Large scale computational screening and experimental discovery of novel materials for high temperature CO₂ capture

Abstract

The implementation of large-scale carbon dioxide capture and storage (CCS) is dependent on finding materials that satisfy several different criteria, the most important being minimising the energy load imposed on the power plant to run the process. The most mature CCS technology, amine scrubbing, leads to a loss of 30% of the electrical work output of the power station without capture, which is far too high for widespread deployment. High-temperature CO₂ absorption looping has emerged as a technology that has the potential to deliver much lower energy penalties, but further work is needed to find and develop an optimal material. We have developed a combined computational and experimental methodology to predict new materials that should have desirable properties for CCS looping, and then select promising candidates to experimentally validate these predictions. This work not only has discovered novel materials for use in high-temperature CCS looping, but analysis of the entirety of the screening enables greater insights into new design strategies for future development.

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Broader context

Given the likelihood of continued fossil fuel use, carbon capture and storage (CCS) technologies become increasingly required, if the world is to reduce atmospheric CO₂ concentrations. The current challenge in implementing widespread CCS is finding materials and processes that achieve acceptable levels of energy efficiency. While much work has been done to optimise the processes involved in CCS, less attention has been given to the discovery and optimisation of new materials. In this work we present a high-throughput computational screening methodology that allows the efficient prediction of the relevant CCS properties for known materials, and succeeds in finding several suitable candidates with improved performance over commonly used materials. Furthermore, extensive experimental testing of candidate materials validates the accuracy of the screening and suggests further iterations for its future use.

Introduction

The challenge of reducing anthropogenic greenhouse gas emissions, particularly that of CO₂, has been the focus of numerous governmental and industrial initiatives in recent years. Despite government policies penalising industry for CO₂ emissions, it is likely that total global emissions of CO₂ will continue to increase over the next decades as countries continue to exploit their natural resource reserves.¹ In this instance, large-scale carbon dioxide capture and storage (CCS) applications represent one of the main initiatives to mitigate the rise of global CO₂ emissions.²

There are a number of different broad categories of proposed materials to separate CO₂ from the flue gas mixture, typically composed of ∼75% N₂, ∼10–15% CO₂, ∼10% H₂O, and ∼3% O₂ at 40 °C and 1 atm. These include physical adsorption processes utilising materials with high internal surface areas such as zeolites, metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs) and activated carbons;^3–6 physical separation processes utilising membrane materials⁷ and high temperature CO₂ absorption looping processes involving the chemical reaction of CO₂ with alkaline earth oxides, notably MgO and CaO and related materials to form solid carbonates.⁸ These high-temperature processes are particularly appealing, given the possibilities of achieving very low energy penalties (∼6–8% with respect to reference power plants without CO₂ capture), the maturity of the technology being reflected in the number of pilot plants using CaO-based sorbents already operating around the world, and the wide range of materials that could possibly be used in such a process.⁹

There is interest in developing new materials for high-temperature CCS, or modifying existing ones, because of the problems with capacity loss encountered when using a pure CaO–CaCO₃ system. In studies using natural limestone, the CO₂ capture capacity of the solid sorbent can decrease by as much as 90 mol% within 5 cycles owing to sintering of the solid particles.¹⁰ Recent research has focussed on the use of various additives to form solid supports and avoid sintering of the limestone, including Al₂O₃, Ca₁₂Al₁₄O₃₃, SiO₂ and MgO.^11–13

Beyond the current approaches, there is a very wide range of basic oxide materials which could potentially be used in developing an optimal process. Practically, synthesising and testing all possible materials is not feasible, and therefore it is desirable to develop a screening methodology based on ab initio computations to identify the most promising candidates for the target application.

There have been a few examples of previous approaches utilising various theoretical methods to screen materials for CO₂ capture. Lin et al. generated a database of potential zeolite-like structures that were subsequently analysed via interatomic potentials to determine their thermodynamic stability.¹⁴ CO₂ absorption isotherms were constructed using molecular simulations, which allowed the calculation of the parasitic energy for the stable materials, corresponding to the penalty imposed on a power plant if fitted with a CCS process using the material (including the energy to compress the gaseous CO₂ for storage). Their screening showed a theoretical limit to the minimum parasitic energy obtainable with zeolitic and zeolitic imidazolate framework (ZIF) materials. A subsequent study applied this parasitic energy metric to a wider class of materials including both experimentally-realised and hypothetical materials, finding that it is suitable for evaluating materials for CCS, as it combines various relevant thermodynamic properties.¹⁵ Several other studies have also been performed on similar materials, albeit on small sets, with some comparison between theoretical and experimental results.^16–18 High-throughput synthesis methods have also been used to find novel ZIF materials for CO₂ capture.⁶

The group of Duan et al.^19–21 have focussed on lithium-based oxide materials for high-temperature CCS applications, using density functional theory (DFT) and phonon calculations to determine the carbonation reaction thermodynamics of these materials, and comparing them with experimental results. Other studies have also outlined a possible screening process based on theoretical DFT and phonon calculations to identify the most suitable materials from a large starting set, with screening performed on smaller sets of alkali-based oxides.^22,23 The limitation of these studies is that they were applied to a relatively small number of materials, many of them already well known experimentally as being promising compounds for CO₂ capture.

The goal of the present study is to theoretically screen thousands of possible carbonation reactions from a very wide range of solid oxide based materials, both with the hope of discovering novel compounds for CCS applications, and that the screening results can help elucidate the underlying principles that can drive future design of CCS materials. Thus, we have utilised the Materials Project database (http://www.materialsproject.org), which contains structural, electronic and energetic data for over 50 000 compounds (as of August 2014) calculated using the Vienna Ab initio Simulation Package (VASP).^24–26 Importantly, the contents of the database are accessible via the REST Materials API,²⁷ which allows users to develop their own screening programs to search the database using an open-source Python library (pymatgen) for materials analysis.²⁸ In this work, a screening methodology was developed to search the database for suitable materials for high-temperature CO₂ capture and to predict the thermodynamic enthalpies for the in silico carbonation reactions of these compounds. To rank these reactions we used an energy penalty concept as used by others,¹⁴ which favours materials whose use for CCS minimises the energy load imposed on a power station. We also screened materials based on their gravimetric CO₂ capacity as a preliminary measure of the material cost per unit of CO₂ captured, and also the net volume change of a material after carbonation as an indication of cycling stability.

