnO 25 wt.% + 75 MnO wt.% + CoO.75 wt.% As CoO. it has As been it has described been describedin Section in 3.1 Section Figure 3.11k, Figure Figure 1k,2h Figure also indicates 2h also thatindicates the 50 that wt.% the MnO 50 wt.% + 50 MnO wt.% + CoO 50 wt.% was CoOmore was stable more during stable the during reduction-oxidation the reduction-oxidation reactions. reactions.

0.01 0.00 75 wt. % MnO+25 wt. % CoO 0.00 410°C -0.02 -0.01 -0.02 -0.04 -0.03 810°C MnO ° 50 wt.% MnO+ 50 wt. % CoO -0.04 528 C

Derivative weight Derivative 25 wt.% MnO+75 wt. % CoO → -0.05 75 wt. % MnO+25 wt. % CoO Mn2O3+CO3O4+O2

-0.06 75 .wt.% MnO+25 wt. % CoO weight Derivative CoO -0.06 Fe3O4 -0.08 -0.07 200 400 600 800 1000 1200 -0.08 Temperature (°C) 200 400 600 800 1000 1200 (° Temperature C) (a) (b)

Figure 2. Cont.

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0.01 0.02 50 wt. % MnO+50 wt . % CoO ° 0.00 380°C 688 C 400°C 0.00 -0.01 MnO+CoO.nΗ2Ο ⋅ -0.02 893°C -0.02 25 wt. % MnO+75 wt. CoO nH2O 527°C

-0.03 Μn O +Co Ο4+Ο2→Mn O +CoO 2 3 3 3 4 902°C MnO+CoO→Μn O +Co O +O -0.04 ⋅ → 2 3 3 4 2 25 wt. % MnO+75 wt. % CoO nH2O Co3O4+Mn2O3+O2 Dervitave weight Derivative weight Derivative -0.04 Dervitave weights -0.05 -0.06 -0.06 200 400 600 800 1000 1200 200 400 600 800 1000 1200 ° Temperature (°C) Temperature ( C) (c) (d) 0.00 ° 0.04 Onset Red 400°C 693 C CoO 0.02 Onset Re 723°C 0.00 MnO.nH O -0.02 2 CoO⋅nH O 830°C -0.02 2 MnO→Mn O 2 3 -0.04

° weight Derivative Dervative weight% 529 C -0.06 845°C MnO→MnO -0.04 2 -0.08 → CoO Co3O4+O2 Dervative weight -0.10 200 400 600 800 1000 1200 200 400 600 800 1000 1200 Temperature (°C) Temperatur (° C) (e) (f) 0.00 0.04 t

Fe3O4 0.02 MnO+CoO.nH O 1061°C -0.02 2 → 0.00 MnO+CoO Mn2O3+Co3O4+O2 Fe O ⋅ nΗ O 2 4 2 1245°C → Mn2O3+Co2O3 +O2 Mn3O4+coO Fe2O3 -0.04 Derivative weigh Derivative -0.02 50 wt. % Mno+50 wt. % CoO Derivative weight Derivative 25 wt. % Mno+75 wt. % CoO -0.04 Phase changes points -0.06 200 400 600 800 1000 1200 1400 200 400 600 800 1000 1200 Temperature (°C) Temperature (°C) (g) (h)

Figure 2. Weight derivative graphs (WDG) for mixed and pure metallic oxides at 30 mL Ar/min and 20 mL O2/min gas Figure 2. Weight derivative graphs (WDG) for mixed and pure metallic oxides at 30 mL Ar/min and 20 mL O2/min gas flowﬂow rates rates subjected subjected to to heating heating from from 30 30 to to 1400 °C◦C (at (at 30 °C/min◦C/min heating heating rate) rate) step step 1 1 redox: redox: (a (a) )WDG WDG for for all all oxides oxides under under the test; (b) WDG for mixed oxides of 75 wt.% MnO + 25 wt.% CoO with temperature; (c) WDG for mixed oxides of 50 the test; (b) WDG for mixed oxides of 75 wt.% MnO + 25 wt.% CoO with temperature; (c) WDG for mixed oxides of 50 wt.% wt.% MnO + 50 wt.% CoO with temperature; (d) WDG for mixed oxides of 25 wt.% MnO + 75 wt.% CoO with temperature; MnO + 50 wt.% CoO with temperature; (d) WDG for mixed oxides of 25 wt.% MnO + 75 wt.% CoO with temperature; (e) t WDG for MnO with temperature; (f) WDG for CoO with temperature; (g) WDG for Fe3O4CoO with temperature; (h) WDG(e) t WDG t for mixed for MnO oxides with of temperature; 25 wt.% MnO (f )+ WDG75 wt.% for CoO CoO and with 50 temperature;wt.% MnO + (50g) wt.% WDG CoO for Fewith3O 4temperatureCoO with temperature; in the first step(h) WDGof redox. t for mixed oxides of 25 wt.% MnO + 75 wt.% CoO and 50 wt.% MnO + 50 wt.% CoO with temperature in the ﬁrst step of redox. 2.3. The Thermal Performance and the Oxygen Uptake of Metallic Oxides 2.3. The Thermal Performance and the Oxygen Uptake of Metallic Oxides 2.3.1. The Thermal Performance of Metallic Oxides 2.3.1. The Thermal Performance of Metallic Oxides Figure 3a shows the enthalpy accumulated for two-step cyclic redox reaction of all Figure3a shows the enthalpy accumulated for two-step cyclic redox reaction of all the the metallic oxides. During the charging and discharging process Fe3O4, MnO, and 50 wt. metallic oxides. During the charging and discharging process Fe O , MnO, and 50 wt.% % MnO + 50 wt. % CoO had large enthalpies, particularly during3 discharging,4 50 wt. % MnO + 50 wt.% CoO had large enthalpies, particularly during discharging, 50 wt.% MnO + MnO + 50 wt. % CoO had a higher enthalpy than other pure and mixed metallic oxides.

