Patent Publication Number: US-2023140129-A1

Title: Thermal battery

Description:
TECHNICAL FIELD 
     This disclosure relates generally to thermal batteries that use a CO 2  sorbent such as CaO/CaCO 3 . 
     BACKGROUND 
     Concentrated solar thermal power (CSP) plants are becoming a widespread renewable energy source supplementing photovoltaics and wind power. First generation CSP plants store thermal energy using the specific heat of molten salt, i.e. a 0.4NaNO 3 -0.6KNO 3  mixture, with an operating temperature between 290° C. and 565° C. The low operating temperature and low energy density (413 kJ/kg) of the molten salt technology results in a high energy storage cost. Thus, higher operating temperatures, efficiencies, and lower costs are desired. A wide variety of materials have been suggested as the successor to molten salt. Emerging thermochemical energy storage technologies show promise, including gas-solid systems such as metal hydrides and metal carbonates, which have high energy densities (651-8397 kJ/kg) making them attractive as thermochemical energy storage (TOES) materials. The quantity of calcium carbonate required to store 1 TJ of energy is only 4% the cost of molten salts on a materials basis. 
     One of the major problems of using CaCO 3  as a TOES material is that CaCO 3  degrades with increasing number of CO 2  release and absorption cycles. For example, the CO 2  capacity in CaCO 3  drops to only ˜8% of its initial capacity after 500 cycles. The cause of the capacity loss may be assigned to a range of events, including a loss of porosity in the formed CaO, sintering of the CaCO 3  due to the temperatures required for CO 2  cycling (e.g. &gt;800° C.), and the limited CO 2  diffusion through CaCO 3 . 
     Various attempts have been made to improve the cyclic stability of CaCO 3 -based TOES materials. A steam reactivation process can be used to replenish the cyclic capacity in CaCO 3  through the formation of Ca(OH) 2 , which can favourably alter the particle morphology. However, the effect of forming Ca(OH) 2  is strongly dependent on temperature, steam content, and must be performed every cycle to maintain decent cyclic capacity. Further, steam reactivation must be performed at relatively low temperatures, usually below 560° C., due to the thermodynamics for Ca(OH) 2  formation, which often involves a further step to decrease the reactor temperature for steam reactivation. Due to these constraints, it is not feasible to operate a CSP thermochemical storage system that requires steam reactivation. 
     It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country. 
     SUMMARY 
     The disclosure provides a method of storing energy. The method comprises heating a material comprising a CO 2  sorbed product and an additive to desorb CO 2  from the material and convert the CO 2  sorbed product to a CO 2  sorbent. The additive is selected to at least partially prevent during heating: sintering of the CO 2  sorbent and/or the CO 2  sorbed product; and the formation of a crust on the material. The crust can minimise or prevent the CO 2  sorbent and CO 2  from reacting with one another to form the CO 2  sorbed product in a subsequent CO 2  absorption step. 
     The disclosure provides a method of storing energy. The method comprises applying heat to material comprising CaCO 3  and an additive to desorb CO 2  from the material to form CaO, the CaO being able to react with CO 2  in a further step to reform CaCO 3 . The additive is selected to at least partially prevents sintering of CaO/CaCO 3  and the formation of a crust that prevents CaO and CO 2  from reacting with one another to form CaCO 3    
     The disclosure provides a method of storing energy. The method comprises applying heat to sorb or desorb CO 2  from material comprising CaO and/or CaCO 3  and an additive. The additive is selected to at least partially prevent sintering of CaO/CaCO 3  and the formation of a crust that prevents CaO and CO 2  from reacting with one another to form CaCO 3 . 
     The disclosure provides a method of storing energy. The method comprises desorbing CO 2  from material comprising CaCO 3  and/or CaO and an additive comprising Zr- or Al-based species, wherein the step of desorbing CO 2  includes heating the material between 600° C. and 1200° C. to convert CaCO 3  to Ca0. The additive is selected to at least partially prevent during heating: (i) sintering of CaO/CaCO 3 ; and (ii) the formation of a crust on the material. The crust minimise or prevent CaO and CO 2  from reacting with one another to form CaCO 3  in a subsequent CO 2  absorption step. 
     The disclosure provides a composition used to sorb and desorb CO 2  in a thermal battery. The composition comprises a form of calcium that is capable of absorbing or desorbing CO 2  to, respectively, form a CO 2  sorbed product or CO 2  desorbed product; an additive having a concentration ranging from about 5 wt. % to about 95 wt. % relative to an amount of the CO 2  sorbed product; wherein the additive at least partially prevents during heating of the composition sintering of the CO 2  sorbed/desorbed product and the formation of a crust that minimises or prevents the CO 2  desorbed product and CO 2  from reacting with one another to form the CO 2  sorbed product. 
     The disclosure also provides a system for storing energy, comprising: a reactor comprising a material that is capable of absorbing or desorbing CO 2  to, respectively, form a CO 2  sorbed product or CO 2  desorbed product, the material having an additive that at least partially prevents during heating: sintering of the CO 2  sorbent/sorbed product; and the formation of a crust on the material, the crust minimising or preventing the CO 2  sorbent and CO 2  from reacting with one another to form the CO 2  sorbed product; and a CO 2  source that is in fluid communication with the reactor to allow a flow of CO 2  between the reactor and CO 2  source during absorption or desorption of CO 2 . 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Embodiments of the disclosure will now be described, by way of example only, with reference to the following non-limiting Figures. 
         FIG.  1    shows a schematic representation of an embodiment of a system of the disclosure; 
         FIG.  2    shows CO 2  absorption data of various CaCO 3 -additive samples over 50 calcination-carbonation cycles at 900° C. for 30 min carbonation and 20 min calcination. 
         FIG.  3    shows CO 2  absorption data comparing the influence of ZrO 2  content and ball-milling parameters at 900° C. for 30 min carbonation and 20 min calcination. 
         FIG.  4    shows CO 2  absorption data comparing the influence of Al 2 O 3  content and extended calcination/carbonation time for up to 50 calcination-carbonation cycles at 900° C. All samples were cycled for 30 min carbonation and 20 min calcination, except the “Ext.” sample that was cycled for 60 min carbonation and 60 min calcination. 
         FIG.  5    shows CO 2  absorption data of CaCO 3 —Al 2 O 3  (20 wt. % bulk) over 500 calcination-carbonation cycles at 900° C. with varying calcination/carbonation times. 
         FIG.  6    shows an expansion of  FIG.  5    from cycle #350 to cycle #500. 
         FIG.  7    shows powder X-ray diffraction (PXD) data of CaCO 3 —ZrO 2  (20 wt. %) of the as-milled sample (bottom) and CO 2  absorbed samples (top) after 50 calcination/carbonation cycles showing the formation of ternary compounds after cycling. Wavelength=1.54056 Å. 
         FIG.  8    shows PXD data of CaCO 3 —Al 2 O 3  (20 wt. %) of the as-milled sample (bottom) and CO 2  absorbed samples (top) after 50 calcination/carbonation cycles showing the formation of ternary compounds after cycling. Wavelength=1.54056 ∈. 
         FIG.  9    shows PXD data of CaCO 3 —Al 2 O 3  (20 wt. %) absorbed after 500 calcination/carbonation cycles. Wavelength=1.54056 Å. 
         FIG.  10    shows in situ PXD data of CaCO 3 —Al 2 O 3  (20 wt. %, bulk) at 917° C. The CO 2  pressure profile is indicated to the right. Carbonation performed for ˜20 min and calcination for ˜30 min in a total of 5 cycles. The bottom of the figure signifies the start of the cycles. Symbols denote: triangle is CaCO 3 ; diamond is Ca0; circle is Al 2 O 3 ; square is Ca—Al—O derived compounds. Wavelength=0.590458 Å. 
         FIG.  11    shows in situ PXD data of CaCO 3 —ZrO 2  (40 wt. %, bulk) at 917° C. The CO 2  pressure profile is indicated to the right. Carbonation performed for ˜20 min and calcination for ˜30 min in a total of 5 cycles. The bottom of the figure signifies the start of the cycles. Symbols denote: triangle is CaCO 3 ; diamond is CaO; circle with addition sign is ZrO 2 ; pentagon is CaZrO 3 . Wavelength=0.590458 Å. 
         FIG.  12    shows scanning electron microscopy data comparing the morphologies of the as-milled (left column) and cycled samples (right column), and energy dispersive spectroscopy showing the elemental distribution of aluminium and zirconium in the respective samples (Al: purple; Zr: yellow). a-b: CaCO 3 ; c-f: CaCO 3 —Al 2 O 3 ; g-k: CaCO 3 —ZrO 2 . 
         FIG.  13    shows an experimental setup based on CO 2 -storage in either A) activated carbon with pressure generated by the thermal profile of the activated carbon (Scenarios 1&amp;2) or B) a carbon dioxide compressor (Scenario 3), from Example Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System. 
         FIGS.  14   a    &amp;  14   b  show Pressure-Composition-Isotherms of the activated carbon in the pressure range utilized in Example Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System. 
         FIG.  15    shows comparison of the different scenarios investigated in Example Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System. 
         FIG.  16    is a ‘zoom-in’ depiction showing the rapid temperature spikes observed upon carbonation in Scenario 3 (Example Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System). 
         FIG.  17    shows a Scanning Electron Microscope (SEM) micrograph of CaCO 3 —Al 2 O 3  sample after thermochemical cycling near 900° C., from Example Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System. 
         FIG.  18    shows a thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) of CaCO 3 —ZrO 2 (13.3 wt %)-Al 2 O 3 (13.3 wt %) from room temperature to 1000° C. (ΔT/Δt=10° C. min −1 ) under argon flow (20 mL min −1 ), from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  19    shows isothermal calcination/carbonation data of a CaCO 3 —ZrO 2 (13.3 wt %)-Al 2 O 3 (13.3 wt %) sample over 50 CO 2  cycles at T˜900° C. and p carbonation ˜5 bar and p calcination &lt;0.8 bar for 20 and 30 minutes, respectively, from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  20    shows a comparison of reaction kinetics between pristine CaCO 3  (green), CaCO 3 —ZrO 2  (20 wt %, red), CaCO 3 —Al 2 O 3  (20 wt %, blue) and the combined CaCO 3 —ZrO 2 (13.3 wt %)-Al 2 O 3 (13.3 wt %, grey), from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  21    shows powder X-ray diffraction data of CaCO 3 —ZrO 2 —Al 2 O 3  samples after 50 CO 2  cycles in the desorbed state, from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  22    shows in situ SR XRD data (λ=0.82502 Å) of CaCO 3 —ZrO 2 —Al 2 O 3  at T=917° C., from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  23    shows Scanning electron microscopy (SEM) pictures and energy-dispersive X-ray spectroscopy (EDS) mapping of as-prepared CaCO 3 —Al 2 O 3 —ZrO 2  and after 50 CO 2  capacity cycles, from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
         FIG.  24    shows SAXS data showing the Porod region (power law slope=−4) from where the specific surface area (SSA) is determined, from Example adding Mixtures of Oxides to Limestone (CaCO 3 ). 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     An embodiment of the disclosure provides a method of storing energy. The method comprises desorbing CO 2  from a material comprising a CO 2  sorbed product and/or a CO 2  sorbent and an additive. The step of desorbing CO 2  includes heating the material to convert the CO 2  sorbed product to the CO 2  sorbent. The additive is selected to minimise during heating: sintering of the CO 2  sorbent/sorbed product; and the formation of a crust on the material. Such a crust can minimise or prevent the CO 2  sorbent and CO 2  from reacting with one another to form the CO 2  sorbed product in a subsequent CO 2  absorption step. 
     The CO 2  sorbent is a species that is capable of absorbing CO 2 . The CO 2  sorbent may be calcium-based. In one embodiment, the CO 2  sorbent is CaO and the CO 2  sorbed product is CaCO 3.  The sorbent may be MgO, BaO and/or SrO, and the sorbed product may be the respective carbonate form e.g. MgCO 3 , BaCO 3  and/or SrCO 3 . The CO 2  sorbent may comprise a mixture of species capable of sorbing/desorbing CO 2 . For example, the CO 2  sorbent/sorbed product may include species such as dolomite (CaMg(CO 3 ) 2 ). The material may initially be provided as CaO that is reacted with CO 2  to form CaCO 3  in a step prior to desorbing CO 2 . The formed CaCO 3  may then be desorbed using the disclosed method. Alternatively, CaCO 3  may be initially provided and then subject to heating to desorb CO 2 . An advantage of initially providing CaCO 3  is that it should not react with any CO 2  or H 2 O present in the environment, for example, during assembly of a thermal battery, making it simpler to calculate the required amount of additive needed. The additive may be added to CaO and/or CaCO 3 . The CO 2  sorbed product and CO 2  sorbent may form different phases. The material may be formed from particles that aggregate together. Aggregation may change a morphology of the material. The particles may be particles of the CO 2  sorbed product and CO 2  sorbent. 
     Throughout this disclosure, the term “sorbing” and “absorption” are used interchangeably, and the terms “desorbing” and “desorption” are used interchangeably. 
     The crust may reduce the permeability of, or be impervious to, CO 2  diffusion at a surface of the material (e.g. particle) to an interior of the material. For example, for a CaO particle, absorption of CO 2  at the surface forms a CaCO 3  crust. If the particle is large enough, such as through growth due to sintering processes, the presence of the crust provides a layered structure having a CaO core and a CaCO 3  crust encapsulating the core. The core-shell or layered structure will generally have a lower CO 2  absorption capacity compared to a non-core-shell structure as the CaCO 3  crust prevents diffusion of CO 2  into the CaO core. Likewise, the CaCO 3  crust may prevent the migration of CaO from the core to a surface of the particle to allow CaO to react with CO 2  at the surface. Put simply, the crust may prevent CaO and CO 2  from reacting with one another. 
     The additive may help to separate the CO 2  sorbed product phase and CO 2  sorbent phases. This separation may help to prevent the CO 2  sorbed product phase and CO 2  sorbent phases from combining and growing in size e.g. by sintering. 
     The additive may prevent the formation of the crust. However, in some embodiments, the additive prevents the formation of a contiguous crust. Put another way, the additive may form a discontinuous crust that is formed from two different species. For example, for CaO/CaCO 3 , the additive may allow the formation of isolated regions of CaCO 3  that are disbursed around the particle. The areas between the disbursed CaCO 3  regions may allow CO 2  to permeate into the particle and/or allow CaO from a core of the material/particle to migrate to a surface of the material/particle to react with any CO 2  present at the surface. The additive may form the areas between the CaCO 3  regions. In an embodiment, during the CO 2  absorption step, the additive allows the CO 2  sorbent to migrate through the material from an inner region to a surface to react with CO 2  present at the surface of the material to form the CO 2  sorbed product. For example, the additive may facilitate Ca 2+  and/or O 2−  mobility through the material. 
     The heating step may be performed at a temperature greater than about 600° C., such as about 800° C. The heating step may be performed at a temperature less than about 1200° C. In an embodiment, the heating step is performed at a temperature ranging from about 800° C. to about 1000° C. In an embodiment, the heating step is performed at a temperature of about 900° C. At these temperatures, species such as CaO tend to sinter. For systems and methods that rely on CaO/CaCO 3  to absorb or desorb CO 2 , sintering can significantly limit the use of CaO/CaCO 3  to absorb or desorb CO 2 . A problem with sintering is that it increases a size of the material/particles. For particles of CO 2  sorbent, increasing the size of the particles beyond a threshold amount generally results in a reduction of the CO 2  absorption capacity of the CO 2  sorbent. Therefore, reducing or eliminating sintering may help to prevent or slow down a decrease in the CO 2  absorption capacity of the CO 2  sorbent during use of the CO 2  sorbent for storing energy. 
