Biotemplating of Al₂O₃-Doped, CaO-Based Material from Bamboo Fiber for Efficient Solar Energy Storage

Abstract

The high-temperature sintering of CaO-based materials leads to the serious decay of energy storage performance during the calcination/carbonation cycle. To overcome the loss in porosity problem, an efficient CaO-based material for thermal energy storage was synthesized using bamboo fiber as the biotemplate. The synthesis parameters (bamboo fiber addition, pyrolysis, Al₂O₃ loading) and the energy storage reaction characteristics of CaO-based energy storage material were optimized on the basis of cyclic calcination/carbonation experiments. The results show that the sacrificed biotemplate enhances the porosity of the synthetic material, denoting improved energy storage density. The cumulative energy storage density of the templated material over 50 cycles is 24,131.44 kJ/kg higher than that of limestone. The carbonation conversion and energy storage density of the templated CaO-based material doped with 5 wt.% Al₂O₃ and 0.5 g bamboo fiber reach 0.75 mol/mol and 2368.82 kJ/kg after 10 cycles, respectively, which is 2.7 times as high as that of original limestone. The maximum apparent carbonation rate of the templated CaO-based materials in the 1st cycle corresponds to a 240% increment compared to limestone. The maximum calcination rate of the synthetic CaO-based material in the 12th cycle remains 93%, as compared with the initial cycle. The microstructure analysis reveals that the hierarchically-stable structure during the cycle is beneficial for a more effective exposure of surface reactive sites for CaO and inward/outward diffusion for CO₂ molecules through CaO. The method using the sacrificed biological template provides an advanced approach to fabricate porous materials, and the composite CaO-based material provides high-return solar energy storage for a potential application in industrial scale.

1. Introduction

With the growth of population and rapid economic development, global energy demand is expected to increase by 50% from 2005 to 2030 [1]. Fossil energy has dominated the world’s energy mode for a long time because of mature technology, high-energy storage-density, convenience for transportation and storage [2]. However, the reserves of fossil energy are limited and fossil energy is non-renewable. Besides, a large amount of exhaust gas and waste generated by the use of fossil energy pollute the air, soil and water. In particular, excessive CO₂ emissions also lead to climate change and the greenhouse effect [3,4]. The further development of society is hindered by the two major problems of environmental pollution and energy depletion. Therefore, the development and utilization of renewable energy has become increasingly urgent, which is also an important way to achieve the goal of “carbon neutrality” [5].

In recent years, solar thermal power generation technology has developed rapidly [6,7]. Solar energy is clean, renewable, abundant and widely distributed [8]. However, solar irradiation is intermittent, dispersive and unstable [9,10]. The large-scale storage of solar energy has become the key to the sustainable supply of energy in the field of solar thermal power generation. Among them, thermochemical energy storage has wide application prospects because of its advantages such as high-energy density, long storage-time, small-storage volume, no heat loss and the easy transportation of materials [11]. Energy storage based on the CaO/CaCO₃ cycle, i.e., the CaL process, has caused wide concern [12]. Further, CaCO₃ begins to decompose after absorbing heat at high temperature, then CaO and CO₂ are generated, during which the heat can be stored. The generated CaO and CO₂ will form CaCO₃ in a carbonator when heat is needed. The reaction can be carried out normally within the temperature range of 700–1000 °C [13].