Catalysts 2021, 11, 362 9 of 22 Catalysts 2021, 11, x FOR PEER REVIEW 9 of 22

50 wt.% CoO had a higher enthalpy than other pure and mixed metallic oxides. Figure3b–d showsFigure the3(b–d) conversion shows the rate conversion efficiency ofrate the effici metallicency oxides.of the metallic Figure3 boxides. shows Figure the conversion 3b shows ratethe conversion of the metallic rate oxides of the during metallic the oxides heating during from roomthe heating to temperature from room 1400 to◦ Ctemperature in the first cycle,1400 °C of redoxin the reaction.first cycle, Figure of redox3c shows reaction. conversion Figure rates 3c shows of all theconversion metallic oxidesrates of during all the themetallic entire oxides cycle redoxduring reaction the entire with cycle time redox and Figurereaction3d with shows time the and conversion Figure 3d rate shows and the the TGAconversion of 50 wt.% rate and MnO the + TGA 50 wt.% of 50 CoO wt.% in MnO the first + 50 cycle wt.% of CoO the in redox the first reaction. cycle of Thus, the redox from Figurereaction.3b–d, Thus, 50 wt.% from MnO Figure +50 3b–d, wt.% 50 CoO wt. had% MnO stable + conversion50 wt. % CoO effect had in thestable redox conversion reaction whileeffect CoOin the had redox unstable reaction conversion while CoO rate had in both unstable cycles conversion of the redox rate reaction. in both Table cycles1 shows of the theredox onset reaction. temperature Table 1 for shows reduction, the onset conversion, temperature enthalpy, for reduction, and phase conversion, change temperature enthalpy, ofand all phase the metallic change oxidestemperature during of the all chargingthe metall stageic oxides in the during first cycle. the charging From that stage table, in the 50 wt.%first cycle. MnO From + 50 wt.% that CoOtable, had 50 wt. the % highest MnO conversion+ 50 wt. % CoO andiron had oxidethe highest had the conversion highest phase and changeiron oxide temperature. had the highest Table2 phase shows change the specific temp energyerature. contents Table 2 forshows all thethe metallicspecific oxidesenergy atcontents 952 ◦C for in bothall the the metallic first and oxides second at 952 reaction °C in steps. both the From first Table and2 ,second at 952 reaction◦C in the steps. first cycleFrom redoxTable reaction,2, at 952 MnO,°C in the CoO, first 50 cycl wt.%e redox MnO +reaction, 50 wt.% MnO, CoO, andCoO, Fe 503O wt.4 had % reductionMnO + 50 towt. oxidation % CoO, and percentage Fe3O4 had ratio reduction of 24.2%, to oxidation 17.7%, 16.7%, percentage and 16%, ratio respectively, of 24.2%, 17.7%, whereas 16.7%, 50 wt.%and 16%, MnO respectively, + 50 wt.% CoO, whereas MnO 50 and wt. Fe %3O MnO4 had + reduction 50 wt. % toCoO, oxidation MnO and of specific Fe3O4 had energy re- densityduction ratioto oxidation 41.5%, 55.2%, of specific 48.2%, energy respectively density duringratio 41.5%, the second 55.2%, redox48.2%, cycle. respectively From Table dur- 2ing, it wasthe second observed redox that thecycle. second From cycle Table redox 2, it reaction was observed had more that energy the second exchange cycle than redox the firstreaction had andhad 25more wt.% energy MnO exchange + 75 wt.% than CoO the had first the leasthad and specific 25 wt.% energy MnO density + 75 ratiowt.% at CoO 952 ◦ hadC (6.7 the kJ/g least and specific 23.1 kJ/g)energy in density the first ratio and at second 952 °C cycle, (6.7 kJ/g respectively. and 23.1 FigurekJ/g) in3 ethe shows first theand endothermicsecond cycle, and respectively. exothermic Figure phases 3e of shows 50 wt.% the MnO endothermic + 50 wt.% and CoO exothermic during redox phases reaction of 50 inwt. the % firstMnO cycle. + 50 wt. % CoO during redox reaction in the first cycle.

200 100 Fe3O4 MnO 150 CoO 50 75 wt. % MnO +25 wt. % CoO 25 wt. % MnO +75 wt. % CoO 100 50 wt. % MnO +50 wt. % CoO 0 50 Enyhalpy (kJ/g) Enyhalpy

Fe O (%) Conversion -50 3 4 MnO CoO 75 wt. % MnO+ 25 wt.% CoO 0 50 wt. % MnO+50 wt. % CoO -100 25 wt. % MnO+75 wt. % CoO -50 0 30 60 90 120 150 180 200 400 600 800 1000 1200 1400 Time (min) Temerature (°C) (a) (b) 7.6 200 weight loss 50 wt. % MnO+50 wt. % CoO conversion 50 wt. % MnO+50 wt. % CoO 100 7.4 150 80 100 7.2 Fe O 3 4 60 50 MnO 7.0 CoO 0 75 wt. % MnO + 25 wt. % CoO 6.8 40

25 wt. % MnO + 75 wt. % CoO Weigtht (mg) 50 wt. % MnO + 50 wt. % CoO Conversion (%) -50 20 6.6 (%) rate Conversion -100 6.4 0 -150 0 50 100 150 200 400 600 800 1000 1200 1400 Time (min) ° Temperature ( C) (c) (d)

Figure 3. Cont.

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C) 16 ×° DSC 8

Exo of 50%Wt Mno +50%Wt CoO -8

Endo of 50%Wt Mno +50%Wt CoO -16 700 750 800 850 900 950 1000 0

Specific heat capacity (J/g capacity heat Specific Temperature ( C) 250 500 750 1000 1250 Temperature (°C) (e)

Figure 3. The thermal performance ofof mixedmixed andand purepure oxideoxide duringduring thethe experimentexperiment at at 30 30 mL mL Ar/min Ar/min andand 2020 mLmL OO22/min/min gas flow ﬂow rates rates subjected subjected to to heating heating from from 30 30 to to1400 1400 °C◦ atC 30 at °C/min 30 ◦C/min heating heating rate and rate cooling and cooling to 800 to °C 800 for◦ twoC for-cyclic two-cyclic redox reactions:redox reactions: (a) The (a oxygen) The oxygen uptake uptake ratio in ratio both in cyclic both cyclic redox redox reactions; reactions; (b) the (b )enthalpy the enthalpy for mixed for mixed all oxides all oxides under under test; test; (c) the conversion performance of all oxides under state (d) conversion performance for all oxides under the test with time; (c) the conversion performance of all oxides under state (d) conversion performance for all oxides under the test with time; (e) specific heat capacity for 50 wt.% MnO + 50 wt.% CoO with temperature. (e) speciﬁc heat capacity for 50 wt.% MnO + 50 wt.% CoO with temperature. Table 1. Physical properties during charging from 25 to 1400 °C with rounding to three digit [55]. Table 1. Physical properties during charging from 25 to 1400 ◦C with rounding to three digit [55]. Onset Temperature Enthalpy (J/g) Phase Changing Material Onset Conversion Reduction (°C) CrystallizationEnthalpy (J/g)Melting TemperaturePhase Changing (°C) Material Temperature Conversion 50 wt.% MnO + 50 wt.% CoO 406 92.340 28 30 Temperature530–895 (◦C) Reduction (◦C) Crystallization Melting 25 wt.% MnO + 75 wt.% CoO 395 87.500 No observation 403 518–905 50 wt.% MnO + 50 wt.% CoO 406 92.340 28 30 530–895 75 wt.% MnO + 25 wt.% CoO 425 87.600 13 8 529–808 25 wt.% MnO + 75 wt.% CoO 395 87.500 No observation 403 518–905 75 wt.% MnOCoO + 25 wt.% CoO720 425 86.00 87.600 No observation 13 284.70 8 529–808850 MnOCoO 400 720 83.800 86.00 No observation 284.7033 525 850–834 FeMnO3O4 1063 400 90.400 83.800 No observation 3328 525–8341245 However,Fe 3theO4 standard enthalpy values1063 for iron(II, III) 90.400oxide, Mn(II) oxide No observation and Co(II) are −1118.4, 28 −248.9 and −237.9 1245 kJ respectively.

