energies Article Improved High Temperature Thermoelectric Properties in Misfit Ca3Co4O9 by Thermal Annealing Arindom Chatterjee 1, *, Alexandros El Sachat 2 , Ananya Banik 3 , Kanishka Biswas 3 , Alejandro Castro-Alvarez 4 , Clivia M. Sotomayor Torres 1,5 , José Santiso 1 and Emigdio Chávez-Ángel 1, * 1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain 2 Institute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, 15341 Athens, Greece 3 New Chemistry Unit, School of Advanced Materials, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur, Bangalore 06484, India 4 Laboratorio de Bioproductos Farmacéuticos y Cosméticos, Centro de Excelencia en Medicina Traslacional, Facultad de Medicina, Universidad de La Frontera, Av. Francisco Salazar 01145, Temuco 4780000, Chile 5 ICREA—Catalan Institute for Research and Advanced Studies, 08010 Barcelona, Spain * Correspondence: arichatterjee1990@gmail.com (A.C.); emigdio.chavez@icn2.cat (E.C.-Á.) Abstract: Ca3 Co4 O9 , a p-type thermoelectric material based on transition-metal oxides, has garnered significant interest due to its potential in thermoelectric applications. Its unique misfit-layered crystal structure contributes to low thermal conductivity and a high Seebeck coefficient, leading to a thermoelectric figure of merit (zT) of ≥1 at 1000 K. Conventionally, it has been believed that thermopower reaches its upper limit above 200 K. However, our thermopower measurements on polycrystalline Ca3 Co4 O9 samples have revealed an unexpected increase in thermopower above 380 K. In this study, we investigate the effects of high oxygen pressure annealing on Ca3 Co4 O9 and provide an explanation based on the mixed oxide states of cobalt and carrier hopping. Our results demonstrate that annealing induces modifications in the defect chemistry of Ca3 Co4 O9 , leading to a decrease in electron hopping probability and the emergence of a thermal activation-like behavior in thermopower. Citation: Chatterjee, A.; El Sachat, A.; These findings carry significant implications for the design and optimization of thermoelectric Banik, A.; Biswas, K.; Castro-Alvarez, materials based on misfit cobaltates, opening new avenues for enhanced thermoelectric performance. A.; Sotomayor Torres, C.M.; Santiso, J.; Chávez-Ángel, E. Improved High Temperature Thermoelectric Keywords: Ca3 Co4 O9 ; thermoelectricity; heike’s limit; cobaltates Properties in Misfit Ca3 Co4 O9 by Thermal Annealing. Energies 2023, 16, 5162. https://doi.org/10.3390/ en16135162 1. Introduction Academic Editors: Tingzhen Ming, Misfit cobaltates, such as Ca3 Co4 O9 or Nax CoO2 , have been the subject of intense Yongjia Wu and Yonggao Yan research in the field of thermoelectrics due to their high thermoelectric performance at elevated temperatures [1,2]. Ca3 Co4 O9 (CCO) is a promising p-type thermoelectric (TE) Received: 31 May 2023 material based on transition-metal oxides. This material exhibits a TE figure of merit, zT ≥ 1 Revised: 30 June 2023 at 1000 K (zT = S2 /(ρ·κ)T, where ρ represents the electrical resistivity (ρ = σ−1 , where σ Accepted: 3 July 2023 is the electrical conductivity), S is the Seebeck coefficient or thermopower, κ is the total Published: 4 July 2023 thermal conductivity, and T is the absolute temperature) [3]. The high zT in this class of compounds is attributed to two distinct reasons. Firstly, the unique misfit-layered crystal structure of the material results in low in-plane [4] and ultralow out-of-plane thermal Copyright: © 2023 by the authors. conductivity [5]. Secondly, the unique electronic band structure of CCO leads to high S Licensee MDPI, Basel, Switzerland. (100–150 µV/K) and low ρ (<10 mΩ.cm) at 300 K [2,6–9]. This article is an open access article The misfit-layered crystal structure of CCO is composed of two subunits: a rocksalt- distributed under the terms and like Ca2 CoO3 (i.e., CaO-CoO-CaO) and a face-shared CoO2 octahedra stacked along their conditions of the Creative Commons c-axis [10,11]. The crystal symmetries of the two subunits differ, causing the cell parameters Attribution (CC BY) license (https:// to not align along all crystallographic axes. A substantial mismatch is observed along the creativecommons.org/licenses/by/ b-axis, where the b-parameter of CoO2 (bCO ) is shorter than the b-parameter of the rocksalt 4.0/). unit (bRS ), and their ratio is referred to as the misfit ratio, q = bCO /bRS . Energies 2023, 16, 5162. https://doi.org/10.3390/en16135162 https://www.mdpi.com/journal/energies Energies 2023, 16, 5162 2 of 13 This class of compounds, as well as layered Nax CoO2 , exhibit an intriguing feature wherein thermopower increases linearly with increasing temperature from 10 to 170 K, a behavior typically observed in metallic conductors [12,13]. Beyond 200 K, thermopower becomes temperature independent for misfit cobaltates [14]. Both theoretical and experi- mental investigations suggest that metallic conduction of travelling charge carriers moving in a broad eg ’ band governs thermopower at lower temperatures (T < 150 K), while localized charge carriers in a narrow a1g band dominate at higher temperatures (T > 200 K) [15]. It is generally assumed that thermopower reaches its high-temperature limit above 200 K, as given by Heike’s formula [16,17]. However, thermopower measurements (T > 380 K) for polycrystalline samples reveal that thermopower further increases with increasing temperature [14,15,18]. Surprisingly, this increase in thermopower is not due to an increase in resistivity as expected from the single parabolic band model, where both quantities are inversely proportional to charge carrier density. In fact, resistivity decreases within 380–750 K. Thus, it remains unclear why thermopower increases above 380 K, even though it is commonly believed that CCO reaches its high-temperature limit above 200 K [14,19,20]. Numerous hypotheses have been proposed in the literature