Article pubs.acs.org/cm
Chemical Decomposition as a Likely Source of Ambient and Thermal Instabilities of Layered Sodium Cobaltate Damjan Vengust,*,† Bostjan Jancar,† Andreja Sestan,† Maja Ponikvar Svet,‡ Bojan Budic,§ and Danilo Suvorov† †
Advanced Materials Department and ‡Department of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia § Laboratory for Analytical Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia ABSTRACT: With the application of an oxygen atmosphere, we synthesized a highly textured sodium cobaltate, Na0.75CoO2. At the same time, we identified its peculiarities that influence the measured parameters to a degree that poses serious questions about this material’s potential for use. We have systematically studied the influence of humidity on the ceramic pellets and identified the conditions under which the material completely deteriorates. By performing microstructural and thermal analyses, coupled with a determination of the evolved gases, we identified the chemical reactions that are involved in this process. In addition, we re-examined the performance of sodium cobaltate under the working conditions and found that the material behaves in a manner different from the expected manner. We have shown, in contrast to many other reports, that the oxygen vacancies do not play a very important role because the changes in the physical parameters can be attributed to the reduction of cobalt and consequently to the formation of CoO inclusions, which increases the amount of sodium in the sodium cobaltate lattice. KEYWORDS: thermoelectric oxide materials, hydrated Na0.75CoO2, oxygen vacancies, CoO
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1997, Terasaki et al.5 obtained a very high Seebeck coefficient (S = 0.1 mV/K) and an unexpectedly low electrical resistivity of 0.2 mΩ cm, at 300 K, from layered NaCo2O4. Shortly afterward, Ohtaki et al. showed that the dimensionless figure of merit zT reaches 0.8 at 1000 K in polycrystalline NaCo2O4,6 and one year later, Fujita obtained a zT value from a single crystal that exceeded unity at 800 K.7 Because a value of zT exceeding unity represents the performance benchmark for a material to be used in applications, research on this type of compound has become increasingly widespread. In parallel with the research on sodium cobaltate, a number of other structurally similar cobaltates have been studied in terms of their thermoelectric applications.8−10 In addition to direct thermoelectric applications, the materials were also studied as potential electrodes for solid oxide fuel cells, where apart from the direct electrical energy generated from the oxidation of the fuel, additional energy can be harvested from the thermoelectric effect.11,12 Sodium cobaltate NaxCoO2 (1/3 < x < 1) exhibits a layered crystal structure in which edge-sharing CoO6 octahedra form CdI2-type layers with Na ions positioned in the prismatic sites between the layers.13 Partial occupation of these prismatic sites leads to complex ordering of the Na ions and vacancies, which results in incommensurately modulated crystal structures.14
INTRODUCTION The growing desire for renewable energy sources, which includes harvesting the waste energy lost through the heat exchange in common heat engines, is driving the research in the field of high-temperature thermoelectric devices. The rapidly growing development of new materials over the past few years has resulted in the discovery of several promising compositions with a high thermoelectric figure of merit, the physical parameter by which the performance of a thermoelectric material can be evaluated, defined as zT = S2σT/κ, where T is the absolute temperature, σ is the electrical conductivity, κ is the thermal conductivity, and S is the Seebeck coefficient. Although there is no theoretical limit for zT, the highest reported values are 2.2 for the p-type semiconducting PbTe intermetallic compound with endotaxial nano inclusions of SrTe1 and 2.4 for a thin-film superstructure containing heavily donor-doped conducting SrTi0.8Nb0.2O3 layers between the insulating SrTiO3 layers forming a two-dimensional electron gas.2 Most state-of-the-art thermoelectric materials, some of which have already been commercially utilized, are intermetallic compounds with figures of merit ranging from 1 to 2. Their use is, however, in most cases limited to temperatures below 700 °C, as they tend to oxidize at higher temperatures.3,4 With regard to these oxides, semiconductors and semimetals have an obvious advantage over these types of materials and are currently the subject of extensive research. Until recently, it was believed that oxides are poor candidates for thermoelectric materials because of their low carrier mobility. However, in © 2013 American Chemical Society