The thermal stability and conversion rate of CaO-based material decrease with an increasing number of cycles at high temperature [14]. Chen et al. [15] found that the absorption efficiency of CaO-based energy storage material was only 35% after 50 cycles. Therefore, many scholars have focused their studies on the improvement of the cyclic energy storage efficiency of CaO. Inert carriers with a high melting point can be added to CaO-based energy storage materials to improve the sintering resistance. These inert carriers mainly include Al₂O₃ [16], MgO [17], SiO₂ [18], CeO₂ [19], TiO₂ [20] and Y₂O₃ [21] etc. Chen et al. [18] used mechanical mixing technology to blend SiO₂ with CaCO₃, then used a thermogravimetric analyzer (TGA) to further analyze the cyclic energy storage performance of the composite. The results showed that when the mass fraction of SiO₂ was 5%, the energy storage performance of the composite was the largest, and the stability was improved by 28%, while the addition of SiO₂ would reduce the calcination conversion of the composite. Bai et al. [20] prepared CaO-based particles doped with 25 wt.% TiO₂. After 20 cycles, the carbonation conversion and the energy density were 57.5% and 1827.7 kJ/kg, almost 1.3 times those of pure Ca(OH)₂ particles. Benitez-Guerrero et al. [22] utilized a mechanical mixing method to prepare CaO/Al₂O₃ composite materials. Ca₄Al₆O₁₃ was formed by the solid reaction of CaO and Al₂O₃, which alleviated the high-temperature sintering of CaO, and improved the heat storage stability of the composite. The effective conversion of the sample with 5 wt.% Al₂O₃ was 0.55 after 20 cycles. Obermeier et al. [23] examined CaO/Al₂O₃ composites prepared from calcium acetate and aluminum nitrate, and found that the heat storage density of the composite with the molar ratio of Ca/Al = 95:5 after 20 cycles was 3.5 times that of the unmodified CaO. Han et al. [24] compared the promotion effects of Al₂O₃, SiO₂ and TiO₂ on the heat storage performance of calcium-based materials, and found that, among the three additives, Al₂O₃ had the most positive influence on enhancing the heat storage stability of the samples. After 50 cycles, the heat storage density of the composite with 5 mol% Al was 1500 kJ/kg, which was equivalent to 87% of the theoretical maximum. Therefore, Al-based inert materials are promising for the stabilization of energy storage performance of CaO-based materials.

The template method can be divided into the soft template method [25,26] and the hard template method [27], according to the different templates selected. Viable template methods on improving porosity have been adopted to prepare calcium-based materials. Wang et al. [28] used carbon spheres as templates to prepare hollow spherical CaO/CaZrO₃ materials, which possessed a large specific surface area and good sintering resistance. Sayyah et al. [29] developed a hollow spherical calcium-based material containing Al₂O₃. After 30 cycles, the collection capacity of the composite reached 0.55 g CO₂/g material, which was five times that of limestone. Ma et al. [30] used carbon microspheres as templates, and the synthetic material with the addition of 5 wt.% Al₂O₃ had relatively stable CO₂ capture performance, with the CO₂ adsorption capacity of 0.37 g/g after 20 cycles. In addition, the pore volume and specific area were 2.62 times and 3.61 times as high as those of carbide slag after 20 cycles, respectively. Compared with traditional preparation methods, the CaO-based energy storage materials prepared by the template method possess controllable pore structure, which is beneficial for predictable energy storage density.

Although the above researches have been carried out on strategies to improve the energy storage performance of calcium-based materials, an understanding of the energy storage mechanism of calcium-based materials, the factors affecting the attenuation of energy storage capacity, and the thermal storage/release rate in the CaL process is still far from complete. In particular, the carbonation stage is basically carried out under pure CO₂ atmosphere and high temperature (>850 °C) during the energy storage stage [31]. The CaCO₃ product layer is rapidly formed and thus hinders the following reaction of inner CaO [32]. Therefore, the pore structure of calcium-based materials should be taken into consideration. In this work, the Al-based inert materials and biological template method mentioned above were adopted to design and fabricate a novel calcium-based energy storage material with a stable and porous structure. Compared to artificial templates, bamboo fiber is mainly composed of high-porosity cellulose, which is abundant and renewable. At the same time, the effects of synthesis parameters such as the template addition, the pyrolysis process and Al loading on the energy storage reaction characteristics of CaO-based materials were evaluated. The theoretical basis of the relationship between the energy storage density and structure property was provided.

2. Experimental

2.1. Experimental Materials

The limestone used in the test was mined from Guangxi Province, China. After crushing and screening, samples with a particle size of <0.125 mm were obtained. The chemical composition of limestone was determined by X-ray fluorescence (XRF), as shown in

Table 1. Chemical composition of limestone (wt.%).

