Elsevier

Journal of Energy Storage

Volume 43, November 2021, 103167
Journal of Energy Storage

Superior thermochemical energy storage performance of the Co3O4/CoO redox couple with a cubic micro-nanostructure

https://doi.org/10.1016/j.est.2021.103167Get rights and content

Highlights

  • Co3O4 nanomaterials with special morphologies were synthesized for heat storage.

  • The capacity of the synthesized Co3O4 exhibited complete redox reversibility.

  • Re-oxidation reaction rate of cubic Co3O4 was twice that of the commercial ones.

  • Thermal hysteresis of cubic Co3O4 was just half that of commercial Co3O4.

  • The synthesized cubic Co3O4 is promising for thermochemical energy storage.

Abstract

Co3O4 has been regarded as one of the most promising materials for redox energy storage due to its high theoretical conversion and complete redox reversibility. However, when undergoing charge-discharge cycles at high temperature, the material becomes sintered. The reversibility of the cobaltous oxides highly deteriorates, and the reoxidation reaction rate notably decreases. In addition, the obvious thermal hysteresis in the redox reaction process also limits the energy storage performance of the material. In this work, Co3O4 with different micro-nanostructured morphologies is synthesized to evaluate its thermochemical energy storage performance. The capacity of the synthesized and commercial Co3O4 is tested by thermogravimetric analysis (TGA). The fresh and recycled oxides are well characterized. Results suggest that all samples show complete reversibility, and the conversion decreases slightly after 30 cycles. The synthesized samples show better performance, especially cubic Co3O4. The reoxidation rate of the commercial sample is approximately 150 μmol/min/g, but that of the cubic sample is approximately 300-200 μmol/min/g in the first five cycles. The oxidation rate of the cubic sample decreases and finally remains at about 180 μmol/min/g from the 5th cycle to the 30th cycle. After 5 charge-discharge cycles, the commercial Co3O4 is severely sintered, while the cubic oxides are only slightly sintered. After 30 redox cycles, the morphology of commercial oxides completely changed, while that of the cubic oxide retains the approximate shape of a cube. Moreover, the thermal hysteresis values of the commercial and cubic Co3O4 are 24.7°C and 10.1°C, respectively, suggesting that the energy loss of cubic oxides is much smaller. The higher exothermic temperature of the cubic Co3O4 redox process also indicates that it can supply high-grade thermal energy.

Introduction

In the contexts of climate change and the dwindling availability of international fossil fuels as an energy supply, the utilization of renewable energy forms, which caters to this era, has quickly developed into a global revolutionary wave. Concentrated solar power (CSP), considered a promising renewable energy source, is the key to solving climate change and the future fossil energy crisis [1,2]. With the global expansion of CSP, from 1984 to 2016, the installed capacity of solar power stations in the world has increased annually [3]. Nevertheless, when developing these CSP sources, there are still some drawbacks, such as their intermittent nature, in which energy is unavailable during off-sun periods; therefore, an efficient energy storage system is required to realize electricity production through the day and night at a global scale. During on-sun hours, the excess energy can be stored by the storage system, and during off-sun hours, the stored energy can be released [4,5].

At present, there are three kinds of energy storage technologies that are used for collecting excess thermal energy in solar fields: sensible energy storage (SES), latent energy storage (LES) and thermochemical energy storage (TCES) [6,7]. SES stores or releases heat depending on the specific heat capacity of materials, and the materials are often molten salts, liquid metals and organic materials [8]. In the LES charging process, solar energy acts as the heat source that initiates a phase change of the energy storage medium; thus, the solar energy is stored in this new phase. In the discharge step, the medium changes back into its original phase. The energy storage performance depends on the latent heat of the medium, and ionic liquids are considered potential LES materials [9]. Even though SES and LES are recognized as useful methods for solar energy storage, their energy densities are not sufficient for some applications. In addition, the energy storage media cannot store energy at ambient temperature, so it is difficult for them to store energy for a long time. In long-term energy storage, energy is usually stored as heat for several months [10]. Based on this requirement, in recent years, TCES has been operated at higher temperatures and has demonstrated a higher energy density with minimal heat loss; thus, TCES has attracted the interest of many researchers [11]. The storage and release of energy depends on the breaking and recombination of chemical bonds. The heat storage density of TCES is 5 and 10 times that of LES and SES, respectively. In the TCES process, both the storage period and transport are theoretically unlimited because there is no thermal loss during storage. However, due to the complexity of TCES technology, it is still a less mature technology and is currently in the experimental stage [12,13]. Once such technology can be realized, it is expected to participate in energy generation dispatchability [14].

