Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox

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Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox materials applied to solar thermochemical energy storage Laurie André, Stéphane Abanades, and Laurent Cassayre ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00554 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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ACS Applied Energy Materials

Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox materials applied to solar thermochemical energy storage

Laurie André1, Stéphane Abanades1, *, Laurent Cassayre2 1

Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS, 7 Rue du Four Solaire, 66120 Font-Romeu, France 2

Laboratoire de Génie Chimique, Université de Toulouse, CNRS, Toulouse, France *

Corresponding author: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract. Thermochemical energy storage (TCES) can be achieved via reversible redox reactions based on metal oxides for solar energy storage application in solar power plants. As cobalt oxide (Co3O4/CoO) and manganese oxide (Mn2O3/Mn3O4) appear as attractive candidates for TCES, an experimental study has been conducted in order to evaluate the potential of mixed oxides from the Co-Cu-O, Mn-Co-O and Mn-Cu-O systems. The addition of Cu to Co3O4 allows accessing promising materials with good cycling stability and high reaction enthalpy for TCES application. As for the Mn-Co-O and Mn-Cu-O systems, the role of the hausmannite phase formation at low Co or Cu contents on the irreversibility of Mnbased mixed oxides is evidenced, thus demonstrating the interest of the addition of a secondary metal oxide to the Mn-based material. For the Mn-Co-O system, low amounts of Mn should be favored, since both the oxygen storage capacity and the reaction enthalpy significantly increase with decreasing Mn contents, and the reversibility is lost above 50 mol% Mn due to the formation of the tetragonal spinel phase (hausmannite), which inhibits further re-oxidation. Among the mixed oxides of the Mn-Cu-O system, the compositions with Cu amounts below 30 mol% cannot be cycled because of the hausmannite phase formation during reduction, similarly to the case of Mn-Co-O system. In contrast, the compositions with Cu amounts in the range 40-80 mol% feature promising redox properties with complete reaction reversibility. The tuning of redox properties by the synthesis of mixed metal oxides shows promises for the development of suitable high temperature TCES materials. Keywords: mixed metal oxide, thermal heat storage, solid-gas reaction, reduction, oxidation, redox cycle, concentrated solar power.

1. Introduction Thermal energy storage (TES) makes possible the continuous production of electricity in solar thermal power plants, during off-sun hours, for example with pressurized air-based solar tower receivers for power generation via gas turbines, by storing energy during on-sun hours in the form of sensible, latent or chemical energy 1. The present study focuses on thermochemical energy storage using solid-gas reversible reactions based on mixed metal oxides. As compared to latent and sensible heat storage, TCES presents interesting advantages such as higher energy storage densities, possible heat storage at room temperature in chemical bonds, in the form of stable and transportable solid materials, and long term


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storage in a large temperature range (up to >1000°C) with a constant restitution temperature. The required high-temperature heat source to perform the charging (endothermic) and discharging (exothermic) steps (Eq. 1 and 2) can be supplied by concentrated solar power (CSP). MO(ox) + Heat MO(red)+ α O2 (g)

(1)

MO(red)+ α O2 (g) MO(ox) + Heat

(2)

Among the potential single metal oxides identified for TCES, cobalt oxide (Co3O4), manganese oxide (Mn2O3) and copper oxide (CuO) present the most interesting properties 2-4. The redox system Co3O4/CoO in particular has demonstrated fast reaction kinetics, complete reaction reversibility and good cycling stability, with an experimental gravimetric energy density of 576 kJ/kg 5, while the enthalpy calculated from tabulated thermodynamic data is reported as 844 kJ/kg 6-7. Main drawbacks of cobalt oxide concern cost and toxicity issues and the high reduction temperature. The gravimetric energy storage density reported for Mn2O3/Mn3O4 is about 110 to 160 kJ/kg 8-9, for a theoretical enthalpy of 202 kJ/kg 9. The notably slow and partial re-oxidation of this material has been reported, sometimes happening in two steps 10, due to strong kinetic limitations inducing poor reaction reversibility and large difference between reduction and oxidation onset temperatures (the oxidation requires either very low cooling rates or extended dwell at an optimum temperature). The CuO/Cu2O system was studied by Hänchen et al. 11 for the separation of oxygen from inert gases. The temperature range to reduce CuO to Cu2O is expected to be between 1030°C and 1134°C in air, suggesting the possibility of using this redox couple for high temperature thermal storage. The reaction kinetics studied in thermogravimetry and reactor tests show that the reaction rate of the re-oxidation strongly depends on the O2(g) concentration injected into the system. The occurrence of sintering during the high temperature reduction is also mentioned. Alonso et al. 12