     The temperature that the heating step is performed at may be selected to minimise or eliminate the formation of species that impede the ability of the particle to sorb or desorb CO 2 . For example, the additive may react with CaO and/or CaCO 3  to form species that impede the ability of the additive to prevent sintering and the formation of the crust. The temperature may be selected to promote favourable Carnot efficiencies whilst minimising the formation of undesirable by-products, such as sintered particles. Heat transfer during sorbing and desorbing CO 2  may be performed under isothermal conditions. 
     The additive may be a metal oxide having at least one metal. The additive may be a ternary compound. The additive may be provided as an additive precursor. The additive precursor may react with the CO 2  sorbent and/or CO 2  sorbed product to form the additive in situ during heating to convert the CO 2  sorbed product to the CO 2  sorbent. When an additive precursor is used, the heating conditions and the number of times sorption and desorption steps are performed may determine the type of additive formed. In an embodiment, the additive precursor includes Al 2 O 3  and/or ZrO 2 . When Al 2 O 3  is used as the additive precursor, the Al 2 O 3  may react with CaCO 3  to form a calcium aluminate species as the additive. When ZrO 2  is used as the additive precursor, the ZrO 2  may react with CaCO 3  to form a CaZrO 3  additive. The additive precursor may comprise Al 2 O 3  and ZrO 2  together. 
     The additive and/or additive precursor may comprise one or more species. For example, the additive may include CaAl 2 O 4 , Ca 3 Al 2 O 6 , Ca 5 Al 6 O 14 , Ca 9 Al 6 O 18 , Ca 12 Al 14 O 33  or a combination thereof. The additive may include a mixture of CaZrO 3  and a calcium aluminate species. In an embodiment, the additive comprises Ca 5 Al 6 O 14 . The Ca 5 Al 6 O 14  may react to form Ca 9 Al 6 O 18 . A mixture of Ca 5 Al 6 O 14  and/or Ca 9 Al 6 O 18  may form a major additive fraction and CaAl 2 O 4  and/or Ca 3 Al 2 O 6  may form a minor additive fraction. The major fraction may comprise &gt;95 wt. % of the additive. The types of species formed may be determined by the thermodynamics of the reagents at the temperature used for sorption/desorption. For example, the major fraction may comprise &gt;95% CaZrO 3  and/or calcium aluminate. The ratio of [Ca 5 Al 6 O 14 ]:[Ca 9 Al 6 O 18 ] may range from about [100]:[0] to about [0]:[100]. The ratio of [Ca 5 Al 6 O 14 ]:[Ca 9 A 16 O 18 ] may start at about [100]:[0] and change to about [0]:[100] over a number of cycles of CO 2  sorption and desorption. For example, when Al 2 O 3  is used as the additive precursor, the Al 2 O 3  may first react with CaCO 3 /CaO to form Ca 5 Al 6 O 14 , and the Ca 5 Al 6 O 14  may then partially convert to Ca 9 Al 6 O 18  to provide a ratio of [Ca 5 Al 6 O 14 ]:[Ca 9 Al 6 O 18 ] over a number of cycles. In an embodiment, a combination of Ca 5 Al 6 O 14  and Ca 9 Al 6 O 18  may comprise &gt;95% of the additive. In an embodiment, a ratio of [Ca 5 Al 6 O 14 ]:[Ca 9 Al 6 O 18 ] is about [50]:[50]. The ratio of [Ca 5 Al 6 O 14 ]:[Ca 9 Al 6 O 18 ] may reach about [50]:[50] after 500 cycles. 
     The additive may act as an “oxygen conductor” that allows oxygen-species such as oxygen ions to migrate from a surface of the material towards or away from an interior of the material. In addition to or in place of the additive may act as an “calcium conductor” that allows calcium-species such as calcium ions to migrate from a surface of the material towards or away from an interior of the material. Reference to calcium in the “calcium conductor” does not limit the ability of the additive to allow only migration of calcium ion, and the additive may allow migration of ions of other metal species such as Mg, Ba and/or Sr. Put another way, the additive may act as an “ion conductor” that allows metal ions to migrate from a surface of the material towards or away from an interior of the material. The additive may also act as “CO 2  sorbent carrier” that allows the CO 2  sorbent, such as CaO, to migrate from an interior of the material towards a surface of the material to allow the CO 2  sorbent to react with CO 2  present at the surface of the material. For example, without being bound by theory, it is thought that when the CO 2  sorbent is CaO and the additive includes Ca 6 Al 6 O 14 , Ca 2+  in the Ca 6 Al 6 O 14  may dissociate to allow a calcium species to react with CO 2  to form CaCO 3 . At the same time, CaO located elsewhere, such as towards an interior of the material, may react with the additive to reform Ca 5 Al 6 O 14 . In this way, the additive may react as a “calcium conductor” that allows conduction of calcium through the material. Another possible mechanism may be that, when Ca 9 Al 6 O 18  acts as the additive, the crystal structure of the additive is built from Al 6 O 18  ‘rings’ with Ca 2+  inside the rings. However, only 72 of possible 80 positions are occupied by Ca 2+  leaving room for Ca 2+ to ‘jump’ between the positions to allow migration of Ca 2+  though the additive and material. Put simply, the additive may not react as such, but may act as a conduit for Ca 2+  ions, feeding them from one location to another as needed. 
     This example is made with reference to calcium and an additive containing aluminium, but the disclosure is not limited to the use of calcium and an additive containing aluminium and other species such as CaZrO 3  could act as the “oxygen conductor” and “CO 2  sorbent carrier”. 
     The method may further comprise a step of sorbing CO 2  onto the CO 2  sorbent to reform the CO 2  sorbed product. The step of reforming the CO 2  sorbed product releases energy. The released energy can be captured and used to power any system that is capable of generating electricity, such as a turbine generator, heat engine, etc. The heat can also be provided to an industrial process (e.g. directly). The step of sorbing and desorbing CO 2  onto/from the material may be carried out under isothermal conditions. 
     The effect of the additive depends on the type of additive and its concentration. In an embodiment, a concentration of the additive ranges from about 5 wt. % to about 95 wt. % relative to the amount of CO 2  sorbed product (e.g. CaCO 3 ). In some embodiments a concentration of the additive ranges from, about 10 wt. % to about 70 wt. %, about 10 wt. % to about 50 wt. %, or about 20 wt. % to about 40 wt. %. When the additive reacts with the CO 2  sorbed product and/or CO 2  sorbent, such as when an additive precursor is used to form the additive, the CO 2  absorption capacity of the material decreases due to the reduced amount of CO 2  sorbent. When the additive reacts with the CO 2  sorbed product and/or CO 2  sorbent, this decrease in the amount of CO 2  sorbent should be taken into consideration when determining the CO 2  absorption capacity. 
     A cycle of desorbing CO 2  from the material and absorbing CO 2  onto the material may be repeated at least 500 times. The time used for CO 2  absorption and CO 2  desorption may be the same. The time for CO 2  absorption and CO 2  desorption may differ. The time used for CO 2  absorption and CO 2  desorption may change with an increasing number of cycles. For example, a time of 1 hour may initially be used for CO 2  absorption and CO 2  desorption, then after a certain number of cycles a time greater than 1 hour may be used for CO 2  absorption and CO 2  desorption. The CO 2  absorption capacity of the material may be changed by changing the CO 2  absorption time. For example, after a number of cycles using a constant CO 2  absorption and CO 2  desorption time, the CO 2  absorption capacity of the material may drop, but absorbing CO 2  for an extended period of time may increase the CO 2  absorption capacity of the material. Providing a step of extended CO 2  absorption may be used to “regenerate” the material to increase the CO 2  absorption capacity. Therefore, an embodiment of the method includes regenerating the material by subjecting the material to an extended CO 2  absorption step. Desorption of CO 2  may be performed under reduced pressure compared to the pressure used to absorb CO 2.  A pressure used for CO 2  absorption and CO 2  desorption may be up to about 5.0 bar. For example, desorption may be performed under vacuum. Pressures above 5.0 bar may be used, for example, up to about 60 bar. However, for such higher pressures, the use of pressure vessels and the like can be required. This can increase the costs of building and maintaining a system used to perform the disclosed method. 
     The pressure used for CO 2  storage can be up to about 100 bar. The CO 2  may be stored in the form of a gas, liquid or in a supercritical state. The CO 2  may also be stored in the form of another metal carbonate. 
     To form the material, CaCO 3  and/or CaO may first be milled before use in the disclosed method. Milling helps to increase a surface area of the material and to disburse e.g. the additive or additive precursor. In an embodiment the material is milled to a size less than 10 μm. The as-milled materials may have the CO 2  sorbed product and additive precursor distributed as discrete regions/particles. However, upon heating, the CO 2  sorbed product/CO 2  sorbent and the formed additive may become evenly distributed. Upon heating, a morphology of the material may change from particulate matter to a porous structure. It should be noted that milling is not required in all embodiments. 
     The CO 2  may be provided as a gas. The CO 2  may be provided as a supercritical fluid. The temperatures and pressures used by the disclosed method prevents the use of liquid CO 2 . The CO 2  may be entrained in a carrier gas and the CO 2  sorbent absorbs the CO 2  entrained in the carrier gas. The carrier gas may act as a heat transfer fluid. When a carrier gas is used to deliver entrained CO 2 , the CO 2  may have a concentration of about 400 ppm. In an embodiment the CO 2  has a purity &gt;95%, such as &gt;99%. In an embodiment, the CO 2  is pure i.e. has a purity 99.95%. The method may be performed in the absence of water. Whilst the presence of water may form hydroxyl/hydroxide species, such as Ca(OH) 2 , at the high temperatures of operation, the formation of hydroxyl/hydroxide species is unlikely. Hence, at such high temperatures, having some moisture in the gas stream will not be an issue, as it will not be reactive at such temperatures, and it may even facilitate good heat transfer. Thus, typically the CO 2  sorbent and CO 2  sorbed product will be free from hydroxyl/hydroxide species. 
     A system used for storing energy is shown in  FIG.  1   . The system is in the form of a thermal battery system  10 . System  10  has a reactor  12  that houses material  14  that can store thermal energy. In one form, the material  14  that can store thermal energy is the disclosed material/particles. In an embodiment, the material  14  is a form of calcium that is capable of absorbing CO 2  or desorbing CO 2  to form a CO 2  sorbed product or CO 2  desorbed product. A CO 2  source  15  is in fluid communication with the reactor via conduits  16  and  18 . Conduits  16  and  18  act as an input/output line to the CO 2  source  15 . However, in an embodiment only one conduit acts as the input/output line to the CO 2  source. A heating system  20  is in thermal communication with the reactor  12 . The heating system  20  can take many forms. In one form, the heating system uses renewable power, such as photovoltaic- or wind-based power, to heat the reactor  12 , but in particular the material. The heating system may include a dish used to concentrate thermal energy from the sun, such as that utilised for a solar-powered Stirling engine. Other forms of heating systems are included within the scope of the current disclosure. Depending on the type of heating system used, a heat exchanger (not shown) may be used in conjunction with a heat transfer fluid to heat the material  14 . The heat transfer fluid may be a gas fed into the powder bed itself, or be the CO 2  working gas. 
     In use of the system  10 , the material  14  is heated to desorb CO 2 . The material  14  may be heated to about 800° C. to about 1000° C. by the heating system  20 . The desorbed CO 2  is then transferred from the reactor  12  to the CO 2  source  15  via the conduits  16  or  18 . It should be appreciated that one of the conduits  16  or  18  acts as an inlet into the CO 2  source  14  and the other of the conduits  16  or  18  acts as an outlet from the CO 2  source  14 . Conduits  16  and/or  18  may be provided with control valves, one-way valves, expansion chambers and/or pumps to assist in removing any CO 2  desorbed in the reactor  12 . 
     The CO 2  source  15  can take many forms. For example, the CO 2  source  15  could be a vessel capable of storing CO 2  either in gas, liquid or supercritical form. The CO 2  source  15  could include or be formed from materials capable of storing CO 2 , such as one or more of: molecular sieves (zeolites), metal organic frameworks, nanomaterials, and activated carbon. Activated carbon has the advantage of being cost effective and readily available. An Example using activated carbon is described below. 
     An advantage of using materials to store CO 2  is that a pressure of the CO 2  in the system can be controlled by adjusting a temperature of the material capable of storing CO 2  instead of using compressors and pumps. The CO 2  source could be a carrier gas that has a component of CO 2 . For example, air having about 400 ppm to about 600 ppm CO 2  could be used as the CO 2  source  15 . When materials capable of storing CO 2  are used as the CO 2  source  15 , the system  10  may have expansion valves located on conduits  16  and/or  18  and heat exchangers in communication with the expansion valves and/or heating system  20  to control the temperature of the CO 2  source  15 . In the embodiment shown in  FIG.  1   , the system  10  has a system for generating electricity  24 . In one embodiment, the system for generating electricity has a steam turbine generator  24  that utilises heat generated by the reactor  12  during absorption of CO 2 . In some forms, instead of a steam turbine generator, the system for generating electricity can employ a heat engine. A heat exchanger  22  is used to transfer heat generated by the reactor  12  to the system for generating electricity  24 . 
     In use of the system  10 , the material is heated so that CO 2  is desorbed from a CO 2  sorbed product such as CaCO 3  to form a CO 2  desorbed product such as CaO. The desorbed CO 2  is transferred to the CO 2  source  15 . The step of desorbing CO 2  stores chemical energy in the material  14  in the reactor as the CO 2  desorbed product. When the energy stored in the reactor  12  is required, CO 2  from the CO 2  source  15  is transferred to the reactor  12  where the CO 2  desorbed product can react with CO 2  to reform the CO 2  sorbed product thereby converting chemical energy to thermal energy. The process of absorption and desorption may be performed isothermally. The process of absorption and desorption may be performed at conditions close to isothermal conditions. For example, the temperature may fluctuate by about 5% on either side of the isothermal temperature. The thermal energy released upon formation of the CO 2  sorbed product is captured by the heat exchanger  22  and is transferred to the system for generating electricity  24 . This process of desorption and absorption forms a cycle that is repeated as many times as energy storage and discharge is required. 
     The system  10  has been described as generating heat for electricity generation, but the system  10  is not limited to generating heat for electricity generation. For example, heat generated by the system  10  could be used for other applications, such as a heat source for example in industrial processes. 
     EXAMPLES 
     Embodiments of the disclosure will now be explained with reference to the following non-limiting Examples. 
     Example—Additive Enhanced Thermochemical Energy Storage Properties of Limestone 
     Methods 
     Sample Preparation 
     Different compositions were formed by mixing 4 g of CaCO 3  with the additives listed in Table 1. 10 mL of ethanol was then added, and the mixtures were ball-milled in stainless steel vials for 2 hours (15×1 min×8 reps; 12×8 mm stainless steel balls). After ball-milling, the samples were dried in an oven at 105° C. for approximately 1 hour to obtain a dry powder. Note, that the above procedure was carried out in an argon-filled glovebox for the sample with the Ni additive, which was dried by applying dynamic vacuum. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Overview of additives used for CaCO 3 -based compositions. 
               
