Standard enthalpy of formation of NaxCoO2 system

Journal of Alloys and Compounds 540 (2012) 62–66

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Standard enthalpy of formation of NaxCoO2 system S. Phapale a, R. Mishra a,⇑, P.K. Mishra b a b

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India Technical Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 6 January 2012 Received in revised form 30 May 2012 Accepted 2 June 2012 Available online 21 June 2012 Keywords: Standard enthalpy of formation Solution calorimetry NaxCoO2 compounds Thermoelectric materials

a b s t r a c t Layered sodium cobalt oxide (NaxCoO2) materials have attracted lot of attention in recent times because of their large thermoelectric power. Information on thermodynamic properties is necessary to predict the stability of these compounds. The present study deals with determination of enthalpy of formation of sodium-cobalt oxides [NaxCoO2 (x = 0.1, 0.2. . .0.9)] using a solution calorimeter. The standard molar heat of dissolutions of NaxCoO2 (x = 0.1, 0.2. . .0.9), NaCl(s), CoCl3(s) and H2O (l) in 0.150 dm3 (1:1 ratio) aqueous HCl solution of concentration 5.96 mol dm3 were measured by an isoperibol solution calorimeter. Using the values of the measured enthalpy of dissolution and other auxiliary data, the standard molar enthalpies   of formation ðDf Hm 298Þ of the compounds were determined. The values of Df Hm 298 for the Na0.1CoO2(s), Na0.2CoO2(s), Na0.3CoO2(s), Na0.4CoO2(s), Na0.5CoO2(s), Na0.6CoO2(s), Na0.7CoO2(s), Na0.8CoO2(s) and Na0.9 CoO2(s) compounds are found to be (598.61 ± 6.03), (637.40 ± 6.04), (676.26 ± 6.04), (716.53 ± 6.05), (758.03 ± 6.06), (800.04 ± 6.07), (841.58 ± 6.08), (883.38 ± 6.09), (924.93 ± 6.10) kJ mol1, respectively. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The increased interest in devices for thermal energy conversion has lead to the search for new efficient thermoelectric materials. A thermoelectric device converts heat into electric energy through the thermoelectric power of the material. In such materials, the temperature gradient drives the diffusion of the charge carriers to the cooler end of the sample and the charge accumulation at the end leads to the development of net electric field, which determines the thermoelectric power. Layered sodium cobalt oxide (NaxCoO2) materials have attracted a lot of attention in recent times because of their large thermoelectric power (100 lV/K at 300 K) [1–3], low electrical resistivity [3,4] and low thermal conductivity [5] indicating its potential application in the field of thermoelectric devices. Enhanced thermopower in sodium cobalt oxide material has been attributed to dominance of spin entropy and that has been demonstrated from the strong dependence of thermopower on the magnetic field [1]. NaxCoO2 has a hexagonal crystal lattice and space group (P63/mmc) with Na and CoO2 alternately stacked along c-axis [6]. The physical properties associated with these compound changes significantly depending upon the level of sodium doping. Variation of Na content in the compound alters the Co+4 to Co+3 ratio and in turn, the extent of spin contribution to thermopower. The thermodynamic information of NaxCoO2 as a function of Na content is therefore, important to evaluate the ⇑ Corresponding author. Tel.: +91 22 2559 2460; fax: +91 22 2550 5151. E-mail address: [email protected] (R. Mishra). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.015

thermodynamic stability of the compound under different environments and to get optimum thermoelectric power output. To our knowledge, recent literature does not provide any data on the thermodynamic properties of NaxCoO2 compounds with varying Na content. In this paper we report the standard molar enthalpy  of formation ðDf Hm 298Þ of Nax CoO2 (where x = 0.1, 0.2. . .0.9) determined using an isoperibol solution calorimeter. 2. Experimental Sodium-Cobalt Oxides NaxCoO2 (x = 0.1, 0.2. . . 0.9) were prepared from high purity Na2CO3 (Aldrich, purity 99.99%) and Co3O4 (Aldrich, purity 99.99%). Thoroughly ground mixtures of Na2CO3 and Co3O4 in appropriate molar proportions were heated in a dynamic vacuum at 773 K for 48 h with an intermediate grinding. The powder samples were made into pellets with a pressure of 15 kg/m2 and were heated at 973 K for 100 h in a platinum boat under a flowing oxygen atmosphere. The compounds were characterized by powder X-ray diffraction (XRD) technique using a Philips X-ray diffractometer (PW-1729) with Cu Ka radiation. Thermal stability of the compounds was determined using a commercial TG-DTA apparatus (Setaram, 92-16.18) in an argon atmosphere at a heating rate of 10 K min1. Na content in the samples was determined by chemical analysis using Flame Photometry method (Systonic Made).