Table 2. SpeciﬁcSpecific energy content inin kJ/gkJ/g of of reduction reduction (charging) (charging) and oxidation (discharging) steps of the six materials.

◦ EnergyEnergy ContentContent inin kJ/gkJ/g atat 952952 °CC O2O/O2/O2 2 RatioRatio of Energy of Energy Uptake Uptake to Energy to MaterialMaterial CycleCycle 1 1 CycleCycle 2 2 ◦ EnergyRelease Release at 952 atC 952 in °C % in % ReductionReduction Oxidation Oxidation ReductionReduction OxidationOxidation CycleCycle 1 1 CycleCycle 22 5050 wt.% wt.% MnO MnO + +50 50 wt.% wt.% CoO CoO 4.4 4.4 26.300 26.300 23.80023.800 57.30057.300 16.70016.700 41.50041.500 2525 wt.% wt.% MnO MnO + +75 75 wt.% wt.% CoO CoO 2.2 2.2 33 33 16.40016.400 7171 6.7006.700 23.10023.100 7575 wt.% wt.% MnO MnO + +25 25 wt.% wt.% CoO CoO 4.2 4.2 32.400 32.400 25.50025.500 7373 12.96012.960 34.90034.900 CoOCoO 6.4 6.4 36.700 36.700 23.90023.900 80.9080.90 17.40017.400 29.50029.500 MnOMnO 6 627.300 27.300 3232 5858 24.20024.200 55.20055.200 Fe3O4 6.4 40 39.500 82 16 48.200 Fe3O4 6.4 40 39.500 82 16 48.200

2.3.2.However, The Oxygen the Storage standard Capacity enthalpy of valuesMetallic for Oxides iron(II, III) oxide, Mn(II) oxide and Co(II) are −F1118.4,igure.4 −shows248.9 the and efficiency−237.9 kJ for respectively. oxygen uptake and release ratio during redox reactions. The uptake and release of oxygen by all the metallic oxides during the two-cycle re- 2.3.2. The Oxygen Storage Capacity of Metallic Oxides dox reaction derived from TGA are shown in Table 3. Fe3O4, 50 wt.% MnO + 50 wt.% CoO, and 25Figure wt.%4 MnOshows + the75 wt.% efficiency CoO forhad oxygen oxygen uptake uptake and values release of ratio2.1, 0.830, during and redox 0.760, reac- re- spectivelytions. The in uptake the first and cycle release and of 0.997, oxygen 0.996, by alland the 0.960, metallic respectively oxides during in the thesecond two-cycle cycle. re-dox reaction derived from TGA are shown in Table3. Fe O , 50 wt.% MnO + 50 wt.% Increase in oxygen uptake implies increase in energy storage.3 Thus,4 this finding indicates CoO, and 25 wt.% MnO + 75 wt.% CoO had oxygen uptake values of 2.1, 0.830, and 0.760, that the aforementioned materials had good energy storage efficiency. For the mixed ox- respectively in the first cycle and 0.997, 0.996, and 0.960, respectively in the second cycle. ides, 50 wt.% MnO + 50 wt.% CoO was found to have good thermochemical energy stor- Increase in oxygen uptake implies increase in energy storage. Thus, this finding indicates age capacity as its oxygen uptake to release ratio values in both cycles were 0.830 and

Catalysts 2021, 11, x FOR PEER REVIEW 11 of 22

0.996. For the pure oxides, iron oxides had the highest oxygen uptake in both cycles (2.1 and 0.997). This result related to the findings in sections 3.1 and 3.2, which revealed that the mass of iron oxide increased during the charging stage. Figure 3 shows the efficiency of oxygen uptake capacity of the pure metallic and mixed oxides. According to that figure and Table 2, iron oxide had the highest oxygen uptake in the first cycle. As revealed, oxygen uptake capacity ratio of 0.86, and 0.96 in the first and second cycles, respectively. Thus, 50 wt.% MnO + 50 wt.% CoO was selected for the next stage, which involved both stability Catalysts 2021, 11, 362 11 of 22 and energy capacity analysis, and Fe3O4 had the oxygen uptake to oxygen release capacity ratio (2.1, 0.997) cycle 1 and cycle 2 during redox, respectively.

Table 3. Thethat oxygen the aforementioned uptake and release materials ratio metallic had good oxides energy during storage redox efficiency. reaction. For the mixed oxides, 50 wt.% MnO + 50 wt.% CoO was found to have good thermochemical energy storage O2 Released (μmol/g) O2 Uptake (μmol/g) O2 Uptake/O2 Released Ratio Materials 1st capacityCycle as its2nd oxygen Cycle uptake 1st Cycle to release ratio2nd Cycle values in both 1st Cycle cycles were 0.830 2nd Cycle and 0.996. 25 wt.% Mno + 75 wt.% For the pure oxides, iron oxides had the highest oxygen uptake in both cycles (2.1 and 2125 1625 1625 1563 0.760 0.960 CoO 0.997). This result related to the findings in Sections 3.1 and 3.2, which revealed that the 50 wt.% Mno + 50 wt.% mass of iron oxide increased during the charging stage. Figure3 shows the efficiency of 2250 1843 1875 1819 0.830 0.990 CoO oxygen uptake capacity of the pure metallic and mixed oxides. According to that figure and 75 wt.% Mno + 50 wt.% Table2, iron oxide had the highest oxygen uptake in the first cycle. As revealed, oxygen 3972 1569 1650 1561 0.420 0.996 CoO uptake capacity ratio of 0.86, and 0.96 in the first and second cycles, respectively. Thus, 100 wt.% CoO 203250 wt.% MnO + 1413 50 wt.% CoO 1475was selected for 1238 the next stage, 0.730 which involved both 0.880 stability 100 wt.% Fe3O4 516and energy capacity1066 analysis,1097 and Fe3O4 had1063 the oxygen uptake2.100 to oxygen release 0.997 capacity 100 wt.% Mno 1372ratio (2.1, 0.997) 391 cycle 1 and cycle 469 2 during redox, 375 respectively. 0.340 0.960

St 2.5 1 cyclic Redox O2 uptake to O2 release nd 2 cyclic redox O2 uptake to O2 release 2.0 A=25 wt. MnO+75 wt. % CoO B=50 wt. % MnO+50 wt. % CoO C=75 wt. % MnO+50 wt. % CoO 1.5 D=100 wt. % CoO E=100 wt. % Fe3O4 F=100 wt. % MnO

Efficiency 1.0

0.5

0.0 ABCDEF Material

Figure 4. TThehe oxygen uptake ratioratio inin bothboth cyclic cyclic redox redox reactions reactions at at 30 30 mL mL Ar/min Ar/min and and 20 20 mL mL O 2/min ◦ ◦ Ogas2/min ﬂow gas rates flow subjected rates subjected to heating to heating from 30from to 140030 to 1400C at °C 30 atC/min 30 °C/min heating heating rate rate and and cooling cool- to ing800 to◦C 800 for °C two-cyclic for two-cyclic redox redox reactions. reactions.