Experimental investigations of the carbonation reaction thermodynamics, cycling stability and morphological changes were performed as a comparison to the theoretical predictions, so as to go beyond the solely theoretical framework used in previous studies. X-ray diffraction (XRD) was used to characterise the structures of the materials both pre- and post-reaction, thermogravimetry and differential scanning calorimetry (DSC) was used to characterise the carbonation enthalpy and cycling capacity, while scanning electron microscopy (SEM) allowed us to characterise the changing surface morphology of the cycled materials. The particular candidate materials were chosen for a number of different reasons. Firstly, materials for which there had been no previous CO₂ capture properties reported were studied to validate the claim that the screening could suggest truly novel materials for CCS applications. Other materials were then selected either as (i) benchmark materials to provide as wide a comparison with other studies, or (ii) because of their predicted performance as CCS materials. The experimental studies showed that the theoretical screening could indeed accurately predict (within error) the carbonation properties of the candidate compounds, validating this method as a tool for the discovery of novel CCS materials. Furthermore, rational design guidelines emerged from the overall screening results, giving valuable insights as to where future research efforts should be aimed to optimise the materials used in CCS.

Methodology

Dataset construction

Given the ground state energies of all the phases present in a given carbonation reaction, it is possible to calculate ΔE_DFT for the reaction, which is equal to ΔH_carbonation at 0 K and zero pressure. The first part of the screening process therefore is to determine this value for all possible reactions within a given subset of compounds. Furthermore, since these reactions will be performed at varying CO₂ partial pressures and temperatures, a single variable can be defined by considering all reactions under varying CO₂ chemical potential (μ_CO2), with a more negative value corresponding to either higher T or lower p_CO2.

The screening focussed on oxide materials that were either binary or ternary compounds in this initial screening, as this not only drastically narrows the phase space within which the search is conducted, but it also excludes compounds with four or more elements that are more likely to phase segregate to simpler binary or ternary compounds during the cycles of carbonation and calcination. Furthermore, materials were limited to those containing elements from the 37 most abundant within the earth's crust, because this realistically reflects concerns about cost and availability of a useful CCS material.‡ The geometry optimised structures and ground state E_DFT of the relevant materials were retrieved from the Materials Project database, having been previously calculated by the Materials Project using VASP.²⁴

The screening comprised 640 unique compounds with a total of 1442 simulated carbonation reactions. To obtain all compounds that matched these initial criteria, quaternary phase diagrams of the form A–M–O–CO₂ were simulated, where M is a non-alkali element, using the Phase Diagram app within the pymatgen library.^30,31 Because the primary interest lies in the evolution of the phases under reaction with CO₂, the approach found in previous studies that generated open phase diagrams with respect to μ_O2^31,32 was adapted to study the phase equilbria under changing μ_CO2.

Open phase diagram construction

In order to construct these phase diagrams, we initially assumed that the system is closed with respect to every element except CO₂, which is non-ideal but reasonable given that p_CO2 > p_O2 during both the carbonation and decomposition reactions. Furthermore, we separately tested each compound for its stability with respect to p_O2, and this was included as an additional filter in our screening, excluding compounds that would oxidise or reduce under conditions similar to those for carbonation. In this case the relevant potential is the CO₂ grand canonical potential which can be defined as:

ϕ(T,P,N_A,N_M,N_O,μ_CO2) = G − μ_CO2N_CO2 = E − TS + PV − μ_CO2N_CO2 (1)

where G is the Gibbs free energy, E is the internal energy, T is the temperature, S is the entropy, P is the pressure, V is the volume, and N_i is the number of atoms of species i in the system. If it is assumed that the change in the PV term is small and primarily cancels between the solid phases relative to the E and TS terms for a reaction that primarily involves solid condensed phases, and ϕ is normalised so that the sum of the A, M and O components is equal to 1, eqn (1) can be rewritten aswhere x_i is the component fraction of i in the ternary phase diagram. To clarify, N_O is the amount of oxygen remaining in a phase after adding or removing stoichiometric amounts of CO₂, for example in the phase Li₂CO₃, N_CO2 = 1 and N_O = 1.

In order to construct these open phase diagrams it is normally necessary to calculate the entropy change in the solid and gas phases due to different vibrational, configurational and electronic excitations. For solid crystalline structures this would normally be approached by calculating the various phonon frequencies (and hence the phonon densities and dispersion curves) for individual phases and using these to derive the total vibrational energy of the crystal at different temperatures. This approach is tractable when it comes to studying a small number of phases and reactions, but in a screening study involving thousands of different compounds, these calculations are too expensive to be feasible.

Fortunately, some assumptions can be employed to simplify the calculation of the phase diagram. Given that all the reactions of interest involve CO₂ gas absorption, it is reasonable to assume that the reaction entropy is dominated by the CO₂ gas entropy, rather than changes due to solid–solid transformations. In fact, it has been previously shown that the Gibbs free energy of solid phase surfaces vary to a very small degree (<10 meV) over a wide range of temperatures (<1500 K) and pressures (<100 atm).^33,34 As such, the change in the CO₂ chemical potential can account for the majority of the effect of temperature on ϕ. At a given temperature T and CO₂ partial pressure p_CO2, the chemical potential can be described as:

where p_CO2 is the CO₂ partial pressure, p₀ is a reference CO₂ partial pressure (0.1 MPa), k is Boltzmann's constant and is the CO₂ entropy, obtained from the NIST-JANAF thermochemical tables.³⁵ If the gas is ideal, the PV term is equal to kT, leaving the chemical potential dependent only on T and p_CO2. The expression for ϕ can be further simplified, given the assumption that the contribution of S_CO2 within μ_CO2N_CO2 dominates compared to S (the entropy of the solid phases): It is possible then to interpret carbonation reactions as the evolution of phases over successive ternary phase diagrams A–M–O with different μ_CO2, essentially tracking the decomposition of a given phase under increasing temperature or CO₂ partial pressure. An example of this can be seen in Fig. 1, showing different projections of the Li–Si–O–CO₂ phase space at constant μ_CO2 slices.

Fig. 1 Ternary Li–Si–O phase diagrams under different μ_CO2 obtained by applying the CO₂ grand potential to the Materials Project database. The physical equivalent of increasing the chemical potential is either an isothermal reaction performed with increasing p_CO2, or an isobaric reaction under decreasing temperature. Vertices that are present in the lefthand phase diagram but are not present in righthand phase diagram represent phases which have decomposed as a result of the change in μ_CO2. The product phases of this decomposition reaction with CO₂ are situated at the vertices at either end of the edge where the original phase was formerly located. For example, Li₄SiO₄ transforms to a mixture of Li₂CO₃ and Li₂SiO₃ with increasing μ_CO2 (see pink shaded area of the phase diagram).

A summary of the assumptions used in simplifying the CO₂ grand canonical potential can be seen in Table 1.

