Enhanced Thermoelectric Performance of Na_0.55CoO₂ Ceramics Doped by Transition and Heavy Metal Oxides

Abstract

Using the solid-state reactions method Na_0.55(Co,M)O₂ (M = Cr, Ni, Zn, W, and Bi) ceramics were prepared and their crystal structure, microstructure, electrophysical, thermophysical, and thermoelectric properties were studied. Doping of Na_0.55CoO₂ by transition or heavy metal oxides led to the increase in the grain size of ceramics, a decrease in electrical resistivity and thermal diffusivity values, and a sharp increase in the Seebeck coefficient, which resulted in essential enhancement of their thermoelectric properties. The largest power factor (1.04 mW/(m·K²) at 1073 K) and figure of merit (0.702 at 1073 K) among the studied samples possessed the Na_0.55Co_0.9Bi_0.1O₂ compound, which also demonstrated the highest values of the Seebeck coefficient (666 μV/K at 1073 K). The obtained results show that the doping of layered sodium cobaltite by different metal oxides allows for improving its stability, microstructure, and functional properties, which proves the effectiveness of the doping strategy for developing new thermoelectric oxides with enhanced thermoelectric performance.

1. Introduction

Waste heat, evolving into the environment by industrial enterprises, cars, and households, can be directly and effectively converted into electrical energy using thermoelectrogenerators (TEGs). The concept of TEGs can be realized with the use of thermoelectric materials, which should possess at the same time both low thermal conductivity (λ) and electrical resistivity (ρ) and high values of the Seebeck coefficient (S) [1,2,3,4,5]. Due to the strong interconnection of the above mentioned properties, the number of thermoelectric materials is strongly restricted. Nevertheless, since the 1960s, a number of effective thermoelectrics have been developed, which are successfully used in the production of thermoelectric devices of different purposes (TEGs, thermoelectric refrigerators (coolers), thermocouples, etc.) [6,7,8]. Traditional thermoelectrics on the base of layered chalcogenides of bismuth and tellurium possess several drawbacks since they contain toxic, rare, and expensive components, are unstable in the air at high temperatures due to oxidation by atmospheric oxygen, etc. Nevertheless, such drawbacks are absent for oxide thermoelectrics [9,10,11,12,13]. One of the well-known representatives of thermoelectrics is layered sodium cobaltite first characterized by Terasaki et al. [14]. Having unique electrotransport properties, layered sodium cobaltite is tested as a possible material for p-branches of TEGs [6,7,15], cathode material for sodium-ion batteries (SIBs), etc. [16,17,18,19]. Its crystal hydrate, Na_xCoO₂·yH₂O, below 4 K undergoes a transition into a superconducting state [20]. The physicochemical and functional properties of Na_xCoO₂ strongly depend on the sodium content (x_Na) [21,22], which also determines the phase structure.

Sodium cobaltite, α-Na_xCoO₂ (0.9 ≤ x ≤ 1.0) possesses an O3 structure, α’-Na_xCoO₂ has an O’3 structure (monoclinically distorted O3 phase), β-Na_xCoO₂ (0.55 ≤ x ≤ 0.6) has a P3 structure, and γ-Na_xCoO₂ (0.55 ≤ x/y ≤ 0.74) has a P2-structure [23]. The letter characterizes the oxygen setting of sodium (octahedral (O) or prismatic (P)), and the number (two or three) shows the quantity of CoO₂ layers in the c period of the Na_xCoO₂ crystal lattice.

Different approaches are used to improve the thermoelectric and other functional properties of Na_xCoO₂: (i) doping strategy (in sublattices of sodium [24,25] or cobalt [26,27,28,29]), (ii) modification strategy (by metals [30,31], reduced graphene oxide [32], etc.), and (iii) microstructure regulation strategy [33,34,35], etc.

In our previous works [36,37], we estimated the possibility of improving the thermoelectric performance of Na_xCoO₂ by its doping for different transition and non-transition metal oxides.

The aim of this study was to examine the effect of Na_xCoO₂ doping by some transition (Cr₂O₃, NiO and ZnO) and heavy metal oxide (WO₃ and Bi₂O₃) on the crystal structure, microstructure, stability, electron transport, and thermophysical and thermoelectric properties of the obtained Na_xCo_0.90M_0.10O₂ (M = Cr, Ni, Zn, W and Bi) complex oxides.