Sample	CaO	MgO	SiO2	Al2O3	Fe2O3	SO3	TiO2	K2O	Na2O	Cl	P2O5	LOI	Others
Calcined limestone [33]	94.77	1.84	2.58	0.24	0.1	--	0.02	0.42	--	--	--	--	0.03
Limestone from Alberta [34]	47.8	2.3	9.05	1.61	1.47	0.59	0.13	0.65	0.07	--	--	38.52	0.01
Limestone in this work	50.69	2.2	2.63	0.55	0.24	0.78	0.02	0.11	--	0.09	0.19	42.4	0.29

. In addition, different species of limestone from literatures are listed as comparison. It can be noticed that the limestone in this work is a general one, and has practical significance for material design. The bamboo powder was sampled from a biotechnology company in Guangdong Province, China. The ash content of the bamboo powder was lower than 0.5 wt.%. The acetic acid (C₂H₄O₂) and aluminum nitrate (Al(NO₃)₃·9H₂O) used in the preparation of composite CaO-based materials were analytical pure reagents (content >99%).

2.2. Sample Preparation Process

The acetic acid solution was used for the pre-treatment of limestone under normal temperature and pressure. The CaO precursor was obtained after the neutralization reaction, filtration and drying processes. According to the CaO/Al₂O₃ mass ratio of 95:5, 3 g of the CaO precursor and a certain amount of Al(NO₃)₃·9H₂O were weighed and mixed in a beaker with 40 mL of deionized water. The loading of materials on the biological template was similar to the previous work [35]. The synthetic parameters were considered with the main focus on the optimization of energy storage performance of CaO-based materials, which was similar to the work from Sher et al. [36]. Different amounts of bamboo fiber (0.5 g, 1 g, 2 g and 7 g) were weighed and added into the beaker. The mixture was put into a water bath with a magnetic stirrer at 80 °C for 1 h. The sample was dried at 60 °C for 12 h. The solid was placed in a muffle furnace and pre-calcined at 600 °C for 1 h. The obtained CaO-based energy storage materials were abbreviated as BF_x-Al (x = 0.5, 1, 2 or 7), where x denoted the addition amount of bamboo fiber in the composite. In addition, the sample without Al₂O₃ loading, denoted as BF_x, was synthesized according to a similar procedure only without the addition of Al(NO₃)₃·9H₂O. The sample with pyrolyzed biotemplate was also prepared, which was denoted as PBF_x-Al. The pyrolysis of the bamboo fiber was performed under N₂ atmosphere through three procedures including heating, temperature stabilizing and temperature lowering stages. The heating rate, aimed temperature and holding time were set as 3 °C/min, 450 °C and 2 h, respectively.

2.3. Cyclic Experiment

The cyclic energy storage/release process was performed using the dual fixed-bed (DFB) including a calciner and a carbonator. The mass flow of gases into the reactors were controlled at 2 L/min by the mass flowmeters. In total, 200 mg of the sample was put into the porcelain boat and sent into the calciner at 850 °C under a pure N₂ gas stream for 10 min. The calcined sample was taken out, put into the dryer for cooling, and weighed using an electronic balance (accuracy of ±0.1 mg). The sample was sent to a furnace of carbonator at 850 °C under the pure CO₂ gas stream for 5 min. The mass of carbonated sample was also recorded. The aforesaid calcination/carbonation processes were repeated 10 times in this experiment. The relevant carbonation conversion (X_N) and the energy storage density (H_N) in the Nth carbonation were adopted to evaluate the energy storage performance of the CaO-based energy storage materials, as shown in Equations (1) and (2) [37]. These parameters were considered to be reliable and the application of various indicators was conducive to cover different experimental conditions [38].

$$X_N=\frac{m_N-m_{\mathrm{cal},N}}{m_0}\cdot \frac{M_{\mathrm{CaO}}}{M_{{\mathrm{CO}}_2}}$$