Table 3. 2.4.The oxygenThe Impact uptake of Gas and Flow release Rate ratio on metallicthe Redox oxides Reaction during redox reaction. From Table 4 the (Fe,Co,Mn)Ox material was tested for different Ar and O2 flow rates O2 Released (µmol/g) O2 Uptake (µmol/g) O2 Uptake/O2 Released Ratio Materials during the heating–cooling processes of the redox reaction. Consequently, 30 mL Ar-20 mL1st O2/ Cyclemin gas flow 2nd Cyclerate was appropriate 1st Cycle for 2nd stable Cycle and high 1st Cycleoxygen uptake. 2nd Figure Cycle 5a 25 wt.% Mno + 75 wt.% CoOshows 2125 the impacts of 1625 applied the 1625different combined 1563 argon and 0.760 oxygen gases flow 0.960 rate on 50 wt.% Mno + 50 wt.% CoO(Fe,Co,Mn)Ox. 2250 From 1843 that Figure 18755a, it was observed 1819 that the cyclic 0.830 reduction reaction 0.990 elic- 75 wt.% Mno + 50 wt.% CoOited by 3972 30 mL Ar-20 1569 mL O2/min 1650gas flow rate 1561had great contributions 0.420 to oxygen 0.996 uptake. 100 wt.% CoO 2032 1413 1475 1238 0.730 0.880 Figure 5b shows the influence of Ar and O2 flow rates on redox reaction kinetics. From 100 wt.% Fe3O4 516 1066 1097 1063 2.100 0.997 100 wt.% Mnothat figure, 1372 it was revealed 391 that keeping 469 Ar gas 375flow rate constant 0.340 and altering the 0.960 oxygen flow resulted in constant O2 uptake in cycle 1 and slightly constant uptake in cycle 2. Fig- ure 5c shows the DSC analysis of (Fe,Co,Mn)Ox redox with time and the endothermic- exothermic2.4. The Impact thermal of Gas process Flow Rate along on thewith Redox phase Reaction change of temperature at 30 mL Ar/min-20 mL OFrom2/min Tablegas flow4 the rate. (Fe,Co,Mn)Ox Figure 5d materialshows the was weight tested loss for differentduring redox Ar and reaction O 2 flow for rates differentduring values the heating–cooling Ar and O2 flow processes rates during of the the redox charging reaction. step. Consequently,From Figure 5d, 30 the mL 30 Ar-20 mL Ar-20mL O 2mL/min O2/ gasmin flow combined rate was gas appropriate flow rates on for (Fe,Co,Mn)Ox stable and high increased oxygen uptake.weight loss Figure when5a shows the impacts of applied the different combined argon and oxygen gases flow rate on (Fe,Co,Mn)Ox. From that Figure5a, it was observed that the cyclic reduction reaction elicited by 30 mL Ar-20 mL O2/min gas flow rate had great contributions to oxygen uptake. Figure5b shows the influence of Ar and O 2 flow rates on redox reaction kinetics. From that figure, it was revealed that keeping Ar gas flow rate constant and altering the oxygen flow resulted in constant O2 uptake in cycle 1 and slightly constant uptake in cycle 2. Figure5c shows the DSC analysis of (Fe,Co,Mn)Ox redox with time and the endothermic-exothermic thermal process along with phase change of temperature at 30 mL Ar/min-20 mL O2/min gas flow rate. Figure5d shows the weight loss during redox reaction for different values Ar and O2 flow rates during the charging step. From Figure5d, the 30 mL Ar-20 mL O 2/min Catalysts 2021, 11, 362 12 of 22

combined gas flow rates on (Fe,Co,Mn)Ox increased weight loss when compared with other combined gas flow rates. Figure5e shows the weight loss for four types of combined Catalysts 2021, 11, x FOR PEER REVIEW 12 of 22 Ar- O2 flow rates for both cycles. Figure5f shows the heat flow curve with the charging temperature. From Figure5e,f, it was revealed that 30 mL Ar-20 mL O 2/min gas flow rate depicted stable redox reaction for the (Fe,Co,Mn)Ox material. compared with other combined gas flow rates. Figure 5e shows the weight loss for four types of combined Ar- O2 flow rates for both cycles. Figure 5f shows the heat flow curve withTable the charging 4. Gas flow temperature. rate and From O2 storage Figure capacity.5e,f, it was revealed that 30 mL Ar-20 mL O2/min gas flow rate depicted stable redox reaction for the (Fe,Co,Mn)Ox material. Materials O2 RELEASED (µmol/g) O2 Uptake (µmol/g) O2 Uptake/O2 Released Ratio Table 4. Gas flow rate and O2 storage capacity. Argon and O2 Flow 1st Cycle 2nd Cycle 1st Cycle 2nd Cycle 1st Cycle 2nd Cycle 20–20Materials 1017O2 RELEASED (μmol/g) 338 O2 Uptake 188 (μmol/g) 159O2 Uptake/O 18.4402 Released Ratio 47.200 Argon and O2 Flow 1st Cycle 2nd Cycle 1st Cycle 2nd Cycle 1st Cycle 2nd Cycle 30–20 1097 388 404 327 36.810 84.270 20-20 1017 338 188 159 18.440 47.200 30–3030-20 11591097 393388 404 293 327 18736.810 25.35084.270 47.690 30–5030-30 11291159 463393 293 314 187 24325.350 27.81047.690 54.465 30-50 1129 463 314 243 27.810 54.465

100 100 Cycl-1 Oxidation-Reduction Cycle-1 90 Cycl-2 Oxidation-Reduction 80 Cycle-2 80 60 70 60 40 50 40 20 30 Ratio oxidation to reduction Ratio Oxgen uptake efficiency (%) 0 20 20-20 30-20 30-30 30-50 20-20 30-20 30-30 30-50 50-50 Ar-O flow rate (ml/min) 2 Flow rate Ar with O2 (a) (b) 0.0 101 100 Exo -0.5 99

g) ⋅ 757° C 98 -1.0 589° C 97 22.5 (J/g) 96 Weight (%) DSC (J/s DSC (30Ar-30O )Nml/min -1.5 60.5 (J/g) 95 2 Endo 44 (J/g) (20Ar-20O )Nml/min ° 94 2 480 C (30Ar-50O )Nml/min 93 2 -2.0 (30Ar-20O2)Nml/min 92 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 Time (s) ° Temperature ( C) (c) (d) 30Ar ml/min,20O ml/min+20Ar ml/min 100 2 20Ar ml/min,20O2ml/min+20Ar m/ml