3. Solution calorimetric measurements The enthalpies of dissolution of NaxCoO2 (x = 0.1, 0.2. . . 0.9) were measured in an isoperibol solution calorimeter operated at 298 K. The construction and operation of the calorimeter are similar to the one described by Athavale et al. [8]. For each of these compounds the samples were weighed and introduced into a glass bulb,

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which were then thermally equilibrated in the calorimetric solution. The solvent used was 0.150 dm3 of aqueous 5.96 mol dm3 HCl. The glass bulb was broken to introduce the sample into the solution when a steady state thermal signal was obtained. The energy equivalent of the calorimeter was determined before and after each measurement by electrical calibration. The performance of the calorimeter was tested by measuring the heat of dissolution of TRIS (Tris (hydroxy methyl) aminomethane) in 0.1 M HCl and spec pure KCl in distilled water. The enthalpies of dissolution of NaxCoO2 (x = 0.1, 0.2. . . 0.9) along with NaCl(s), CoCl3(s) and H2O(l) were measured in similar fashion. Using these experimental values and other auxiliary data from literature the standard molar enthalpies of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) were determined.

Na0.9CoO2 Na0.8CoO2

Relative Intensity(a.u)

Na0.7CoO2 Na0.6CoO2 Na0.5CoO2 Na0.4CoO2 Na0.3CoO2 Na0.2CoO2 Na0.1CoO2

4. Results and discussions The analysis of the mass changes observed in the samples after completion of heating cycles indicates a complete loss of CO2 moiety with negligible loss of sodium oxide. Thermogravimetric analysis of NaxCoO2 samples indicated single step decomposition due to loss of Na2O in all the samples. Decomposition temperatures for the samples were determined from the on-set temperature of the mass loss versus temperature plot. The decomposition temperature, the observed and the expected mass loss are given in Table 1. The sodium content of the samples was also analyzed chemically using flame photometry and the results of the chemical analysis are also given in Table 1. From the mass loss and chemical analysis data, the compositions of the samples are found to be close to the expected compositions. Fig. 1 gives the XRD patterns of the compounds NaxCoO2 (x = 0.1, 0.2 . . . 0.9). No lines due to the starting components Na2 CO3, Co3O4 or any other impurity phases were found. In literature, varying structures viz., hexagonal (P63/mmc), rhombohedral (R3m), monoclinic (C2/m) and orthorhombic (Pnmm) have been reported with different sodium contents [6,7]. However, the basic structures of all the compositions remain closely similar except the variation in stacking sequence of the CoO6 octahedral layers or position and occupation of Na in the lattice. It has been observed that the position of Na also varies with the composition. Recently, Huang et al. has shown that throughout the composition range of Na, a two layer hexagonal lattice can explain the structure [6], although other studies indicate a three layer rhombohedral or monoclinic structure [7]. In this work we have considered only two layer hexagonal stacking (P63/mmc) in the entire range of

10

20

30

40

50

60

70

80

2θ (degree) Fig. 1. XRD plots of NaxCoO2(x = 0.1, 0.2. . . 0.9).

compositions. The cell parameters obtained agrees well with the reported data [6] and the observed lattice parameters are given in Table 1. Further we mentioned here that for compositions x = 0.4 to 0.7, the appearance of two reflections suggests the presence of two hexagonal lattice with small difference in the c-parameters. This can be attributed to a two different in Na positions as mentioned by Huang et al. [6]. Here, the exact structural analyses have not been carried out to identify the symmetry of the lattice. The enthalpy of dissolution of TRIS and KCl in 0.1 M HCl(aq) and distilled water were found to 29.75 ± 0.02 kJ mol1 and 17.22 ± 0.02 kJ mol1 respectively, against the reported values of 29.77 ± 0.03 kJ mol1 [9] and 17.24 ± 0.02 kJ mol1 [10] respectively, indicating the accuracy of the enthalpy measurements. The reproducibility of the measurements was checked by taking several measurements of each sample. The molar heats of dissolution for NaxCoO2 (x = 0.1, 0.2. . . 0.9) in the HCl(aq) when plotted against their respective molar concentration show that there is no significant dilution effect and the experiments were carried out effectively under infinite dilution conditions in each case. So in each case the average of five values were taken. The results of the enthalpies of dissolution of NaxCoO2(s) (x = 0.1, 0.2. . . 0.9), NaCl(s), CoCl3(s) and H2O(l) are given in Table 2. The thermochemical cycles from which the standard molar