2. Materials and Methods

Ceramic samples of Na_0.60Co_0.90M_0.10O₂ (M = Cr, Ni, Zn, W, and Bi) materials were prepared by the solid-state reactions method following the reaction scheme:

0.9Na₂CO₃ + 0.3/x M_xO_y+ 0.9 Co₃O₄ + (0.75 − 0.15 y/x) O₂ = 3 Na_0.60Co_0.90M_0.10O₂ + 0.9 CO₂

(1)

Powders of Na₂CO₃ (99%), Co₃O₄ (99%), Cr₂O₃ (99%), NiO (99.99%), WO₃ (99.99%), and Bi₂O₃ (99.99%) were mixed in proportions of Na:Co:M = 0.6:0.9:0.1. The excess amount of Na₂CO₃ was introduced in the batch to compensate for the losses of the Na₂O in the ceramics due to its high-temperature treatment and to obtain the samples of the following composition Na_0.55(Co,M)O₂ (M = Cr, Ni, Zn, W, and Bi) [21,38]. Weighed batches were mixed with C₂H₅OH (~3–5 wt.%) and then milled in a PM 100 Retsch planetary mill (material of beakers and grinding balls is ZrO₂) for 90 min at 300 rpm. Obtained slurries were dried and pressed in the form of tablets with a diameter of 19 mm and height of 2–3 mm. Afterward, the as-obtained samples were calcined in air for 12 h at a temperature of 1133 K. After calcination, the samples were crushed in an agate mortar, then milled in the planetary mill and pressed in the form of bars with dimensions of 5 × 5 × 30 mm³ and tablets having a diameter of 15 mm and height of 2–3 mm, which were sintered in air for 12 h at a temperature of 1203 K [36].

According to [39,40], the oxygen sublattice of Na_xCoO₂ layered cobaltite contains a negligible quantity of vacancies, so the oxidation degree of cobalt in these phases is controlled only by the content of sodium (x_Na) in them (${\mathrm{Na}}_{\mathrm{x}}^{+1}{\mathrm{Co}}_{1-\mathrm{x}}^{+4}{\mathrm{Co}}_{\mathrm{x}}^{+3}{\mathrm{O}}_2$).The average oxidation degree of cobalt and real sodium content in the obtained ceramic materials were determined by means of iodiometric [38] and reverse potentiometric titration [41] (see Supplementary Information).

The phase identification of the samples and calculation of their lattice parameters were performed by means of X-ray diffraction analysis (XRD) using Bruker D8 XRDAdvance X-ray diffractometer CuKα radiation (λ = 1.5406Å). The values of the coherent scattering area for the Na_0.55Co_0.90M_0.10O₂ ceramics were calculated using the Debye–Sherrer equation (D_S) (2) [42] and the size-strain (D_SS) method (3) [43]:

D_S = (0.9λ)/(β × cosΘ),

(2)

(d × β × cosΘ) = (0.9λ/D_SS)(d² × β × cosΘ) + (ε/2)²

(3)

where d is the interplanar distance, nm, β is the full width of the reflex at its half maxima, rad, Θ is the diffraction angle, ° and ε is the microstrain.

The degree of crystallographic orientation of the grains of Na_0.55Co_0.90M_0.10O₂ ceramics was estimated via the Lotgering factor [3,44]:

f = (p − p₀)/(1 − p₀)

(4)

where p = ΣI(00l)/ΣI(hkl) (ΣI is the sum of the counts of X-ray diffraction peaks of the synthesized samples) and p₀ = ΣI₀(00l)/ΣI₀(hkl) (ΣI₀ is the sum of the counts of X-ray diffraction peaks of the randomly oriented sample). Here p₀ was calculated from the peak intensities of the reference phase (JCPDC #00-030-1182)).

The apparent density of synthesized ceramics (d_EXP) was determined from the mass and geometrical dimensions of the samples, and their total porosity (Π_t) was calculated as

Π_t = (1 − d_EXP/d_XRD) × 100%·

(5)

where d_XRD is the X-ray density of the samplesin g/cm³.

