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Raman evidence of the formation of LT-LiCoO2 thin layers on NiO in molten carbonate at 650°C
Applied Surface Science 225 (2004) 356–361
Raman evidence of the formation of LT-LiCoO2 thin layers on NiO in molten carbonate at 6508C L. Mendozaa, R. Baddour-Hadjeanb,*, M. Cassira, J.P. Pereira-Ramosc a
Ecole Nationale Supe´rieure de Chimie de Paris, Laboratoire d’Electrochimie et de Chimie Analytique (UMR 7575 CNRS), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b LADIR, CNRS UMR 7075, 2 rue Henri Dunant, 94320 Thiais, France c LECSO, CNRS UMR 7582,2 rue Henri Dunant, 94320 Thiais, France Received 3 October 2003; received in revised form 22 October 2003; accepted 22 October 2003
Abstract The structural evolution of thin layers of Co3O4 elaborated on nickel-based substrates in the Li2CO3–Na2CO3 carbonate eutectic at 650 8C as a function of time immersion is reported. Raman microspectrometry has been applied in order to provide more information on the nature of the protective cobalt oxide layers. The typical Raman fingerprint of the LT-LiCoO2 compound has been obtained, with four well defined bands at 449, 484, 590 and 605 cm1, while XRD data are unable to distinguish the layered phase (HT) from the spinel one (LT). The mechanical stability of such films does not exceed 10 h in direct contact with the molten carbonate bulk at 650 8C; nevertheless, these conditions are much more corrosive than in a molten carbonate fuel cell (MCFC). # 2003 Elsevier B.V. All rights reserved. Keywords: LT-LiCoO2 coating; Molten carbonate; Fuel cell; Raman spectroscopy; Surface analysis
1. Introduction The molten carbonate fuel cell (MCFC) performance and lifetime are greatly dependent on the slow corrosion, dissolution and precipitation of LixNi1xO formed on the nickel cathode. The substitution or protection of this cathode is one of the key problems in order to ensure the technological and commercial development of this high temperature fuel cell [1–3]. One of the solutions is the elaboration of NiO-based materials containing other oxides less soluble and
*
Corresponding author. Tel.: þ33-1-49781155; fax: þ33-1-49781323. E-mail address: [email protected] (R. Baddour-Hadjean).
with electrochemical performances close to that of LixNi1xO, this allows to combine the high conductivity and the good electrocatalytic properties of LixNi1xO with the stability of protective coatings. Among the compounds NiO–MO, LiCoO2, less soluble in carbonate melts and with acceptable electrochemical properties, appears as the best candidate [4]. Indeed, the use LiCoO2 as a thin protective layer is of particular interest due to a better mechanical resistance and low cost [4–11]. Furthermore, LiCoO2 has also been extensively studied as a cathode material for rechargeable Li-ion batteries [12–15]. Very few works have been dedicated to the in situ formation of LiCoO2 in molten carbonates and its structural characterisation. Yamada and Uchida have obtained LiCoO2 from the oxidation of metallic cobalt depos-
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.026
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
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ited on a gold substrate in Li2CO3–K2CO3 at 650 8C [16]. Yang and Kim have investigated the oxidation behaviours of Ni–Co alloys in the same melt and demonstrated the formation of LiCoO2 from open circuit potential measurements and X-ray diffraction experiments [17,18]. Recently, the high temperature LiCoO2 phase characterised by an hexagonal symmetry has been detected by Raman spectroscopy on dense or porous nickel samples after the oxidation of Co also in the same molten carbonate eutectic at 650 8C [19]. In a recent work, thin layers of Co3O4 were elaborated on nickel-based substrates in order to protect the nickel cathode, i.e. by lowering its dissolution at 650 8C in Li2CO3–Na2CO3, one of the second generation MCFC electrolyte. The technique proposed consists in an electrochemical potentiostatic deposition which has been proved to be an efficient, cheap and low cost method [20]. The purpose of the present paper is to provide more information on the nature and stability of cobalt oxides layers when exposed to a second-generation electrolyte, Li2CO3–Na2CO3 at 650 8C. Here our interest is to get a local picture of the structural changes taking place at the cathode interface using Raman spectroscopy, these data being completed by XRD experiments.
lysis is performed at constant potential of 0.50 versus SCE. The electrolysis time was 8 h and the solution was magnetically stirred during electrodepositing. A platinum foil counter electrode (1 cm2) and a SCE (Hg|Hg2Cl2) reference electrode in a separate compartment filled with 0.5 mol l1 NaNO3 were used for the deposition. Brown deposits were obtained. After aqueous electrodepositing, the films were rinsed with de-ionised water. For either structural characterisation or oxidation in molten carbonates, the samples were annealed during 4 h in air at 500 8C.