Received: September 16, 2013 Revised: November 8, 2013 Published: November 12, 2013 4791
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RESULTS AND DISCUSSION After calcination in air up to 870 °C and sintering in an oxygen atmosphere at 970 °C, the cross-sectional view of the Na0.75CoO2 ceramics revealed a high degree of texturing, confirmed by the X-ray diffraction (XRD) pattern recorded from the polished surface (Figure 1). The X-ray pattern in
The occupation of interlayer sites determines the average oxidation state of the Co ions, which influences the electronic correlations and spin states within the CoO2 layers, thus giving rise to a unique combination of electrical and magnetic properties. In Na0.5CoO2, for example (x = 0.5), half of the cobalt atoms are found in the Co3+ state and the other half in the Co4+ state; for this reason, the material behaves as a chargeordered insulator. If x is between 2/3 and 3/4, the material is classified as a Curie−Weiss metal, with a particularly high thermoelectric power above 800 K. There are, however, huge discrepancies between the reported values of the thermoelectric properties of sodium cobaltates. On the basis of the previously reported crystal chemistry of the system, at least three reasons can be foreseen. The first is highly anisotropic grain growth, which results in platelike grains. Depending on the processing conditions, this results in different degrees of texturing. This can in turn influence the directional dependence of the electrical and thermal conductivity. The second reason, which in the literature is often correlated with changes in the thermoelectric properties, could be associated with the oxygen vacancies in the CoO 2 layers.15−17 The formation of oxygen vacancies, however, has been disputed in the literature.18,19 The third reason could be the high mobility of the sodium ions between the CoO2 layers, which renders the layered sodium cobaltate prone to a reaction with atmospheric water.20,21 To resolve the reported inconsistencies, we studied the chemistry of sodium cobaltate with respect to the chemical sensitivity of the CoO2 layers to an oxygen partial pressure at elevated temperatures and the reactivity of the interlayer sodium with the environment.
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Article
EXPERIMENTAL SECTION
Figure 1. (a) SEM-BE image of the cross-sectional microstructure of ceramics and (b) X-ray diffraction patterns of the powdered sample (black) and the cross-sectional surface (red) of the layered sodium cobaltate with the nominal Na0.73CoO2 composition.
The samples were prepared by solid-state reaction using stoichiometric amounts of reagent powders of Na2CO3 and Co3O4 (both from SigmaAldrich, 99.9% pure). After being weighed, the powders were homogenized under absolute ethanol in a planetary ball mill using 10 mm yttria-stabilized zirconia (YSZ) balls. The homogenized powders were then dried, isostatically pressed into disk-shaped samples, and calcined in an atmosphere of air between 820 and 870 °C. Before being sintered, the powders were ball milled under absolute alcohol using 3 mm YSZ balls, and the sintering was performed in an atmosphere of 1 MPa of oxygen at 970 °C. The prepared samples were then first analyzed by X-ray powder diffraction, conducted with a PANalytical Empyrean instrument. The data were collected using Cu Kα radiation in the 2θ range from 10° to 60° with a step of 0.013° and a counting time of 88.995 s, using a diffracted beam monochromator for the line detector to remove the effect of the Co fluorescence. To determine the stoichiometry of the sintered samples, a quantitative chemical analysis was performed. The sodium was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), the cobalt by complexometric titration, and the amount of oxygen in the samples by high-temperature pyrolysis using an Elementar vario EL cube. The microstructures were examined using a Jeol JSM-7600F scanning electron microscope. The mass changes and the gases released during the heating of the samples were analyzed with a Jupiter 449 simultaneous thermal analysis (STA) instrument, coupled with a mass spectrometer (MS) using an air or argon atmosphere. The electrical conductivity was measured under an air atmosphere using a Keithley 2002 multimeter and a standard four-probe setup, prior to which gold electrodes were printed on the bar-shaped samples with approximate dimensions of 4 mm × 4 mm × 25 mm.