The open porosity of the samples (Π_o) was determined by weighing the samples of Na_0.55Co_0.9M_0.1O₂ ceramics saturated by ethylacetate for 45 min under a vacuum of 25–30 Pa and introduced into the liquid ethylacetate following the equation:

Π_o = ((m₁ − m)/(m₁ − m₂)) × 100%

(6)

where m is the mass of the dry ceramic sample g; m₁ and m₂ are the masses of the sample saturated by the liquid ethylacetate at weighing in the air and introduced into the liquid ethylacetate, respectively.

The microstructure and chemical composition of the Na_0.55Co_0.90M_0.10O₂ samples were studied using scanning electron microscopy (SEM) by a JEOL JSM–5610LV scanning electron microscope equipped with an EDX JED–2201 X-ray energy dispersive chemical analysis system.

The electrical resistivity (ρ) and the Seebeck coefficient (S) of the sintered ceramics were measured in the air within a temperature interval of 323–1073 K following the procedure described in [36].

The thermal diffusivity (η) of the layered cobaltites was studied in the helium atmosphere within the temperature interval of 323–1073 K by means of an LFA 457 MicroFlash device (NETZSCH). The thermal conductivity of Na_0.55Co_0.9M_0.1O₂ sintered ceramics was calculated by the equation

λ = η × d_EXP × Cp

(7)

where Cp is heat capacity, calculated by the Dulong–Petit law. This approach can be used for the estimation of the Cp values of ceramics examined. As in [45], it was found that the heat capacity of the layered sodium cobaltite near 300 K becomes slightly dependent on temperature and reaches a plateau.

The phonon (λ_ph) and electronic (λ_el) parts of the thermal conductivity of ceramics were roughly approximated by Equations (8) and (9)

λ = λ_el + λ_ph

(8)

λ_el = (L × T)/ρ

(9)

where L is Lorentz number (L = 2.45 × 10^–8 W × Ω × K^–2).

The values of power factor (P) and figure of merit (ZT) of the Na_0.55Co_0.9M_0.1O₂ layered oxides were calculated as

P = S²/ρ

(10)

and

ZT = (P × T)/λ.

(11)

3. Results and Discussion

The sodium content in the as-obtained samples (x_Na), determined by iodometry and reverse potentiometry, was about 0.55 for all the samples under study (

Table 1. Values of sodium content (x_Na), the oxidation state of cobalt (Co^+z), lattice constants (a, c, c/a, V), Lotgering factor (f), size of the coherent scattering area obtained using the Scherrer equation (Ds), and the size-strain method (D_SS), microstrain (ε), and X-ray density (d_XRD) of ceramic samples Na_0.55Co_0.90M_0.10O₂ (M = Cr, Ni, Zn, W, and Bi).

M	xNa	z	a, Å	c, Å	c/a	V, Å3	f	Ds, nm	DSS, nm	ε × 104	dXRD, g/cm3
Cr	0.552	3.50	2.840	10.91	3.841	76.21	0.82	73	61	3.285	4.48
Co	0.545	3.45	2.824	10.99	3.892	75.87	0.35	48	46	3.521	4.54
Ni	0.559	3.61	2.824	10.95	3.879	75.66	0.60	62	53	1.257	4.55
Zn	0.548	3.61	2.849	10.91	3.830	76.70	0.64	71	61	0.680	4.51
W	0.551	3.17	2.837	10.89	3.839	75.91	0.89	78	64	3.448	5.08
Bi	0.553	3.28	2.839	10.61	3.842	76.13	0.87	65	56	2.833	5.17

). The average oxidation state of cobalt in Na_0.55Co_0.90M_0.10O₂ (M = Cr, Co, Ni, Zn, W, and Bi) layered cobaltites varied within 3.17–3.61 (Table 1), increasing in the case of the substitution of the acceptor character (Ni or Zn instead of Co) and decreasing at the substitution of the donor character (W or Bi instead of Co).

The obtained samples of Na_0.55Co_0.90M_0.10O₂ (M = Cr, Ni, Zn, W, and Bi), like Na_0.55CoO₂ base cobaltite, possessed a hexagonal structure, which corresponds to the structure of γ-Na_xCoO₂ phase [46,47] (Figure 1a). Additionally, the reflexes of the impurity phase, Co₃O₄, were identified in their powder diffractograms.