to explain the observed en- hanced S in CCO. These include spin-state transitions in Co3+ ions, partial localization of electrons in eg ’ orbital, and charge associated with Co-t2g occupancy during structural transitions [13]. However, ab initio calculations have demonstrated that the energy re- quired for the first excited spin in a Co atom is incompatible with the energy scale for a phase transition at 400 K [21]. Instead, they suggest that partial charge localization may be responsible for the observed increase in S. First-principles calculations on CCO indicate that both Ca2 CoO3 and CoO2 subunits contribute to the transport, while spectroscopy experiments reveal that the 3d orbital of CoO6 octahedra in the CoO2 layer participates in conduction [8]. Further analysis of the crystal structures at higher temperatures suggested that the structural transition occurs due to the arrangement of CoO layer in the RS sub-unit, which weakly modifies the CoO2 layer, thereby explaining the resistivity drop but not the increased S [22]. Despite numerous proposed explanations, the fundamental mechanism that drives the observed increase in the parameter “S” beyond 380 K remains unclear. This study examines the impact of high oxygen pressure annealing on the electronic transport and thermoelectric properties of polycrystalline pellets of CCO. Specifically, we have demonstrated that the temperature-independent behaviour of thermopower in as- synthesized samples can be altered to exhibit a normal thermal activation-like behaviour through annealing. To explore these effects, we conducted electronic transport and thermoelectric property measurements on as-synthesized and annealed samples over a wide temperature range (310–750 K). Our analysis revealed that the thermopower of the as-synthesized samples remained constant over a temperature range of 320–380 K, and then increased with increas- ing temperature from 380 to 750 K. In contrast, the annealed samples did not exhibit any saturation in thermopower, but rather displayed a thermal activation-like behaviour across the entire range of temperatures studied. To explain these observations, we propose a plausible mechanism based on the hop- ping probability of electrons based on the Heike’s limit of the Seebeck coefficient. Our results suggest that high oxygen pressure annealing may alter the defect chemistry of CCO, resulting in a reduction in the hopping probability of electrons and the emergence of a thermal activation-like behaviour in thermopower. These findings have important implications for the design and optimization of CCO-based thermoelectric materials. 2. Experimental Methods Synthesis: Ca3 Co4 O9 polycrystalline pellets were synthesized by conventional high temperature solid state reactions [23,24]. Stoichiometric mixtures of Co3 O4 and CaCO3 were homogenized using an agate mortar to obtain a homogeneous powder. The powder was pressed into a pellet using a stainless-steel die under uniaxial pressures, followed by calcination and sintering in a high O2 atmosphere. The calcined pellet was finely ground, Energies 2023, 16, 5162 3 of 13 pressed into a new pellet, and sintered in a tubular furnace at 900 ◦ C for 20 h in a 100 sccm oxygen flow. The density of cold-pressed samples was found to be approximately 87% relative to the theoretical value. To enhance the density to above 90%, which is necessary for achieving reliable electronic, thermoelectric, and thermal transport properties [25–28], spark-plasma sintering was conducted. Specifically, the sintering process was carried out at a uniaxial press of ~40 MPa at 923 K temperature. The final density achieved after spark-plasma sintering was approximately 91.8% relative to the theoretical value. Thermoelectric measurements: The resulting spark plasma-sintered pellets were pre- pared for the thermoelectric characterization through two means: (i) rectangular bars of dimensions 10 × 2.5 × 2 mm3 were fabricated for the measurement of S and ρ through the standard 4-probe method using a LINSEIS instrument (Selb, Germany), under a controlled helium atmosphere (0.1 MPa) between 310 and 750 K, and (ii) disks of diameter 8 mm and thickness of 3 mm were cut for the measurement of thermal diffusivity using laser flash techniques in a NETZSCH LFA instrument (Selb, Germany) under a controlled nitrogen atmosphere between 300 and 750 K. To optimize the heat fluxes, both sides of the disks were coated with carbon. The total thermal conductivity was calculated using the relation κtotal = D·Cp ·d, where D, Cp , and d are the thermal diffusivity, specific heat capacity, and density of the pellet, respectively. Estimation of the ratio of particles to sites using Heikes equation: The continous increase of the S in CCO materials was estimated using the Heike’s formula given as follows [16]: N !   kB NA kB x ST →∞ = − ln = − ln (1) e 1 − NN e 1−x A where x denotes the ratio of number of particles (N) to sites (NA ) and e is the absolute value of elementary electron charge. x was estimated by using the measured Seebeck of each temperature, i.e., exp(−α) e·S x= , α= (2) 1 + exp(−α) kB Estimation of the activation energy: The activation energy (EA ) of the system was de- termined using the small polaron hopping model (or Arrhenius type model) as follows [29]:   σ0 EA σ(T ) = exp − (3) T kB T where σ(T) represents the electrical conductivity at temperature T, σ0 is a pre-exponential factor (or residual resistivity), and kB is the Boltzmann constant. By taking the natural loga- rithm of the electrical conductivity multiplied by the measured temperature (ln(σ ·T)) and plotting it against the reciprocal of temperature (1/T), the slope of this linear relationship provides a direct measure of the activation energy as follows: EA 1 ln(σ · T ) = − + ln(σ0 ) (4) kB T Structural characterization: The samples underwent structural characterization us- ing X-ray diffraction (XRD) (from PANalytical X’pert Pro MPD diffractometer, Malvern, UK). The XRD analysis allowed for the determination of the crystal structure and lattice parameters of the material. Additionally, elemental analysis was performed using an EDX detector (Quanta 650 FEG, Hillsboro, OR, USA) to determine the chemical composition of the material. The quantification and oxidation states of cobalt ions were also determined using X-ray photoelectron spectroscopy (XPS). XPS measurements were conducted using a SPECS PHOIBOS150 hemispherical analyzer (Berlin, Germany) equipped with a monochro- matic X-ray source operating at 1486.6 eV and a power of 300 W. The synthesized samples were polycrystalline, and hence, no preferred orientations were observed in their crystal parameters of the material. Additionally, elemental analysis was performed using an EDX detector (Quanta 650 FEG, Hillsboro, OR, USA) to determine the chemical composition of the material. The quantification and oxidation states of cobalt ions were also determined using X-ray photoelectron spectroscopy (XPS). XPS measurements were conducted using Energies 2023, 16, 5162 4 of 13 a SPECS PHOIBOS150 hemispherical analyzer (Berlin, Germany) equipped with a mono- chromatic X-ray source operating at 1486.6 eV and a power of 300 W. The synthesized samples were polycrystalline, and hence, no preferred orientations were observed in their crystal structure. structure. Therefore, Therefore, anisotropies anisotropies werewere not not foundfound in the in the thermal thermal andand electronicproperties electronic properties of of the samples.the samples. Annealing: Annealing: ToTo investigate thethe investigate effect of annealing effect on the of annealing onthermoelectric properties the thermoelectric of properties of the the Ca3Co4O9 polycrystalline pellets, the as-synthesized sample was measured in a Ca3 Co4 O9 polycrystalline pellets, the as-synthesized sample was measured in a controlled controlled helium atmosphere up to 710 K and then cooled down to room temperature helium atmosphere up to 710 K and then cooled down to room temperature with the same with the same controlled atmosphere. The heating–cooling cycle from 300 to 710 K usually controlled takes atmosphere. a few hours. Then, theThe heating–cooling sample was annealed atcycle 1173 Kfrom 300min for 120 to 710 at 20Ksccm usually takes a few oxygen hours. flow. We Then, call the this sample sample was annealed atThe “air-annealed”. 1173 K for 120 min thermoelectric at 20 sccm oxygen measurements of air- flow. We call this sample annealed sample were“air-annealed”. performed inThe thermoelectric the same measurements controlled helium atmosphere of air-annealed within 300– sample were 710 K, performed and cooled down in theassame controlled before. helium atmosphere The “air-annealed” within 300–710 sample was annealed again at K, and cooled 1173 Kdown for 120as min before. at 100The “air-annealed” sccm oxygen flow. sample wassample We call this annealed again at 1173 K Then, “oxygen-annealed”. for 120 min at transport 100 sccm properties oxygen flow. of oxygen-annealed We call this samplesample were measured inThen, “oxygen-annealed”. the same heliumproperties transport atmosphere as before. By of oxygen-annealed performing sample transport measurements were measured under different in the same helium annealing atmosphere as before. By conditions, performing wetransport aimed to gain insights into the measurements changes under in the electronic different annealingand thermoelectric conditions, we aimed to properties of Ca 3Co4O9 as a function of annealing conditions. After annealing, no gain insights into the changes in the electronic and thermoelectric properties of Ca3 Co4 O9 significant variations in the XRD patterns were observed, suggesting that the material as a function of annealing conditions. After annealing, no significant variations in the retained its crystal structure and stability under the given annealing conditions. Figure 1 XRD patterns were observed, suggesting that the material retained its crystal structure and presents a flow chart illustrating the distinct stages involved in the measurements process stability of the Ca3Counder the given annealing conditions. Figure 1 presents a flow chart illustrating 4O9 compounds. the distinct stages involved in the measurements process of the Ca3 Co4 O9 compounds. Figure Figure1.1. Flow chart outlining Flow chart outlining the thesequential sequentialsteps stepsundertaken undertaken throughout throughout thethe experimental experimental investigation. investigation. 3. Results 3.1. Structural and Elemental Characterizations The inset of Figure 2a shows a schematic representation of CCO system, which serves to elucidate its structural arrangement and layered nature. The crystal structure of CCO exhibits a misfit-layered configuration, characterized by the presence of alternating rocksalt layers (Ca2 CoO3 ) and face-shared octahedra layers (CoO2 ). This figure further highlights the three-dimensional stacking of these layers and their unique arrangement. Comple- menting this illustration, Figure 2a,b present the powder X-ray diffraction (XRD) pat- terns obtained from the as-synthesized CCO sample. Notably, the XRD pattern closely resembles those observed in previous studies on polycrystalline CCO samples synthe- sized by other researchers [30,31]. The peak assignment was carried out based on the The inset of Figure 2a shows a schematic representation of CCO system, which serves to elucidate its structural arrangement and layered nature. The crystal structure of CCO exhibits a misfit-layered configuration, characterized by the presence of alternating rocksalt layers (Ca2CoO3) and face-shared octahedra layers (CoO2). This figure further highlights the three-dimensional stacking of these layers and their unique arrangement. Energies 2023, 16, 5162 Complementing this illustration, Figure 2a,b present the powder X-ray diffraction (XRD) 5 of 13 patterns obtained from the as-synthesized CCO sample. Notably, the XRD pattern closely resembles those observed in previous studies on polycrystalline CCO samples synthesized by other researchers [30,31]. The peak assignment was carried out based on work the workofofHira Hira etetal. al.