Table 1 The form of the CO₂ grand canonical potential, and the assumptions used to obtain the simplified expressions used in the screening

ϕ	Assumptions
E − TS + PV − μCO2NCO2	Exact form
E − TS − μCO2NCO2	PV small relative to E − TS
E − μCO2NCO2	S small relative to SCO2 (contained within μCO2)
	CO2 behaves as an ideal gas
	E = E^DFT at 0 K, ECO2 fitted to experimental values

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Correction for CO₂ molecule

It is also necessary to obtain a more accurate estimation of the energy of the CO₂ molecule found in the database, because the database entry for CO₂ is based on solid, rather than gaseous CO₂. More pertinently, solid–gas reactions in density functional theory are subject to non-cancellation of errors that often require empirical adjustment.^36,37 To this end we calibrated the database energy by comparing ΔE_DFT and ΔH_experimental at 293 K for a small set of basic binary alkali oxides, and a number of ternary Li-based oxides (Fig. 2). The average error between the theoretical and experimental values was added as a correction to E_CO2, and the corrected value was used for the screening. This correction will also account for some part of the solid entropy change between 0 and 293 K not taken into account in the calculations.

Fig. 2 Errors between ΔE_DFT and ΔH_experimental at 293 K for selected binary and ternary oxides. The average error was used as a correction to E_CO2 in the Materials Project database.

Comparison to other theoretical studies

There are previous theoretical results derived from a similar level of DFT theory to model ΔE_DFT for the carbonation reactions of binary oxides and some different reaction pathways for Li-based ternary oxides.^19,20,23,38 To validate the calculated energies in the Materials Project database, we compiled these previous results and compared the respective ΔE_DFT for the various reactions. The respective values for ΔE_DFT are shown in Fig. 3.

Fig. 3 Comparison between the calculated ΔE_DFT for this work (blue), and with the studies of Duan et al. (red).^19,20,23,38 There is a small but constant difference between the two studies, with the Materials Project ΔE_DFT being 21 kJ mol_CO2⁻¹ lower than the previous study on average.

It can be clearly seen that there is a small but constant overestimation (more negative) of ΔE_DFT in this work compared to the previous studies. The main source of deviation between the two studies is the different values of E_CO2 used: −2191 kJ mol⁻¹ and −2219 kJ mol⁻¹ for our study and the work of Duan et al. respectively,²² which would lead to our more negative ΔE_DFT.

Energy penalty calculations

To evaluate the potential use of these compounds in CCS applications, the overall parasitic energy penalty that would be imposed on a model power plant through the use of this material was considered, following the approach in previous studies.¹⁴ Further details of these calculations are available in the ESI.†

Synthesis of candidate materials

Pure polycrystalline samples of the candidate materials were prepared using a solid-state method. The starting reagents and synthesis conditions are summarised in Table 2. Prior to weighing, CaCO₃ was dried at 1000 K overnight to ensure there was as little moisture present in the compound as possible. The stoichiometric mixtures were ground using an agate mortar and pestle, except for the Mg₆MnO₈ starting materials, which were mixed thoroughly with a high energy ball mill (SPEX SamplePrep 8000M) for 1 hour. Phase purity of the samples was monitored by room-temperature X-ray powder diffraction (XRD), using a Panalytical Empyrean diffractometer utilising Cu K_α radiation. The Li₅FeO₄ and Li₆CoO₄ samples were seen to visibly react over the course of a day in air, as denoted by a distinct change in colour, owing to reaction with CO₂ in the air. XRD and TGA measurements confirmed the presence of Li₂CO₃ in these samples as a result of their reaction.

Table 2 Synthesis conditions and starting materials for the compounds in the present study

Compound	Starting materials	Reaction programme	Atmosphere
Ca4Nb2O9	CaCO3 + Nb2O5	1173 K for 12 h, 1648 K for 12 h × 2	Air
Li5FeO4	Li2O + Fe2O3	973 K for 12 h	Argon
Li6CoO4	Li2O + CoO	973 K for 12 h	Argon
Li4SiO4	LiCO3 + SiO2	1173 K for 12 h	Air
Li5AlO4	Li2O + Al2O3	973 K for 12 h	Air
Mg6MnO8	MgO + MnO2	1173 K for 12 h	Air
Na3SbO4	Na2CO3 + Sb2O3	923 K for 12 h, 1173 K for 12 h, 1223 K for 12 h	Air

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Thermogravimetry

The carbonation and calcination reactions of the candidate materials were investigated using a thermogravimetric analyser (TGA/DSC 1, Mettler Toledo) operating at atmospheric pressure. In each experiment, a sample of ∼20 mg of powder was placed in a 70 μL Al₂O₃ or Pt crucible, supported on a cantilever-type balance. Gases were fed to the reaction chamber through three gas ports, viz. reactive gas, purge gas and protective gas. The reaction chamber was electrically heated by a tubular furnace surrounding the balance. Both the protective gas and the purge gas were N₂, and were fed to the TGA reaction chamber with a flow rate of 50 mL min⁻¹. The reactive gas was a stream of pre-mixed N₂ and CO₂, fed by a capillary so that the gas could flow over the top of the crucible. The partial pressure of CO₂ at the surface of the solid sample was adjusted by varying the mix of N₂ and CO₂ in the reactive gas, while keeping a constant overall flow rate of 50 mL min⁻¹.

For the experiments on differential scanning calorimetry (DSC), the samples were first heated to 873 K under a flow of N₂, before being exposed to a pure stream of CO₂ for 10 minutes at a constant temperature. Integration of the heat flow curves over this time interval with a baseline set to the heat flow prior to carbonation gave the heat accumulated in the sample during carbonation.

The actual CO₂ concentration at the gas–solid interface was calibrated against the well-understood thermodynamic CaO/CaCO₃ carbonation equilibrium. For example, when a carbonated sample of pure CaO (98 wt%) was slowly heated in a specific mixture of CO₂ and N₂, the temperature at the onset of CaCO₃ decomposition was recorded and the corresponding CO₂ partial pressure in contact with the solid phase was determined from the phase diagram of the CaO–CaCO₃–CO₂ system. In the temperature-programmed decomposition (TPD) experiments, the samples were heated from 323 K to either 973 K (Li₆CoO₄ and Li₅FeO₄) or 1223 K (Ca₄Nb₂O₉, Mg₆MnO₈ and Na₃SbO₄) under a specific partial pressure of CO₂, i.e. a specific N₂/CO₂ ratio. The equilibrium temperature corresponding to a given partial pressure of CO₂ was determined by the temperature at which the material started to decompose (after possibly carbonating at a lower temperature), determined by the zero of the first derivative of the mass curve.