This phase forms due to the partial degradation of the ceramic surface by its interaction with the CO₂ and H₂O vapors present in an atmosphere according to the reactions (12) and (13):

Na_0.55CoO₂ + 0.275 CO₂ = 0.275 Na₂CO₃ + 1/3 Co₃O₄ + 0.196 O₂

(12)

Na_0.55CoO₂ + 0.275 H₂O = 0.55 NaOH + 1/3Co₃O₄ + 0.196 O₂

(13)

The reflex intensities of the Co₃O₄ phase for Na_0.55Co_0.90M_0.10O₂ compounds were essentially smaller than those for the base Na_0.55CoO₂ phase (Figure 1a), and the content of Co₃O₄ oxide in these materials estimated using methods described in [48], was equal to 30 mol.% for Na_0.55CoO₂, 16 mol.% for M = W, 15 mol.% for M = Cr, and Ni, 12 mol.% for M = Zn, and 8 mol.% for M = Bi. The obtained results indicated the increase in the chemical stability of the layered sodium cobaltite in an atmosphere of wet air at its doping by transition or heavy metal oxides due to the increase of the metal–oxygen interactions in their structure.

The values of the a lattice constant of the Na_0.55Co_0.90M_0.10O₂ solid solution slightly increased, but the c lattice constant decreased compared to the parent phase of Na_0.55CoO₂ (Table 1). As a result, the values of volume of the unit cell of Na_0.55Co_0.90M_0.10O₂ cobaltites at the variation of their cationic composition retained almost unchanged, but their axial ratio essentially decreased at the partial substitution of cobalt by other metals in Na_0.55CoO₂. Such a substitution slightly affects the size of the unit cell of layered sodium cobaltite but results in its significant deformation—contraction in the direction of the c axis (perpendicular to the conducting –[CoO₂]– layers) and enlarging in the direction of the a axis (parallel to the conducting –[CoO₂]– layers).

The coherent scattering area of the Na_0.55Co_0.90M_0.10O₂ materials obtained using different approaches (Figure 1b) is essentially large (up to 29–63% and 15–35% according to the results of the Sherrer and size-strain methods, respectively) (Figure 1c, Table 1) than for the base-layered sodium cobaltite phase. In turn, the microstrain values of the Na_0.55Co_0.90M_0.10O₂ ceramics were slightly (M = Cr and W) or essentially (M = Ni, Zn, and Bi) smaller in comparison to the parent Na_0.55CoO₂ cobaltite (Table 1).

The Lotgering factor values of the ceramics increased from 0.35 for Na_0.55CoO₂ (moderate orientation [44]) to 0.60–0.64 for Na_0.55Co_0.90M_0.10O₂ (M = Ni and Zn) (good orientation [44]) and 0.82–0.89 for Na_0.55Co_0.90M_0.10O₂ (M = Cr, W, and Bi) (very good orientation [44]) (Table 1). Therefore, the doping of Na_0.55CoO₂ by different metal oxides increased the grain crystallographic orientation degree of the ceramics (its texturing degree). The most prominent effect among samples synthesized in this work was observed for Na_0.55Co_0.90W_0.10O₂.

The apparent density values of the Na_0.55Co_0.90M_0.10O₂ ceramics varied within 3.43–3.72 g/cm³ and decreased at the partial substitution of Co by Cr, Ni, or Zn and increased at the substitution of Co by W and Bi (

Table 2. Values of apparent density (d_EXP), total (Π_t) and open (Π_o) porosity, electrical resistivity (ρ₁₀₇₃), Seebeck coefficient (S₁₀₇₃), power factor (P₁₀₇₃), and figure of merit (ZT₁₀₇₃) of Na_0.55Co_0.9M_0.1O₂ (M = Cr, Ni, Zn, W, and Bi) ceramics.

M	dEXP, g/cm3	Πt, %	Πo, %	104 × ρ1073, Ω × m	S1073, μV/K	P1073, mW/(m × K2)	ZT1073
Cr	3.48	17	22	3.44	540	0.846	0.565
Co	3.65	18	18	4.82	285	0.169	0.233
Ni	3.43	22	23	5.13	608	0.721	0.622
Zn	3.59	21	17	3.03	421	0.584	0.523
W	3.92	17	17	5.20	620	0.740	0.643
Bi	3.72	21	17	4.28	666	1.038	0.702

). The open porosity values of ceramics varied within 17–22% and were close to each other for the samples having different cationic compositions. The closed porosity of the ceramics was negligible (Table 2).