[32]. [32].TheThe analysis analysis of theof XRDthedata XRD datathereveals reveals absencethe absence of detectable of detectable impurity impurity phases. phases. Inset:schematic Figure2.2.Inset: Figure schematic representation representation of the of the misfit-layered misfit-layered crystal crystal structure structure of Ca3of CoCa 4O93 Co4 O9 illustrat- ing the arrangement illustrating the arrangementof of the thetwo twosubunits: Ca2CoO3, subunits: Ca2CoO3, which which consists consists of CaO-CoO-CaO of CaO-CoO-CaO layers, layers, and a and a face-shared face-shared networkofofCoO network CoO2 octahedra. (a) XRD of as-synthesized sample full range and (b) 2 octahedra. (a) XRD of as-synthesized sample full range and (b) zoom zoom around 10 to 40 degrees. The peak assignment was carried out based on the work of Hira et around 10 to 40 degrees. The peak assignment was carried out based on the work of Hira et al. [32]. al. [32]. Figure Figure 3a,b3a,b showshow the analysis the EDX EDX analysis of the as-synthesized of the as-synthesized sample and sample andofSEM picture of SEM picture theCCO the CCO powder powder showing showing a particle a particle size few size around around few micrometers. micrometers. EDX analysisEDX analysis reveals reveals theelemental the elemental composition composition ofas-synthesized of the the as-synthesized sample.sample. The spectrum The spectrum obtained shows the obtained shows characteristic peaks corresponding to calcium, cobalt, and oxygen atoms presented in the CCO compound. The carbon peak shown at low energy corresponds to the carbon tape used to hold the sample. Figure 4 gives essential insights into the elemental composition and oxidation states of the material under investigation. In Figure 4a, the XPS spectrum clearly demonstrates the presence of all three elements, namely Ca, O, and Co [33]. In order to gain a deeper understanding of the oxidation states of cobalt ions, a high-resolution XPS measurement was conducted targeting the Co-2p region (770–810 eV), as shown in Figure 4b–d. The raw spectrum (black line) is presented at the top, while the baseline-corrected spectrum is displayed at the bottom of Figure 4b. Notably, two prominent peaks corresponding to Co 2p3/2 and 2p1/2 are clearly discernible, accompanied by two additional satellite peaks located at relatively higher binding energies. These satellite peaks are commonly attributed to the presence of mixed valence states of cobalt ions [29]. By performing meticulous fitting of the Co-2p spectrum (Figure 4c,d), three distinct peaks with noticeable variations in their binding energies, full width at half maximum (FWHM), and area under the curve were identified. Based on these distinctive characteristics, we confidently assigned these peaks to the presence of Co2+ , Co3+ , and Co4+ ions [34]. It is noteworthy that the observed Energies 2023, 16, 5162 6 of 13 Energies 2023, 16, x FOR PEER REVIEW 6 of 14 shift of binding energies towards higher energies with increasing oxidation states aligns with expectations. The coexistence of mixed valence states, including Co2+ ions in the the rockcharacteristic peaks salt layer and corresponding a combination of Coto3+calcium, 4+ ions and and Cocobalt, oxygen in the atomslayer, hexagonal presented in not only the CCO compound. provides insights intoThe the carbon peak material’s shown at low composition energy but also corresponds enhances to the carbon our understanding of tape used to hold its oxidation the[34]. states sample. Energies 2023, 16, x FOR PEER REVIEW 7 of 14 Figure Figure3.3.(a) (a)Elemental Elementalanalysis analysisby byenergy energydispersive dispersiveX-ray X-rayspectroscopy spectroscopy(EDX). (EDX).(b) (b)SEM SEMpicture pictureofof the theCCO CCOpowder. powder. Figure 4 gives essential insights into the elemental composition and oxidation states of the material under investigation. In Figure 4a, the XPS spectrum clearly demonstrates the presence of all three elements, namely Ca, O, and Co [33]. In order to gain a deeper understanding of the oxidation states of cobalt ions, a high-resolution XPS measurement was conducted targeting the Co-2p region (770–810 eV), as shown in Figure 4b–d. The raw spectrum (black line) is presented at the top, while the baseline-corrected spectrum is dis- played at the bottom of Figure 4b. Notably, two prominent peaks corresponding to Co 2p3/2 and 2p1/2 are clearly discernible, accompanied by two additional satellite peaks lo- cated at relatively higher binding energies. These satellite peaks are commonly attributed to the presence of mixed valence states of cobalt ions [29]. By performing meticulous fit- ting of the Co-2p spectrum (Figure 4c,d), three distinct peaks with noticeable variations in their binding energies, full width at half maximum (FWHM), and area under the curve were identified. Based on these distinctive characteristics, we confidently assigned these peaks to the presence of Co2+, Co3+, and Co4+ ions [34]. It is noteworthy that the observed shift of binding energies towards higher energies with increasing oxidation states aligns with expectations. The coexistence of mixed valence states, including Co2+ ions in the rock salt layer and a combination of Co3+ and Co4+ ions in the hexagonal layer, not only provides insights into the material’s composition but also enhances our understanding of its oxida- tion states [34]. Figure4.4.XPS Figure XPSspectrum spectrumofofas-sythesid as-sythesidsample. sample.