2. Experimental
2.3. Characterisation techniques
2.1. Electrochemical deposition
The Raman spectra were recorded at room temperature using micro-Raman system with a Dilor XY spectrometer including charge coupled device (CCD) detector. An argon ion laser (514.5 nm) was used as the excitation source. The spectra were measured in back-scattering geometry. The resolution was about 1 cm1. A 50X objective was used to focus the laser light on sample surface to a spot size of 5 mm2 and the laser power was kept to 1 mW to avoid any degradation of the electrode. XRD experiments were carried out with a Siemens D5000 type diffractometer using a Co Ka1 radiation ˚ ) and a back graphite monochromator. (l ¼ 1:789 A The diffraction pattern was scanned by steps of 0.028 (2y) with a fixed counting time (2.3 s) between 168 and 808. The composition of the deposits was examined with energy-dispersive spectroscopy (EDS).
Cobalt oxide films were formed by potentiostatic electrolysis at 25 8C on 10 mm 10 mm 0:5 mm nickel foils after oxidation (Johnson Mathey, of analytical purity 99.994%) [20]. The oxidation treatment of the nickel foils consisted in their immersion in the molten carbonate at 650 8C during 2 h [21]. The deposition was performed in a four-compartment Tacussel glass cell with 40 ml of a 0.1 mol l1 Co(II) solution prepared from Co(NO3)26H2O (Fluka, with a purity of 99.99% analytical reagent grade chemical) in a 0.5 mol l1 solution of NaNO3 (Merck) with a pH of 4. The solution was de-aerated for 30 min prior to electrochemical deposition, then 4 ml of 1 mol l1 NaOH solution was added to the Co(II) solution and the final pH was 7.40. A blue precipitate of Co(OH)2 was formed. In the second step, an electro-
2.2. Oxidation in molten carbonates at 650 8C The oxidation treatment of the nickel foils recovered by the Co3O4 deposit consisted in their immersion in a cell containing (Li0.52Na0.48)2CO3 eutectic melt (Fluka, of analytical purity 99.9%) at 650 8C contained in an alumina Al23 reactor (250 mm 60 mm) sealed hermetically by a stainless steel cover with a Viton O-ring, saturated with a mixture of air/CO2 70/30 (mol%). The immersion time in the carbonate melt was between 15 min and 48 h. After immersion, the films were rinsed with de-ionised water, and before the morphological and structural characterisation, the films were dried at 100 8C for 1 h.
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
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3.2. Raman results and discussion Fig. 2a shows the Raman spectrum of a nickel foil after in situ oxidation in molten carbonate melts Li2CO3–Na2CO3 (52–48 mol%). A poor signal is observed, with a broad contribution of the NiO vibrations at 400, 560 and 730 cm1, as proved by comparison with a pure nickel oxide sample (Fig. 2b). A further heat treatment of the electrochemical Co3O4 deposit leads to the well defined Co3O4 vibrational fingerprint with Raman bands at 480, 520, 618 and 688 cm1 (Fig. 3b). The oxidation products of the nickel foils recovered by the Co3O4 deposit results have been followed by Raman microspectrometry as a function of the immersion time in the carbonate melt. After a very short time (15 min), it is clear that the Co3O4 fingerprint (Fig. 3b) has practically completely disappeared and a new system is evidenced corresponding to the formation of a LiCoO2 film, as indicated by the four well defined Raman bands observed at 449, 484, 590 and 605 cm1 (Fig. 3c).