According to the SEM results, the grains of the Na_0.55CoO₂ ceramics were plate-like, with dimensions (l) of 5–10 μm and thicknesses of 2–4 μm (average dimension (l_av) of about 8 μm and aspect ratio (AR) of about 2.7). The microstructure of the Na_0.55Co_0.90M_0.10O₂ materials was similar to the Na_0.55CoO₂ one but they differed in the size and aspect ratio (form) of the grains (Figure 2b–f). The grain size of the Na_0.55Co_0.90M_0.10O₂ (M = Cr, Ni, and Zn) ceramics was varied, within 8–15 μm with a thickness of 3–6 μm (l_av (AR) was about 11 μm (2.8), 13 μm (2.6), and 9μm (1.8) for M = Cr, Ni, and Zn, respectively). Doping of layered sodium cobaltite ceramics with bismuth or tungsten oxides results in an increase of its grain size by up to 12–21 μm and a decrease of the thickness of the grains by up to 1–3 μm. For Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂, the l_av (AR) values were equal to about 13 μm (7.5) and 16 μm (8.0), respectively. As a result, the anisotropy of grains of layered sodium cobaltite ceramics increased at its doping by heavy metal oxides containing highly charged metal ions (Bi⁵⁺, W⁶⁺).

According to the EDX results (see Supplementary Information, Table S1) the cationic composition of the cobalt sublattice of the Na_0.55Co_0.90M_0.10O₂ compounds was close to the aim one, and the distribution of elements within the volume of ceramics was close to uniform (Figure 3).

As can be seen from Figure 4a, Na_0.55CoO₂ ceramics possess a metallic conductivity character (∂ρ/∂T > 0), which changed into a semiconducting one (∂ρ/∂T < 0) near 520 K, like the conductivity crossover in layered calcium cobaltite of Ca₃Co₄O_9+δ [49]. The electrical resistivity values of the Ni-doped sample are close to the Na_0.55CoO₂ ones, but Na_0.55Co_0.90Ni_0.10O₂ demonstrates only the semiconducting conductivity character within the whole studied temperature range. The other Na_0.55CoO₂ substitutes, like the parent phase, demonstrated conductivity crossover, but their temperature shifts to the higher values (for example, to ~720 K for Na_0.55Co_0.90Cr_0.10O₂ or ~920 K for Na_0.55Co_0.90W_0.10O₂ (Figure 4a)). The doping of layered sodium cobaltite by different metal oxides results in a decrease its electrical resistivity values (Figure 4a,d, Table 2) due to the donor character of doping (W or Bi for Co), decreasing the grain boundaries density (increase of grains size), which serve as charge carriers scattering regions, and also due to the decreasing of high-resistive Co₃O₄ phase content in the samples [50] (Figure 1a). Note that the electrical resistivity values of all samples are getting closer at a high temperature. Therefore, ρ values of Na_0.55Co_0.90M_0.10O₂ (M = Ni and W) at 1073 K are slightly larger, and for Na_0.55Co_0.90M_0.10O₂ (M = Cr, Zn, and Bi), they are comparatively smaller than those of Na_0.55CoO₂ (Figure 4d and Table 2).

The positive sign of the Seebeck coefficient (Figure 4b) of all the studied samples points out that holes are the main charge carriers, and they are p-type conductors. The change in the values of the Seebeck coefficient at high temperatures is practically linear (Figure 4b). This dependence follows Equation (14), usually used for the description of the thermoelectric properties of metals and degenerated semiconductors [3,4]:

S = [(8π²k_B²)/(3eh)] × m* × T × (π/3n)^2/3

(14)

where k_B is the Boltzmann constant, m* is the density of a state’s effective mass, h is the Planks constant, e is the charge of the electron, T is the absolute temperature, and n is the charge carrier’s concentration.