Table 1 Chemical, thermal and XRD analysis data for NaxCoO2 (x = 0.1, 0.2. . . 0.9) compounds. NaxCoO2

Chemical analysis

Thermal analysis

XRD analysis

Wt% of Na2O (observed)

Wt% of Na2O (expected)

Decomp. Temp. (K)

% Mass loss (observed)

(Expected)

Cell parameters (observed) (°A)

Na0.1CoO2

3.06

3.32

1147

2.41

2.46

Na0.2CoO2

6

6.49

1140

4.76

4.82

Na0.3CoO2

8.84

9.5

1131

6.99

7.05

Na0.4CoO2

11.63

12.38

1123

9.09

9.19

Na0.5CoO2

14.37

15.13

1137

11.01

11.23

Na0.6CoO2

17.22

17.76

1143

12.91

13.18

Na0.7CoO2

18.85

20.27

1151

14.89

15.04

Na0.8CoO2

21.78

22.68

1153

16.51

16.83

Na0.9CoO2

23.51

24.99

1154

18.01

18.54

a = 2.893(2) c = 11.397(1) a = 2.818(1) c = 11.313(2) a = 2.820(3) c = 11.230(2) a = 2.822(1) c = 11.170(2) a = 2.823(2) c = 11.110(1) a = 2.820(2) c = 11.010(9) a = 2.832(1) c = 10.869(3) a = 2.861(2) c = 10.712(4) a = 2.869(1) c = 10.690(3)

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Table 2 The molar enthalpies of dissolution of NaxCoO2 (x = 0.1, 0.2. . . 0.9) NaCl(s) and CoCl3(s) in 0.150 dm3 aqueous solution of 5.96 mol dm3 HCl at T = 298. K; m denotes the mass of the sample dissolved; DH is the measured energy change per unit mass and DsolHm is the molar enthalpy of solution. Solute

Sample code

Na0.1CoO2(s) Mol.wt = 93.23

S1

Na0.2CoO2(s) Mol.wt = 95.53

S2

Na0.3CoO2(s) Mol.Wt = 97.83

S3

Na0.4CoO2(s) Mol.wt = 100.13

S4

Na0.5CoO2(s) Mol.wt = 102.43

S5

Na0.6CoO2(s) Mol.wt = 104.73

S6

Na0.7CoO2(s) Mol.wt = 107.03

S7

Na0.8CoO2(s) Mol.wt = 109.33

S8

Na0.9CoO2(s) Mol. wt = 111.63

S9

NaCl(s) Mol.wt = 58.5

CoCl3(s) Mol. wt = 165.43

H2O(l) Mol.wt = 18.00

m (solute) (g)

D (J/g)