30Ar ml/min,30O2Nml/min+20Ar m/ml 0

98 50Ar ml/min,50O2 ml/min+20Ar m/ml

-1 96

Weight (%) 94 -2 300C/min

Smooth DSC (mw/mg) DSC Smooth 200C/min 92 100C/min -3 0 30 60 90 120 150 180 0 150 300 450 600 750 900 Time (s) Temperature (°C) (e) (f)

Figure 5. The impacts of different Ar and O2 combined flow rates on the redox reaction for the (Fe,Co,Mn)Ox: (a)O2 uptake efficiency with Ar-O2 flow rates; (b) impact of gas flow rates on redox reaction; (c) DSC analysis with phase change temperature picks and enthalpies; (d) the impact of gas flow rates on weight loss during heating in the first cycle, and (e) the weight loss with time for both cyclic redox reactions including the impacts additional 20 mL/min Ar protective gas; (f) smooth DSC analysis in terms of heat flow curve against the variation in charging temperature. Catalysts 2021, 11, 362 13 of 22

2.5. The Effect of Heating Rate on Redox Reaction Heating rate is one of the determinant factors that affect the redox reaction of metallic oxides. Figure6a shows the smooth DSC analysis of the effect of different heating rates on the (Fe,Co,Mn)Ox material as a function of time. As observed from Figure6a, during the charging process, heating rate at 30 ◦C/min that was applied during redox shifts to negative than other heating rates at the time of 87 min and the flow heat increase sharply at 100 min. The redox reaction rate at 10 ◦C/min dominated that of 20 ◦C/min for about 143 min. However, from 143 to 243 min reaction time, the redox reaction at 20 ◦C/min dominated that of 10 ◦C/min heating rate. From 243 to 300 min reaction time, redox reaction at 20 ◦C/min heating rate was lower than that of 10 ◦C/min heating rate. Since 30 ◦C/min heating rate has impacted the TGA machine, there had been additional 20mL/min argon flow during charging at 1400 ◦C and discharging at 800 ◦C. However, the addition of Ar gas affected the decomposition rate of the (Fe,Co,Mn)Ox material, as it shifted the enthalpy to the negative region as shown in Figure6b. From Figure6b, comparing the heat flow rates (10, 20, 30 ◦C/min) at 951 ◦C, the (Fe,Co,Mn)Ox material had 1.6 kJ/g, −0.99kJ/g, and −4.65 kJ/g, respectively. From 400 to 1400 ◦C during charging, the 20 ◦C/min heating rate affected the metallic oxides smoothly and without altering enthalpy, whereas 10 ◦C/min heating rate diverged to the highly positive enthalpy region and 30 ◦C/min rating diverged to the highly negative enthalpy region. Figure5c shows the derivative weight during the charging process from 30 to 1400 ◦C. From that Figure5c, the phase change indicator temperature is shown on the critical curves for the different values of heating rates. For 30 ◦C/min heating rate case, the phase change values temperature for the redox were 261, 778, and 1160 ◦C; at 20 ◦C/min heating rate the values were 371, 822, and 1112 ◦C, and at 10 ◦C/min heating rate for the values were 324, 800, and 1077 ◦C. Thus, the phase change temperature values for the metallic oxides subjected to 20 ◦C/min heating rate were generally higher than those at 10 ◦C/min heating rate. In the early stage of the redox reaction, the onset temperature corresponded to heating rate in the order 30 < 10 < 20 ◦C/min. Figure5e shows the weight loss with corresponding charging temperature-axis and it revealed that decomposition reaction at 30 ◦C/min heating rate was higher than those of 10 ◦C/min and 20 ◦C/min heating rates. Figure6d shows the weight loss with corresponding charging temperature-axis and it revealed that decomposition reaction at 30 ◦C/min heating rate was higher than those of 10 ◦C/min and 20 ◦C/min heating rates. Figure6e shows the conversion rate for the (Fe,Co,Mn)Ox material at different heating rates. It revealed that increase in temperature at 30 ◦C/min heating rate attained a conversion rate of 1.25 at 964 ◦C, which sharply decreased to 1 at 1029 ◦C. Figure6f shows the conversion rate the whole redox reaction process. It revealed that conversion rate increased sharply for about 91 min of the redox process started, and sharply decreased when it reached at 100 min then became constant until to 236 min, increased at 258 min and decreased at 266 min and then became constant afterwards. It was also be deducted from that figure that the conversion rates during 10 ◦C/min and 20 ◦C/min heating rates overlapped except between 206 to 208 min. The conversation rate during 10 ◦C/min heating rate was higher than that during 20 ◦C/min heating rate. In addition, from 287 to 320 min, the conversion rate during 20 ◦C/min heating rate was higher than that during 10 ◦C/min heating rate. Catalysts 2021, 11, 362 14 of 22 Catalysts 2021, 11, x FOR PEER REVIEW 14 of 22

12 10 30°C/min 8 5 20°C/min °C, 1.6kJ/g) 10°C/min °C, -0.99 (951 951 4 0 C,-4.65) (951° 0 -5

-4 30°C/min Enthalpy(kJ/g) -10 20°C/min Smooth DSC (mW/mg) DSC Smooth -15 -8 10°C/min 0 50 100 150 200 250 300 -20 Time (min) 200 400 600 800 1000 1200 1400 Temperature (°C)

(a) (b) 30°C/min,5.887mg 20°C/min (964, 1.25) 0.004 20°C/min,6.965mg 1.2 10°C/min,6.8405mg 10°C/min 30°C/min 0.000 822°C 1160°C (1029, 1) 0.8 -0.004 DTG 0.4 -0.008 Conversionrate

261°C 800°C -0.012 324°C 371°C 778°C 0.0 1077°C 1112°C 200 400 600 800 1000 1200 250 500 750 1000 1250 Temperature (°C) Temperature (°C) (c) (d) 101 10°C/min 100 1.2 20°C/min 99 30°C/min 98 0.8 97 20°C/min 0.4 Weight (%) Weight 96 10°C/min Conversionrate 30°C/min 95 94 0.0 93 200 400 600 800 1000 0 50 100 150 200 250 300 Temperature ( ° C) Time (min)

(e) (f)