Figure 1 reveals the absence of (00l) peaks, which indicates that the prevailing orientation of the grains is that with the [001] pointing along the pressing direction of the green powders before sintering. The observation indicates that no special treatment, such as the polymerized complex method,22 during the preparation of the powder is required to obtain highly oriented ceramics after sintering. Such a high degree of spontaneous texturing is a consequence of the platelike morphology of the calcined particles, governed by the layered crystal structure of the sodium cobaltate, which can be achieved by a simple solid-state reaction method. Furthermore, backscattered electron images combined with EDS indicate that the ceramic is a single-phase material. It is well-known that in layered cobaltates the transfer of charge between the interlayer species and the two-dimensional CoO2 layers takes place, resulting in a high concentration of electron holes with a high mobility within these layers.23 Consequently, the textured ceramics are bound to exhibit a directional dependence of the electrical properties. In the case of the oxygen atmospheresintered Na0.75CoO2 with a cross-sectional microstructure, such as that shown in Figure 1, we measured the room-temperature electrical conductivity along the direction of preferred growth to be 412 S/cm, whereas in the perpendicular direction, it was 179 S/cm. This large difference corresponds to a high degree of 4792
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texturing and, as in the single crystal,5 reveals the anisotropic nature of this material. There has been much discussion regarding the stoichiometry of bulk ceramic sodium cobaltate, mostly based on research with similar compounds, which suggests that sodiumcontaining species exhibit a high vapor pressure and are thus prone to sublimation during processing at elevated temperatures. As a consequence, different calcination procedures, such as “rapid heat up”,24 have been suggested. We performed a chemical analysis utilizing ICP-AES, complexometric titration, and a pyrolysis technique to determine the stoichiometry and found that the loss of sodium during calcination in air up to 870 °C and sintering under an oxygen atmosphere, if present, is only negligible. The results of the analysis are listed in Table 1.
The amount of water and carbon dioxide released from Na0.73CoO2 varies with respect to the conditions to which the material is exposed, which can be seen from the TG curves shown in Figure 3. The curves were recorded at a rate of 5 °C/
Table 1. Elemental Composition (with standard deviations) of Sodium Cobaltate Ceramics Determined by ICP-AES, Complexometric Titration, and Pyrolysis
Figure 3. TG/EGA of powder with a nominal Na 0.73 CoO 2 composition: first run after synthesis (black), second run performed immediately after the first run (blue), and second run after the powder had been stored in a desiccator for 48 h (red).
w(Co)
w(O)
w(Na)
w(C)
53.5 ± 0.2
30.5 ± 0.2
15.1 ± 0.5
0.24 ± 0.05
min up to 800 °C, followed by an isotherm of 3 h. The black curve represents data for the material after calcination; the blue curve represents data for the same material, but with the second TG run performed immediately after the sample had been cooled from the first run, and the red curve represents data from the TG run from the same material stored in a desiccator for 48 h after the second TG run. The results of the thermal analysis imply that after the H2O and CO2 are released, the material becomes stable and does not lose any weight up to ∼930 °C, where it starts losing O2. Furthermore, Figure 3 suggests that CO2 and water from the atmosphere are absorbed independently by the layered cobaltate because the material stored in the desiccator releases only CO2 upon heating. It has not yet been resolved how the CO2 