Surface measurements

The surface morphologies of the solid samples at various stages of the cycling experiments (viz. fresh, fully carbonated and fully regenerated) were examined ex situ by scanning electron microscopy (SEM). The samples were inspected in a field-emission gun SEM (Hitachi S-5500) with a secondary electron detector.

The porosity and specific surface area (SSA) of the candidate materials were determined using volumetric sorption measurements (TriStar3000 analyzer, Micromeritics) in N₂ at 77 K. The SSA was calculated using Brunauer–Emmett–Teller (BET) analysis using N₂ sorption.³⁹ Pore size distribution and pore volumes were determined by applying the Barrett–Joyner–Halenda (BJH) model using a 55 point adsorption–desorption isotherm.

Results and discussion

Summary of screened materials

The screening of the Materials Project database for compounds within the A–M–O–CO₂ open phase diagrams found 640 distinct compounds able to undergo reactions with CO₂, with a total of 1442 distinct carbonation reactions. The difference in these numbers is due to some compounds being able to undergo several different decomposition pathways with increasing CO₂ chemical potential. A breakdown of these numbers into separate alkali metal compounds can be seen in Table 3.

Table 3 Total number of compounds and associated carbonation reactions found in the original screening, and after screening out compounds that either react with CO₂ below 293 K, or oxidise before carbonation

	Original screening		After filtering
Alkali metal	Compounds	Reactions	Compounds	Reactions
Ba	91	195	60	121
Ca	74	164	32	46
K	101	261	73	191
Li	57	118	50	103
Mg	48	84	19	32
Na	92	249	71	187
Rb	81	175	60	135
Sr	75	165	59	122
Non-alkali	21	31	8	9
Total	640	1442	432	946

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Two subsequent filters were applied to the results to screen realistically for materials that could be practically used for high temperature CO₂ absorption looping applications. The first filter removed materials whose decomposition reactions occurred below 293 K, as practically a CCS process would have to occur at room temperature or above. The second filter used a similar open phase diagram construction as used previously in the original screening process, except this time with the open element being O₂, to screen the materials for their stability at the very low p_O2 levels expected in the calciner, the reaction chamber where the carbonated materials are decomposed to release gaseous CO₂ and reform the original material (p_O2 < 0.01). Those materials that would reduce or even oxidise under these conditions were removed from the screening, as they would transform before the carbonation reaction had a chance to occur. The remaining distinct compounds, along with the number of distinct possible carbonation reactions, can be seen in Table 3.

These results show that the methodology was able to find materials that undergo carbonation reactions across all the different alkali metals used, and is certainly the largest number of solid oxide type materials ever to be considered for CCS applications in a single study. Interestingly, while a reasonable number of compounds are found in the database for the most abundant alkaline earth metals, Ca and Mg, these compounds are disproportionally removed by the filtering process. Potentially the stability of the binary oxides CaO and MgO compared to the other alkali binary oxides could simply result in fewer stable tertiary phases with these elements.

Calculated properties of screened materials

Using the relative energies of the reactant and product phases in each theoretical carbonation reaction it was possible to compute a value for ΔH_carbonation.³⁰ For a single compound, the multiple reactions essentially describe the same decomposition process, followed to different endpoints, with the order following increasing ΔH_carbonation. Experimentally, we only observed the products of the first decomposition reaction, even on increasing T, and therefore the results could be further restricted to one carbonation reaction for each distinct compound, the reaction with the most negative ΔH_carbonation.

In evaluating the theoretical energy penalties of the screened materials, the most commonly used high temperature CCS material, CaO, was used as a benchmark for comparison (with a calculated energy penalty of 41.9 kJ (mol CO₂)⁻¹). For reference, process engineering studies using CaO as a solid absorbent in a postcombustion CCS setup found this process imposes a 6–8% energy penalty as compared to plants without CCS,^40–42 and of that penalty, between 3–10% is attributed to the energy to calcine CaCO₃ to CaO. A commonly accepted goal for an economically-viable CCS process set by the US Department of Energy is to have an energy penalty of 5% or less, so if we assume that our calculated energy penalty mostly accounts for the calcination energy, this would mean finding a material in our screening with E_p < 35 kJ (mol CO₂)⁻¹. This is still only a rough approximation, as any promising candidate would also need to be subjected to in-depth process investigation to determine its realistic energy penalty in a CCS process.

From the stoichiometry of the reaction, it is also possible to determine the CO₂ gravimetric capacity (g_CO2absorbed/g_sorbent) of each theoretical carbonation reaction, as well as the volume change for the reaction (derived from the change in the unit cell volume of the optimised structures). There is some evidence to suggest that changes in volume contribute to the cycling instability of CaO, with large volume changes possibly causing pore closure and particle sintering and resulting in the rapid fading of the CO₂ capture efficiency over the first 10–20 cycles.^10,43–45 Therefore, the compounds were screened based on the volume change with carbonation as a preliminary indication of the stability of the compound over multiple cycles.

These different parameters were plotted for each of the screened reactions in Fig. 4. There is a direct correlation with ΔH and E_p, as expected considering it is a dominant part of the Q_loss and Q_recovered terms used to determine the overall energy penalty (for more details see the ESI†). However, for less exothermic carbonation reactions, the magnitude of these terms is similar to that of the specific heat capacity term. The approximate specific heat capacities of the compounds used in the screening were calculated using the Dulong–Petit law,⁴⁶ this being a good approximation for solid state materials at high temperature, and also being trivial to calculate as it only depends on the molar mass (MM) of the compound. In the lower energy penalty part of the plot, some materials are seen with similar E_p, but with ΔH that differ by as much as 0.5 eV. In this region some control is possible over the balance between ΔH and MM to achieve a given E_p, and given that compounds with a lower MM will normally have a higher gravimetric CO₂ capacity, this means it is possible to select compounds on the basis of this parameter. Conversely, for a process requiring a smaller ΔH to reduce heat flow in and out of the system, it is possible to select compounds with a higher MM to achieve the same overall E_p.

Fig. 4 Overview of results from the screening of the Materials Project database for compounds that undergo carbonation. Each data point represents the carbonation reaction for each distinct compound with the most negative ΔH. The calculated energy penalty, E_p, for each reaction is plotted against ΔH_carbonation (top), CO₂ gravimetric capacity (middle) and volume change (bottom). The plots on the left are for the complete 640 distinct compounds in the screening, and the plots on the right are for specific compounds of interest (including the five candidate materials).