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NiO(200)
NiO(111)
Co3 O4 (400)
Co3 O4 (311)
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LiCoO2 (003)
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Co3 O4 (111)
Fig. 1a shows the XRD pattern of the electrochemical deposit on Ni foil after heat-treatment at 500 8C during 4 h. It reveals the intense lines of the nickel substrate and the less intense lines of NiO, formed at the interface. In addition, one can recognise the weak and broad (1 1 1), (3 1 1) and (4 0 0) lines of the Co3O4 compound, which confirms the electrochemical deposition of the Co3O4 oxide at the surface of the nickel substrate [20]. Further immersion in the molten carbonate melt during 15 min leads to the XRD pattern presented in Fig. 1b. At this stage of immersion time, we observe the disappearance of the Co3O4 diffraction lines and the emergence of the most intense (0 0 3) line of the LiCoO2 compound. The presence of this latter does not allow to discriminate between the LTand HT-LiCoO2. Indeed, determination of the LiCoO2 type can only be obtained from the presence of broad or well resolved peaks for the (0 0 6), (0 1 2) and (0 1 3), (1 2 0) lines. This prompts us to use the Raman spectroscopy which allows to get specific fingerprints for each structure.
Intensity /arb. units
3.1. XRD data
50
60
70
2θ/°
Fig. 1. XRD pattern obtained on a Ni foil recovered by a cobalt oxide deposit: (a) after heat-treatment at 500 8C in air during 4 h; (b) after 15 min of exposure to Li2CO3–Na2CO3 (52–48 mol%) at 650 8C in air/CO2 (70/30).
As mentioned earlier, LiCoO2 is reported to have two closely related structural modifications, layeredtype HT and spinel LT-LiCoO2 [22]. While the HTLiCoO2 phase with an hexagonal unit cell and a layered structure can be described by alternate pure Li and Co planes separated by oxygen layers, the LTLiCoO2 with cubic structure is characterised by a partial disorder between the Liþ and Co3þ ions. The distinction between these two phases by XRD is not easy because their powder pattern are almost identical, as demonstrated by Rossen et al. [23]. Conversely, Raman spectroscopy can be a useful tool to differentiate these two structures [24]. The factor group analysis on the space group R-3m and Fd3m for the HT and LT phases, respectively, predict two and four Raman bands, respectively. Comparison of the Raman spectra of our in situ LiCoO2 film with the conventional HT-LiCoO2 compound is reported in Fig. 4. Two strong Raman bands at 487 and 597 cm1 are observed for the HT-LiCoO2 (Fig. 4b) and four Raman bands are observed at 449, 484, 590 and 605 cm1 for our LT-LiCoO2 (Fig. 4a), in agreement with the assignment of the spinel LTLiCoO2 structure [24]. It is interesting to note that all the LiCoO2 films synthesised in molten carbonate media have been reported as HT compounds with the layered structure [19].
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
500
359
560
Intensity (a.u.)
400
730
a
b
300
400
500
600
700
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-1
Wavenumber (cm ) Fig. 2. Raman spectra of the (a) Ni/NiO substrate; (b) a NiO pure sample.
For longer immersion times, the evolution of the Raman spectrum is illustrated in Fig. 5. The typical response of LT-LiCoO2 is maintained up to 6 h of immersion (Fig. 5a–c). However, the improved resolution of the overall Raman spectrum suggests an increase of the crystallisation with time. Beyond 590 605 484
Intensity (a.u.)
449
c 688
480 520 618
b a
300
400
500
600
700
800
-1
Wavenumber (cm )
Fig. 3. Raman spectra of (a) Ni/NiO substrate; (b) the heat-treated cobalt oxide deposit; (c) the oxidised deposit in molten carbonate electrolyte after 15 min immersion time.
12 h of immersion, the loss of the Raman LiCoO2 fingerprint (Fig. 5d) probably indicates the LT-LiCoO2 film is not mechanically stable. An EDS analysis confirms a significant loss of the cobalt content on the substrate, which decreases by 80% after 48 h of time immersion. The disappearance of the (0 0 3) XRD line is in accordance with the loss of LiCoO2 film. In fact, the Co3O4 electrochemical deposit encounters a chemical transformation leading to LiCoO2 formation according to this reaction [25]. Co3 O4 þ 3Liþ þ 2CO3 2 ! 3LiCoO2 þ 2CO2 þ e In the molten carbonate melt at 650 8C, a growth and crystallisation process of the grains takes place. Such a reaction induces an important volume expansion, as suggested by the very different density values: 6.11 and 2.5 (from [21]) for Co3O4 and LiCoO2, respectively, which can account for the poor mechanical stability of the deposit after chemical transformation. This phenomenon has already been observed, by Belhomme [21] and Yacisi and Selman [26] after in situ oxidation of metallic Co in LiCoO2. This effect is probably responsible for the loss of the mechanical
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L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
605 590
Intensity (a.u.)