from the atmosphere reacts with layered sodium cobaltates. There are, however, two most likely mechanisms. CO2 can incorporate into the oxygen sublattice forming oxycarbonate, as has been observed in some perovskite materials,26 or it can react with interlayer sodium and oxygen, forming sodium carbonates. Water molecules, on the other hand, can either intercalate between the layers or react with the interlayer sodium to form NaOH, as will be discussed later. The loss of oxygen, which for Na0.73CoO2, according to TG/EGA, under an air atmosphere takes place above 930 °C, has often been associated with the formation of oxygen vacancies. The stoichiometric formula was rewritten as NaxCoO2.00−δ, where δ did not exceed 0.16 with regard to the TGA.16 When the material was reheated under the elevated oxygen partial pressure, the total mass was regained. Because the material did not show any clear structural changes that could be resolved by X-ray diffraction, no additional experiments were performed to clarify the exchange of oxygen.14−16 In the same period of time, some other group performed an accurate structural analysis, i.e., nuclear magnetic resonance (NMR), on Na0.66CoO2 in which the expected shift in the resonance spectra, which would correspond to the presence of oxygen vacancies in the CoO2 sublattice, was not observed.18 Recently, Casolo et al. published a theoretical paper in which they showed, by using first-principle thermodynamic equations and the density functional theory (DFT), that the formation of oxygen vacancies in Na0.75CoO2 is highly unlikely, merely because their formation energy is too large.19 Because the charge transport properties depend solely on the oxygen
In contrast to other research groups that were measuring the elemental composition,16,17 we also determined the concentration of oxygen and carbon using pyrolysis.25 If we neglect for a moment the presence of carbon (and water as will be explained shortly), the stoichiometry of oxygen-sintered sodium cobaltate with the nominal Na0.75CoO2 composition was determined to be Na0.73±0.02Co1.00±0.01O2.10±0.01. It is known that layered sodium cobaltate exists over a broad range of sodium concentrations, with x ranging from virtually 0 to 1. Different concentrations of interlayer sodium need to be charge-compensated by the CoO2 layers, which can be by either the adaptation of the average oxidation state of the cobalt or the nonstoichiometry on the oxygen sublattice. The 5% excess of oxygen determined by the pyrolysis can be for this type of material more plausibly ascribed to various species intercalated between the CoO2 layers; in particular, the detection of carbon implies the presence of carbonate in the structure. To further elucidate this, we performed a set of thermogravimetric/ evolved gas analysis (TG/EGA) experiments. Figure 2 shows
Figure 2. TG curve of Na0.73CoO2 recorded at a rate of 5 °C/min between room temperature and 1000 °C, showing the release of gases as determined by the evolved gas analysis.
the TG curve of the calcined Na0.73CoO2 recorded from room temperature to 1000 °C with a heating rate of 5 °C/min, where three distinct regions with a loss of mass can be seen. The analysis of the evolved gases revealed a minor loss of H2O up to ∼200 °C, followed by the release of CO2 between ∼330 and ∼780 °C, and the loss of O2 above ∼930 °C. 4793
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Figure 4. (a) SEM-BE image of the microstructure of the sample with a nominal Na0.73CoO2 composition annealed under an Ar atmosphere. (b) Comparison of X-ray diffraction patterns after annealing in air (black) and Ar (red) and being reoxidized in O2 (blue). (c) TG curves recorded under an Ar atmosphere (black) and rerun under an O2 atmosphere. (d) SEM-BE image of the microstructure of the sample reoxidized under an O2 atmosphere.