Selection of candidate materials

Our experimental studies focussed on five materials with previously unreported CCS properties: Mg₆MnO₈, Ca₄Nb₂O₉, Na₃SbO₄, Li₅FeO₄ and Li₆CoO₄. Mg₆MnO₈ has a much lower theoretical E_p than CaO and a comparably high gravimetric CO₂ capacity, making it a promising material for future applications. Ca₄Nb₂O₉ and Na₃SbO₄ were chosen because they had similar, but lower E_p values than that of CaO from the screening, and as such they represent useful comparisons to the benchmark material, especially in comparing their cycling stability and the effect of adding a third element to the binary Ca–O phase diagram (in the case of Ca₄Nb₂O₉). Li₅FeO₄ and Li₆CoO₄ were chosen for their very high theoretical gravimetric CO₂ capacities, and for comparison to previous studies on similar Li ternary oxides.^47–51 It should be noted, however, that Na₃SbO₄ and Li₆CoO₄ are unlikely to find use in CCS applications owing to the cost of Sb and the toxicity of Co, although Ca₄Nb₂O₉, Mg₆MnO₈ and Li₅FeO₄ are made from more abundant and safe elements and are thus particularly appealing. The relevant parameters obtained from the Materials Project screening for these materials are presented in Table 4.

Table 4 Theoretical screening results for CaO and the preliminary candidates in the experimental analysis. The respective carbonation reactions relating to the tabulated values are CaO + CO₂ → CaCO₃; Ca₄Nb₂O₉ + 2CO₂ → 2CaCO₃ + Ca₂Nb₂O₇; Mg₆MnO₈ + 5CO₂ → 5MgCO₃ + MgMnO₃; Na₃SbO₄ + CO₂ → Na₂CO₃ + NaSbO₃; 0.5Li₅FeO₄ + CO₂ → Li₂CO₃ + 0.5LiFeO₂; and 0.33Li₆CoO₄ + CO₂ → Li₂CO₃ + 0.33CoO

Compound	E p (kJ (mol CO2)^−1)	ΔHcarbonation (kJ mol^−1)	CO2 capacity (gCO2/gsorbent)	Volume change (%)
CaO	41.9	−179.5	0.78	124.4
Mg6MnO8	9.7	−105.6	0.67	91.7
Ca4Nb2O9	37.9	−153.9	0.18	37.3
Na3SbO4	40.7	−167.6	0.17	37.4
Li5FeO4	44.7	−208.7	0.57	59.9
Li6CoO4	44.6	−217.4	0.80	104.9

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Initial post-carbonation structural characterisation

XRD patterns acquired of the as-synthesised Mg₆MnO₈, Ca₄Nb₂O₉, Na₃SbO₄, Li₅FeO₄ and Li₆CoO₄ samples confirmed that all these materials were mostly a single pure phase, with some small impurity phases present in the case of Na₃SbO₄. BET surface area analysis was also performed on the materials, showing them to have similarly low surface areas, consistent with oxides sintered at high temperatures (results available in the ESI†). Further XRD patterns were then taken of these samples after being carbonated under p_CO2 = 0.37 bar at 773 K (Mg₆MnO₈), 973 K (Li₅FeO₄ and Li₆CoO₄) or 1223 K (Na₃SbO₄ and Ca₄Nb₂O₉) in a thermogravimetric analyser (TGA) to identify the phases that formed upon carbonation, and to see if these match with the phases predicted by the screening. Carbonation temperatures were identified by an initial TGA experiment to determine the range over which the samples carbonated and regenerated. For Na₃SbO₄, Li₅FeO₄ and Li₆CoO₄, phases present in the pre- and post-carbonation samples could be indexed and their structures and cell parameters were refined using Rietveld refinement, as shown in Fig. 5. For Li₅FeO₄ a significant amount of the original material remained after carbonation (19% by weight), indicating that the reaction had not gone to completion.

Fig. 5 Indexed XRD diffractograms (λ = 1.54056 Å) and refinements of the as-synthesised Na₃SbO₄, Li₅FeO₄ and Li₆CoO₄ samples, and after carbonation at 973 K (Li₅FeO₄ and Li₆CoO₄) or 1223 K (Na₃SbO₄) under p_CO2 = 0.37 bar. The experimental data points are shown in red, with the pattern obtained from the Rietveld refinements overlaid in black, and the difference curve below.

Unfortunately in the case of Mg₆MnO₈ and Ca₄Nb₂O₉, even after multiple attempts to carbonate the samples at different temperatures and over longer time periods, no evidence for reaction could be found in their XRD diffractograms. Subsequent TGA experiments showed that the materials increased their weight by a very small amount close to the detection limit of the instrument (∼0.01 mg) when heated under flowing CO₂, which likely is undetectable by diffraction. As these samples were essentially unreactive with CO₂ they were excluded from any further analysis.

Carbonation experiments

In order to determine ΔH_carbonation and ΔS_carbonation, the equilibrium constants of the respective carbonation reactions as a function of temperature were measured experimentally by TPD in a TGA under various CO₂ partial pressures, with a typical trace for Na₃SbO₄ shown in Fig. 6. As described in previous studies,²⁹ the equilibrium curve for a carbonation reaction can be fitted by an exponential curve of the form:where p_CO2 is the CO₂ partial pressure in atm, T is calcination temperature in K, and A and B are constants specific to each reaction, which can be obtained by plotting a logarithmic curve of 1/T measured at different p_CO2.

Fig. 6 Temperature programmed decomposition (TPD) experiment for Na₃SbO₄ at p_CO2 = 0.27 bar. The mass gain corresponds to the carbonation reaction Na₃SbO₄ + CO₂ → Na₂CO₃ + NaSbO₃. Calculation of the zero point of the first derivative of the mass curve gives the temperature of the onset of decomposition.

From these it is possible to calculate ΔH_carbonation and ΔS_carbonation from these fitting constants, assuming that ΔH_carbonation and ΔS_carbonation are approximately constant:

K_p = exp(ΔS_r/R)·exp(−ΔH_r/RT) (6)

A = exp(ΔS_r/R) (7)

B = ΔH_r/R (8)

The fits for the constants A and B for Na₃SbO₄ and Li₄SiO₄ (as an example of a promising material studied previously for CCS⁴⁹) are shown in Fig. 7, and then compared against the corresponding values obtained from the Materials Project screening in Table 5. Attempts to perform a similar TPD analysis for Li₅FeO₄ and Li₆CoO₄ failed because the phases formed in the carbonation reactions do not decompose before the melting point of Li₂CO₃ at ∼993 K, even though the temperature of decomposition was predicted to be ∼805 K at p_CO2 = 1 from the screening. TPD traces of Li₅FeO₄ and Li₆CoO₄ are given in Fig. 8, showing the mass gains as the materials react with CO₂ to form Li₂CO₃ and LiFeO₂ or CoO respectively. Upon further heating, however, there is no mass loss, and therefore the same method of determining the carbonation equilibrium constant cannot be used. It is also seen in these TGA traces that the experimental CO₂ capacity is much less than the theoretical value as obtained from the stoichiometry of the carbonation reaction. One reason for this may be that a large percentage of the sample had already reacted with CO₂ in the air prior to the experiment, as it was not possible to keep the samples in vacuum when transferring in and out of the TGA itself. Furthermore, longer isothermal experiments should be performed in future studies to better elucidate the combined effect of thermodynamics and kinetics on the rate of these reactions.