            
           
           
               
               
               
               
            
               
                   
                 Wt. %  
                 Mol. %  
                 Vol. %  
               
               
                 Additive 
                 additive 
                 additive 
                 additive 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 C (graphite; 98-99%) 
                 20 
                 67.6 
                 23.0 
               
               
                 Al 2 O 3   
                 10, 20, 40 
                 9.8, 19.7, 39.6 
                 7.1, 14.6, 31.4 
               
               
                 13 nm, 99.8% 
                   
                   
                   
               
               
                 SiO 2   
                 20 
                 29.4 
                 20.3 
               
               
                 10-20 nm, 99.5% 
                   
                   
                   
               
               
                 Fe 2 O 3   
                 20 
                 13.5 
                 11.5 
               
               
                 &lt;50 nm 
                   
                   
                   
               
               
                 Ni 
                 20 
                 29.9 
                  7.1 
               
               
                 &lt;100 nm, ≥99% 
                   
                   
                   
               
               
                 ZnO 
                 20 
                 23.5 
                 10.8 
               
               
                 dispersion, 40 wt. %  
                   
                   
                   
               
               
                 in EtOH, &lt;130 nm 
                   
                   
                   
               
               
                 ZrO 2   
                 20, 40 
                 16.9, 35.1 
                 10.7, 24.1 
               
               
                 &lt;100 nm 
                   
                   
                   
               
               
                 Zeolite Y, Na 
                 20 
                 13.4 
                 16.9 
               
               
                 5.1:1 SiO 2 :Al 2 O 3  molar ratio 
                   
                   
                   
               
               
                 Zeolite Y, H 
                 20 
                 13.4 
                 16.9 
               
               
                 80:1 SiO 2 :Al 2 O 3  molar ratio 
                   
                   
                   
               
               
                 Zeolite Mordenite, Na 
                 20 
                 13.4 
                 16.9 
               
               
                 13:1 SiO 2 :Al 2 O 3  molar ratio 
                   
                   
                   
               
               