(a) (a)Full Fullrange rangeand andzoom zoomaround aroundCo-2p Co-2poxidation oxidationstate: (b)state: raw (b) andraw and baselined baselined data, (c)data, (c) baselined baselined and and fitted fitted spectrum, spectrum, (d) zoom(d) to zoom to highlight highlight the of the goodness goodness the of the fitted spectrum. fitted spectrum. 3.2. 3.2.Electronic ElectronicTransport Transport Properties and Thermoelectric Properties and ThermoelectricPower Power Figure Figure5a,c show 5a,c the simultaneous show the simultaneousmeasurements of resistivity measurements (ρ(T)) and(ρ(T)) of resistivity thermopower and thermopower (S(T)) (S(T))of as a function as temperature. a function of temperature. ρ(T)adisplays ρ(T) displays a non-monotonic non-monotonic behavior behavior for the as- for the as-synthesized synthesized sample. sample. Initially, Initially, within within the temperature the temperature range K, range of 300–500 of it300–500 decreasesK, itfrom decreases from 21 to 18 mΩ·cm. However, beyond this range, it continuously 21 to 18 mΩ·cm. However, beyond this range, it continuously increases to 22.6 mΩ increases to·cm 22.6 mΩ·cm at 710 K. In contrast, S(T) exhibits a positive value of 140 mV/K at 710 K. In contrast, S(T) exhibits a positive value of 140 mV/K at 330 K, indicating the at 330 K, indicating the prevalence of p-type charge carriers in the samples. Moreover, it remains temperature independent between 310 K and 380 K but increases beyond this range to reach a maximum value of 194 mV/K at 710 K, which is consistent with previous reports [18,35,36]. The air-annealed sample exhibited a similar behavior of resistivity (ρ) within the temperature range of 330–380 K when compared to the as-synthesized sample. Energies 2023, 16, 5162 7 of 13 prevalence of p-type charge carriers in the samples. Moreover, it remains temperature independent between 310 K and 380 K but increases beyond this range to reach a maximum value of 194 mV/K at 710 K, which is consistent with previous reports [18,35,36]. The air-annealed sample exhibited a similar behavior of resistivity (ρ) within the temperature range of 330–380 K when compared to the as-synthesized sample. However, resistivity de- creased from 22 to 17 mΩ.cm with increasing temperature up to 710 K, displaying a thermal activation-like behavior (see Figure 5b). In contrast, the absolute value of thermopower and its temperature dependence remained comparable to the as-synthesized sample, implying that high-temperature conductivity was mainly improved by annealing process. In stan- dard band gap materials, electronic resistivity and thermopower decrease with increasing temperature. However, in the as-synthesized sample, we observed that the thermopower Energies 2023, 16, x FOR PEER REVIEW 8 of 14 increases with temperature while the resistivity decreases. This observation suggests that electronic resistivity and thermopower may be governed by different mechanisms. Figure 5. (a) Electrical resistivity of as-synthesized (red square), oxygen-annealed (blue diamond), Figure 5. (a) Electrical resistivity of as-synthesized (red square), oxygen-annealed (blue diamond), and air-annealed CCO samples as function of tempertature. (b) Linear fitting of ln(σ·T) versus 1000/T and air-annealed CCO samples as function of tempertature. (b) Linear fitting of ln(σ·T) versus for each 1000/T sample. for each sample.(c,d) (c,d)Seebeck coefficient Seebeck coefficient of set of the theofset of samples samples as function as function of temperature. of temperature. ToTo investigate investigatethis further, we performed this further, high temperature we performed transport measurements high temperature transport measurements of ofthe theoxygen-annealed oxygen-annealed sample. sample.The The results showsshows results a significant drop in ρ drop a significant to 14 mΩ.cm, in ρ to 14 mΩ.cm, while while their temperature dependence remain constant between 300 K and 380 K. The their temperature dependence remain constant between 300 observed reduction in resistivity in our sample could be attributed to an increase in the K and 380 K. The observed reduction density in resistivity of charge in our sample carriers, an improvement couldmobility, in charge be attributed to an increase or a combination of both. in the density of Itcharge carriers, is plausible that an theimprovement annealing process in charge at highmobility, or a combination oxygen pressure resulted inofthe both. It is plausible compensation of oxygen that the annealing vacancies, process leading at high to the introduction oxygen of extra in pressure resulted charge carriers the compensation of oxygen (i.e., holes or Co vacancies, +4 ions) in the CCO lattice. Oxygenation may occur within the rock salt leading to the introduction of extra charge carriers (i.e., holes or Co+4 ions) in type Ca2CoO3-δ layer [37], and the charge neutrality is then balanced by producing a higher the CCO lattice. Oxygenation may occur within the rock salt type Ca2 CoO3-δ layer [37], ratio of Co+4/Co+3 ions in CoO2 via charge transfer from the rock salt layer to the CoO2 sub- and[38]. unit theIncharge contrast,neutrality the S of theissample then balanced decreased to by+110 producing µ V/K at a 330higher K, but ratio its of Co+4 /Co+3 ions in CoO temperature 2 via charge dependence remainedtransfer from relatively theThis stable. rock salt layeris to observation the CoO consistent with2 sub-unit [38]. In contrast, the S of the sample decreased to +110 µ V/K at 330 K, but its temperature the general understanding that in the majority of thermoelectric materials, the electronic conductivity dependence is directly remainedproportional to the concentration relatively stable. Thisofobservation charge carriers,iswhile the ther- with the general consistent mopower is inversely proportional to it. Thus, this observation further supports the hy- understanding that in the majority of thermoelectric materials, the electronic