Fig. 7 Plot of ln(K_p) against the inverse decomposition onset temperatures derived from TPD experiments for Na₃SbO₄ and Li₄SiO₄. The linear fit to determine the constants A and B relating to K_p is also shown on each plot.

Table 5 Experimentally derived values and predicted values from the Materials Project screening of ΔH_carbonation and ΔS_carbonation for CaO, Na₃SbO₄ and Li₄SiO₄. The screening values are based on the following reactions: CaO + CO₂ → CaCO₃, Na₃SbO₄ + CO₂ → Na₂CO₃ + NaSbO₃ and Li₄SiO₄ + CO₂ → Li₂CO₃ + Li₂SiO₃

	Experimental		Screening
Compound	ΔHr (kJ mol^−1)	ΔSr (J mol^−1 K^−1)	ΔHr (kJ mol^−1)
CaO	−170 ± 5	152 ± 5	−179.5
Na3SbO4	−175 ± 26	150 ± 23	−167.6
Li4SiO4	−149 ± 58	149 ± 59	−167.9

---

Fig. 8 TGA traces for Li₅FeO₄ and Li₆CoO₄ under p_CO2 ≈ 0.1 bar.

For the materials studied, it is seen that there is very good agreement between the theoretical and experimental values for ΔH_carbonation. In the case of CaO, this is to be expected because of the CO₂ energy correction applied which was derived from experimental values for binary oxide carbonation reactions. But for Na₃SbO₄ and Li₄SiO₄, these results validate the accuracy of the screening process and its ability to predict correctly the carbonation equilibrium for both current promising CCS materials and also for previously unstudied materials. Fairly large errors are often seen in TPD fitting, especially because of the sensitivity between any small amount of scatter in the TGA data (which can be due to instrumental factors) and the enthalpy and entropy values that are eventually derived.

Determination of carbonation enthalpy by DSC

For materials whose carbonation products melt before decomposing, such as Li₅FeO₄ and Li₆CoO₄, DSC experiments provide an alternative way to determine ΔH_carbonation without melting the material. Integrated heat curves for the candidate materials, CaO and Li₅AlO₄ (as an example of a another similar material studied previously for CCS^51,52) derived from DSC are shown in Fig. 9. By taking a TGA trace simultaneously, the mass change of the sample can be measured over the same time interval, and calculating the CO₂ absorbed in moles allows ΔH_carbonation in kJ mol⁻¹ to be derived:

Fig. 9 DSC obtained heat flow and cumulative heat traces for CaO, Li₅FeO₄, Li₆CoO₄ and Li₅AlO₄ samples, studied isothermally at 823 K under p(CO₂) = 1 bar. There is a lag of ∼60–100 s before carbonation begins due to the change of reactive gas flow in the TGA between pure N₂ and pure CO₂. The difference in lag between the trace for Li₅AlO₄ and the other samples is due to the use of a slightly different gas flow setup which uses valves to change between gases more quickly.

The results are shown in Table 6 for experiments performed isothermally at 873 K.

Table 6 Values of ΔH_carbonation obtained from DSC and TPD experiments compared with literature and the predicted values from the Materials Project screening. All values are in kJ mol⁻¹

Compound	DSC	TPD	Screening	Literature
	ΔHr	ΔHr	ΔHr	ΔHr
CaO	−157 ± 5	−170 ± 5	−179.5	−169 ± 4^53
Li5FeO4	−197 ± 5		−208.7
Li6CoO4	−158 ± 5		−217.4
Li5AlO4	−195 ± 5		−212.8

---

Comparing the ΔH_carbonation for CaO obtained from DSC, −157 kJ mol⁻¹, with the known experimental value of −169 kJ mol⁻¹,⁵³ it is seen that while the DSC method tends to underestimate ΔH_carbonation by ∼10%, it can still be used as an approximate experimental guide. For Li₅FeO₄ and Li₅AlO₄ there is a reasonably good agreement between the experimental and theoretical values, within error. For Li₆CoO₄ the theoretical value is ∼30% higher than that obtained from the DSC experiment. Further thermogravimetric tests to gain a more accurate experimental value of ΔH_carbonation are ongoing, specifically TPD experiments under more precisely determined p(CO₂) and ramp rates, which will allow greater confidence when comparing to the values obtained from the Materials Project.

CO₂ cycling capacity studies

To further investigate the cycling performance of the novel CCS material Na₃SbO₄, the sample was heated between 923 K and 1173 K for 40 minutes each to sequentially carbonate and decarbonate the material. The switches between temperatures were achieved by heating and cooling at constant rates of 20 K min⁻¹ and −20 K min⁻¹, respectively, all under a constant CO₂ partial pressure of 0.1 bar in contact with the solid sample. The results of the cycling experiments were analysed by estimating the CO₂ uptake (g_CO2/g_sorbent) by the sorbent in each cycle according to:where m_max,i and m_min,i are the maximum and minimum mass of the sample in the ith cycle respectively. The profile of CO₂ uptake, as a function of cycle number, is plotted in Fig. 10a.

Fig. 10 (a) Gravimetric CO₂ capacity as a function of cycle number for Na₃SbO₄, measured by TGA under p(CO₂) = 0.1 bar. The full and dashed lines show the predicted capacity, based on the stoichiometry of the carbonation reaction. Na₃SbO₄ initially absorbs CO₂ at close to full theoretical capacity, but its capacity quickly fades to between 25–35% of that of the first cycle. (b) Maximum and minimum sample masses measured during each cycle for Na₃SbO₄.

