Matthew T.
Dunstan
a,
Anubhav
Jain
b,
Wen
Liu
c,
Shyue Ping
Ong
d,
Tao
Liu
a,
Jeongjae
Lee
a,
Kristin A.
Persson
e,
Stuart A.
Scott
f,
John S.
Dennis
g and
Clare P.
Grey
*a
aDepartment of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK. E-mail: cpg27@cam.ac.uk
bEnergy Technologies Area, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd, Berkeley, CA 94720, USA
cCambridge Centre for Advanced Research and Education in Singapore, Nanyang Technological University, 1 Create Way, Singapore 138602, Singapore
dDepartment of NanoEngineering, University of California San Diego, 9500 Gilman Drive, Mail Code 0448, La Jolla, California 92093-0448, USA
eDepartment of Materials Science and Engineering, University of California Berkeley, 210 Hearst Mining Building, Berkeley, CA 94720, USA
fDepartment of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK
gDepartment of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge, CB2 3RA, UK
First published on 21st January 2016
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.
Broader contextGiven 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. |
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 50000 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.
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 EDFT 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 μO231,32 was adapted to study the phase equilbria under changing μCO2.
ϕ(T,P,NA,NM,NO,μCO2) = G − μCO2NCO2 = E − TS + PV − μCO2NCO2 | (1) |
(2) |
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 pCO2, the chemical potential can be described as:
(3) |
(4) |
A summary of the assumptions used in simplifying the CO2 grand canonical potential can be seen in Table 1.
Fig. 2 Errors between ΔEDFT and ΔHexperimental at 293 K for selected binary and ternary oxides. The average error was used as a correction to ECO2 in the Materials Project database. |
Fig. 3 Comparison between the calculated ΔEDFT 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 ΔEDFT being 21 kJ molCO2−1 lower than the previous study on average. |
It can be clearly seen that there is a small but constant overestimation (more negative) of ΔEDFT in this work compared to the previous studies. The main source of deviation between the two studies is the different values of ECO2 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 ΔEDFT.
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 |
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.
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.
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 |
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 pO2 levels expected in the calciner, the reaction chamber where the carbonated materials are decomposed to release gaseous CO2 and reform the original material (pO2 < 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.
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 Ep < 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 (gCO2absorbed/gsorbent) 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 Ep, as expected considering it is a dominant part of the Qloss and Qrecovered 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 Ep, 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 Ep, 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 Ep.
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 |
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.
(5) |
From these it is possible to calculate ΔHcarbonation and ΔScarbonation from these fitting constants, assuming that ΔHcarbonation and ΔScarbonation are approximately constant:
Kp = exp(ΔSr/R)·exp(−ΔHr/RT) | (6) |
A = exp(ΔSr/R) | (7) |
B = ΔHr/R | (8) |
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 |
For the materials studied, it is seen that there is very good agreement between the theoretical and experimental values for ΔHcarbonation. 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.
(9) |
The results are shown in Table 6 for experiments performed isothermally at 873 K.
Compound | DSC | TPD | Screening | Literature |
---|---|---|---|---|
ΔHr | ΔHr | ΔHr | ΔHr | |
CaO | −157 ± 5 | −170 ± 5 | −179.5 | −169 ± 453 |
Li5FeO4 | −197 ± 5 | −208.7 | ||
Li6CoO4 | −158 ± 5 | −217.4 | ||
Li5AlO4 | −195 ± 5 | −212.8 |
Comparing the ΔHcarbonation 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 ΔHcarbonation 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 ΔHcarbonation 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.
(10) |
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 gCO2/gsorbent 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.
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.
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 ΔHcarbonation and hence the largest Ep, while Ca and Mg based compounds have a much lower Ep generally. In particular, there seem to be many Mg-based materials with relatively low Ep 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.
The range of ΔHcarbonation and Ep 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 ΔHcarbonation and decreases Ep. 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.
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 ΔHcarbonation 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 ΔHcarbonation, 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.
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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