                 BaCO 3   
                 9.5 
                 5  
                  6.1 
               
               
                   
               
            
           
         
       
     
     Sieverts&#39; Method/Pressure-Composition-Isotherm 
     Samples were introduced in a SiC sample cell, which was attached via Swagelok parts to a Hy-Energy PCTpro E&amp;E. The sample was heated to ˜900° C. (ΔT/Δt=5° C. min −1 ) at p(CO 2 )=10 −2  bar, hence decomposing (desorbing) the sample. Subsequently, cycling of the sample was initiated at isothermal conditions (˜900° C.) with an absorption at p(CO 2 )˜5 bar for 30 mins in a 46.3 cm 3  volume, followed by desorption at p(CO 2 )˜10 −2  bar for 20 mins in a 206.7 cm 3  volume. A total of 50 cycles was collected for all samples. Finally, the samples were absorbed and cooled under p(CO 2 )˜5 bar. The cycling was extended to 500 cycles for the CaCO 3 —Al 2 O 3  (20 wt. %) sample using the same conditions as described above except calcination/carbonation times were varied. 
     Powder X-Ray Diffraction 
     X-ray diffraction (XRD) on powdered samples was performed on a Bruker D8 Advance diffractometer equipped with a CuK α1,12  source in flatplate geometry mode. Data were collected using a Lynxeye PSD detector from 15-70° 2θ at 0.02° steps. 
     In situ Synchrotron Radiation Powder X-Ray Diffraction 
     In situ time-resolved Synchrotron Radiation Powder X-ray Diffraction (SR-PXD) data were collected at the Powder Diffraction beamline at the Australian Synchrotron, Melbourne, Australia on a Mythen microstrip detector at λ=0.590458 Å. Powdered samples were loaded into quartz capillaries (i.d.=0.5 mm, o.d.=0.6 mm), which were attached to a gas system enabling control of CO 2  pressure. The samples were heated by a heat blower to 950° C. at ΔT/Δt=6° C. min −1  while rotating during data acquisition. Temperature calibration was performed with NaCl and Ag. 
     Scanning Electron Microscopy 
     Scanning electron microscopy (SEM) and energy dispersive spectroscopy were performed using a Tescan Mira3 field emission SEM with an Oxford Instruments X-Max SDD X-ray detector and AZtec software. The SEM images were collected using a backscattered electrons detector, an accelerating voltage of 15 kV, an aperture size of 30 μm, and a working distance of ˜15 mm. SEM samples were prepared by embedding powdered samples in an epoxy resin and polished using colloidal silica, which were eventually sputter coated with a 10 nm thick carbon layer. 
     Results &amp; Discussion 
     An overview of the samples prepared is given in Table 1. Generally, the CO 2  absorbing capacity of a CaO/CaCO 3  system decreases dramatically within the first 10 cycles (30 min carbonation, 20 min calcination). Eventually most of the samples have a CO 2  absorbing capacity of ˜14% after 50 calcination/carbonation cycles, which is less than when pure CaCO 3  is used as the starting (sorbed) material, which reaches a capacity of ˜17%,  FIG.  2   . However, a few samples showed promise to improve the cyclic stability of the CaCO 3 . The SiO 2  and NaY sample react in a similar way with CaCO 3  to form spurrite (Ca 5 (SiO 4 ) 2 CO 3 ), and during cycling they both stabilise at ≈16-20% capacity, which is slightly better than for pure CaCO 3 . Graphite addition aids in a slower capacity loss, though, after 50 cycles the CO 2  absorbing capacity is similar (˜20%) to samples when SiO 2  and NaY are used as the additive. 
     Addition of 20 wt. % ZrO 2  retains the capacity at ˜80% within the first 10 cycles, but a steady degradation of the sample is observed and at the end of 50 cycles the capacity is reduced to ˜55%. Similarly, the addition of 20 wt. % Al 2 O 3  results in a steady capacity degradation and after 50 cycles it reaches ˜49%. 
     To further improve the CO 2  capacity, samples of varying weight ratios of ZrO 2  and Al 2 O 3  were investigated. As evident from  FIG.  3   , addition of 40 wt. % ZrO 2  is superior to only 20 wt. %, as the addition of 20 wt. % ZrO 2  indicates a continuous capacity decrease but such capacity decrease is not observed with an addition of 40 wt. % ZrO 2 . Additionally, extended ball-milling (10 hours) of the 20 wt. % ZrO 2  sample, which should result in smaller particles at the initiation of calcination/carbonation, eventually follows the same trend as for the same sample, but ball-milled for only 1 hour. Hence, an initially smaller particle size does not seem to play a role in determining CO 2  absorbing capacity when the sample is cycled multiple times. 
       FIG.  4    compares different ratios of Al 2 O 3  additive added to CaCO 3  and the influence of starting from bulk or nanoparticle Al 2 O 3  (10-20 nm based on TEM, Sigma-Aldrich). The optimum ratio is found to be 20 wt. % of either bulk or nanoparticle Al 2 O 3  with a CO 2  capacity of ˜49% after 50 cycles. Similar to samples having ZrO 2 , the initial particle size of Al 2 O 3  does not seem to have an influence on the CO 2  absorbing capacity. Eventually, the CaCO 3 -20 wt. % Al 2 O 3  sample was cycled 500 times under varying calcination/carbonation times, see  FIG.  5    and  FIG.  6   . The capacity retention after 500 cycles is &gt;80% when carbonation times are extended to &gt;12 hours. Additionally, the response time is fast and capacity remains moderate as lowering the calcination/carbonation times to 30 and 20 min, respectively, still accounts for 50% capacity. Hence, the CaCO 3 -20 wt. % Al 2 O 3  system shows remarkable energy storage properties, i.e. response time and capacity, which make an embodiment of this system suitable for energy storage applications, such as a thermal battery. 
     Comparison of powder X-ray diffraction (PXD) data of the as-milled samples and the carbonated samples after 50 calcination/carbonation cycles at 900° C. reveals that the additive reacts to form further additive products, which may partly explain the decreasing CO 2  absorption capacity of the samples.  FIG.  7    shows the PXD data of the CaCO 3 —ZrO 2  (20 wt. %) and  FIG.  8    shows the PXD data of the CaCO 3 —Al 2 O 3  (20 wt. %), which highlights that a reaction between CaCO 3 /CaO and the additive has occurred according to the following respective reaction schemes: 
       CaO(s)+ZrO 2 (s)→CaZrO 3 (s)
 
       5CaO(s)+3Al 2 O 3 (s)→Ca 5 Al 6 O 14 (s)
 
     Minor quantities of side products, such as CaAl 2 O 4  and Ca 3 Al 2 O 6  are also observed after 50 cycles for the samples that include Al 2 O 3 . These results show that in some embodiments the Al 2 O 3  acts as an additive precursor that reacts with CaO during heating and during calcination/carbonation to form the additive. 
     Extending the cycling studies to 500 cycles ( FIG.  9   ) for samples having Al 2 O 3  reveals the conversion of minor fractions of CaAl 2 O 4  and Ca 3 Al 2 O 6  into primarily Ca 6 Al 6 O 14  and Ca 9 Al 6 O 18  whilst a small fraction of Mayenite (Ca 12 Al 14 O 32 , &lt;3 wt. % from Rietveld refinement) is also observed. 
     The ZrO 2  and Al 2 O 3  enhanced CaCO 3  systems show cyclic capacities that are greatly enhanced compared to other tested additives ( FIGS.  2 - 6   ). A simple additive that only restricts CaO/CaCO 3  sintering is not as effective to capacity retention. An important difference is the properties of the as-formed ternary oxides, CaZrO 3  and Ca x Al y O z . Some features that differentiate these ternary oxides from other additives include their ability to conduct ions at high temperatures and their ability to enable a higher CO 2  diffusion through the material. Closely related Ca 12 Al 14 O 33  (Mayenite) is reported to be an oxide ion conductor, and the layered structure of Ca 6 Al 6 O 14  is hypothesised to facilitate Ca 2+  mobility. O 2−  and Ca 2+  migration through the additive structure can thus improve reaction kinetics and be beneficial in retaining the CO 2  capacity. The possibility of CaO migration is assigned to the low intrinsic defect formation energy of 1.61 eV to create a Ca-site Schottky type disorder in CaZrO 3 . In  FIG.  10   , in situ XRD data of CaCO 3 -Al 2 O 3  shows the initial decomposition of CaCO 3  as Bragg reflections from CaO increases in intensity. As CO 2  gas is applied to the system, the CaO Bragg reflections decrease rapidly in intensity. Throughout the CO 2  absorption and desorption cycling, Ca—Al—O containing compounds continuously form, although in minor fractions. It is assumed that the forming products are the same as the ones observed in ex situ PXD data ( FIGS.  8  and  9   ), but the few, low intensity, Bragg reflections makes identification unreliable. 
       FIG.  11    shows the in situ data of CaCO 3 —ZrO 2  (40 wt. %, bulk). The immediate formation of CaZrO 3  and fast depletion of ZrO 2  (within ˜1 hour/1 cycle) is evident, and the amount of CaZrO 3  quickly reaches ˜65 wt. % of the sample (theoretically 67.1 wt. %). Furthermore, the crystallite size of CaCO 3  and CaO doubles and triples, respectively, over the 5 cycles applied here, which eventually may result in a capacity decrease due to presence of large crystallites. However, the evident formation and consumption of CaO shows the quick response of the system to calcinate/carbonate. 
     Table 2 shows the relative amounts, extracted from Rietveld refinements, of CaCO 3 , CaO, CaZrO 3  and ZrO 2  during carbonation and calcination. The quantity of CaZrO 3  quickly increases then stabilises at about 77 wt. % whilst at the same time the amount of ZrO 2  decreases then stabilises at about 1 wt. %. The ratio of [CaCO 3 ]:[CaO] changes from about [27 wt. %]:[0.4 wt. %] to about [10 wt. %]:[13 wt. %] during carbonation and calcination. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Parameters extracted from Rietveld  
               
               
                 refinement, ZrO 2  (40 wt %) sample. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 CaCOs [wt. %]  
                   
                   
                   
               
               
                 Cycle# 
                 (crystallite size, nm) 
                 CaO 
                 CaZrO 3   
                 ZrO 2   
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 0 (carbonated) 
                   46 (115) 
                 3 (50) 
                  15 (6.7) 
                 36 
               
               
                 1 (calcinated) 
                   14 (422) 
                 13 (161) 
                   65 (19.6) 
                 8 
               
               
                 1 (carbonated) 
                 28.8 (245) 
                 0.4 
                 65.8 (23) 
                 5 
               
               
                 2 (calcinated) 
                   10 (565) 
                 13 (171) 
                   74 (24) 
                 3 
               
               
                 2 (carbonated) 
                   26 (227) 
                 0.4 
                   71 (26) 
                 2.6 
               
               
                 3 (calcinated) 
                 9   
                 12 (159) 
                   77 (27) 
                 2 
               