FigureFigure 6. The 6. The impact impact of heatof heat ﬂow flow rate rate during during redox redox reactions: reactions: (a )(a Changes) Changes in in smooth smooth DSC DSC with with time; time; ((bb)) changeschanges inin enthalpyen- thalpy with charging temperature in cycle 1; (c) phase change indicator graph during cycle 1; (d) changes in conversion with charging temperature in cycle 1; (c) phase change indicator graph during cycle 1; (d) changes in conversion rate with rate with charging temperature in cycle 1; (e) changes in TGA with charging temperature in cycle 1 and (f) changes in chargingconversion temperature rate during in cycle the whole 1; (e) changescharging– indischarging TGA with time charging for both temperature redox cycles. in cycle 1 and (f) changes in conversion rate during the whole charging–discharging time for both redox cycles. 2.6. The EDS and SEM analysis of the (Fe,Co,Mn)Ox Material 2.6.EDS The of EDSthe (Fe,Co,Mn)Ox and SEM analysis Material of the (Fe,Co,Mn)Ox Material EDS of the (Fe,Co,Mn)Ox Material Figure 7a shows the EDS spectrum of the reduced elements of O, Mn, Fe, and Co with weightFigure percentages7a shows of the 41.78%, EDS spectrum10.14%, 38.23%, of the and reduced 9.84% and elements element of compositions O, Mn, Fe, and of Co with71.59%, weight 5.06%, percentages 18.77%, and of 4.58%, 41.78%, respectively. 10.14%, 38.23%, Excluding and impurities 9.84% and and element oxygen, compositions Figure of7b 71.59%, shows the 5.06%, reduced 18.77%, EDS spectru and 4.58%,m of Mn, respectively. Fe, and Co with Excluding weight impuritiespercentages and17.36%, oxygen, Figure65.57%,7b andshows 17.06% the element reduced compositions EDS spectrum of 17.76%, of Mn, 65. Fe,97%, and and Co with16.27%, weight respectively. percentages 17.36%,Figure 65.57%,7c shows andthe EDS 17.06% mapping element images compositions of the reduced of 17.76%,(Fe,Co,Mn)Ox 65.97%, oxide and and 16.27%, Figures respectively. Figure7c shows the EDS mapping images of the reduced (Fe,Co,Mn)Ox oxide and Figure7d–g show the EDS mapping pictures of O, Mn, Fe, and Co. From the EDS mapping images, there were black spots seen on the surface of the material, which was created due to the interaction of the applied ﬁeld and missing the electrons atoms of that element. During Catalysts 2021, 11, x FOR PEER REVIEW 15 of 22

Catalysts 2021, 11, 362 15 of 22

7d–g show the EDS mapping pictures of O, Mn, Fe, and Co. From the EDS mapping images, there were black spots seen on the surface of the material, which was created due to ◦ ◦ thethe redox interaction reaction of withthe applied applied field temperature and missing at the thermal electrons charging atoms of step that of element. 30 C to During 1400 C, thethe added redox weight reaction was with 100 applied wt.% temperature Fe3O4 on 50 at wt.% thermal MnO charging + 50 wt.% step CoO. of 30 However,°C to 1400 °C, from thethe EDS added weight weight analysis was 100 spectrum wt.% Fe distribution,3O4 on 50 wt.% the MnO untreated + 50 wt.% weight CoO. percentage However, of from Fe-O (38.28%)the EDS was weight higher analysis than thespectr sumum of distribution, the weight the percentage untreated of weight Co-O andpercentage Mn-O (10.14%of Fe-O + 9.84%)(38.28%) and was the treatedhigher weightthan the percentage sum of the of Feweight (65.57%) percentage and Mn-Co of Co-O (17.36% and + 17.06%).Mn-O From(10.14%+9.84%) the TGA analysis and inthe Sections treated 3.1 weight and 3.2 pe, itrcentage was deduced of Fe that (65.57%) the reduction and Mn-Co of CoO and(17.36%+17.06%). MnO was faster From than the Fe 3TGAO4. Furthermore,analysis in Sections Fe-O 3.1 was and not 3.2, well it was reduced deduced except that for the the ◦ ◦ 0.31%reduction weight of lossCoO due and to MnO dehydration was faster until than it Fe reached3O4. Furthermore, 1200 C. Nevertheless, Fe-O was not at well 1200 re-C, Fe-Oduced was except reduced for the by 1.31%0.31% fromweight its loss initial due weight to dehydration as compared until it to reached a maximum 1200 °C. reduction Nev- ofertheless, MnO (9.18%) at 1200 and °C, CoO Fe-O (6.27%). was reduced This by might 1.31% be from due its to initial the availability weight as compared of 65% of to Fe-O a duringmaximum redox reduction and that halfof MnO of its (9.18%) weight and was CoO Mn-Co, (6.27%). which This was might because be due the to redox the availa- reaction decreasedbility of the65% weightof Fe-O of during Mn-Co redox more and than that that half of theof its Fe-O. weight However, was Mn-Co, the EDS which spectrum was andbecause mapping the resultsredox reaction both revealed decreased the the uniform weight distribution of Mn-Co more of composed than that elements of the Fe-O. in the reducedHowever, reaction. the EDS Figure spectrum8 shows and mapping the EDS spectrumresults both and revealed mapping the uniform analysis distribution of the fresh (Fe,Co,Mn)Oxof composed material. elements It in was the revealed reduced that reac thetion. percentage Figure 8 shows of weight the distributionEDS spectrum spectrum and ofmapping Fe, Mn, andanalysis Co wereof the 24.04,fresh (Fe,Co,Mn)Ox 3.3, and 72.67, material. and weight It was percentagerevealed that of the atomic percentage weight compositionof weight distribution were 24.37%, spectrum 72.5%, of and Fe, 3.12%,Mn, an respectively.d Co were 24.04, Figure 3.3,8 andb shows 72.67, the and electronic weight percentage of atomic weight composition were 24.37%, 72.5%, and 3.12%, respectively. image of the fresh (Fe,Co,Mn)Ox mixture before the reduction process. Figure8c–f show Figure 8b shows the electronic image of the fresh (Fe,Co,Mn)Ox mixture before the reduc- the EDS mapping images of the fresh Fe, O, Mn, and Co. Unlike the reduced EDS spectrum tion process. Figures 8c–f show the EDS mapping images of the fresh Fe, O, Mn, and Co. weight analysis in Figure7a, the EDS spectrum observation in Figure8a particularly for Co, Unlike the reduced EDS spectrum weight analysis in Figure 7a, the EDS spectrum obser- had less percentage weight distribution and it had an uneven weight distribution when vation in Figure 8a particularly for Co, had less percentage weight distribution and it had observed over the EDS mapping image in Figure8c. From Figure8c, Fe had more evenly an uneven weight distribution when observed over the EDS mapping image in Figure 8c. distributed weights and Mn was less evenly distributed. However, Co had uneven, less From Figure 8c, Fe had more evenly distributed weights and Mn was less evenly distrib- weight,uted. andHowever, scattered Co had form uneven, of weight less distribution weight, and and scattered white-colored form of spotsweight scattered distribution on the surfaceand white-colored of the EDS maps spots indicate scattered the on presencethe surfac ofe of Co the and EDS Mn, maps while indicate the black the presence points spots of indicateCo and the Mn, existence while the of black oxygen. points spots indicate the existence of oxygen.

(a) (b)

Figure 7. Cont.