DsolHm/kJ mol1

0.0209 0.0371 0.0379 0.0674 0.0437

90.42 90.85 90.53 90 90.31

0.0514 0.0572 0.0296 0.0392 0.0385

157.23 158.27 157.44 157.75 157.23

0.023 0.0328 0.0389 0.041 0.039

220.08 219.97 219.77 219.77 219.77

0.0383 0.0512 0.0319 0.0232 0.0453

251.17 251.37 251.27 251.27 251.07

0.0305 0.0395 0.0529 0.0422 0.0502

257.44 257.35 257.35 257.44 257.25

0.0333 0.0446 0.0265 0.0442 0.0504

253.51 253.51 253.41 253.41 253.32

0.0302 0.0812 0.0555 0.0346 0.0394

258.34 258.25 258.34 258.15 258.24

0.0377 0.0366 0.0494 0.0331 0.0372

258.3 258.3 258.21 258.21 258.12

0.0678 0.0678 0.0678 0.0678 0.0683

262.74 262.56 262.74 262.65 262.83

0.1022 0.227 0.2453 0.2304 0.2364

142.39 142.73 142.22 142.05 142.22

0.0566 0.0301 0.052 0.0678 0.0227

323.4 322.8 322.19 322.8 321.59

1 1 1 1 1

303.33 316.11 233.88 222.22 313.88

8.43 8.47 8.44 8.39 8.42 Avg. = 8.43 ± 0.03 15.02 15.12 15.04 15.07 15.02 Avg. = 15.05 ± 0.04 21.53 21.52 21.5 21.52 21.5 Avg. = 21.51 ± 0.01 25.15 25.17 25.16 25.16 25.14 Avg. = 25.16 ± 0.01 26.37 26.36 26.36 26.37 26.35 Avg. = 26.36 ± 0.01 26.55 26.55 26.54 26.54 26.53 Avg. = 26.54 ± 0.01 27.65 27.64 27.65 27.63 27.64 Avg. = 27.64 ± 0.01 28.24 28.24 28.23 28.23 28.22 Avg. = 28.23 ± 0.01 29.33 29.31 29.33 29.32 29.34 Avg. = 29.33 ± 0.01 8.33 8.35 8.32 8.31 8.32 Avg. = 8.33 ± 0.01 53.5 53.4 53.3 53.4 53.2 Avg. = 53.4 ± 0.10 5.46 5.69 4.21 4 5.65 Avg. = 5.0 ± 0.8

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Table 3 Reaction scheme for the standard molar enthalpy of formation of NaxCoO2 (x = 0.1, 0.2. . .0.9), (sln = 0.150 dm3 of 5.96 mol dm3 HCl) DfH298(NaxCoO2,s) = 0.5[DfH298(NaxCoO2,s) = 0.5[DHi(0.1,0.2. . .0.9) + 2x.DH2 + DH3 + DH4 + DH5 + 2x. DH6 + 6DH7 + 6DH8 + 3DH9 + 3DH10]. Reaction

D Hi

DHm (kJ mol1)

Ref.

2NaxCoO2(s)+6HCl(sln) = 2xNaCl(sln) + CoO(s) + [Co(H2O)6]Cl3(sln) + 3H2O(sln) + (3–2x)/ 2Cl2(g)

DHi1 (i = S1, S2, S3, S4, . . . , S9)

This work

Na NaCl(s) + sln = NaCl(sln)

D H2

S1: 8.43 ± 0.03 S2: 15.05 ± 0.04 S3: 21.51 ± 0.01 S4: 25.16 ± 0.01 S5: 26.36 ± 0.01 S6: 26.54 ± 0.01 S7: 27.64 ± 0.01 S8: 28.23 ± 0.01 S9: 29.33 ± 0.01 8.33 ± 0.01

CoCl3(s) + sln = [Co(H2O)6]Cl3(sln)

D H3

183.4 ± 1.8

Co(s) + 0.5 O2(g) = CoO(s) Co(s) + 1.5 Cl2(g) = CoCl3(s) Na(s) + 0.5 Cl2(g) = NaCl(s) HCl(g) = 0.5 H2(g) + 0.5Cl2(g) HCl(sln) = HCl(g) + sln H2(g) + 0.5 O2(g) = H2O(l) H2O(l) + sln = H2O(sln)

D H4 D H5 D H6 D H7 D H8 D H9 DH10

238.91 ± 0.5 163.59 ± 0.1 412.63 ± 0.1 148.5 ± 0.6 92.31 ± 0.2 285.9 ± 0.8 5.0 ± 0.8

x.Na(s)+Co(s)+ O2(g) = NaxCoO2(s)

Df Hm 298



0.5[Si + 2x.DH2+DH3 + DH4 + DH5 + 2x.DH6 + 6DH7 + 6DH8 + 3DH9 + 3DH10]

enthalpies of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) have been derived are given in Table 3. The scheme for the chemical reaction between NaxCoO2 and the solution (5.96 M HCl) was assumed to be 2Nax CoO2 ðsÞ þ 6HClðslnÞ ¼ 2xNaClðslnÞ þ CoOðsÞ þ ½CoðH2 OÞ6 ðslnÞþ ð32xÞ Cl2 ðslnÞ. 2