449
484
a
597 487
b
300
400
500
600
700
800
-1
Wavenumber (cm ) Fig. 4. Raman spectra of (a) our LiCoO2 film; (b) HT-LiCoO2.
adherence, which was evidenced by Raman experiments showing the disappearance of LiCoO2. However, a dissolution process of LiCoO2 in molten carbonate melt cannot be discarded [4,27]: LiCoO2 þ 32 CO2 ! Liþ þ Co2þ þ 32 CO3 2 þ 14 O2
The spinel phase of LiCoO2 (LT-LiCoO2) is obtained in the present experiments due to the low synthesis temperature and to the crystallographic phase of the precursor, where structural filiations take place between the electrodeposited Co3O4 and the LT-LiCoO2 formed in the molten carbonate melt at 650 8C. In fact, the 590 605
484
Intensity (a.u.)
449 a
b
c d 300
400
500
600
700
800
Wavenumber (a.u.) Fig. 5. Raman evolution as a function of immersion time in molten carbonate electrolyte: (a) 15 min; (b) 1 h; (c) 6 h; (d) 12 h and more.
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
Co3O4 and LT-LiCoO2 have very similar cubic lattice parameters (a), 0.8085 and 0.7995 nm, respectively [28,29]. Both compounds belong to the same space group: Fd3m. Recently, Shao-Horn et al. [29] have obtained the LT-LiCoO2 from the solid-state reaction of Co3O4 with Li2CO3 at 400 8C. Adhikary et al. [35] have used the same precursor at 300 8C in order to prepare the LT-LiCoO2 without traces of the high temperature phase. Conversely, when the synthesis temperature is high enough (more than 800 8C) and the precursors are: CoCO3 and Li2CO3 or Li2O [30,31], LiOH and CoCO3 [32,33] or Li2CO3 and CoCO3 [34], the HT-LiCoO2 is obtained.
4. Conclusion In this study, the structural characterisation of Co3O4 thin layers on nickel substrate was performed after direct exposure to the molten carbonate bulk under the usual MCFC cathode conditions. XRD experiments allowed to check the oxidation process of Co3O4 into LiCoO2 without indication on the cubic (LT) or hexagonal (HT) structure. Using Raman microspectrometry, it was possible to give an experimental evidence of the formation of a LT-LiCoO2 film, as indicated by the four well defined Raman bands observed at 449, 484, 590 and 605 cm1. The formation of LT-LiCoO2 thin films in molten carbonate at 650 8C has never been reported before in the literature; however, under the specific conditions used in this work, the mechanical stability of the cobalt oxide film did not exceed 12 h at 650 8C. These results are encouraging with respect to the protective role of LT-LiCoO2, if we consider that in a MCFC device, the molten carbonate electrolyte being immobilised in a solid inert structure of LiAlO2, the corrosive conditions are significantly less severe in comparison with the experiments performed directly in the bulk.
References [1] C. Yuh, J.R. Selman, J. Electrochem. Soc. 138 (1991) 3649. [2] K. Yamada, T. Nishina, I. Uchida, J.R. Selman, Electrochim. Acta 38 (1993) 2405.