(1)

$$H_N=1000\cdot \frac{(m_N-m_{\mathrm{cal},N})\cdot \mathrm{\Delta }{H}^0}{M_{{\mathrm{CO}}_2}\cdot m_0}$$

(2)

where m_N and m_cal,N are the masses of the CaO-based energy storage material after the Nth carbonation and calcination, respectively, g; m₀ is the initial mass of the material, g; M_CO2 and M_CaO are the molar masses of CO₂ and CaO, respectively, and g/mol; △H⁰ is the standard enthalpy change of the reaction between CaO and CO₂, 178 kJ/mol. H_N refers to the maximum heat that can be released per unit mass of the CaO-based energy storage materials, kJ/kg.

The multicyclic carbonation conversion was fitted according to the semi-empirical equation [39,40], as shown in Equation (3):

$$X_N=X_{\mathrm{r}}+\frac{X_1}{k(N-1)+{(1-X_{\mathrm{r}}/X_1)}^{-1}}$$

(3)

where X₁ and X_r are the initial and residual carbonation conversions, and mol/mol; k is the attenuation factor.

2.4. Characterization

Surface morphologies were explored using a Regulus 8100 scanning electron microscope (SEM) and Octane Elect energy dispersive spectroscopy (EDS). The thermal carbonation and decomposition behaviors of the samples were studied using the HTC-3 thermo-gravimetric analyzer (TGA). About 6 mg of the sample was heated to 850 °C at a rate of 20 °C/min in a pure CO₂ stream (120 mL/min). This temperature was maintained for 15 min in pure CO₂ and a further 40 min in a pure N₂ stream. The changes of temperature and relative mass were recorded to determine the actual reaction kinetics of CaO-based energy storage materials.

The carbonation rate (u_N,t) and calcination rate (ν_N,t) were evaluated according to Equations (4) and (5), respectively. The decomposition ratio of the sample (D_N,t) was calculated, as shown in Equation (6):

$$u_N{}_{,t}=\frac{d(m_{N,t}^{}-m_{N,t-1}^{})}{m_0^{}dt}\frac{M_{\mathrm{CaO}}}{M_{{\mathrm{CO}}_2}}\begin{array}{cc}& \end{array}(0<t<t_1)$$

(4)

$$v_N{}_{,t}=\frac{d(m_{N,t}^{}-m_{N,t-1}^{})}{m_0^{}dt}\frac{M_{\mathrm{CaO}}}{M_{{\mathrm{CO}}_2}}\begin{array}{cc}& \end{array}(t_1\le t\le t_2)$$

(5)

$$D_N{}_{,t}=\frac{m_{N,\mathrm{t}1}^{}-m_{N,t}^{}}{m_{N,\mathrm{t}1}^{}-m_{N,\mathrm{t}2}^{}}\begin{array}{cc}& \end{array}(t_1\le t\le t_2)$$

(6)

where m_N,t is the mass of the sample at t min of the Nth cycle, g; t₁ and t₂ represent the end of the carbonation and calcination stages, respectively.

3. Results and Discussion

3.1. Effect of Biotemplate Addition on Energy Storage Performance of CaO-Based Materials

The influence of a bamboo fiber addition on the carbonation conversions of CaO-based energy storage materials is shown in Figure 1. According to the experimental results, the X₁₀ of composite CaO-based energy storage material with 2 g bamboo fiber is 0.53, which is 11% and 93% higher than those of acid-treated limestone and original limestone. In addition, the fitting curves of Equation (3) are obtained to provide the long-term energy storage capacities of the materials, and the parameters are summarized, as shown in

Table 2. Fitting parameters for multicyclic energy storage according to Equation (3).