The formation of the reaction products were individually established by different experimental methods. The presence of Na+ ion was checked by a flame test which showed a golden yellow color. The formation of CoO(s) was verified by analysis of the precipitate. The yield of the CoO(s) was quantitatively determined which was in agreement with the calculated value. The XRD pattern of the CoO(s) residue annealed at 400 °C for 3 days in a sealed quartz ampoule matches with standard pattern (JCPDS file no 421300). The formation of a [Co(H2O)6]Cl3(sln) complex was confirmed by comparing the UV–VIS absorption spectra for the solutions containing dissolved NaxCoO2 compounds and CoCl3. The absorption maxima (400 nm) for the solutions containing NaxCoO2 compounds and CoCl3 samples overlap with each other indicating the formation of a [Co(H2O)6]3 complex in both the cases. Similarly the evolution of Cl2(g) was proved by collecting the evolved gas in a vacuum container and analyzing the species in a quadruple mass spectrometer. The experimentally measured enthalpy values were combined with other auxiliary data such as the standard enthalpies of formation of NaCl(s), CoO(s) and CoCl3(s), H2O(l), HCl(g), HCl(aq) from the literature [11,12] to derive the standard molar enthalpies of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) at 298 K. The standard molar enthalpy of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) at 298 K are found to be (598.61 ± 6.03), (637.40 ± 6.04), (676.26 ± 6.04), (716.53 ± 6.05), (758.03 ± 6.06), (800.04 ± 6.07), (841.58 ± 6.08), (883.38 ± 6.09), (924.93 ± 6.10). The standard enthalpy of formation of NaCoO2 obtained from extrapolation of enthalpy versus composition in Na composition range x = 0.1 to x = 0.9 is found to be 968.8 kJ mol1. The above data on standard enthalpies of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) compounds are first measurements of this series. These standard enthalpy of formation of NaxCoO2 (x = 0.1, 0.2. . . 0.9) compounds at 298 K with respect to the component oxides calculated using the standard enthalpies of formation data obtained from this work and values of standard enthalpies of formation for CoO(s) [13] and Na2O(s) [14] following the equation 2x Na2 OðsÞ þ CoOðsÞ þ ð2x ÞO2 ðgÞ ¼ Nax CoO2 were found 4 to be 339.8, 357.6, 375.6, 394.9, 415.5, 436.6, 457.2,

This work This work 11 8 8 8 8 8 This work This work

478.1 and 498.7 kJ mol1 for x = 0.1, x = 0.2. . . 0.9, respectively. It could be observed that the enthalpy of formation of the compounds in the series from elements and as well as from the oxides are found to be increase with increase in Na content. The extra stability in higher Na containing sample could be attributed to the increase in ionic bonding character between Na+ ions and the oxide layers on either side of Na ion. A similar increasing trend in the enthalpy of formation of LixCoO2 (x = 0, 0.25, 0.5 and 1.0) with increase in Li content was reported by Wang et al. [15]. The standard enthalpy of formation of LiCoO2 reported by the authors was 678.2 kJ mol1 which is comparatively less than the value for NaCoO2 obtained in this experiment. The higher enthalpy of formation of NaCoO2 compared to LiCoO2 can be explained by the bigger size of Na+ ion which interact more strongly compared to smaller Li+ ion. Q. Huang etal. [6] have reported phase changes in NaxCoO2 series materials with paramagnetic metal at (xNa 0.3) that changes to charge-ordered insulator at (xNa 0.5) to Curie-Weis metal at (xNa 0.7) and finally to a weak–moment magnetically ordered state at (xNa > 0.75). However, the values for enthalpy of formation of NaxCoO2 (x = 0.2. . . 0.9) determined in this work do not reflect the formation of these ordered phases. This could be explained by the fact that the extent of energy involved in the rearrangement of atoms resulting above mentioned phase transitions may be too less compared to the overall changes in the enthalpy of formation by increasing the Na content. To summarize, we have determined standard enthalpies of formation of NaxCoO2 in the range x = 0.1 to 0.9 at room temperature. The enthalpy of formation of the compound increases with Na content. The lattice parameter ‘a’ while remains almost unchanged, the ‘c’ parameter progressively decreases. The higher value of enthalpy of formation, for samples with higher Na content, is due to increase in ionic bonding character between Na+ ions with the oxide layers on either side of Na+ ion. To our knowledge, this data of standard enthalpy of formation is the first reported data for the above compounds and could be useful in computational chemistry. Acknowledgement Authors thank Dr. D. Das, Head, Chemistry Division, Bhabha Atomic Research Centre, Mumbai for his guidance and support in carrying out this work.

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