               
                 3 (carbonated) 
                   25 (234) 
                 0.5 
                   73 (28) 
                 1.5 
               
               
                 4 (calcinated) 
                 9.5 
                 12 (147) 
                   77 (29) 
                 1.5 
               
               
                 4 (carbonated) 
                   25 (204) 
                 0   
                 73.5 (26) 
                 1.5 
               
               
                 5 (calcinated) 
                 9.4 
                 12 (125) 
                 77.4 (27) 
                 1.2 
               
               
                 5 (carbonated) 
                   25 (206) 
                 0.5 
                 73.5 (27) 
                 1 
               
               
                   
               
            
           
         
       
     
     Scanning electron microscopy was used to analyse the particle morphology of as-milled and cycled samples, see  FIG.  12   . The as-milled CaCO 3  consists of finely divided particles in the size range 2-8 microns ( FIG.  12   a   ). The morphology significantly changes into a worm-like, porous structure after CO 2  absorption/desorption cycling ( FIG.  12   b   ). The porosity of the structure after cycling should promote permeation of CO 2  through the structure, but the interconnected particle morphology of CaCO 3  ( FIG.  12   b   ) seems to retard carbonation. Energy dispersive spectroscopy (EDS) reveals minor impurities of iron (Fe), which originates from the ball-milling, and MgO/MgCO 3  impurities from the commercial grade CaCO 3 . The as-milled CaCO 3 —Al 2 O 3  sample ( FIG.  12   c   ) has particles similar to that of bulk CaCO 3  (i.e.  FIG.  12   a   ) but also has very small particles 100 nm), which is assigned to the hardness of Al 2 O 3  that may assist in creating smaller particles during milling. After cycling, the bulk CaCO 3  turns into a maze of particles ( FIG.  12   d   ), but the presence of alumina as an additive, or additive precursor to form e.g. Ca 5 Al 6 O 14  and Ca 9 Al 6 O 18 , may allow the separation of the CaO/CaCO 3  particles by the Al 2 O 3 . Both the CaCO 3  and CaCO 3 —Al 2 O 3  sample morphology is porous, which allows beneficial CO 2  diffusion through the particle structure. Furthermore, the as-milled CaCO 3 —Al 2 O 3  has specific regions with aluminium, i.e. Al 2 O 3  ( FIG.  12   e   ), whereas the cycled sample ( FIG.  12   f   ) has aluminium more evenly distributed throughout the same areas as it formed an additive e.g. Ca 5 Al 6 O 14 . In the CaCO 3 —ZrO 2  sample ( FIG.  12    g), the Zr appears to be well distributed in both the as-milled sample as ZrO 2  ( FIG.  12   j   ) and after cycling as CaZrO 3  ( FIG.  12   k   ). It is noted that the ZrO 2  contains a small impurity of Hf. 
     Perspectives/Outlook/Thermal Battery 
     A cost comparison of the proposed TOES materials, CaCO 3 , and the state-of-the-art molten salt technology is provided in Table 3. The $3000/tonne price of ZrO 2  makes the price per terajoule in the CaCO 3 —ZrO 2  (40 wt. %) system significantly more expensive than the state-of-the-art molten salts. However, Al 2 O 3  is more abundant and thus cheaper, i.e. $376/tonne. Hence, the materials cost can be reduced by ˜95% per terajoule electrical energy produced if the molten salt is replaced with CaCO 3 —Al 2 O 3  (20 wt. %). The operating pressure of &lt;6 bar CO 2  reduces the engineering challenges and costs, while the CO 2  may be stored in a zeolite or activated carbon by physisorption, which removes the energy demanding compression of the CO 2  gas during storage. Supercritical CO 2  may be utilised as the heat transfer fluid at 900° C., which makes it compatible with, e.g. the Rankine-Brayton combined cycle or the Stirling engine. The latter is highly efficient at 900° C. (theoretically η˜72%). Overall, the high energy density storage material and small footprint may enable the utilisation in Stirling dishes, which are dispatchable and may thus be ideal for remote areas with a requirement for power, e.g. minesites. Further, the thermal battery enables seasonal storage of a wide variety of renewable energy from, e.g. wind mill farms, photovoltaics, and CSP plants. The disclosed thermal battery, may maintain a 90% capacity up to 500 cycles, comparable to Li-ion batteries, which typically reach a capacity of 80%, defined as the batteries cycle life, after 1000 to 4500 cycles, corresponding to a lifespan between 7 and 20 years. Finally, the disclosed thermal battery may hold important safety features: (i) the chemical reactions are limited by equilibrium pressure, which prevents the reactions from running wild; (ii) hot, corrosive fluids, e.g. molten salt, is not present; and (iii) the compounds are not flammable. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Cost comparison of materials 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Molten Salt 
                 CaMg(CO 3 ) 2  ⇄  
                   
                 CaCO 3  ⇄ 
                 CaCO 3  ⇄ 
               
               
                   
                 (40 NaNO 3 : 
                 MgO +  
                 CaCO 3  ⇄ 
                 CaO + CO 2  +  
                 CaO + CO 2  +  
               
               
                   
                 60 KNO 3 ) 
                 CaCO 3  + CO 2   
                 CaO + CO2 
                 40 wt. % ZrO 2   
                 20 wt. % Al 2 O 3   
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Enthalpy ΔH (kJ/mol) 
                 39.0 
                 125.8 
                 165.5 
                 165.5 a   
                 165.5 a   
               
               
                 Molar Mass (g/mol) 
                 94.60 
                 184.40 
                 100.09 
                 100.09 a   
                 100.09 a   
               
               
                 Density (g/cm 3 ) 
                 2.17 
                 2.85 
                 2.71 
                 2.71 a   
                 2.71 a   
               
               
                 Capacity (wt. % CO 2 ) 
                 — 
                 23.9 
                 44.0 
                 44.0 a   
                 44.0 a   
               
               
                 Gravimetric Energy Density (kJ/kg) 
                 413 
                 682 
                 1657 
                 1657 a   
                 1657 a   
               
               
                 Volumetric Energy Density (MJ/m 3 ) 
                 895 
                 1944 
                 4489 
                 4489 a   
                 4489 a   
               
               
                 Operating Temperature Range (° C.) 
                 290-565 
                 ~590 
                 900 
                 900 
                 900 
               
               
                 Carnot Efficiency (%) 
                 46 
                 65 
                 74 
                 74 
                 74 
               
               
                 Estimated Practical Efficiency (%) 
                 27 
                 41 
                 49 
                 49 
                 49 
               
               
                 Mass Required (tonnes) 
                 9100 
                 3598 
                 1228 
                 6385 
                 3250 
               
               
                 Volume Required (m 3 ) 
                 4194 
                 1262 
                 453 
                 1863 
                 1124 
               
               
                 Materials Cost ($/tonne) b   
                 630 
                 50 
                 10 
                 1206 
                 83 
               
               
                 Total Materials Cost Required ($) 
                 5,733,289 
                 179,887 
                 12,298 
                 7,700,530 
                 270,397 
               
               
                   
               
               
                   a Relates only to the active CaCO 3  part of the sample; 
               
               
                   b as of 2019. 
               
            
           
         
       
     
     Conclusion 
     A series of twelve CaCO 3 -additive systems have been systematically investigated. The CaCO 3  is observed to decompose between 763° C.-851° C. depending on the additive, which is attributed to the size effects after ball-milling. Neat CaCO 3  has a capacity retention of ˜15% after 50 calcination/carbonation cycles. However, addition of ZrO 2  (40 wt. %) or Al 2 O 3  (20 wt. %) shows the remarkable ability to enable an 80% CO 2  retention over &gt;100 cycles with fast kinetics and where the sample is fully calcined/carbonated within 1 hour. Hence, the disclosed system may be suitable for use as a thermochemical energy storage material in a thermal battery operating at 900° C., which offers higher Carnot efficiency compared to other forms of thermal batteries, while the materials cost and footprint is significantly lowered, e.g. 95% of the materials cost. 
     Example—Scale-up of a CaCO 3 —Al 2 O 3  (16.7 wt %) System 
     A scale-up of the CaCO 3 -Al 2 O 3  (16.7 wt %) system to 3.2 kg of material was investigated in three different configurations:
         (i) activated carbon was utilised as a CO 2  storage material and thermally controlled to regulate the calcination and carbonation reaction through a generated pressure gradient. Furthermore, the thermodynamic equilibrium pressure of CaCO 3  was varied in a first scenario through lowering of the temperature to 850° C. upon carbonation and raising it to 950° C. upon calcination.   (ii) the sample temperature is kept constant at 900° C. while utilising the activated carbon storage method.   (iii) the activated carbon was substituted with a compressor to achieve a significant under/overpressure upon calcination/carbonation, i.e. 0.5 bar and 5-6 bar, respectively, compared to the 1 bar equilibrium pressure at 900° C.       