studied the CuO/Cu2O redox system, under air and under argon, using a rotating solar

reactor. The conversion rate reported for the reduction step is 80% under argon at 900°C and 40% under air at 1000°C. However, the conversion rate for the oxidation step is as low as 9% in air at 700°C. These results show the impact of the experimental conditions, the sintering effect and the morphology of the material on its reactivity and stability. The development of alternative TCES materials is needed, and room for improvement yet remains regarding



reaction kinetics, reversibility, loss-in-capacity over cycles or sintering of the materials, in addition to developing cost effective and affordable materials for large scale application. When considering the optimization of materials reactivity, some improvements of the redox performances may be achieved by controlled morphology through synthesis and stabilization with inert material to alleviate sintering effects. Another attractive method is to modify the properties of the pure oxides with the addition of a second transition metal oxide, with the aim of tuning reaction kinetics, cycling stability and reaction temperature 3, 5, 13. In the present work, binary mixing between Co, Mn and Cu oxides are considered. The incorporation of copper in cobalt oxide has been studied in the context of the separation of oxygen from air by means of mixed oxides. Motuzas and Diniz da Costa 14 demonstrated the decrease of the temperature difference between the reduction and oxidation steps of the mixed oxide Co0.8Cu0.2Ox up to 125°C compared to 250°C for pure CuO, for an oxygen exchange capacity of 5% by mass. The addition of copper to cobalt oxide is shown to promote the exchange of oxygen, but a large proportion of copper in the mixed oxide also results in a lower oxygen storage capacity. The material is described as stable when subjected to several redox cycles at high temperature (up to 1000°C in air). Block et al. 15 studied and compared several binary oxide systems, including the Co-Cu-O system for 15 Cu contents between 3 and 97 mol%. All the compositions show reaction temperatures around 865°C for the reduction step, which is about 50°C lower than the reduction temperature of pure Co3O4 and about 180°C lower than the reduction temperature of pure CuO, and thus confirms that the oxygen mobility is exacerbated. The Co-Mn-O system was also considered for applications in thermochemical storage 15-16. The performances of mixed oxides (Mn1-xCox)3O4 (with 0≤x(Co)≤0.08, 0.93≤x(Co)≤1 and x(Co)=0.47) were equal to or lower than those of the pure oxides (Co3O4 and Mn2O3) 16. The addition of Mn to cobalt oxide is detrimental to the properties of pure cobalt oxide when the addition of manganese exceeds 1.8%, suggesting that pure oxides show more interest in thermochemical storage than mixed oxides in this system. Block et al. 15 studied the compositions x(Mn)= 0.1 and 0.9 but the mixed oxide with 90% Mn showed no reduction when heated to more than 1000°C (exact temperature is not specified) under 20% O2/N2 atmosphere. However, mixed oxides with compositions between 0.2≤x(Mn)≤0.9 have been barely studied and their interest in thermochemical storage remains to be evaluated.


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The Mn-Cu-O system has also been the subject of some recent studies 15, 17, 18 reporting that the addition of Cu to Mn2O3 affects the reaction temperature and improves the kinetics of redox reactions of the material. However, the Mn-Co and Mn-Cu are complex systems that were scarcely studied with only a few random compositions tested and without interpreting the trends over the whole compositional range in light of the available phase diagrams. The addition of Mn or Cu to cobalt oxide aims at alleviating the toxicity and cost issues, and lowering the reduction temperature. The addition of Co or Cu to manganese oxide aims at improving the reaction reversibility and oxidation kinetics. The objective of the present work is to evaluate, by extensive experimental measurements based on thermal analysis techniques, the influence of oxide compositions in the Co-Cu-O, Mn-Co-O, and Mn-Cu-O systems, in order to reach optimal performances and suitability for TCES application, such as high reaction enthalpy, complete reaction reversibility, performance stability upon redox cycling, reaction temperature tuning, and low temperature hysteresis between the reduction and oxidation steps.