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[3] J.B.J. Veldhuis, F.C. Eckes, L. Plomb, J. Electrochem. Soc. 139 (1992) L6. [4] T. Fukui, H. Okawa, T. Tsunooka, J. Power Sources 71 (1998) 239. [5] R.C. Makkus, K. Hemmes, J.H.W. de Wit, J. Electrochem. Soc. 141 (1994) 3429. [6] S.T. Kuk, Y.S. Song, K. Kim, J. Power Sources 83 (1999) 50. [7] T. Fukui, S. Ohara, H. Okawa, T. Hotta, M. Naito, J. Power Sources 86 (2000) 340. [8] K. Takizawa, A. Hagiwara, J. Electrochem. Soc. 148 (9) (2001) A1034. [9] A. Durairajan, H. Colon-Mercado, B. Haran, R. White, B. Popov, J. Power Sources 104 (2002) 157. [10] J. Han, S.-G. Kim, S.P. Yoon, S.W. Nam, T.-H. Lim, I-H. Oh, S.-A. Hong, H.C. Lim, J. Power Sources 106 (2002) 153. [11] S.-G. Kim, S.P. Yoon, J. Han, S.W. Nam, T.-H. Lim, S.-A. Hong, H.C. Lim, J. Power Sources 112 (2002) 109. [12] T. Ohzuku, A. Ueda, J. Electrochem. Soc. 144 (1997) 2780. [13] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558. [14] Y.M. Chiang, Y.I. Jang, H. Wang, B. Huang, D.R. Sodoway, P. Ye, J. Electrochem. Soc. 145 (1998) 887. [15] R.J. Gummow, D.C. Liles, M.M. Thackeray, Mater. Res. Bull. 28 (1993) 235. [16] K. Yamada, I. Uchida, Chem. Lett. (1994) 299. [17] B.Y. Yang, K.Y. Kim, Electrochim. Acta 43 (1998) 3343. [18] B.Y. Yang, K.Y. Kim, Electrochim. Acta 43 (1999) 2227. [19] S.T. Kuk, Y.S. Song, S.I. Suh, J.Y. Kim, K. Kim, J. Mater. Chem. 11 (2001) 630. [20] L. Mendoza, V. Alvin, M. Cassir, A. Galtayries, J. Electroanal. Chem. 548 (2003) 95. [21] C. Belhomme, Ph.D. Thesis, University of Paris VI, 2000. [22] B. Garcia, J. Farcy, J.P. Pereira-Ramos, J. Electrochem. Soc. 144 (1997) 1179. [23] E. Rossen, J.N. Reimers, J.R. Dahn, Solid State Ionics 62 (1993) 53. [24] W. Huang, R. Frech, Solid State Ionics 86–88 (1996) 395. [25] P. Tomcyck, J. Wyrwa, M. Mosialek, J. Electroanal.Chem. 463 (1999) 78. [26] Y.S. Yacisi, J.R. Selman, Solid State Ionics 124 (1999) 149. [27] K.-I. Ota, Y. Takeishi, S. Shibata, H. Yoshikate, N. Kamiya, J. Electrochem. Soc. 142 (1995) 3222. [28] W.L. Smith, A.D. Hobson, Acta Cryst. B29 (1973) 362. [29] Y. Shao-Horn, S.A. Hackney, A.J. Kahaian, M.M. Thackeray, J. Solid State Chem. 168 (2002) 60. [30] A. Lundblab, B. Bergman, Solid State Ionics 96 (1997) 173. [31] A. Lundblab, B. Bergman, Solid State Ionics 96 (1997) 183. [32] M.M. Thackeray, J. Electrochem. Soc. 142 (1995) 2558. [33] Y.M. Chiang, Y.I. Jang, H. Wang, B. Huang, D.R. Sodoway, P. Ye, J. Electrochem. Soc. 145 (1998) 887. [34] C. Lagergen, A. Lundblad, B. Bergman, J. Electrochem. Soc. 141 (1994) 3429. [35] K. Adhikary, M. Takahashi, S. Kikkawa, Mater. Res. Bull. 33 (1998) 1845.