     Scenarios (i) and (iii) showed a 64% energy capacity retention at the end of a 10th cycle. The decrease in capacity was assigned to the formation of Mayenite, Ca 12 Al 14 O 33 , which was considered an unwanted by-product. 
     Finally, a 316 L stainless-steel reactor was investigated to establish corrosion issues when treated under CO 2  atmosphere and 850° C. for approximately 1400 hours. X-ray diffraction reveal oxidation of the exterior of the reactor to Fe 2 O 3 , while the interior seems intact as Fe 3 O 4 . 
     Sample Preparation 
     3011 g of CaCO 3  (Sigma-Aldrich, &gt;99.0%) was hand-mixed with 603.71 g of Al 2 O 3  (Sigma-Aldrich, Puriss. 98%), i.e. in a 16.7 wt % ratio, before it was poured into a 10 L plastic container and shaken thoroughly together. The powder was then mixed/milled continuously for 1 hour in batches of ˜250 g in a custom-made 650 mL 316 stainless-steel vial containing 55 stainless-steel balls, o.d.=10 mm and m=465.2 g, using a Glen Mills Turbula T2C shaker mixer operating at 160 RPM. 
     Powder X-Ray Diffraction 
     In-house powder X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer equipped with a CuKα 1,2  source in flat-plate geometry mode. Data were collected using a Lynxeye PSD detector in the 2θ-range 10-80° in steps of 0.02°. The phases were identified using the EVA Bruker software and the International Centre for Diffraction Data (ICDD) PDF4 database. The diffraction peaks were quantitatively analysed by the Rietveld method using the Bruker TOPAS Version 5 software. 
     Scanning Electron Microscopy 
     Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed on a Tescan Mira3 FESEM coupled with an Oxford Instruments X-Max SDD X-ray detector and AZtec software. SEM images were collected using a secondary electron (SE) and backscattered electron (BSE) detector, an accelerating voltage of 20 kV, and a working distance of ˜15 mm. SEM samples were prepared by either depositing powders onto a stub or embedding samples in an epoxy resin, which was polished using colloidal silica. Eventually the samples were sputter-coated with a 10 nm thick carbon layer before imaging ( FIG.  17   ). 
     Thermal Conductivity Analysis 
     A TPS 500S (Hot Disk, Thermtest) was used for thermal analysis, featuring a double nickel spiral sensor, laminated by electrically insulating Kapton. A C7577 (2 mm radius) sensor was employed and calibrated using stainless steel, polystyrene, and NaCl standards. A piece of the solid material obtained from the reactor, was polished to achieve a smooth surface, and thermal properties were measured 10 times on two different areas of a sample. 
     Pressure-Composition-Isotherm (PCI) 
     Pressure-composition-isotherm experiments were performed on activated carbon to determine the CO 2  storage properties of the material. The sample, 2.0252 g of activated carbon, was placed into a stainless-steel high-temperature sample cell, which was attached to a custom-made Sieverts&#39; apparatus. An absorption and a desorption curve were obtained under isothermal conditions at T=20, 60, 100, and 120° C. by increasing/decreasing the pressure in steps of 1 bar (+/−0.5 bar) between p(CO 2 )=1-20 bar at each temperature. The activated carbon was, in each pressure step, kept at the determined pressure for 30 minutes to reach an equilibrium ( FIGS.  14   a  and  14   b   ). 
     Design of Experimental Setup for CO 2  Cyclic Capacity Measurements 
     A custom-made 316L stainless-steel reactor (2″ tubing, i.d. 4.5 cm, length 138 cm, V=2194.8 cm 3 ) was filled with 3197g of material, i.e. CaCO 3 —Al 2 O 3  (16.71 wt %). This resulted in 1792 g of active material, i.e. CaCO 3 , when the reaction with Al 2 O 3  was completed, see reaction scheme 2, and allowed for 788 g of CO 2  to be cycled. The stainless steel reactor was placed in a furnace (Furnace Technologies, model P44) capable of maintaining 900° C. for long periods of time, and connected to a custom made gas system. Thermocouples were installed at the gas inlet end of the reactor, in the middle, and at the far end of the gas inlet to monitor the temperature in different areas of the reactor as function of carbonation/calcination. The gas system was built from standard Swagelok connections and consisted of a CO 2  inlet, blow-off, and vacuum outlet, which allowed the system to be evacuated for moisture and to load it with CO 2  gas. The absolute pressure and differential pressure across an orifice (diameter 0.75 mm) between the CO 2  storage gas bottles and the stainless-steel reactor was measured by a Rosemount pressure transmitter (3051SMV). The measured flow rate allowed calculations of the mass flow of CO 2  and thus the CO 2  capacity of the CaCO 3  over multiple cycles, through the equation: 
     
       
         
           
             
               
                 
                   
                     q 
                     m 
                   
                   = 
                   
                     
                       
                         C 
                         
                           
                             1 
                             - 
                             
                               β 
                               4 
                             
                           
                         
                       
                       · 
                       ε 
                       · 
                       
                         π 
                         4 
                       
                     
                     ⁢ 
                     
                       
                         d 
                         2 
                       
                       · 
                       
                         
                           2 
                           ⁢ 
                           Δ 
                           ⁢ 
                           
                             p 
                             · 
                             
                               ρ 
                               1 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     eq 
                     ⁢ 
                         
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
     where C is the discharge coefficient, β is the ratio between the orifice and pipe diameter (0.16), ε the expansibility factor (˜1), d is the orifice diameter (0.075 cm), Δp the differential pressure in Pa, and ρ is the gas density (ref.  I. Bell, NIST Reference Fluid Thermodynamic and Transport Properties Database  ( REFPROP )  Version 9- SRD  23,  National Institute of Standards and Technology ). 
     In scenario 3, an installed pressure transmitter (Rosemount 3051S) on the gas bottle side was used to calculate the amount of moles of CO 2  in the gas bottles, based on the real gas law, pv=nRTZ, and eventually the mass of CO 2  in and out of the bottles. Once connected to the gas system, the CaCO 3  reactor was heated to 150° C. in vacuo for 48 hours to eliminate moisture in the system whilst tubing was thoroughly heated with a heat gun. Three different scenarios were evaluated where the CO 2  storage and reactor temperature were varied: 
     CO 2  Storage in Activated Carbon: 
     In Scenario 1 and 2, ˜20 kg of activated carbon was used for controlling the CO 2  pressure by varying the temperature of the activated carbon between 20° C. (lowering CO 2  pressure resulting in calcination of the CaCO 3 ) and 110° C. (increasing CO 2  pressure resulting in carbonation of CaO), which uptook/released about -4.1 wt % CO 2 , see  FIGS.  14   a    and  14   b.  The activated carbon was kept inside aluminium bottles (gas bottle 1 and 2, V=14.7 L each), which were heated/cooled by a Huber CC-505 water bath set to 15 or 120° C. in 12 h intervals (3 hours for heating or cooling and 9 hours maintaining the set temperature). Thermocouples were placed on the outside of the gas bottles, which were further insulated by 200 mm wool. Similar to the CaCO 3  reactor, vacuum was applied to the activated carbon vessels while heating them to 125° C. for 48 hours to eliminate the presence of moisture in the system. 
     Scenario 1: The temperature of the activated carbon was varied between 20 and 110° C., while the temperature of the material was varied between 850 and 950° C. (1 hour for heating/cooling and 11 hours at the set temperature) to create the largest driving force for calcination/carbonation. 
     Scenario 2: The temperature of the activated carbon was varied between 20 and 110° C., while the temperature of the material is kept constant at ˜900° C. 
     Before initiating scenario 3, the CaCO 3 /Al 2 O 3 -material was fully ‘charged’ by applying ˜4 bar of CO 2  pressure for ˜10 days. 
     CO 2  storage in Pressure Vessels: 
     Scenario 3: A CO 2  compressor (HASKEL 86990), a pressure transmitter (Rosemount 3051S), and a pressure regulator was installed between the CO 2  gas storage bottles and the pressure transmitter/reactor, see  FIG.  13   , which were utilised to keep the calcination pressure below 0.7 bar and apply ˜5-6 bar on the sample for carbonation. 
     Results &amp; Discussion 
     Energy Storage Capacity 
     Scenario 1: This scenario represented a large thermodynamic driving force for calcination and carbonation due to the temperature fluctuation of the furnace, i.e. the CaCO 3 —Al 2 O 3  material. Despite the large driving force, the CO 2  capacity, i.e. the energy capacity, degraded over the 10 cycles from 732 g CO 2  (of the theoretically 788 g CO 2 , 92.9%) to 483.7 g CO 2  (61.4%) on calcination. Although, the last four cycles fluctuated slightly around 500 g of CO 2  (63-64%). The decrease in capacity to around 63% was explained by the two different kinetic regions observed, where the slowest kinetics appeared to be dominating above 350 g of CO 2  absorbed. Self-heating of the material may degrade the capacity during rapid CO 2  absorption. Thus, within the timeframe of the experiment, the sample was not able to absorb more than 63% of the full capacity. The slow reaction kinetic region may be overcome by applying a CO 2  overpressure (as highlighted in scenario 3), but the pressure provided from the activated carbon may simply not be sufficient to provide adequate kinetics, despite the decrease in equilibrium pressure at 850° C. 
     Scenario 2: The influence of maintaining the sample (CaCO 3 /Al 2 O 3 ) temperature constant at 900° C. was reflected in the absorption kinetics, which appeared slower, probably due to the higher equilibrium pressure at 900° C. (compared to absorption at 850° C.) and thus the smaller overpressure that may restrict the reaction kinetics. The CO 2  cycling was initiated with a limited carbonation that reached 383.68 g CO 2  (46.79%) and a subsequent calcination ending at 420.72 g CO 2  (53.39%), which indicated a fraction of CO 2  left from the first scenario. The CO 2  capacity gradually dropped throughout the cyclic measurements ending at a CO 2  release of 276.64 g (35.11%). In comparison with scenario 1 the second scenario displayed slower reaction kinetics, which was assigned to the smaller driving force created when maintaining the sample temperature at 900° C. This further caused the CO 2  capacity to gradually decrease. 
     Scenario 3: The full CO 2  capacity of the material was restored before initiating this scenario. The first calcination showed full CO 2  desorption, i.e. 788 g of CO 2  was released. Subsequently, 750 g of CO 2  (˜95.2% capacity) was absorbed during the first carbonation, whereas the capacity slightly decreased throughout the 10 cycles. The calcination curves plateaued, whereas the carbonation curves did not finish within the timeframe. Again, this was assigned to the second reaction kinetics regime, which was much slower compared to the initial process. The capacity ended up with a carbonation at 499.3 g CO 2  (63.4%), which is similar to Scenario 1. A comparison of the different scenarios is shown in  FIG.  15   . In  FIG.  15   , the top row shows the variation in gas bottle temperature, the middle row shows the variation in furnace temperature applied to the stainless-steel reactor, while the bottom row shows the CO 2  capacity in each scenario. 
     Thus, the applied CO 2  over/under pressure during carbonation/calcination had a similar effect compared to variations in the calcination/carbonation temperature. However, the CO 2  over-pressure resulted in rapid temperature spikes during absorption (increase up to 22° C. on the outside of the reactor, see  FIG.  16   ), which were not observed in the two previous scenarios due to the CO 2  pressure being close to equilibrium pressure, hence the reaction kinetics were slower. 
     Sample Composition from X-Ray Diffraction 
     The sample composition was evaluated after Scenario 3 by X-ray diffraction at three different spots in the reactor, i.e. at the reactor gas inlet, the reactor middle, and the far end of the reactor. At this point the CO 2  capacity has degraded to ˜64%. Powder samples were prepared by grinding the compacted samples. Furthermore, the stainless-steel from the reactor was investigated to establish any degradation from being in CO 2  atmosphere over a long period of time (approx. 1400 hours) at T˜900° C., see Table 4. Interestingly, the reactor inlet possessed the largest amount of CaCO 3  and CaO, 46.0(2) and 10.1(9) wt %, respectively, and the lowest amount of the by-product Mayenite, Ca 12 Al 14 O 33  (33.4(2) wt %), although this contributed to almost a third of the sample composition. In the middle of the reactor the content of Mayenite was 49.8(2) wt %, whereas the content of CaCO 3  and CaO was 32.1(2) and 18.1(1) wt %, respectively. At the end of the reactor, 42.0(2) wt % of Mayenite was observed with only 49.3(2) wt % of CaO. The expected compound, Ca 5 A 16 O 14 , was only identified at the inlet and in the uncompacted powder at the end of the reactor (10.6(3) and 7.2(2) wt %, respectively), whereas it was not observed in the middle of the reactor. The composition results reveal that the capacity decrease observed was partly due to the formation of large fractions of Mayenite throughout the reactor due to the consumption of CaO (reaction scheme 2) compared to the expected product (reaction scheme 1). 
       5CaCO 3 (s)+3Al 2 O 3 (s)→Ca 5 Al 6 O 14 (s)+5CO 2 (g)   (1)
 
       7Ca 5 Al 6 O 14 (S)+CaO(S)→3Ca 12 Al 14 O 33 (s)   (2)
 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Sample composition (wt %) extracted from Rietveld refinement of PXD data. 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Stainless 
                 Stainless 
               