unlikely to form because their formation energy is too large.19 The formation of the CoO secondary phase also results in an increase in the concentration of sodium ions between the CoO2 layers, which strengthens the cohesion forces and compresses the unit cell along the [001] direction. This is evident from the shift of the (001) diffraction peaks in the X-ray spectra shown in Figure 4. Furthermore, an increase in the concentration of sodium within the Na0.73CoO2 structure must result in a decrease in the average oxidation state of the Co within the CoO2 layers. The reaction that involves a loss of oxygen from sodium cobaltate, which occurs below 800 °C under an Ar atmosphere and above 930 °C in air, can be expressed as
sublattice, the change in conductivity and Seebeck coefficient was not attributed to the generation of oxygen vacancies; they suggested, instead, that the formation of some unknown complex secondary phases takes place, which alter the electronic states in Na0.75CoO2. To look into the loss of oxygen in more detail, we performed TG and annealing experiments under an argon atmosphere. In Figure 4, the TG/EGA runs recorded with a heating rate of 5 °C/min up to 800 °C followed by a 3 h isothermal period are shown. The black TG curve shows the loss of mass that occurs in three segments. According to the EGA, apart from H2O and CO2, the material now also loses O2 below 800 °C. Performing a rerun within the thermal analysis furnace using pure oxygen as the atmosphere, we observed a weight gain, as indicated by the red TG line in Figure 4. An annealing experiment performed under the same conditions as the TG revealed, as can be seen from the X-ray and microstructural analyses shown in Figure 4, that the argon atmosphere results in the formation of CoO as the second phase. Reheating under an oxygen atmosphere reverses the reaction, and single-phase Na0.73CoO2 is again formed. This finding, in contrast to other reports, indicates that in Na0.73CoO2 sodium cobaltate compensates for the loss of oxygen by forming a secondary phase CoO. The material can be reoxidized, during which time the CoO inclusions disappear from the microstructure. The reoxidation process in a pure O2 atmosphere starts already at temperatures between 300 and 400 °C and follows the continuous exponential plot (red curve in Figure 4c). These findings indicate that the loss of oxygen results in the formation of the secondary phase and not the oxygen vacancies, which is consistent with the NMR18 in a very similar experiment and confirms the thermodynamically based DFT calculations, which reveal that the oxygen vacancies are
Na 0.73CoO2 ↔ Na 0.73Co1 − x O2 − 2x + xCoO + 0.5O2
(1)
The average oxidation state (OS) of the Co within the layered structure can be expressed in terms of x, which is the molar fraction of Co that is reduced to the +2 oxidation state.
OS(Co) =
3.27 − 4x 1−x
(2)
As is evident from the TG curves (Figure 4), roughly 2% of the initial mass is regained after the Na0.73CoO2 that is exposed to heating in an Ar atmosphere is rerun in pure O2. The 2% of the mass gained back according to eq 2 indicates that ∼13% of cobalt, initially in the +3.27 average oxidation state, has been reduced to CoO and subsequently reoxidized, while the average oxidation state of the cobalt within the CoO2 layers did not drop below +3.16. The formation of CoO reduces the cumulative oxidation state of the Co in the system, which in the literature is often regarded as the oxidation state within the CoO2 layers and is attributed only to an increase in the fraction of Co3+ ions.15,16 Our findings imply that any interpretation of the experimental results or the prediction of properties based 4794
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Na0.73CoO2 layered cobaltates should not be used under an air atmosphere above 930 °C. Another issue associated with the reactivity of sodium cobaltate with the atmosphere is its hygroscopicity. The first reports of the possibility of ambient humidity reacting with this type of material were published by Foo et al.19,20 Their systematic study was performed on the superconducting oxyhydrate, which contained different amounts of water molecules between the CoO2 layers, forming several distinct Na0.3CoO2·yH2O (y = 0, 0.1, 0.3, 0.6, or 1.3) structural polytypes. They found that at room temperature the material restructures from an anhydrous type to given polytypes, depending on the ambient humidity. In our case, the ceramics with the nominal Na0.73CoO2 composition showed chemical sensitivity to the ambient conditions by whitening on the surface. An inspection of the surface by SEM revealed the presence of whisker crystals growing from the surface of the sodium cobaltate grains (Figure 7).