2. Materials synthesis and characterization Mixed oxides of Co-Cu (x(Cu) = 0, 0.03, 0.1, 0.2, 0.25, 0.3, 0.4, 0.6, 0.8, 1), Mn-Co (x(Mn) = 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.7, 0.8, 0.9, 1), and Mn-Cu (x(Cu) = 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 0.9, 1) were synthesized using a modified Pechini method 19, from the corresponding metal nitrates (>98% purity) mixed with citric acid and ethylene glycol (>99% purity) in aqueous solution. The Pechini synthesis was selected because it yields porous materials with high surface reactivity. The powders can thereby undergo rapid sintering and reach a stabilized structure directly obtained after high-temperature treatment. This in turn allows highlighting the positive impact of the metal addition on improving the mixed oxides redox properties and stability with a restricted number of cycles. The obtained powders were then calcined at 750°C for 4h and characterized with X-ray diffraction (XRD) at room temperature, using a PANanalytical XPert Pro diffractometer (CuKα radiation, λ=0.15418 nm). XRD measurements of θ-θ symmetrical scans were made over an angular range of 10 to 80°. The step size and the time per step were fixed at 0.01° and 20 s respectively. The contribution from Kα2 was removed and the X-ray diffractograms were recorded and studied using the PANanalytical software. The instrumental function was



determined using a reference material (SRM 660, lanthanum hexaboride, LaB6 polycrystalline sample) and can be expressed by a polynomial function 20. The powders morphology was observed by Scanning Electron Microscopy (SEM), using a FESEM Hitachi S4800 microscope. For a few samples, the particle size distribution was measured by laser diffraction in a Malvern Mastersizer 3000. Simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), using a Netzsch STA 449 F3 System, were conducted on the synthesized mixed oxides powders in order to assess the influence of oxides composition on the oxygen storage capacity (OSC), reaction temperature, reaction reversibility, redox performance stability and heat storage capacity. About 40 mg of powder was placed into an alumina crucible and then analyzed in TGA coupled with DSC. The heating and cooling rate was generally 10°C/min for the reduction and re-oxidation steps, and redox cycles were performed in a 20%O2/Ar atmosphere (10 Nml/min for O2 and 40 Nml/min for Ar). The reaction enthalpy was quantified by integrating the DSC peaks observed during reduction (endothermal) and oxidation (exothermal). The reported data (OSC, temperatures, and enthalpies) were averaged over the course of three reduction/oxidation cycles for each material.

3. Results and discussion 3.1. Investigation of the Co-Cu-O system Based on the phase diagram compiled by Driessen et al. 21 and additional experimental data, Zabdyr et al. 22 have proposed an assessment of the thermodynamic parameters of the Co-CuO system. We implemented this model in the FactSage software 23. It includes the thermodynamic description of two solid solution phases Co(Cu)O and Cu(Co)O as well as three solid components Cu2CoO3, Co3O4 and Cu2O. The model was used to plot the Co-Cu phase diagram in air (Fig. 1).


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1100

Co(Cu)O + Cu2O Cu2CoO3 + Cu2O Co(Cu)O

Co(Cu)O

1000

+ Cu2CoO3 cycle Tmax

Temperature (°C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu2CoO3 + Cu(Co)O

Cu(Co)O

900 Co(Cu)O + Cu(Co)O Co3O4(s) + Co(Cu)O 800

700 cycle Tmin

Co3O4 + Cu(Co)O 600 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x(Cu)

Figure 1. Calculated Co-Cu-O phase diagram in air (model from 22). The red dots correspond to the transition temperatures measured in this study.

The phase diagram evidences that the Cu content should not change the phase assemblage of the starting material calcined at 750 °C: at equilibrium, it is a mixture of Co3O4 and CuO phases, with minor solubility of oxides into each other. Accordingly, the materials synthesized within the Co-Cu-O system are composed of a mixture of Co3O4 and CuO, as illustrated by the diffractograms presented in Fig. 2.



Figure 2. XRD analysis of synthetic Co-Cu oxides after calcination at 750°C.

Redox activity and oxygen storage capacity (OSC) TGA experiments show the mass variation (Δm, corresponding to the OSC) during redox reactions (Fig. 3 for x(Cu) up to 0.25, Fig. S1 for higher Cu contents). A mass loss ascribed to O2 release during reduction is measured during heating whereas a mass gain (O2 capture during oxidation) is measured during cooling. A good reversibility is observed for all the mixed Co-Cu oxides when cycled between 700 and 950°C at 10°C/min, with a total reduction step and a high conversion rate for oxidation (average of 96% for all the mixed oxides). However, as compiled in Fig. 4, the OSC decreases as the Cu content increases from x(Cu) = 0.03 to x(Cu) = 0.8. For pure CuO, which is converted into Cu2O, a higher reduction temperature (1032°C) was measured for this phase transition, and a large OSC (about 10%) was recorded.


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Figure 3. TGA of mixed Co-Cu oxides with x(Cu) equal to (a) 0.03, (b) 0.1, (c) 0.2, (d) 0.25. The last reduction step was performed in Ar to recover the materials in the reduced state

∆m - Max. ∆m Red - Average ∆m Ox - Average [15] ∆m Red [15] ∆m Ox Calculated

10

8

∆m (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

2

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x(Cu) Figure 4. Oxygen storage capacity (∆m) of mixed Co-Cu oxides cycled between 700°C and 950°C in air (squares and triangles: this study; circles: data from 15; full line: calculated with the model from 22); data for x(Cu)=1 correspond to a maximum cycle temperature of 1100°C.