               
                   
                 Material at 
                 Material at 
                 Material at 
                 Material 
                 Steel 
                 Steel 
               
               
                   
                 Reactor 
                 Reactor 
                 Reactor 
                 Compacted at 
                 reactor 
                 reactor 
               
               
                 Compound 
                 Inlet 
                 middle 
                 End 
                 Reactor End 
                 inside 
                 Outside 
               
               
                   
               
               
                 CaCO 3   
                 46.0(2) 
                 32.1(2) 
                 — 
                 49.3(2) 
                  1.6(3) 
                 — 
               
               
                 CaO 
                 10.1(9) 
                 18.1(1) 
                 45.6(1) 
                  3.0(8) 
                 — 
                 — 
               
               
                 Ca 5 Al 6 O 14   
                 10.6(3) 
                 — 
                  7.2(2) 
                 — 
                 — 
                 — 
               
               
                 Ca 12 Al 14 O 33   
                 33.4(2) 
                 49.8(2) 
                 47.2(1) 
                 42.0(2) 
                 — 
                 — 
               
               
                 Ca(OH) 2   
                 — 
                 — 
                 — 
                  5.8(1) 
                 — 
                 — 
               
               
                 Fe 3 O 4   
                 — 
                 — 
                 — 
                 — 
                 98.4(3) 
                 — 
               
               
                 α-Fe 2 O 3   
                 — 
                 — 
                 — 
                 — 
                 — 
                 58.4(7) 
               
               
                 γ-Fe 2 O 3   
                 — 
                 — 
                 — 
                 — 
                 — 
                 41.6(7) 
               
               
                   
               
            
           
         
       
     
     Throughout the experiment it was noted that the exterior of the 316L Stainless Steel tube was flaking. Logically this was understood to be oxidation of the tube at high temperature. XRD analysis verified that the flakes were composed of α-Fe 2 O 3  (Hematite—58.4(7)) and γ-Fe 2 O 3  (Maghemite—41.6(7) wt %. In contrast, the interior of the tube had also undergone reaction with the XRD identifying Fe 3 O 4  (Magnetite) as well as some CaCO 3  and a minor unknown phase. 
     Conclusions
         A 3 kg scale-up system was demonstrated utilising three different running scenarios.   Varying the sample temperature between 850 and 950° C. created a thermodynamic driving force, which proved as good as varying the applied CO 2  pressure between 0.5 and 5-6 bar during calcination/carbonation, respectively, while keeping the sample at 900° C.   Although, the energy capacity decreased over the 10 cycles applied here at real-life conditions (12 h calcination and carbonation), reaching a level of ˜60%. The capacity drop was assigned to either excessive self-heating during CO 2  absorption and/or the excessive formation of Mayenite, Ca 12 Al 14 O 33 , which has previously been observed at temperatures above 1000° C., but the formation may be possible at a 950° C. operating temperature.   Making CO 2  compression superfluous increases the overall energy efficiency of the system when removing the energy penalty of compression (8-20% of the overall energy balance).   Activated carbon proved sufficient as a CO 2  storage system       

     Example—Adding Mixtures of Oxides to Limestone (CaCO 3 ) 
     The effect of adding both Al 2 O 3  and ZrO 2  to limestone (CaCO 3 ) to enhance the cyclic stability and reaction kinetics of endothermic CO 2  release and exothermic CO 2  absorption was investigated. 
     EXPERIMENTAL 
     Sample Preparation 
     Two individual samples were produced by mixing CaCO 3  (Sigma-Aldrich, &gt;99.0%) with Al 2 O 3  (nanopowder, 13 nm (TEM), 99.8% purity; 20 wt %, i.e. ˜4 g CaCO 3  and ˜1 g Al 2 O 3 ) and ZrO 2  (Sigma-Aldrich, nanopowder, &lt;100 nm; 40 wt %, i.e. ˜3 g CaCO 3  and ˜2 g ZrO 2 ). 10 mL of ethanol (CH 3 CH 2 OH) was added to the samples, which were then ball-milled in stainless steel vials for 2 hours (15 min milling×1 min pause×8 reps; 12×8 mm stainless steel balls). After ball milling, the samples were placed in an oven at 105° C. for approximately 1 hour to obtain a dry powder. Finally, the two samples were hand ground together in a 2:1 ratio (Al 2 O 3 :ZrO 2  sample; 0.8 and 0.4 g, respectively) to obtain 1.2 g of CaCO 3 —Al 2 O 3 —ZrO 2  sample with ˜13.3 wt. % of each additive. 
     Thermogravimetric and Differential Scanning Calorimetry Analysis 
     Thermogravimetric and simultaneous differential scanning calorimetry (TG-DSC) analysis was performed on a Mettler Toledo DSC 1 instrument as shown in  FIG.  18    The samples were heated from room temperature to 1000° C. (ΔT/Δt=10° C. min −1 ) under an argon flow (20 mL min −1 ) in alumina crucibles. 
     Sieverts Experiments 
     The CaCO 3 —Al 2 O 3 —ZrO 2  sample (0.2979 g) was loaded into a SiC sample cell, which was attached via Swagelok parts to a custom-made manometric Sieverts apparatus (https://doi.org/10.1016/j.jallcom.2019.02.067.). The sample was heated to ˜895° C. (ΔT/Δt=5° C. min −1 ) at p(CO 2 )=10 −2  bar, thus decomposing the sample. Subsequently, cycling of the sample was initiated at isothermal conditions (˜895° C.) with carbonation at P average,carbonation (CO 2 )˜5.2 bar(±0.6 bar) for 30 minutes in a 55.8 cm 3  volume, followed by calcination at p average,calcination (CO 2 )˜0.75 bar for 30 minutes in a 206.1 cm 3  volume. A total of 50 cycles of isothermal CO 2  absorption and desorption were collected. Finally, the sample was carbonated and cooled to room temperature under p(CO 2 )˜5 bar. The data had been scaled according to reaction (3) and (4) occurring, leaving a 40.7 wt % CaCO 3  quantity, which was the active component: 
       CaO(s)+ZrO 2 (s)→CaZrO 3 (s)   (3)
 
       5CaO(s)+3Al 2 O 3 (s)→Ca 5 Al 6 O 14 (s)   (4)
 
     Thus, the fractional capacity in  FIGS.  19  and  20    were based on 1 mole of CO 2  being released/absorbed according to reaction scheme 5. 
       CaCO 3 (s)⇄CaO(s)+CO 2 (g)ΔH 890° C. =165.7 kJ; ΔS 890° C.= 143.0 J/K; ΔG 890° C.≈ 0 kJ   (5)
 
     Powder X-Ray Diffraction 
     In-house powder X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer equipped with a CuKα 1,2  source in flat-plate geometry mode ( FIG.  21   ). Data were collected using a Lynxeye PSD detector in the 2θ-range 15-60° in steps of 0.02°. 
     In Situ Synchrotron Radiation Powder X-Ray Diffraction 
     In situ time-resolved Synchrotron Radiation Powder X-ray Diffraction (SR XRD) data were collected at the Powder Diffraction beamline at the Australian Synchrotron, Melbourne, Australia on a Mythen microstrip detector at λ=0.825018 (ref. https://doi.org/10.1063/1.2436201, https://doi.org/10.1016/S0168-9002(02)02045-4) . The powdered sample was loaded into a quartz capillary (i.d.=0.5 mm, o.d.=0.6 mm), which was attached to a gas system enabling control of CO 2  pressure. The sample was inserted into a hot air-blower operating at 917° C. while oscillating during data acquisition. After five CO 2  cycles, the sample was cooled to room temperature at ΔT/Δt=50° C. min −1  under p(CO 2 )=5 bar. Temperature calibrations were performed using the well-known thermal expansion of NaCl and Ag (ref. https://doi.org/10.1107/S1600576715011735, https://doi.org/10.1063/1.1901803) ( FIG.  22   ). 
     Scanning Electron Microscopy 
     Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed on a Tescan Mira3 FESEM coupled with an Oxford Instruments X-Max SDD X-ray detector and AZtec software. SEM images were collected using a backscattered electron (BSE) detector, an accelerating voltage of 20 kV, and a working distance of ˜15 mm. SEM samples were prepared by placing powder onto double-sided conductive carbon tape on a 12.6 mm aluminium sample mount. Excess powder was removed by a passing a light flow of argon gas over the sample. Samples were then sputter-coated with a 3 nm thick platinum layer before imaging. 
     Small-Angle X-Ray Scattering 
     Small angle X-ray scattering (SAXS) data was collected on a Bruker Nanostar instrument equipped with an Excillium MetalJet source (GaK α , λ=1.3402 Å). Sample powders were pressed between polymer films in transmission geometry and measured under vacuum. Data were background subtracted and put onto an absolute scale using a NIST SRM3600 glassy carbon standard (ref. https://doi.org/10.1107/S0021889803002279). Specific surface area (SSA) was calculated from the high-q Porod region (power law slope=−4) using the Unified model in the Irena software package for Igor Pro (WaveMetrics) (ref. https://doi.org/10.1107/S0021889895005292, https://doi.org/10.1107/S0021889809002222). This was calculated through: 
     
       
         
           
             
               
                 
                   SSA 
                   = 
                   
                     B 
                     
                       2 
                       ⁢ 
                       
                         πδΔρ 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     eq 
                     . 
                         
                     2 
                   
                   ) 
                 
               
             
           
         
       
         
         
           
             where B is Porod&#39;s constant refined in the Unified fit, δ is the crystallographic density, and Δρ 2  is the scattering contrast between the powder (density of 3.05 g/cm 3 , linear attenuation coefficient of 223 cm −1 ) and vacuum. 
           