conductivity pothesis that the annealing process introduced additional holes in the CCO material, is directly thereby proportional enhancing its electricalto the concentration of charge carriers, while the thermopower is conductivity. inversely proportional Moreover, the activationtoenergy it. Thus, (EA) this was observation furtherthe determined thorough supports analysis the hypothesis that the of the slope obtained from plotting the natural logarithm of the temperature dependence of resistivity multiplied by the temperature (ln(T·σ)) against the reciprocal of temperature (1/T) (see Figure 5b and Equation (4)). In a system characterized by coherent charge carriers, the obtained EA values from both curves are expected to exhibit close Energies 2023, 16, 5162 8 of 13 annealing process introduced additional holes in the CCO material, thereby enhancing its electrical conductivity. Moreover, the activation energy (EA ) was determined thorough the analysis of the slope obtained from plotting the natural logarithm of the temperature dependence of resis- tivity multiplied by the temperature (ln(T·σ)) against the reciprocal of temperature (1/T) (see Figure 5b and Equation (4)). In a system characterized by coherent charge carriers, the obtained EA values from both curves are expected to exhibit close resemblance. However, any significant deviation between these values suggests the possibility of incoherent trans- port as a function of temperature. This implies that the carriers in the system are localized in nature and primarily follow a hopping transport mechanism. Furthermore, the activation energy (EA ) obtained from the temperature dependence of resistivity ln(σ·T) and thermopower S(T) as a function of 1/T was analysed using Equation (4), as shown in Figure 5b,d. In a system with coherent charge carriers, the EA obtained from both curves should be very similar to each other. If not, that might indicate a sign of incoherent transport as a function of temperature, implying that carriers are localized in nature and mainly obey hopping transport mechanism [14,39]. The localized nature of charge carriers at high enough temperature produces temperature-independent thermopower, as explained by the Heike’s formula (or sometimes referred to as the Mott- Heikes formula, see Equation (1)) [40]. Figure 6 shows the schematic representation of the electronic structure of misfit cobal- tates. The 3d orbitals of the Co atom in the CoO2 layer experience splitting into t2g and eg energy states due to the octahedral crystal field. Moreover, the t2g orbitals undergo further splitting into a narrow out-of-plane a1g band and a wide in-plane eg’ band due to the triangular distortion present in the CoO2 layer [21]. The conduction mechanism at lower temperatures (T < 200 K) is dominated by the eg’ band, which has a broad width [8]. Conversely, at relatively higher temperatures (T > 200 K), the narrow a1g band dominates the conduction mechanism [21]. Due to its narrow bandwidth, the charge carriers become localized, and the conduction mechanism is governed by the hopping of charge carriers from one site to another. It is noteworthy that ρ decreases slightly or remains temperature- independent for all samples within the range of 310–750 K. Limelette et al. [41] proposed the opening of a pseudogap, which is caused by the disappearance of the quasiparticle resonance. Hence, a slight increase in the charge carrier concentration (n) within the range of 310–750 K or a fixed number can be considered. At temperatures below 10 K, previ- ous studies proposed the inclusion of a spin-entropy contribution to the thermopower in misfit [42] and Nax CoO2 cobaltates [43,44]. The S shows a linear increase in the range of 20–180 K, indicating metallic conduction, followed by a temperature-independent behavior in the range of 180–380 K, suggesting that S reached its high temperature limit [21,45], as previously discussed. At extremely high temperatures (T→∞), the band width, W, becomes narrow, and consequently, charge carriers are localized. Thus, in this limit (when kB T» W, where kB is the Boltzmann constant), S becomes a function of a pure thermodynamic parameter, the configurational entropy, which is related to the degeneracy of the charge carriers over the available sites. Within this regime, Heike’s formula was derived as a mathematical expression for S, considering that ‘N’ number of particles are distributed over ‘NA ’ sites, where two particles cannot occupy the same site (Equation (1)). Although this formula was modified by including the spin-orbit degeneracy [45] as an extra contribution to S, it remains unclear [46]. Intriguingly, our measurements suggest that S continues to increase with temperature from 380 to 750 K, and therefore, the actual high temperature limit for CCO remains unresolved. A structural phase transition at around 400 K has been reported in CCO, as evi- denced by lattice parameters (a, b1 , and b2 ), resistivity, and specific heat exhibiting a hysteresis behavior with a magnetic field, indicating the possible presence of magnetic ordering [22,36,43]. Furthermore, magnetic phase transitions have been observed at higher temperatures. For instance, a transition from low-spin Co3+ (LS-Co3+ ) to intermediate-spin [21,45], as previously discussed. At extremely high temperatures (T→∞), the band width, W, becomes narrow, and consequently, charge carriers are localized. Thus, in this limit (when kBT» W, where kB is the Boltzmann constant), S becomes a function of a pure thermodynamic parameter, the configurational entropy, which is related to the degeneracy of the charge carriers over the available sites. Within this regime, Heike’s Energies 2023, 16, 5162 formula was derived as a mathematical expression for S, considering that N’ number of 9 of 13 particles are distributed over NA’ sites, where two particles cannot occupy the same site (Equation (1)). Although this formula was modified by including the spin-orbit degeneracy [45] as an extra contribution to S, it remains unclear [46]. Intriguingly, our Co 3+ (IS-Co3+ )suggest measurements is postulated to occur to that S continues at increase 400 K [47,48], while another with temperature fromphase 380 to transition 750 K, from and therefore, the paramagnetic LS-Co 3+ high actual temperature limit to paramagnetic for CCO high-spin remains (HS) stateunresolved. was reported at 500 K [41]. Energies 2023, 16, x FOR PEER REVIEW 10 of 14 eg eg 3d A structural phase transition at around 400 K has been reported in CCO, as evidenced by lattice parameters (a, b1, andab1g2), resistivity, and EF specific heat exhibiting a hysteresis behavior with a magnetic field, indicating the possible presence of magnetic ordering t2g e'g [22,36,43]. Furthermore, magnetic phase transitions have been observed at higher temperatures. For instance, a transition from low-spin Co3+ (LS-Co3+) to intermediate-spin Co3+ (IS-Co3+) is postulated to occur at 400 K [47,48], while another phase transition from paramagnetic Figure LS-Co Figure 6.6.Schematic Schematic 3+ to paramagnetic representation representation high-spin of electronic structure of electronic (HS) of Castate structure 3Co Owas of 4Ca 9. Coreported 3 4 O9 . at 500 K [41]. The investigation of the temperature-dependent behavior of the S beyond 380 K, via Heike’s The investigation formula, is a subjectof the temperature-dependent of interest, given its potential to behavior of theinsights offer valuable S beyond into380 K, viatransition the Heike’s formula, of partialisLS-Co a subject +3 to of aninterest, IS-Co+3 given its potential configuration to offer at elevated valuable insights temperatures. Equation (1) indicates into the transition that SLS-Co of partial +3 to an IS-Co is temperature +3 configuration independent and solely reliant temperatures. at elevated on the parameter Equation x. (1)Ifindicates Heike’s formula that S is is valid between temperature 200 and 710 independent and K,solely then the rise on reliant of Sthe can be parameter explained x. If Heike’sby the decrease formula of x. The is valid variation between 200ofand x’ between 710 K, then 310 and 710 K the rise ofwas S cancalculated be explained from by thethedecrease measuredofS,x.using Equation (2), The variation as shown of ‘x’ between in 310 Figure and7a.710 TheKresults indicate that was calculated from the for as-synthesized measured S, using and air-annealed, Equation (2), as x’ should shown in decrease Figure 7a. from The0.16 to 0.09 results withinthat indicate thefor as- temperature synthesizedrange of 310–710 K ‘x’ and air-annealed, to should explain decrease the increasefrom of0.16 S from 140 within to 0.09 to 195 µV/K. The the temperature decrease of x’ with increasing temperature is possible if range of 310–710 K to explain the increase of S from 140 to 195 µV/K. The decrease either N’ decreases or NA of ‘x’ increases. Based on ρ (T), we assumed that either [n] increases slightly with increasing temperature is possible if either ‘N’ decreases or NA increases. Based on or remains almost constant. ρ (T), weInassumed either scenario, NA must that either increase toslightly [n] increases augmentorS.remains For the purpose of simplicity, almost constant. In either we have considered N as a constant N = [n] × Vunit cell, where Vunit cell is the volume of the scenario, NA must increase to augment S. For the purpose of simplicity, we have considered unit cell. N as a constant N = [n] × Vunit cell , where Vunit cell is the volume of the unit cell. Figure7.7.High Figure Hightemperature temperature limit limit of of thermopower thermopower andand Heike’s Heike’s formula. formula. (a) Calculated (a) Calculated x (i.e., x (i.e., ratioratio of ofparticle particletotosite) site)from from measured measured SS from from300 300toto750 750K.K.Solid Solidlines linesareare guide to the guide eyes. to the (b) Hopping eyes. (b) Hopping of ofLS-Co LS-Co electrons in +4+4 electrons in LS-Co LS-Co+3background +3 background at at 300–350 300–350K and K and(c) hopping of LS-Co (c) hopping +4 in the of LS-Co +4 inLS-Co +3 the LS-Co+3 and IS-Co+3 +3 background at 550 K (or T > 380 K). and IS-Co background at 550 K (or T > 380 K). In the temperature range of 300–350 K, the conduction mechanism in the system can be explicated by the a1g electron hopping between the LS-Co+3 and LS-Co+4 states, as Energies 2023, 16, 5162 10 of 13 Energies 2023, 16, x FOR PEER REVIEW 11 of 14 In the temperature range of 300–350 K, the conduction mechanism in the system can be explicated by the a1g electron hopping between the LS-Co+3 and LS-Co+4 states, as depicted in Figure 7b. We assume that a y% of LS-Co +3 undergoes a spin-state transition depicted in Figure 7b. We assume that a y% of LS-Co +3 undergoes a spin-state transition from LS to IS-Co+3 , and the remaining (1−y)% stays as LS-Co+3 by reaching an equilibrium from LS to IS-Co+3, and the remaining (1−y)% stays as LS-Co+3 by reaching an equilibrium +3 ions at higher temperatures (see Figure 7c). Therefore, the presence of both LS and IS-Co at higher temperatures (see Figure +4 7c). Therefore, the presence of both LS and IS-Co+3 ions changes changes the probabilityof the probability ofCoCo+4 electron electron (or(or hole) hole) hopping. hopping. For For temperatures temperatures above above 380 K,380 K, aa1g electrons of LS-Co +3 can hop within the a state of LS-Co+4 , while the e electrons of 1g 1g electrons of LS-Co can hop within the a1g state of LS-Co , while the eg electrons of IS- +3 +4 g IS-Co +3 +4 , as indicated by arrows. Co+3 ionsions cancan alsoalso hophopinto into the ethe eg states g states of LS-Coof LS-Co +4, as indicated by arrows. After +4 After the spin-state spin-statetransition, transition,two twosituations situations cancan bebe highlighted: highlighted: (i) holes (i) holes