Sample	X1	Xr	k	Residual Error
Limestone	0.64	0.11	0.27	0.01
Acid-treated limestone	0.93	0.003	0.09	0.06
BF2	0.87	0.01	0.07	0.03

. The results show that the residual errors are lower than 0.06. Therefore, the data of multicyclic carbonation conversion can be well-fitted by the model. BF₂ shows the more stable energy storage performance. For example, the decay rate of BF₂ is verified to be 5% lower than that of acid-treated limestone. The cumulative energy storage densities of different materials over 50 repeated cycles are compared, as shown in Figure 2. Compared with the other two samples, BF₂ exhibits higher energy storage density; a value that is 3911.88 kJ/kg and 24,131.44 kJ/kg higher than those of treated and untreated limestone. With the presence of bamboo fiber in the preparation environment, the sacrificed biomass could further induce the enhanced porosity of the modified composite on the basis of microstructure. Sun et al. [41] found powerful evidence on the relationship between the improved pore structure of CaO and the more active carbonation reaction. Bamboo fiber used as a biotemplate to prepare CaO-based energy storage materials can effectively improve the heat capacity in the cycle.

3.2. Effect of Support Loading on Energy Storage Performance of CaO-Based Materials

In order to verify the effect of adding Al₂O₃, the templated CaO-based materials with and without Al₂O₃ are compared, as shown in Figure 3. It is observed that when 5 wt.% Al₂O₃ is added to the templated CaO-based material, the carbonation conversion is 0.82 in the initial cycle, which is 5% lower than that without Al₂O₃. The result is attributed to the decreased CaO content in the composite material. The carbonation conversion of BF₂-Al after 10 cycles retains 76% of its initial value, corresponding to a 125% increment compared to BF₂. The X₁₀ of BF₂-Al is 2.3 times as high as that of original limestone. It can be seen from the experimental analysis that using Al₂O₃ as the inert support in the composite CaO-based energy storage material can significantly alleviate the sintering problem during the calcination/carbonation cycle, thereby improving the carbonation conversion of CaO.

3.3. Comparison of Energy Storage Performances of Different Templated CaO-Based Materials with Doped Al₂O₃

In the case of templated CaO-based materials with Al₂O₃ added, the energy storage performances with original biomass and biochar from pyrolysis are compared, as shown in Figure 4. It can be observed that the energy storage performance of the CaO-based material with biochar as the template is far worse than that of the CaO-based energy storage material with bamboo fiber. The X₁ of PBF₂-Al is only 0.59, and X₁₀ drops to 0.23. The data is even lower than that of limestone. It is speculated that this is due to the poor impregnation effect of the biochar template, where the calcium precursor could not be effectively dispersed around the biochar. There is no obvious change on the pore structure of the limestone with only the introduced ash of bamboo fiber. The analysis suggests that bamboo fiber as the template is more suitable for preparing CaO-based energy storage materials. Sher et al. [42] regarded biorenewable nanocomposites formed from biomaterials, e.g., cellulose and lignin, as important parts in energy storage applications, including batteries and supercapacitors. Although sustainable technologies for the direct conversion of sunlight into electricity have been developed, the process has poor efficiency [43]. The results obtained here are encouraging to achieve enhanced thermal energy efficiency for application in the generation of high-quality heat and cleaner electricity.

The carbonation conversions of the composite CaO-based energy storage materials, with different addition amounts of biomass, are shown in Figure 5. It is apparent that the X₁₀ of the templated CaO-based materials decrease with the increase of bamboo fiber content. The X₁₀ of BF_0.5-Al, BF₁-Al, BF₂-Al and BF₇-Al are 0.75, 0.69, 0.63 and 0.28, respectively. This phenomenon is due to the increased ash content as the addition amount of bamboo fiber increases. The energy storage density of BF_0.5-Al reaches 2368.82 kJ/kg after 10 cycles, which only drops by 12% compared with the 1st cycle. This result shows that the templated CaO-based materials with 0.5 g bamboo fiber have not only high-energy storage capacity, but also have strong anti-sintering ability. The experimental results of the carbonation conversions of composite CaO-based energy storage materials from literatures are demonstrated in

Table 3. Summary of various synthetic CaO-based energy storage materials in references.