Raman evidence of the formation of LT-LiCoO2 thin layers on NiO in molten carbonate at 6508C L. Mendozaa, R. Baddour-Hadjeanb,*, M. Cassira, J.P. Pereira-Ramosc a
Ecole Nationale Supe´rieure de Chimie de Paris, Laboratoire d’Electrochimie et de Chimie Analytique (UMR 7575 CNRS), 11 rue Pierre et Marie Curie, 75231 Paris Cedex 05, France b LADIR, CNRS UMR 7075, 2 rue Henri Dunant, 94320 Thiais, France c LECSO, CNRS UMR 7582,2 rue Henri Dunant, 94320 Thiais, France Received 3 October 2003; received in revised form 22 October 2003; accepted 22 October 2003
Abstract The structural evolution of thin layers of Co3O4 elaborated on nickel-based substrates in the Li2CO3–Na2CO3 carbonate eutectic at 650 8C as a function of time immersion is reported. Raman microspectrometry has been applied in order to provide more information on the nature of the protective cobalt oxide layers. The typical Raman fingerprint of the LT-LiCoO2 compound has been obtained, with four well defined bands at 449, 484, 590 and 605 cm1, while XRD data are unable to distinguish the layered phase (HT) from the spinel one (LT). The mechanical stability of such films does not exceed 10 h in direct contact with the molten carbonate bulk at 650 8C; nevertheless, these conditions are much more corrosive than in a molten carbonate fuel cell (MCFC). # 2003 Elsevier B.V. All rights reserved. Keywords: LT-LiCoO2 coating; Molten carbonate; Fuel cell; Raman spectroscopy; Surface analysis
1. Introduction The molten carbonate fuel cell (MCFC) performance and lifetime are greatly dependent on the slow corrosion, dissolution and precipitation of LixNi1xO formed on the nickel cathode. The substitution or protection of this cathode is one of the key problems in order to ensure the technological and commercial development of this high temperature fuel cell [1–3]. One of the solutions is the elaboration of NiO-based materials containing other oxides less soluble and
*
Corresponding author. Tel.: þ33-1-49781155; fax: þ33-1-49781323. E-mail address: [email protected] (R. Baddour-Hadjean).
with electrochemical performances close to that of LixNi1xO, this allows to combine the high conductivity and the good electrocatalytic properties of LixNi1xO with the stability of protective coatings. Among the compounds NiO–MO, LiCoO2, less soluble in carbonate melts and with acceptable electrochemical properties, appears as the best candidate [4]. Indeed, the use LiCoO2 as a thin protective layer is of particular interest due to a better mechanical resistance and low cost [4–11]. Furthermore, LiCoO2 has also been extensively studied as a cathode material for rechargeable Li-ion batteries [12–15]. Very few works have been dedicated to the in situ formation of LiCoO2 in molten carbonates and its structural characterisation. Yamada and Uchida have obtained LiCoO2 from the oxidation of metallic cobalt depos-
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.10.026
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
357
ited on a gold substrate in Li2CO3–K2CO3 at 650 8C [16]. Yang and Kim have investigated the oxidation behaviours of Ni–Co alloys in the same melt and demonstrated the formation of LiCoO2 from open circuit potential measurements and X-ray diffraction experiments [17,18]. Recently, the high temperature LiCoO2 phase characterised by an hexagonal symmetry has been detected by Raman spectroscopy on dense or porous nickel samples after the oxidation of Co also in the same molten carbonate eutectic at 650 8C [19]. In a recent work, thin layers of Co3O4 were elaborated on nickel-based substrates in order to protect the nickel cathode, i.e. by lowering its dissolution at 650 8C in Li2CO3–Na2CO3, one of the second generation MCFC electrolyte. The technique proposed consists in an electrochemical potentiostatic deposition which has been proved to be an efficient, cheap and low cost method [20]. The purpose of the present paper is to provide more information on the nature and stability of cobalt oxides layers when exposed to a second-generation electrolyte, Li2CO3–Na2CO3 at 650 8C. Here our interest is to get a local picture of the structural changes taking place at the cathode interface using Raman spectroscopy, these data being completed by XRD experiments.
lysis is performed at constant potential of 0.50 versus SCE. The electrolysis time was 8 h and the solution was magnetically stirred during electrodepositing. A platinum foil counter electrode (1 cm2) and a SCE (Hg|Hg2Cl2) reference electrode in a separate compartment filled with 0.5 mol l1 NaNO3 were used for the deposition. Brown deposits were obtained. After aqueous electrodepositing, the films were rinsed with de-ionised water. For either structural characterisation or oxidation in molten carbonates, the samples were annealed during 4 h in air at 500 8C.