in LS-Co in LS-Co +4 can +3 +3 canhophop inin the background backgroundof ofboth bothLS-Co LS-Co and +3 and IS-Co, resulting IS-Co +3 , resultingin aninincrease an increase in hopping in hopping probability, probability,and and (ii) the the NNAAsites sitesisisalso also increased increased as easg energy eg energy statesstates participate participate in hopping. in hopping. Consequently, Consequently, ititcan canbe beargued arguedthat thatthetheenhanced enhanced hopping hopping probability probability ofof electrons electrons at at higher higher temperatures temperatures reducesreduces the resistivity, the resistivity, and the and the increased increased number number of NAofreduces NA reduces the of the value x,value which of could x, which could eventually eventually increase the increase Seebeck thecoefficient Seebeck coefficient with temperature. with temperature. However, this isHowever, merely athis is merelyand hypothesis, a hypothesis, and further further experiments areexperiments necessary toare necessary obtain bettertoinsights obtain into better the insightsmechanism transport into the transport at higher mechanism temperatures. at higher temperatures. For comparison, factors (S2 /ρ or For comparison, power σSpower 2 ) of factors (S2/ρ or σS2) ofand the as-synthesized the annealed as-synthesized and are samples annealed plotted samples togetherare in plotted Figuretogether 8a. All the in Figure 8a. All the samples displayed a nearly identical samples displayed a nearly identical power factor from 210 to 600 K. The 20-sccm power factor from 210 to 600oxygen- K. The 20-sccm oxygen-annealed sample at 710 K exhibited a considerable annealed sample at 710 K exhibited a considerable enhancement. The thermal conductivity enhancement. The ofthermal conductivitysample the air-annealed of the air-annealed is displayedsample in Figureis displayed 8b. Both in Figureup heating 8b.and Bothcolling heatingdown up and colling down measurements showed quite measurements showed quite similar results and constant temperature depedence. The similar results and constant temperature depedence. The quite low κ of the misfit cobaltates is mainly attributed to the quite low κ of the misfit cobaltates is mainly attributed to the phonon scattering at the phonon scattering at the CoO2 and RS interfaces and the highly ordered RS layer [49]. CoO2 and RS interfaces and the highly ordered RS layer [49]. However, the lower κ for However, the lower κ for polycrystalline samples reflects the important contribution of polycrystalline samples reflects the important contribution of phonon scattering at the grain phonon scattering at the grain boundaries [9]. boundaries [9]. Figure Figure 8. (a) Power 8. (a) Powerfactor factorofofas-synthesized as-synthesized (red (red square), square), oxygen-annealed oxygen-annealed (blue(blue diamond), diamond), and air- and air- annealed CCO annealed CCOsamples samplesasasfunction functionofoftempertature. tempertature.(b) (b) Measured Measured thermal thermal conductivity conductivity of of air- air-anealed sample and funtion of temperature. (c) Thermoelectric figure of merit of the air-annealed sample. Energies 2023, 16, 5162 11 of 13 4. Conclusions This study focused on the preparation and thermoelectrical characterization of poly- crystalline bulk Ca3 Co4 O9 samples subject to different annealing processes subject to different oxygen pressures. The thermoelectric properties of both as-synthesized and an- nealed samples are observed to be highly stable within the temperature range of 310–750 K. The influence of oxygen pressure on the electronic resistance and thermoelectric power was investigated through post-annealing at high temperatures. The results demonstrated that the oxygen-annealed sample exhibited metallic resistivity, lower resistance, and the absence of temperature-independent thermopower. The spin-state transition of cobalt ions was identified as a crucial factor affecting the hopping probability of localized carriers and its potential correlation with temperature-dependent thermopower. Furthermore, the air-annealed (atmosphere with low O2 pressure) sample showcased a notable 23% enhance- ment in power factor at 750 K compared to the as-synthesized sample. These findings shed light on the intricate relationship between oxygen pressure, cobalt ion spin states, and the thermoelectric properties of Ca3 Co4 O9 , contributing to the understanding and advancement of thermoelectric materials for various applications. Author Contributions: Conceptualization, C.M.S.T., J.S. and E.C.-Á.; data curation, A.C., A.B. and K.B.; formal analysis, A.C., A.B., K.B. and E.C.-Á.; funding acquisition, C.M.S.T. and J.S.; investigation, A.C., A.E.S., A.B., A.C.-A. and E.C.-Á.; methodology, A.C.; resources, C.M.S.T. and J.S.; software, A.C.-A.; supervision, C.M.S.T. and J.S.; validation, A.E.S.; writing—original draft, A.C. and E.C.-Á.; writing—review and editing, A.C., A.E.S., A.C.-A. and E.C.-Á. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by MINECO ref. MAT2016-77100-C2-1-P; CNRS-CSIC PICS Project ref. 261091, the EU for funding through project H2020-MSCA-RISE-2014 ref. 645658, and the AGAUR agency for 2017SGR. ICN2 is funded by the CERCA program/Generalitat de Catalunya and by the Severo Ochoa program of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, grant no. SEV-2017-0706). C.M.S.T. and E.C.-Á. acknowledge support from Spanish Ministry MINECO/FEDER: FIS2015-70862-P PHENTOM. A.E.S. acknowledges funding from the EU- H2020 research and innovation program under the Marie Sklodowska Curie Individual Fellowship THERMIC (Grant No. 101029727). Data Availability Statement: Raw data can be provided by the corresponding and first author (A.C.) upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. References 1. Brinks, P.; Huijben, M. Thermoelectric Oxides. 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