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The equilibrium was computed at various compositions and temperatures using the thermodynamic model. These calculations provide the theoretical mass loss (i.e. oxygen storage capacity), ∆m, as defined by (Eq. 3). This quantity is directly comparable to the mass change measured by TGA.

∆ % = 100.

(3)

where Tmin and Tmax are referring to the minimal and maximal temperature of the thermochemical cycle. An excellent agreement is observed with regard to OSC data and equilibrium calculations for the whole composition range (Fig. 4), which implies that equilibrium is reached during the cycles. The continuous and regular evolution of OSC with composition is because cobalt is the only redox active component in the mixed oxides at these temperatures, with redox state switching between +II and +III, while copper stays +II. Thus, the higher the amount of Cu, the lower the mass of active redox species, which implies a lower exchange of oxygen. The phase diagram (Fig. 1) indicates the existence of a Cu2CoO3 phase for x(Cu)>0.38 and T>915°C, but this phase was never detected by XRD. The phases identified by XRD (Fig. S2) after TGA (last reduction step performed at 950°C in Ar) are a mixture of (Co,Cu)O and (Cu,Co)O. However, the formation of Cu2CoO3 from CuO and CoO does not release any oxygen, and thus does not affect the oxygen storage properties. The results obtained here confirm that adding copper to cobalt oxide reduces the capability of the material to store and release oxygen. The reaction associated to mixed Co-Cu oxides with 0 200 kJ/kg) were achieved for the whole composition range. However, addition of Cu to cobalt oxide reduces the reaction enthalpy and thus the volumetric/gravimetric energy storage density. The redox transition temperatures for these mixed oxides decreased compared to the pure oxides, making thus possible to reduce the operating temperature and to adapt the storage system to different conditions of application.



As for the Mn-Co-O system, only the compositions with x(Mn)≤0.5 showed reversible reactions involving the (Co,Mn)O phase. All transition temperatures were higher than those of the pure oxides, and the stability and energy storage capacity of the mixed oxides were lower than that of pure Co3O4. The mixed oxides with x(Mn)=0.4 and 0.5 compositions require high reduction temperatures under air to observe the formation of the (Co,Mn)O phase from the spinel phase. As for compositions with higher Mn contents, the formation of the hausmannite phase in the Mn-Co-O system is definitely detrimental to the reaction reversibility. Consequently, the addition of cobalt to Mn3O4 in order to avoid the formation of the hausmannite phase represents an interesting option to improve the performances of manganese oxide. Anyhow, pure cobalt oxide remains more performant than the mixed oxides of the Mn-Co-O system, because both the OSC and the energy storage capacity decrease drastically with Mn addition from x(Mn)=0.05 to x(Mn)=0.5. Finally, the mixed oxides of the Mn-Cu-O system were shown to be interesting candidates in the range of 0.4≤x(Cu)≤0.8, because the tetragonal spinel (hausmannite) phase formation is avoided. In contrast, poor reaction reversibility was observed at low Cu content because of the low re-oxidation capability of hausmannite, which confirms the beneficial effect of Cu addition for enhancing the cyclability of Mn2O3, similarly to the Mn-Co-O system. Therefore, the reaction reversibility of Mn-Co-O and Mn-Cu-O systems is strongly improved with the addition of Co and Cu above 40 and 30 mol%, respectively. In addition, the measured reaction enthalpies are higher than that of pure Mn2O3. It is also evidenced that the energy storage capacity of materials is directly proportional to the oxygen storage capacity for each mixed oxide system, thereby unveiling that the abilities of materials for both oxygen exchange and energy storage are strongly correlated.

Supporting Information. XRD analysis of cycled materials, TGA of materials studied within the Co-Cu-O, Mn-Co-O and Mn-Cu-O systems, SEM analysis of fresh and cycled powder of Co-Cu-O with x(Cu)=0.2, granulometric analysis of the Co-Cu mixed oxide powder, measured enthalpies versus mass loss for mixed oxides of the Co-Cu-O and Mn-Co-O systems, temperatures for the reduction step of mixed Co-Cu, Mn-Co and Mn-Cu oxides under Ar.


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Acknowledgements: This study was performed in the framework of the STAGE-STE European project (FP7, project N° 609837).

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Experimental investigation of Co-Cu, Mn-Co and Mn-Cu redox

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