Doping of Na_0.55CoO₂ by different transition or heavy metal oxides results in the sharp increase of its Seebeck coefficient, which is more pronounced at high temperatures (Figure 4b,e and Table 2). This is probably caused by an increase in the configurational entropy that is provided by the presence of Co in different charge (Co²⁺, Co³⁺, and Co⁴⁺) and spin (high-, intermediate- and low-spin states) states [51]. The highest S values among the studied samples demonstrated that Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂ complex oxides (620 and 666 μV/K at 1073 K, respectively) are 2.18 and 2.33 times larger than for the parent Na_0.55CoO₂ phase (Figure 4e and Table 2). These phases are prospective materials for p-legs of ceramic (oxide) thermocouples. It should be noted that similar results were obtained when studying the Seebeck coefficient of cobalt-substituted derivatives of Ca₃Co₄O_9+δ layered cobaltite [52].

Power factor values of Na_0.55CoO₂ derivatives are essentially larger compared to the parent phase due to their low electrical resistivity and high Seebeck coefficient (Figure 4c,f and Table 2). They increased as the temperature increased and reached the maximum value of 0.846 and 1.038 mV/(m·K²) for Na_0.55Co_0.90Cr_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂ ceramics, respectively. That is 5.01 and 6.14 times larger than for the undoped sodium cobaltite (Table 2 and Figure 4e).

Both the thermal diffusivity and thermal conductivity values of Na_0.55Co_0.10M_0.10O₂ ceramics decreased as the temperature increased (Figure 5a,b) and increased at the doping of Na_0.55CoO₂ by different metal oxides (Figure 5d,e), probably mainly due to the increase of the ceramics′ grains size, resulting in a decrease of the density of the grain boundaries, serving as effective regions of phonons scattering [37]. The electronic part of the thermal conductivity of the ceramics was relatively small (λ_el/λ = 0.04–0.07) and increased with temperature increase. The phonon part dominated and decreased at temperature gain (Figure 5c).

Figure of merit values of the studied ceramics sharply increased with the temperature increase and doping of layered sodium cobaltite by different transition and heavy metal oxides (Figure 6 and Table 2). The maximum thermoelectric performance was demonstrated for the Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂ complex. Their ZT values reached 0.643 and 0.702 at 1073 K, which is 2.76 and 3.01 times larger than for the parent Na_0.55CoO₂ phase.

Thus, the obtained results show the effectiveness of the doping strategy used in this work to enhance the thermoelectric characteristics of layered sodium cobaltite and give the possibility of recommending Na_0.55Co_0.90M_0.10O₂ (M = W, Bi) ceramics as the prospective material for p-branches of high-temperature thermoelectric devices used for the effective and direct conversion of heat into electrical energy.

4. Conclusions

Summarizing the results of the present contribution, partial substitution of Co by different transitions (Cr, Ni, and Zn) or heavy metals (W and Bi) in Na_0.55CoO₂ (up to 10 mol.%) retains the crystal structure of this phase unchanged and slightly affects its lattice constants, but increases the chemical stability, coherent scattering area, grain size, and grain orientation degree of Na_0.55Co_0.90M_0.10O₂ solid solutions compared to the unsubstituted Na_0.55CoO₂ phase. Such a substitution leads to the decrease of the values of electrical resistivity of Na_0.55Co_0.90M_0.10O₂ ceramics, and an increase in their thermal diffusivity, thermal conductivity, and the Seebeck coefficient values. The maximum S values possess Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂ materials (620 and 666 μV/K at 1073 K, respectively), which are 2.18 and 2.33 times larger, than for the base Na_0.55CoO₂ phase. As a result, the power factor and figure of merit values of Na_0.55Co_0.90M_0.10O₂ solid solutions are essentially higher than those for Na_0.55CoO₂ sodium cobaltite, and the maximum thermoelectric performance demonstrates Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂ compounds, where ZT values at 1073 K reach 0.643 and 0.702, which is 2.76 and 3.01 times larger than those for the Na_0.55CoO₂ phase. The obtained results demonstrate the effectiveness of the doping strategy for the enhancement of the thermoelectric performance of layered sodium cobaltite derivatives. Materials possessing the best thermoelectric characteristic among the studied samples (Na_0.55Co_0.90W_0.10O₂ and Na_0.55Co_0.90Bi_0.10O₂) can be used as p-branches of high-temperature thermoelectric modules and generators, as well as p-legs of ceramic (oxide) thermocouples.