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Catalysts 2021, 11, x FOR PEER REVIEW 16 of 22

(e) (f)

(g)

FigureFigure 7. EDS 7. EDS analysis analysis of reducedof reduced (Fe,Co,Mn)Ox: (Fe,Co,Mn)Ox: ( a(a)) EDS EDS spectrum; spectrum;(g) ((b) the treated and and reduced reduced spectrum, spectrum, (c ()c electronic) electronic imageimage ofFigure the of (Fe,Co,Mn)Oxthe 7. EDS (Fe,Co,Mn)Ox analysis material; of reducedmaterial; (c) (Fe,Co,Mn)Ox: EDS (c) EDS mapping mapping (a picture) EDS picture spectrum; of Fe, of Mn, Fe, (b Co,)Mn, the and treatedCo, O; and ( dand) O; EDS reduced(d mapping) EDS spectrum, mapping of O; (( ecof) electronic EDSO; (e mapping) EDS mapping of Mn; (f) EDS mapping of Fe; (g) EDS mapping of Co. of Mn;image (f) EDS of mapping the (Fe,Co,Mn)Ox of Fe; (g )material; EDS mapping (c) EDS of mapping Co. picture of Fe, Mn, Co, and O; (d) EDS mapping of O; (e) EDS mapping of Mn; (f) EDS mapping of Fe; (g) EDS mapping of Co.

(a)( a) (b(b) )

(c)( c) (d(d) ) Figure 8. Cont.

Catalysts 2021, 11, 362 17 of 22 Catalysts 2021, 11, x FOR PEER REVIEW 17 of 22

Catalysts 2021, 11, x FOR PEER REVIEW 17 of 22

(e) (f)

FigureFigure 8. EDS 8. analysisEDS analysis the freshthe fresh (Fe,Co,Mn)Ox (Fe,Co,Mn)Ox material: material: ((aa)) EDSEDS spectrum; (b ()b electronic) electronic image image of the of the(Fe,Co,Mn)Ox (Fe,Co,Mn)Ox material; (c) EDS mapping of O; (d) EDS mapping of Fe; (e) EDS mapping of Mn; (f) EDS mapping of Co. material; (c) EDS mapping of O; (d) EDS mapping of Fe; (e) EDS mapping of Mn; (f) EDS mapping of Co.

(e) 2.7. The SEM Analysis (f) 2.7. The SEM Analysis Figure 8. EDS analysis the fresh (Fe,Co,Mn)OxThe SEM material: images ( aof) EDSthe (Fe,Co,Mn)Oxspectrum; (b) electronic are shown image inof theFigures (Fe,Co,Mn)Ox 9 and 10. Figure 9 shows material; (c) EDS mapping of O; the(dThe) EDS morphology SEMmapping images of Fe; of ( e)(Fe,Co,Mn)Ox EDS of themapping (Fe,Co,Mn)Ox of Mn;material (f) EDS mappingafter are shownthe of Co.two-cycle in Figures reaction9 and process. 10. FigureThe 9 shows the morphology of (Fe,Co,Mn)Ox material after the two-cycle reaction process. 2.7. The(Fe,Co,Mn)Ox SEM Analysis catalyst was air-dried and then characterized. The (Fe,Co,Mn)Ox material Thewas (Fe,Co,Mn)Ox composed of catalysthighly agglomerated was air-dried polygonal-shaped and then characterized. structures, which The had (Fe,Co,Mn)Ox highly The SEM images of the (Fe,Co,Mn)Ox are shown in Figures 9 and 10. Figure 9 shows material was composed of highly agglomerated polygonal-shaped structures, which had the roughmorphology surfaces of (Fe,Co,Mn)Ox(cracked and materialweathered after sectio the ns).two-cycle Figure reaction 9a–d deprive process. rough The surface fea- (Fe,Co,Mn)Oxhighlytures, rough which catalyst surfacescomprised was air-dried (cracked small andand andthen large characterized. weathered crystal-like Thestructures sections). (Fe,Co,Mn)Ox that Figure coincided material9a–d with deprive the EDS rough wassurface spectrumcomposed features, results,of highly which suggesting agglomerated comprised that pol theygonal-shaped small small and structures large structures, crystal-like belong which to hadcobalt structures highly and manganese that coincided withroughfragments thesurfaces EDS (cracked while spectrum the and large weathered results, structures suggestingsectio ns).were Figure iron that 9a–d fragments. the deprive small roughFigure structures surface 10a–d fea- belong shows the tocobalt mor- and tures,manganesephology which comprised of fragments (Fe,Co,Mn)Ox small while and materiallarge the crystal- large afterlike structures the structures two-cycle were that redox coincided iron reaction fragments. with thefor EDS different Figure 10 applieda–d shows spectrum results, suggesting that the small structures belong to cobalt and manganese thevoltage morphology and dimensions. of (Fe,Co,Mn)Ox Figure 10a–d material also afterrevealed the large two-cycle structures redox with reaction smoother for sur- different fragmentsfaces andwhile less the agglomerated large structures structures. were iron fragments.Figure 10c Figure shows 10a–d hexagonal shows shapethe mor- whereas Figure phologyapplied of voltage (Fe,Co,Mn)Ox and dimensions. material after the Figure two-cycle 10a–d redox also reaction revealed for different large structures applied with smoother 10d shows a porous structure, which is important for energy storage application. voltagesurfaces and and dimensions. less agglomerated Figure 10a–d also structures. revealed large Figure structures 10c showswith smoother hexagonal sur- shape whereas facesFigure and 10 lessd agglomerated shows a porous structures. structure, Figure 10c which shows is hexagonal important shape for whereas energy Figure storage application. 10d shows a porous structure, which is important for energy storage application.

(a) (b) (a) (b)

(c) (d) (c) (d) Figure 9. SEM micrographs of the (Fe,Co,Mn)Ox material. (a) Image acquired using a 10 kv accel- × µ erating voltage at 20,000 length of 2 m; (b) image acquired using a 10 kv accelerating voltage at 10,000× length of 5 µm; (c) image acquired using a 10 kv accelerating voltage at 1000× length of 50 µm and (d) image acquired a 10 kv accelerating voltage at 5000× length of 10 µm. Catalysts 2021, 11, x FOR PEER REVIEW 18 of 22

Figure 9. SEM micrographs of the (Fe,Co,Mn)Ox material. (a) Image acquired using a 10 kv Catalysts 2021, 11, 362accelerating voltage at 20,000× length of 2 μm; (b) image acquired using a 10 kv accelerating 18 of 22 voltage at 10,000× length of 5 μm; (c) image acquired using a 10 kv accelerating voltage at 1000× length of 50 μm and (d) image acquired a 10 kv accelerating voltage at 5000× length of 10 μm.