         
       
    
     Results &amp; Discussion 
     Thermal Analysis 
     Thermal analysis of as-prepared CaCO 3 —Al 2 O 3 —ZrO 2  revealed a small mass loss (2.6 wt. %, see  FIG.  18   ) between room temperature and 250° C., which was assigned to evaporation of moisture from the sample, as no reaction was expected in this temperature range. At ˜650° C., a one-step endothermic decomposition initiated with a total mass loss of 31.3 wt % completed by ˜850° C., which was close to the theoretical mass loss expected due to CO 2  release (32.2 wt %). The observed decomposition temperature was typical for CaCO 3  and further reactions with ZrO 2  and Al 2 O 3  occurred with the decomposition product, CaO (reaction scheme 3 and 4), without further mass loss (ref. https://doi.org/10.3390/app9214601, https://doi.org/10.1039/DOTA03080E). 
     Thermochemical CO 2  Pressure Cycling 
     The CaCO 3 —Al 2 O 3 —ZrO 2  sample had a minor initial drop in the expected CO 2  capacity between the first and second cycle (from 82.5% to 71.4%, see  FIG.  19 ,  1    mole CO 2 =100% theoretical capacity, which was calculated from reaction scheme (3) and (4) as 40.7% CaCO 3  was the active portion of the sample). However, the system recovered and showed promise by stabilising at 80% capacity and maintaining it throughout the remaining cycles. The full thermochemical capacity (i.e. 1 mol of CO 2 ) was calculated based on the initial amount of CaCO 3  and assuming that the additives fully reacted with the CaCO 3  (reaction 3 and 4), where the cyclable CaCO 3  content reached 40.7 wt. % of the entire sample mass. The thermochemical system showed rapid reaction kinetics as the 8% capacity was charged/discharged within 30/20 minutes, respectively.  FIG.  20    compares the reaction kinetics of pristine CaCO 3 , CaCO 3 —Al 2 O 3  (20 wt %), and CaCO 3 —ZrO 2  (20 wt %), along with the CaCO 3 —Al 2 O 3 —ZrO 2  sample (ref. https://doi.org/10.1039/DOTA03080E). In  FIG.  20   , the calcination/carbonation (des/abs) cycle is given on the x-axis: t abs =30 min, p abs ˜5.2 bar, t des =20 min, p des ˜0.75 bar. The data was corrected according to reaction schemes (3) and (4) taking place. The capacity exceeding 1 mol of CO 2  indicated that the initial reactions (3) and (4) were incomplete at this stage. 
     The ternary CaCO 3 —Al 2 O 3 —ZrO 2  system showed rapid absorption kinetics throughout all 50 cycles. In particular, the CO 2  desorption kinetics in the ternary CaCO 3 —Al 2 O 3 —ZrO 2  system became superior to the other systems as cycling increased, overcoming a previous kinetic degradation issue observed in the binary systems. Finally, the cyclic capacity after 50 cycles was ˜81% for CaCO 3 —Al 2 O 3 —ZrO 2  compared to ˜78% and ˜68% for the CaCO 3 —Al 2 O 3  and CaCO 3 —ZrO2 samples, respectively (ref. https://doi.org/10.1039/DOTA03080E). 
     Composition 
     The composition of CaCO 3 —Al 2 O 3 —ZrO 2  after 50 cycles (in the desorbed state) was identified by XRD (see  FIG.  21   ). The diffraction pattern revealed that CaCO 3  was present, which was assigned to partial CO 2  absorption during cooling of the sample. In  FIG.  21   , markers: CaCO 3  (brown dot); CaO (blue dot); CaZrO 3  (clover); Ca 3 Al 2 O 6  (green diamond); Ca 5 Al 6 O 14  (purple spade); Ca 9 Al 6 O 18  (red diamond); Ca 12 Al 14 O 33  (Mayenite, orange diamond); unknown (question mark). Rietveld refinement of the Powder X-ray diffraction data. Y obs : red; Y calc  : black, and Y diff : blue. hkl Markers from the top to the bottom: CaCO 3  (top), CaO, CaZrO 3 , Ca 3 Al 2 O 6 , Ca 5 Al 6 O 14 , Ca 9 Al 6 O 18 , and Ca 12 Al 14 O 33  (bottom). R wp ˜9.17%. 
     Furthermore, the decomposition product CaO was present along with the expected reaction products CaZrO 3  and Ca 5 Al 6 O 14  but also by-products Ca 3 Al 2 O 6  and Ca 9 Al 6 O 18 . The compounds Ca 3 Al 2 O 6  and Ca 5 Al 6 O 14  were intermediates on the pathway to form Mayenite, i.e. Ca 12 Al 14 O 33 , which was also identified in the sample (ref. https://doi.org/10.3390/ma12010084). The decrease in capacity, i.e. to the retained 80% over 50 cycles, was assigned to the side reaction producing Mayenite (and partially Ca 9 Al 6 O 18 ), which was only observed in a limited amount in a previous study with better capacity retention (ref. https://doi.org/10.1039/DOTA03080E). 
     The in situ SR XRD data ( FIG.  22   ) initially showed the rapid formation of CaO at 917° C., which indicated the decomposition of CaCO 3  under 1 bar of CO 2  pressure. In  FIG.  22    the pressure profile is indicated to the right of the figure. Carbonation was performed for ˜20 min and calcination for ˜30 min in a total of 5 cycles. The bottom of the figure signifies the start of the cycles. Intensity was indicated as blue: low and red: high. Markers: CaCO 3  (circle); CaZrO 3  (triangle); CaO (diamond); CaAl 2 O 4  (pentagon); Ca 3 Al 2 O 6  (square); Ca 5 Al 6 O 14  (bowtie): Ca 12 Al 14 O 33  (star). The reaction can be observed most significantly by intensity increases/decreases for the CaO Bragg reflections as a function of the CO 2  pressure change, although, small alterations in the intensity of the Bragg reflection from CaCO 3  at 2θ=19.05° are also visible. Hence, CaO Bragg reflections quickly appear/disappear when CO 2  pressure is released/applied. Furthermore, the gradual but continuous formation of CaZrO 3  throughout the experiment was also evident in parallel with several Ca—Al—O compounds being observed, i.e. CaAl 2 O 4 , Ca 3 Al 2 O 6 , Ca 5 Al 6 O 14 , and Ca 12 Al 14 O 33 . Mayenite, Ca 12 Al 14 O 33 , seemed to be formed in a large fraction, which may be due to the difference in temperature between the synchrotron measurements and the cycling measurements, 917 vs. 900° C., respectively, which may influence thermodynamics and/or reaction kinetics of its formation. 
     Morphology and Specific Surface Area 
     The morphology of the CaCO 3 —Al 2 O 3 —ZrO 2  sample before and after cycling was evaluated through scanning electron microscopy (SEM), see  FIG.  23    (EDS mapping colour code: Al: blue; Zr: green; Ca: red). From the EDS mapping it was evident that the as-prepared sample had calcium and aluminium finely dispersed throughout. However, large particles containing zirconium, i.e. ZrO 2  (˜4-10 μm) were present, likely due to its resistance to comminution given its extreme hardness (8-8.5 Mohs). The morphology did not change significantly after cycling, although, elemental zirconium was more dispersed, which was assigned to the formation of CaZrO 3  rather than ZrO 2 . This finding also matched XRD results, where ZrO 2  was absent after cycling, hence the large Zr-rich particles (6-10 μm) must be CaZrO 3 . Calcium and aluminium was still finely dispersed throughout the entire sample, which agreed with the observed formation of Ca—Al —O compounds. The multicomponent CaCO 3 —Al 2 O 3 —ZrO 2  proved to have a very different morphology to CaCO 3  samples containing only one of the additives, i.e. CaCO 3 —Al 2 O 3  and CaCO 3 —ZrO 2  (ref. https://doi.org/10.1039/DOTA03080E). The binary systems formed an interconnected molten-like morphology after cycling with some degree of porosity, which is not observed for the ternary mixture. 
     Comparison of the specific surface areas measured by SAXS (data presented in  FIG.  24   ) indicated the large influence of the porous ZrO 2  added in the as-prepared samples, as the ternary sample had a much larger surface area than the as-milled CaCO 3 —Al 2 O 3 (20 wt %) sample, but similar to the CaCO 3 —ZrO 2 (40 wt %) sample. Despite the observations by SEM, where the ternary CaCO 3 —Al 2 O 3 —ZrO 2  system seemed to lack porosity in comparison with the CaCO 3 —Al 2 O 3 (20 wt %) and CaCO 3 —ZrO 2 (40 wt %), the SAXS data for the ternary system showed that the surface area was approximately 2-6 times larger after cycling compared to the binary samples, see Table 5. The increased surface area was likely to play a key role in the observed higher cyclic CO 2  capacity retention, allowing CO 2  to be released and absorbed from a significantly greater portion of the material more easily. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Comparison of the specific surface area between the binary systems  
               
               
                 CaCO 3 —Al 2 O 3 (20 wt %) and CaCO 3 —ZrO 2 (40 wt %) and the ternary  
               
               
                 system CaCO 3 —Al 2 O 3 (13.3 wt %)—ZrO 2 (13.3 wt %). 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Specific 
                   
                   
                   
               
               
                   
                 Surface 
                   
                   
                   
               
               
                   
                 Area 
                 # CO 2   
                   
                   
               
               
                 Sample 
                 (m 2  · g −1 ) 
                 cycles 
                 Comments 
                 Ref 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 CaCO 3 —Al 2 O 3   
                 3.7(4) 
                 0 
                 As-milled 
                 https://doi.org/10.1039/D0TA03080E 
               
               
                 (20 wt %) 
                   
                   
                   
                   
               
               
                 CaCO 3 —Al 2 O 3   
                 1.2(1) 
                 50 
                 Absorbed 
                 https://doi.org/10.1039/D0TA03080E 
               
               
                 (20 wt %) 
                   
                   
                   
                   
               
               
                 CaCO 3 —ZrO 2   
                  52(5) 
                 0 
                 As-milled 
                 https://doi.org/10.1039/D0TA03080E 
               
               
                 (40 wt %) 
                   
                   
                   
                   
               
               
                 CaCO 3 —ZrO 2   
                 3.1(3) 
                 50 
                 Absorbed 
                 https://doi.org/10.1039/D0TA03080E 
               
               
                 (40 wt %) 
                   
                   
                   
                   
               
               
                 CaCO 3 —ZrO 2   
                  50(5) 
                 0 
                 As-prepared 
                 This Example. 
               
               
                 (13.3 wt %) —   
                   
                   
                   
                   
               
               
                 Al 2 O 3   
                   
                   
                   
                   
               
               
                 (13.3 wt %) 
                   
                   
                   
                   
               
               
                 CaCO 3 —ZrO 2   
                   6(1) 
                 50 
                 Absorbed 
                 This Example. 
               
               
                 (13.3 wt %)— 
                   
                   
                   
                   
               
               
                 Al 2 O 3   
                   
                   
                   
                   
               
               
                 (13.3 wt %) 
               
               
                   
               
            
           
         
       
     
     Conclusions 
     The combination of adding both ZrO 2  and Al 2 O 3  to CaCO 3  had a positive effect as a cyclic stability of &gt;80% was achieved at rapid calcination/carbonation times, i.e. 20 and 30 min, respectively. The reaction kinetics were fast, especially during carbonation. However, the calcination kinetics improved as the sample was cycled. The cyclic stability was hypothesised to arise from a synergetic effect of having both CaZrO 3  and Ca—Al—O compounds present in the sample, by preventing sintering of CaO/CaCO 3  particles and improving reaction kinetics, even better than the individual binary systems. This effect was highlighted by the larger specific surface area observed in the ternary system after cycling and the different morphology of the cycled material. However, the excessive presence of large zirconium-rich particles suggested that a smaller quantity of ZrO 2  may be sufficient in achieving the same benefits whilst lowering the overall price of the system. This Example suggested that combining different active properties of additives/catalysts can enhance the cyclic stability of a metal carbonate even further. This opened up multiple new pathways for optimising the thermochemical energy storage properties of metal carbonates. 
     In the claims which follow and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments. 
     It will be understood to persons skilled in the art of the current disclosure that many modifications may be made without departing from the spirit and scope of the disclosure.