The results show that Na₃SbO₄ carbonates at close to full theoretical capacity on the first cycle, but shows capacity fading upon further cycling. In particular, the capacity of Na₃SbO₄ rapidly decreases even by the second cycle, which showed approximately half the CO₂ uptake compared to the first cycle. It settles to a gravimetric capacity of ∼0.035 g_CO2/g_sorbent after 24 cycles, which is roughly half the gravimetric capacity displayed by CaO under similar conditions.¹⁰

Further analysis of the maximum and minimum sample masses measured during each cycle reveals that the reduction in capacity is almost entirely due to the decreasing maximum sample mass measured in each cycle, compared to the minimum sample mass which stays relatively constant (Fig. 10b). The minimum mass corresponds to the mass of the fully regenerated sample, and as such these results indicate that the sample is regenerating fully upon each cycle. The reduction in the maximum sample mass shows that the capacity fading is primarily due to the reduced amount of carbonate being formed in each cycle.

These results indicate that volume change might not be sufficient to indicate materials which might have improved cycling stability, and that capacity fading is a more complex process than can be described by a single variable.

Morphological studies of materials upon cycling

To further investigate the reasons for the degradation of CO₂ capacity experienced by Na₃SbO₄ upon cycling, the sample was observed using SEM across a number of carbonation cycles, as shown in Fig. 11.

Fig. 11 SEM images of Na₃SbO₄ samples under two different magnifications (left: ×500, right: ×1500): heated to 1223 K under N₂ (A) and (B); fully carbonated (C) and (D); and cycled 20 times, then fully regenerated under N₂ (E) and (F). Upon extended cycling Na₃SbO₄ shows a drastic decrease in both surface area and available porosity, which is likely responsible for its reduction in cycling capacity. The drastic reduction in surface area can be seen especially in the cycled material (F) as compared to the unreacted material (B), indicating that this is a key parameter influencing the cycling stability of Na₃SbO₄.

The SEM results indicate that, like CaO and MgO, the capacity of Na₃SbO₄ to absorb CO₂ is limited by the extent of carbonation, which is a strong function of the surface morphology.^10,54 The fresh particles (images (a) and (b)) show a large amount of available surface area, allowing the maximum amount of reaction with gaseous CO₂. It is much more difficult to distinguish two different phases in the carbonated samples (images (c) and (d)); instead, there appears to be a phase formed with a different morphology and reduced surface area and porosity. This is presumably Na₂CO₃, which sinters because the sample was close to its melting point. The images of the fully regenerated particle after 20 cycles (images (e) and (f)) show that the morphology mirrors that of the carbonated phase, indicating that this sintering persists even upon regenerating the sample, and that multiple cycles appear to lock in this reduction of surface area and porosity. This would seem to explain the reduction in carbonation capacity of these cycled particles as there is less available surface area for CO₂ to react with. These results indicate that thermodynamic parameters influencing the ability to fully regenerate the original material, as well as finding ways to control the surface morphology of the sorbents over many cycles, are key to finding a material with a stable cycling capacity. Both parameters influence the overall behaviour of a sorbent, and only focussing on a single aspect is insufficient to define a material's ability to withstand multiple carbonation cycles without losing capacity.

Discussion

Regarding gravimetric CO₂ capacity, CaO has one of the highest capacities amongst the compounds screened, due to it having a very small molecular mass (MM) that is able to react with 1 mol of CO₂ (and which makes it particularly desirable for use in CCS). Similarly, the other binary oxides screened also had very high gravimetric capacities due to their small MM. The addition of ternary elements that do not directly take part in the carbonation reaction and simply lead to the formation of secondary reaction products would be thought to result in such ternary materials generally having very low capacities. However, there are some ternary compounds that show comparable capacities, such as the compounds with very high alkali metal content Li₅FeO₄, Li₆CoO₄ and Mg₆MnO₈. While it is important to have a high theoretical CO₂ capacity, in practice a low E_p is more desirable in terms of reducing costs, so a lower capacity can be offset with a lower E_p to still produce a material that is promising for CCS applications.

If we take volume change to be one indicator of a compound's resistance to capacity fading upon cycling, the spread of results confirms that there is a wide composition space in which to find more optimal materials with improved properties compared to CaO. In particular, CaO had one of the highest volume increases upon carbonation (124%) out of all the materials screened, with only the binary compounds NiO, MgO and Li₂O having a larger volume expansion. However, a large volume change is not always determinental, as seen in materials such as Ba₄Sb₂O₉ which displays a large change in volume upon reaction, but also did not fade in capacity over 100 cycles.²⁹

Fig. 12 further filters the screened compounds to include only those where A = Na, Li, Mg or Ca, to make a more direct comparison of our screening results with the kinds of materials that have been suggested previously in the literature, composed of these metals. It is clear from Fig. 12 that the carbonation equilibrium is greatly influenced by the choice of metal atom, with Na-based compounds having the most negative ΔH_carbonation and hence the largest E_p, while Ca and Mg based compounds have a much lower E_p generally. In particular, there seem to be many Mg-based materials with relatively low E_p that could have a desirable carbonation equilibrium. For example, the screening found two ternary Mg-based compounds with very high gravimetric capacity: Mg₂SiO₄ or olivine, which has already been researched extensively as a geological CCS material⁵⁵ and Mg₆MnO₈, which unfortunately when investigated in the present study was found to be extremely unreactive. The poor reactivity of Mg₆MnO₈ could be due to either poor intrinsic kinetics or mass transport in the material. Similar to MgO, these materials carbonate at much lower temperatures than Ca-based materials, explaining the much slower reaction kinetics (which are compounded by a generally smaller enthalpic driving force for the carbonation reaction). Further studies are intended to optimise this material's reactivity potentially through producing a sample with smaller particles that might improve its carbonation kinetics, although grinding a material to increase its reactivity introduces a further energy cost to the overall process.

Fig. 12 Screened materials containing Na, Li, Mg or Ca as their alkali atom. Each data point represents the carbonation reaction for each distinct compound with the most negative ΔH. The calculated energy penalty, E_p, for each reaction is plotted against ΔH_carbonation (top), CO₂ gravimetric capacity (middle) and volume change (bottom). The respective binary oxides are circled for clarity.

The range of ΔH_carbonation and E_p within members of the same alkali or alkaline-earth metal group seems directly related to the relative stability of the corresponding binary alkali metal oxides, which are the reactive species in all of the carbonation reactions (regardless of what other metal atoms might be present), and in the majority of cases the addition of a ternary element increases ΔH_carbonation and decreases E_p. This would suggest that there is a good thermodynamic reason for pursuing ternary alkali compounds as novel CCS materials and moving beyond using simple binary oxides.

In terms of gravimetric CO₂ capacity, obviously the screening results reflect the fact that the binary oxides have the highest capacities, which is why they are so attractive for further development. However, there are a suite of ternary compounds with significant capacity, especially amongst Li-based compounds. Some of these have already been explored, such as Li₂ZrO₃,⁴⁷ Li₅AlO₄⁵¹ and Li₄SiO₄,⁴⁹ but this screening has found many others, such as Li₆CoO₄, Li₆MnO₄ and Li₅FeO₄, that could be the subject of further research.