Precursor and Methods	Reaction Conditions	Cycle No.	XN (mol/mol)	Reference
Calcium citrate hydrate and aluminum acetylacetonate, space-confined chemical vapor deposition method	Calcination: 750 °C, 100% N2, 10 min; carbonation: 850 °C, 100% CO2, 5 min	20	0.75	[6]
Calcium hydroxide and titanium dioxide, extrusion-spheronization method	Calcination: 750 °C, 100% N2, 10 min; carbonation: 850 °C, 100% CO2, 6 min	20	0.575	[20]
Limestone and powdered nanoalumina, mechanical mixing method	Calcination: 900 °C, 70% CO2/30% air, 5 min; carbonation: 850 °C, 100% CO2, 5 min	20	0.55	[22]
Limestone and ZrO2, mechanical milling method	Calcination: 1000 °C, 100% CO2; carbonation: 850 °C, 100% CO2	11	0.35	[23]
Ca(NO3)2·4H2O and rice husk, biotemplate method	Calcination: 725 °C, 100% He, 5 min; carbonation: 850 °C, 100% CO2, 5 min	20	0.34	[44]
Ca(NO3)2, Dy(NO3)3 and Al(NO3)3, Pechini method	Calcination: 1000 °C, 100% CO2; carbonation: 850 °C, 100% CO2, 30 min	40	0.51	[45]
Limestone and Al(NO3)3·9H2O, template method	Calcination: 850 °C, 100% N2, 10 min; carbonation: 850 °C, 100% CO2, 5 min	10	0.75	This work

. Cost-effective treatments have been investigated to enhance the CaO multicycle stability, such as mechanical mixing, extrusion-spheronization, the biotemplate method, etc. Compared to other synthetic materials, the templated CaO-based materials with bamboo fiber exhibits high residual carbonation conversion. Biomaterials as the pore-forming additives fulfil the demands of efficient and innovative materials for energy storage applications.

3.4. Reaction Kinetic Evaluation and Morphological Characterization

To detect the endothermic and exothermic nature of the reactions, TGA tests are performed on BF_0.5-Al and limestone sampled from different cycles of DFB experiment after the 1st and 11th calcination. The comparison of the carbonation kinetics, along with time and the programmed temperature, is shown in Figure 6. As can be observed, apparent carbonation occurs at about 8 min with 40 °C, as shown in Figure 6a. During the first 30 min, the 1st carbonation stage of BF_0.5-Al and limestone are nearly complete. Then, X_N increases slowly during the following time. Finally, the X_N of BF_0.5-Al are 46% and 73% higher than those of limestone in the 1st and 11th cycles, respectively. The carbonation rates under relatively high temperature (>400 °C) are analyzed, as shown in Figure 6b. The u_N,t of BF_0.5-Al and limestone in the 1st carbonation reach a peak at 25 min at around 570 °C, after which they decrease gradually. The corresponding u_1,25 of BF_0.5-Al is 0.07 mol·mol⁻¹·s⁻¹, which is 2.4 times as high as that of limestone. After 10 cyclic processes, the maximum u_N,t of BF_0.5-Al is still 40% higher, compared with the data of limestone. The apparent carbonation rate of CaO has a close link to the CO₂ diffusion from the physical perspective, and the reactivity of CaO surface from the chemical perspective. In agreement with previous results, it is demonstrated that the superior structure skeleton of synthetic Ca/Al-based materials obtained by the biotemplating method improves the carbonation kinetics.

The calcination details at a constant temperature (850 °C) are given in Figure 7. The CaCO₃ is almost completely decomposed (D_N,t > 99%) to CaO within 10 min for both BF_0.5-Al and limestone, as shown in Figure 7a. In addition, the calcination rates exhibit obvious peak values, as shown in Figure 7b. The BF_0.5-Al shows apparently higher calcination rates, compared with limestone. For example, the maximum ν_10,t of BF_0.5-Al is 0.15 mol·mol⁻¹·s⁻¹, which retains around 93% of the initial value in the 1st calcination, and the data is 1.4 times that of limestone. The calcination includes the CaCO₃ decomposition and the outward diffusion of CO₂. Sher et al. [46] also proved that the mass transfer rates were in near agreement with the reactivities of material under a similar reaction environment. The effective CO₂ diffusion in return contributes to the faster CaCO₃ decomposition. In conclusion, the kinetic results reflect the good porosity of BF_0.5-Al for the cyclic heat storage/release application.