Salt hydrates, carbonates, hydroxides, ammonia, and organics are the most typical thermochemical energy storage materials used in different TCES systems [4]. In various reaction systems, such as the salt hydrates and carbonate reaction system, the reverse reactions involve H2O and CO2. When H2O participates in the TCES reactions, it will be converted into steam, suggesting that part of the energy will be consumed. CO2 storage also requires some extra energy. Therefore, neither of these reactions is beneficial due to reducing the energy efficiency of the system [15]. Metal oxides do not need to consider this problem, and there is no need to consume heat due to phase changes and product separation. Air, which serves as the reactant in the oxidation reaction, is also selected as the medium for heat transfer. The TCES of metal oxides is realized by the following reversible redox reactions, which consist of two steps (reduction and oxidation) and can be used to directly produce electricity via the air Brayton cycle [16]:Charge(onsun)MOx+ΔHMOxy+y2O2(g)Discharge(offsun)MOxy+y2O2(g)MOx+ΔH

The reduction reaction corresponds to energy storage (heat charge), and the heat required for the reduction reaction is provided by the excess heat in the solar power plant. The oxidation reaction corresponds to the energy release (heat discharge). When the solar energy is insufficient, the heat is released through the oxidation reaction to achieve peak regulation.

The redox couples of pure metal oxides, including Co3O4/CoO, Mn2O3/Mn3O4, BaO2/BaO, CuO/Cu2O and Fe2O3/Fe3O4, have been researched for TCES. Among them, the Co3O4/CoO pair is widely regarded as a potential redox couple for TCES due to its complete reversibility and high density in energy storage behavior. The redox onset temperature of the Co3O4/CoO system is approximately 900°C, and the theoretical conversion rate is 6.64%, which is approximately twice that of the Mn2O3/Mn3O4 redox couple. The enthalpy of the redox reaction is 844 kJ/kg, which is much higher than that of other metal oxides [4,17]. The reaction formula is as follows:Co3O43CoO+12O2gΔH=844KJ/Kg

This redox reaction involves the transition between Co3O4 and CoO. Co3O4 is a cubic spinel structure, and the valence states of Co are 2+ and 3+. CoO also has a cubic structure with Co valence states of 2+. The reduction reaction rate during the charge process depends on the rate of heat transfer, while the oxidation reaction rate during the discharge step is influenced by the rate of oxygen diffusion. Furthermore, the sintering of materials owing to the higher reaction temperature affects the reaction rate, especially the reoxidation rate. As the 32nd rarest element in the Earth's crust, Co is more difficult to obtain than other elements [5], so the price and rarity of Co3O4 is the main drawback for its utilization. However, it should be noted that the reoxidation rate and theoretical conversion rate of CoO are much higher than those of other pure metal oxides for every single cycle [18], which means that in regard to time, Co3O4 can complete the oxidation reaction faster and release more energy.

Nonetheless, after long-term charge and discharge cycling, the material sinters due to the high temperature. The two main processes of sintering are particle densification and coarsening, which make the diffusion resistance of oxygen on the material surface increases and leads to the cycling stability and reoxidation rate of the materials decreasing [19,20]. Thus, the key to improving the performance of energy storage is to enhance the cycling stability and reoxidation rate [5]. Agrafiotis et al. found that the porous foamed structure of Co3O4 has good sintering resistance, and it could maintain its structural stability and redox reversibility after 30 cycles [19]. Pagkoura et al. found that the addition of CeO could improve the redox kinetics of Co3O4; however, its structure would be destroyed after long-term cycling [21]. Alfonso et al. compared the redox performance of microporous Mn3O4 with Mn3O4 powder and found that microporous Mn3O4 had a faster reoxidation rate. In addition, they chemically modified the sample with several metal oxides and demonstrated that doping with Fe promoted the fastest redox rate and highest cycling stability [20]. For this reason, more effort should be made to enhance the anti-sintering capability of redox couples.