Volume change is perhaps the least important parameter to optimise for, as it is still not fully understood how large a role it plays in determining the cycling stability of a material. However, given the influence volume change has on the morphology of cycled materials and their subsequent reactivity, it is still a worthwhile measurement to include in our methodology. Furthermore, our results show that the binary oxides have the largest volume change upon cycling, further underscoring the necessity to pursue development of ternary alkali oxides as materials of interest which might have more moderate volume changes. Melting points of the phases involved, especially the carbonate phases, may also be an important parameter to include in future screening studies. Having a lower melting point generally leads to the cycled materials having lower remaining available porosity due to the earlier onset of sintering at the Tamman temperature (generally defined to be half the melting point of the material) and hence a smaller cycling capacity.

A further insight that cannot be ignored is the role of kinetics in finding a suitable CCS sorbent. The apparent failure of Mg₆MnO₈ and Ca₄Nb₂O₉, despite their very promising predicted capabilities, shows that any relevant future large scale screening approach must consider a way to build in reaction kinetics into its rational design algorithm. In this particular case the discrepancies between theory and experiment manifest from different causes for Mg₆MnO₈ and Ca₄Nb₂O₉. In the case of Mg₆MnO₈, similar Mg-based minerals such as olivine and even the binary oxide MgO are known to have very poor carbonation kinetics in the solid state, requiring radically different approaches to improve their performance such as including H₂O in the gas stream.^56,57 For Ca₄Nb₂O₉, it is likely that the much higher sintering temperature (1648 K) leads to a loss in available surface area for reaction, and hence leads to the slower overall carbonation kinetics.

One idea is to adapt an approach used for framework materials that characterises the internal surface volume and connected porosity of materials from their structure to estimate the kinetic barriers to gas diffusivity in these materials.⁵⁸ Conversely, it also means that more in-depth work to optimise promising materials may be needed to overcome these kinetic issues, lest excellent materials be missed after initial properties testing.

An additional factor that may also influence the actual performance of the screened materials is that of H₂O in the reactant gas stream. All our experiments and screening were performed in dry conditions, but this is not the case in most flue gases, which typically contain ∼10% H₂O. Previous studies on similar materials to those found in our screening, most notably Li₄SiO₄⁵⁹ and Li₅AlO₄,⁶⁰ found that the addition of H₂O to the CO₂ gas stream results in a drastic decrease in carbonation reaction temperature. Such a decrease would result in a lower calculated energy penalty when compared to the results from our screening in dry conditions. Therefore, many of the materials studied in this work may in fact show even better performances in more realistic reactor conditions, and future work is planned to include H₂O in both the theoretical and experimental approaches outlined here.

Finally, given our recent work on novel CCS material, Ba₄Sb₂O₉,²⁹ a material that was previously not characterised and therefore not in the Materials Project database, underlines the importance of continually working to expand the database with new compositions, either from theory or experiment. Predicting new compositions and structures based on ternary oxides containing the abundant alkali metals Na, Mg and Ca using the Structure Predictor module already present in the Materials Project system⁶¹ is a direction considered for further study.

Conclusions

Through the use of open phase diagrams and the Materials Project, the largest set to date of oxide compounds were screened for their theoretical thermodynamics of carbonation, as well as their theoretical gravimetric CO₂ capacity and volume change upon carbonation. The results were validated both against standard experimental values for known carbonation reactions, and against similar previous DFT studies, and good agreement was found with both. A single parameter, energy penalty, was used to assess the relative efficiency of using a compound in a model CCS process, and to provide a way to rank materials in terms of their suitability for further study.

Using a variety of thermogravimetric techniques, experimental validation was provided for a small set of candidate materials suggested by the Materials Project screening. For some of the materials, such as Na₃SbO₄ and Li₅FeO₄, the experimental values of ΔH_carbonation were in good agreement with those calculated from the screening procedure, indicating that the output of that screening can predict the carbonation thermodynamics of materials from a theoretical standpoint. This experimental validation is vital to developing a robust screening procedure that is able to predict materials that function under realistic reaction conditions.

Furthermore, the screening results give new insights to implement in rational design approaches towards finding optimal CCS materials. Firstly, the use of ternary alkali metal oxide compounds is found to be advantageous both in being able to achieve lower energy penalties due to less negative ΔH_carbonation, but also in avoiding very large volume changes that occur when carbonating the binary oxide compounds, which could lead to cycling instability through sintering and pore clogging.

The alkali earth metals Mg and Ca are found to be generally more favourable than compounds containing Li and Na mainly due to their lower energy penalty, and there are many compounds found in the screening that are suitable for further study. However, subsequent experimental results found that Mg-based materials in particular display poor reaction kinetics, and require further optimisation in order to be used. These results show that large scale screening processes can employ a reasonably efficient level of DFT theory to achieve accurate results that give real information into overall trends that are important to designing novel functional materials.

Cycling experiments on Na₃SbO₄ showed that this material suffers similar capacity fading as seen in the CaO–CaCO₃ system, despite it having much lower predicted volume expansion upon carbonation. SEM studies suggest that the decrease in available surface area of the cycled particles compared to those in the fresh sample contributes to this capacity fading, and that some amount of the carbonate phase does not regenerate in later cycles. Future studies on a wider range of compounds will hopefully assist in understanding the underlying parameters influencing the stability of these compounds over many cycles of carbonation.

These results show that large scale screening processes can employ a reasonably efficient level of DFT theory to achieve accurate results that give real information into overall trends that are important to designing novel functional materials. As more sophisticated high-throughput methods are devised it will be increasingly possible to target functional materials with an array of complex and useful properties all before entering the laboratory.

Acknowledgements

M. T. Dunstan acknowledges funding from the Cambridge Commonwealth Trusts, Trinity College, Cambridge and is a recipient of a STFC Futures Early Career Award. M.T. Dunstan, S.A. Scott, J.S. Dennis and C.P. Grey acknowledge funding from EPSRC Grant No. EP/K030132/1. A. Jain, S.-P. Ong and K. Persson gratefully acknowledge support as well as infrastructure and Materials Project data through the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Project Center Grant No. EDCBEE. W. Liu acknowledges funding from NRF, Singapore under its CREATE programme.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ee03253a

‡ Sb was also included in the screening, despite not being within the most abundant elements, because of our work on a novel CCS material, Ba₄Sb₂O₉.²⁹

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