on theoretical calculations should take the presence of cobalt in the 2+ oxidation state and the excess of sodium in the sodium cobaltate structure into consideration. Because the average oxidation state of Co within the CoO2 layers influences the electronic correlations and spin states, it in turn influences the power factor, and furthermore, the presence of inclusions within the microstructure influences the thermal conductivity. M. Banobre Lopez showed that with an increase in the sodium concentration in this layered structure by the separate synthesis of different stoichiometries, the Seebeck coefficient increases, while the electrical conductivity decreases.15 With this argument, we can equally likely explain the results of P. H. Tsai, who found very similar changes in the thermoelectric properties were attributed to the formation of oxygen vacancies.16 As can be seen from Figure 5, when the molar fraction of Co reduced
Figure 5. Average oxidation state of cobalt within the CoO2 layers of sodium cobaltate and the cumulative oxidation state of cobalt as a function of the fraction of cobalt reduced to the +2 oxidation state.
to a +2 oxidation state would exceed 0.27, the oxidation state of Co within the CoO2 layers would decrease below +3 and the layered structure would decompose. Such a decomposition can be seen from the X-ray diffraction collected from the material with the nominal Na0.73CoO2 composition annealed at 950 °C for 10 h (Figure 6). It is therefore important to control the
Figure 7. (a) SEM-SE image of the pellet surface of ceramics with the nominal Na0.73CoO2 composition exposed to ambient conditions for 1 week and (b) comparison of the X-ray diffraction patterns of the pellet surface (red) and the interior (black).
X-ray diffraction showed that the whiskers are sodium carbonate hydrates with crystal structures similar to those of the minerals trona [Na3(CO3)(HCO3)·2H2O] and thermonatrite (Na2CO3·H2O), which suggests chemical reactions with ambient H2O and CO2. A chemical reaction between water and sodium cobaltate was previously studied by Sakurai et al.27 By submersing powdered material in water, they found that interlayer sodium ion are partially exchanged for hydronium ions, which results in a unit cell expansion along the [001] direction and keeps the oxidation state of the cobalt within the CoO2 layers constant. The following reaction was proposed:
Figure 6. X-ray diffraction pattern of powder with the nominal Na0.73CoO2 composition annealed at 950 °C for 10 h.
atmosphere during the high-temperature processing of theses materials. For the synthesis of high-density ceramics based on the nominal Na0.73CoO2 composition, for instance, a temperature of >950 °C is required, because of which an oxygen atmosphere needs to be utilized to prevent the reduction of the cobalt and the formation of the secondary phase. Furthermore, these findings suggest that thermoelectric materials based on 4795
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Chemistry of Materials NaxCoO2 + 2y H 2O → Nax − y(H3O)y CoO2 + y H 2O
Article
(3)
The growth of sodium carbonate hydrate whiskers from the cobaltate grains can be interpreted as being a consequence of the reactions between NaOH, formed due to interlayer ion exchange, and atmospheric CO2 and H2O: 2NaOH + CO2 → Na 2CO3 ·H 2O
(4)
3NaOH + CO2 → Na3(CO3)(HCO3) ·2H 2O
(5)
To elucidate how the chemical properties of the ceramic could change, we exposed the Na0.73CoO2 ceramic pellets to a very humid atmosphere with a relative humidity of >95% at 30 °C. The pellets immediately became wet on the surface, and after a few days, their structural integrity severely deteriorated (Figure 8).
Figure 8. Images of ceramic pellets with the nominal Na0.73CoO2 composition: (a) as synthesized and (b) exposed to a relative humidity of 95% at 30 °C.
Figure 9. (a) X-ray diffraction and TG/EGA of the outer and inner layer of ceramics with the nominal Na0.73CoO2 composition after exposure to high humidity. (b) Structural changes and two distinct evaporation peaks of water indicate that a considerable amount of the hydrated phase of sodium cobaltate was formed.
After the relative humidity had been decreased, the disintegration process slowed, and below 65%, the ceramic pellet remained dry and did not lose its integrity. Although the outer layer was covered by sodium carbonate whiskers, the interior, which was inspected by SEM, did not differ from the freshly prepared material. From the aging, which was performed for 720 h, we could estimate that the depth of penetration of the humidity was