(a) (b)

(e)

Figure 10. SEMFigure micrographs 10. SEM micrographs of the treated of (Fe,Co,Mn)Oxthe treated (Fe,Co,Mn)Ox material: (a) material: Image acquired (a) Image using acquired a 10 kv using accelerating a 10 voltage at 10,000× lengthkv accelerating of 5µm; (b) voltage image acquired at 10,000× using length a 10 of kv 5μ acceleratingm; (b) image voltage acquired at 2000 using× lengtha 10 kv of accelerating 20 µm; (c) image acquired voltage at 2000× length of 20 μm; (c) image acquired using a 10 kv accelerating voltage at 5000× using a 10 kv accelerating voltage at 5000× length of 10 µm; (d) image acquired using a 10 kv accelerating voltage at 20,000× length of 10 μm; (d) image acquired using a 10 kv accelerating voltage at 20,000× length of 2 μm, length of 2 µm, and (e) image acquired using a 10 kv accelerating voltage at 3000× length of 10 µm. and (e) image acquired using a 10 kv accelerating voltage at 3000× length of 10 μm. 3. Material and Methods 3 Material and Methods 3.1. Material Synthesis and Characterization 3.1. Material Synthesis and Characterization Pure oxides namely CoO, MnO, Fe3O4, and mixed oxides with proportion of 25 wt.% Pure oxidesCoO namely + 75 CoO, wt.% MnO, Fe 503O wt.%4, and CoO mixed + 50 oxides wt.% with MnO, proportion and 75 wt.% of 25 CoO wt.% + 25 wt.% MnO CoO + 75 wt.%were MnO, synthesized. 50 wt.% CoO The + proportion 50 wt.% MnO, was taken and by75 weight.wt.% CoO First, + 25 30 mgwt.% each MnO of the six metallic were synthesized.oxides The was proportion prepared, was it taken is, 30 mgby weight. of CoO, First, 30 mg 30 of mg MnO, each 30of mgthe ofsix Fe metal-3O4, 7.5 mg of CoO lic oxides was prepared,+ 22.5 mg it of is, MnO 30 mg (31.7), of CoO, 15 mg30 mg of CoOof MnO, + 15 30 mg mg of of MnO Fe3O4 and, 7.5 22.5mg of mg CoO of CoO + 7.5 mg + 22.5 mg of MnOof MnO. (31.7), 42.3 15 mg mol% of CoO of MnO, + 15 40mg mol% of MnO of CoO, and 22.5 12.8 mg mol% of CoO of Fe +3O 7.54, 10.6mg of mol% of MnO + 30 mol% of CoO, 10 mol% of CoO + 31.7 mol% of MnO and 20 mol% of CoO + 21 mol% of MnO. The purity of metallic oxide was 99.99%. The powders were heated at a range of 30 to 1400 ◦C for about 180 min. Catalysts 2021, 11, 362 19 of 22

3.2. Experimental Procedure for Thermochemical Redox Cycling The experimental result data were collected and analyzed using transition temperature during reduction-oxidation process and the, oxygen uptake capacity, oxygen release and reaction enthalpies were obtained by using TGA and DSC analysis. The different metallic oxide samples were exposed to the test of TGA and DSC, EDS and SEM analysis. The redox reaction cycles were performed in O2/Ar atmosphere (30 mL/min Ar and ◦ 20 mL/min O2), between 30 to 1400 C for each metallic oxide. In the reduction-oxidation steps, the heating rate was 30 ◦C/min and the cooling step was 20 ◦C/min for the re- oxidation. The charging and discharging processes for all materials were performed in two series cycles. The metallic oxides were heated from 30 to 1400 ◦C at a heating rate of 30 ◦C/min (charging), cooled from 1400 to 800 ◦C at 20 ◦C/min heating rate (discharging) then heated to 1400 ◦C (charging) and held at that temperature for 2 min followed by cooling to 800 ◦C and held at that temperature for 30 min and then heated from 800 to 1400 ◦C at 30 ◦C/min (charging) and held at that temperature for 2 min and then cooled to 800 ◦C at 20 ◦C/min (discharging). These processes were conducted for achieving stability as well as to analyze the oxygen uptake and release of oxygen, weight loss, and phase changing temperature in the redox reactions.

3.3. Thermodynamic Calculations The metallic oxides experimental data were analyzed using OriginPro software (2018a, Northampton, MA, USA). For the calculations, using the OriginPro software and extracted data from the TGA and DSC excel, the oxygen uptake, oxygen release, the weight loss, the onset reduction temperatures, the conversion rate, the phase change temperatures, the gas ﬂow rates, and the heat rate, which were applied during redox reactions, were analyzed and calculated. During the experiment, the additional argon gas for protection were considered. The gaseous components, O2, and Ar were considered, with ideal mixing properties. The total pressure was always 1 atm. The phase change diagrams and phase changing temperatures were plotted and calculated. The calculations provide the theoretical mass loss (i.e., oxygen storage capacity) and rate of conversion of metallic oxides in the trial of the tests, which were calculated as follows. From

∆w(%) η = × 10, 000 µmol/g (3) 32

m − m(T) χ = o (4) mo − m∞ win(mols) − wO2 (mols) χcon = (5) win(mols) + wO2 (mols) From the non-parametric kinetics (NPK), which originally was developed by Ser- rra et al. [56–58] where, η is the O2 storage or release capacity, ∆(w%) is the weight loss, χ is the conversion rate, mo is initial weight, m(T) is the weight at temperature (T), m∞ the residue weight at the end of reaction, χcon is conversion efﬁciency, win is the weight of

metallic oxides set for redox, wO2 is the weight of metallic oxides changes to oxygen.

4. Conclusions

This study utilized different proportions of MnO, CoO, and Fe3O4 to produce a metallic oxide material with high thermal storage performance using a two-cycle redox reaction. Proper moisture of MnO and CoO can stabilize the weak reaction rate of Mn and fast reaction of CoO as well as enhance cyclic stability and oxygen uptake ratio. The combination of 50 wt.% MnO + 50 wt.% CoO produced a material with high cyclic stability, high oxygen uptake ratio, and high energy conversion performance. The mixing of 100 wt.% Fe3O4 with 50 wt.% MnO + 50 wt.% CoO resulted in decreased oxygen uptake ratio, increased stability performance, increased the reaction temperature and decreased the gap in temperature for reduction-oxidation steps for the material thereby reducing the Catalysts 2021, 11, 362 20 of 22

sensible energy loss during the heating and cooling steps of the material production process. The phase change and decomposition of iron oxide occurred at the very high temperatures. Increasing the oxygen exchange capacity on iron oxides during redox reaction had a clear adverse effect on the thermal performance of MnO and CoO, and thereby leading to decrease in thermal enthalpy in the process. Thus, the development of new models covering large multi-component systems could further be used in selecting other transition metals to enhance the properties of mixed metal oxides for thermochemical energy storage application technology.

Author Contributions: Formal analysis, B.J.; Software, J.Z. and J.A.; Supervision, Q.H. and B.G.L.; Writing—original draft, Y.G.D.; Writing—review & editing, E.B.T. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by the National Natural Science Foundation of China (No. 51976044; 51950410590), China Postdoctoral Science Foundation Fund (2019M651284), and Fundamental Re- search Funds for the Central Universities (HIT.NSRIF.2020054) Conﬂicts of Interest: The authors declare no conﬂict of interest and the funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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