In addition to sintering, there is another drawback that limits the application of the Co3O4/CoO redox couple. The so-called thermal hysteresis is an interesting phenomenon, in which the onset temperatures of the reduction and oxidation reactions are different from each other; typically, the onset temperature of the oxidation reaction is usually higher than that of the reduction reaction. Thermal hysteresis is one of the most remarkable characteristics of energy storage materials; therefore, narrowing the redox hysteresis can improve the charge-discharge efficiency [22]. However, thus far, we have not seen any studies that have focused on the thermal hysteresis of the Co3O4/CoO redox couple. Alfonso et al. narrowed the redox hysteresis of Mn-based redox couples by Cu-Fe doping and found that 10% Fe+5% Cu had the most obvious improvement and that the thermal hysteresis decreased from 225°C to 98°C [22,23]. Even though the thermal hysteresis of the Co3O4/CoO redox couple is only a few tens of degrees, it is also necessary to narrow the temperature difference to enhance the charge-discharge efficiency.

Morphological modifications of Co3O4 have been proposed to obtain complete reversibility and fast redox kinetics. Special micro-nanostructured materials, such as hollow sphere Co3O4 and nanorod Co3O4, have been widely used in the energy field [24,25] because micro-nanostructured materials with special morphologies can inherit the advantages of nanomaterials. These micro-nanostructured materials with special morphologies exhibit interesting properties, such as better mass transfer properties and larger surface areas and pore volumes, which are called integral microstructure synergies. These properties have been proven in previous studies [26], [27], [28]. Therefore, for TCES, micro-nanostructured materials may also exhibit good cycling performance.

In the present study, to improve the redox cycling stability and reaction rate and lower the thermal hysteresis, Co3O4, with different morphologies, is synthesized by a hydrothermal method. The TCES performances of the materials are evaluated by thermogravimetric analysis (TGA), and the characteristics of the physical and chemical properties of the fresh and recycled samples are determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), the Brunauer-Emmett-Teller (BET), and Raman spectroscopy techniques.

Section snippets

Materials preparation

Co3O4, with different morphologies, was synthesized by a hydrothermal method. Taking cubic Co3O4 as an example, cobalt acetate tetrahydrate (2.38 g), triethanolamine (2.0 g) and CO(NH2)2 (3.00 g) were dissolved in 100 mL of deionized water under continuous stirring, and then a red and transparent solution was obtained. The solution was placed into a Teflon autoclave and kept at 180°C for 12 h. The product was separated by centrifugation and washed several times with ethanol and deionized water

Characterization results of the energy storage materials

Different morphologies of the samples could be observed in the SEM results. The SEM images of the Co3O4 precursor (CoCO3) [27,28] are shown in Fig. 1. All precursor particles presented a special shape, and the precursor particles grew into different morphologies based on the addition of different organic solvents. For the CUB-Gp (Fig. 1(a) and (b)), the size of the cube was about ∼10 μm. It was also observed that the CUB-Gp particle was formed by accumulation of dense square sheets. As seen in

Conclusions

Micro-nanostructured Co3O4 materials with a cubic morphology exhibited good redox performance, promoting their use in thermochemical energy storage applications. All samples demonstrated complete redox reversibility even after 30 cycles. It was confirmed over 30 charging-discharging cycles that the reoxidation rate decreased due to sintering, but the reoxidation rate and thermal hysteresis of the synthesized samples were greatly improved compared to those of the commercial sample. In

CRediT authorship contribution statement

Lei Liu: Investigation, Data curation, Writing – original draft. Zijian Zhou: Writing – review & editing, Investigation, Conceptualization. Changqing Wang: Investigation. Jie Xu: Visualization. Hongqiang Xia: Visualization. Guozhang Chang: Conceptualization. Xiaowei Liu: Conceptualization, Supervision, Resources. Minghou Xu: Conceptualization, Supervision, Resources.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This study was supported by the National Natural Science Foundation of China (51906078 and 51922045) and the Foundation of the State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2021-K75). Additionally, the Analytical and Testing Centers at Huazhong University of Science & Technology are appreciated.

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