The microstructure test shows that the calcined limestone exhibits plain surfaces and BF_0.5-Al obtained by the biotemplating method exhibits more hierarchical structures, as compared between Figure 8a,b. Valverde et al. [10] demonstrated that a less porous structure corresponded to a decrease in the heat released from exothermic reaction. Ma et al. [30] proved that the better pore characteristics favored the better contact between CaO particles and CO₂, resulting in the higher carbonation degree. The sintering of limestone after 10 calcination/carbonation cycles can be observed with the agglomeration of CaO particles denoting coarsening, as shown in Figure 8c. Moreover, the surface of limestone is broken into smaller pieces due to the local densification. The grain growth due to sintering problem generates physical barriers to CO₂ diffusion, which leads to the decreased carbonation conversion and energy storage density. Further, BF_0.5-Al shows slight densification and is less affected by the cyclic reactions at high temperature, as shown in Figure 8d. The result can be attributed to the difference in pore distribution during synthesis and the even dispersion of Al₂O₃, as shown in Figure 9. The calculated mass ratio of CaO to Al₂O₃ in BF_0.5-Al is 94.4:5.6, according to the EDS analysis in

Table 4. The quantitative result of elements in BF_0.5-Al from EDS analysis.

Element	Weight (%)	Atomic (%)
Ca	45.34	25.16
O	52.68	73.22
Al	1.98	1.63

, which is consistent with the fabrication parameter mentioned above. This result explains the theory of the rapid kinetically-controlled calcination and carbonation stage of BF_0.5-Al, since the stable hierarchical structure increases the accessible active sites for inward CO₂ adsorption, and surface area for outward CO₂ diffusion during calcination.

4. Conclusions

This work is focused on highly efficient and stable CaO-based materials for the thermochemical energy storage (TCES) of concentrated solar power (CSP), based on the CaL process. The composite material was prepared using limestone, aluminum nitrate and bamboo powder. The effects of synthesis parameters on the energy storage performance of CaO-based materials were evaluated. According to the experimental results, the sacrificed biotemplate enhances the porosity of CaO-based materials, and thus the carbonation rate. Biochar from pyrolysis and an excessively high addition of biomass are detrimental to the carbonation conversion. When the mass ratio of CaO:Al₂O₃ is 95:5, and the amount of bamboo powder added is 0.5 g, the carbonation conversion reaches 0.75 after 10 cycles, which is 2.7 times the data for natural limestone. The corresponding energy storage density of the composite is 2368.82 kJ/kg, with a slight drop of 12% compared with the 1st cycle. The templated CaO-based material shows apparently higher carbonation and calcination rates, compared with limestone. Combining the kinetic results with morphological characteristics, it can be concluded that the good porosity of BF_0.5-Al, and the uniform distribution of Al₂O₃ support are beneficial for the cyclic heat storage/release application. The long-term cumulative energy storage density demonstrates that CaO-based materials fabricated by the biotemplating method allow for an economically and ecologically-effective manner for solar energy storage.

The utilization of a biorenewable template can meet the quality expectations with porous materials, which is expected to play an important part in the production of advanced materials. In future studies, the better filtration of various biomaterials would further improve the energy storage efficiency of CaO. It would also be useful to extend understanding on the relationship between microstructure property and the reaction kinetics of materials. In addition, a comprehensive assessment on the implications for the industrial-scale deployment of our results is beyond the scope of this work. More intelligent models should be developed to ensure a reliable techno-economic evaluation of synthetic CaO-based energy storage materials in practical applications.