2. Experimental
2.3. Characterisation techniques
2.1. Electrochemical deposition
The Raman spectra were recorded at room temperature using micro-Raman system with a Dilor XY spectrometer including charge coupled device (CCD) detector. An argon ion laser (514.5 nm) was used as the excitation source. The spectra were measured in back-scattering geometry. The resolution was about 1 cm1. A 50X objective was used to focus the laser light on sample surface to a spot size of 5 mm2 and the laser power was kept to 1 mW to avoid any degradation of the electrode. XRD experiments were carried out with a Siemens D5000 type diffractometer using a Co Ka1 radiation ˚ ) and a back graphite monochromator. (l ¼ 1:789 A The diffraction pattern was scanned by steps of 0.028 (2y) with a fixed counting time (2.3 s) between 168 and 808. The composition of the deposits was examined with energy-dispersive spectroscopy (EDS).
Cobalt oxide films were formed by potentiostatic electrolysis at 25 8C on 10 mm 10 mm 0:5 mm nickel foils after oxidation (Johnson Mathey, of analytical purity 99.994%) [20]. The oxidation treatment of the nickel foils consisted in their immersion in the molten carbonate at 650 8C during 2 h [21]. The deposition was performed in a four-compartment Tacussel glass cell with 40 ml of a 0.1 mol l1 Co(II) solution prepared from Co(NO3)26H2O (Fluka, with a purity of 99.99% analytical reagent grade chemical) in a 0.5 mol l1 solution of NaNO3 (Merck) with a pH of 4. The solution was de-aerated for 30 min prior to electrochemical deposition, then 4 ml of 1 mol l1 NaOH solution was added to the Co(II) solution and the final pH was 7.40. A blue precipitate of Co(OH)2 was formed. In the second step, an electro-
2.2. Oxidation in molten carbonates at 650 8C The oxidation treatment of the nickel foils recovered by the Co3O4 deposit consisted in their immersion in a cell containing (Li0.52Na0.48)2CO3 eutectic melt (Fluka, of analytical purity 99.9%) at 650 8C contained in an alumina Al23 reactor (250 mm 60 mm) sealed hermetically by a stainless steel cover with a Viton O-ring, saturated with a mixture of air/CO2 70/30 (mol%). The immersion time in the carbonate melt was between 15 min and 48 h. After immersion, the films were rinsed with de-ionised water, and before the morphological and structural characterisation, the films were dried at 100 8C for 1 h.
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
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3.2. Raman results and discussion Fig. 2a shows the Raman spectrum of a nickel foil after in situ oxidation in molten carbonate melts Li2CO3–Na2CO3 (52–48 mol%). A poor signal is observed, with a broad contribution of the NiO vibrations at 400, 560 and 730 cm1, as proved by comparison with a pure nickel oxide sample (Fig. 2b). A further heat treatment of the electrochemical Co3O4 deposit leads to the well defined Co3O4 vibrational fingerprint with Raman bands at 480, 520, 618 and 688 cm1 (Fig. 3b). The oxidation products of the nickel foils recovered by the Co3O4 deposit results have been followed by Raman microspectrometry as a function of the immersion time in the carbonate melt. After a very short time (15 min), it is clear that the Co3O4 fingerprint (Fig. 3b) has practically completely disappeared and a new system is evidenced corresponding to the formation of a LiCoO2 film, as indicated by the four well defined Raman bands observed at 449, 484, 590 and 605 cm1 (Fig. 3c).
0 20
a 30
40
NiO(220)
NiO(200)
NiO(220)
NiO(200)
NiO(111)
Co3 O4 (400)
Co3 O4 (311)
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NiO(111)
LiCoO2 (003)
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Co3 O4 (111)
Fig. 1a shows the XRD pattern of the electrochemical deposit on Ni foil after heat-treatment at 500 8C during 4 h. It reveals the intense lines of the nickel substrate and the less intense lines of NiO, formed at the interface. In addition, one can recognise the weak and broad (1 1 1), (3 1 1) and (4 0 0) lines of the Co3O4 compound, which confirms the electrochemical deposition of the Co3O4 oxide at the surface of the nickel substrate [20]. Further immersion in the molten carbonate melt during 15 min leads to the XRD pattern presented in Fig. 1b. At this stage of immersion time, we observe the disappearance of the Co3O4 diffraction lines and the emergence of the most intense (0 0 3) line of the LiCoO2 compound. The presence of this latter does not allow to discriminate between the LTand HT-LiCoO2. Indeed, determination of the LiCoO2 type can only be obtained from the presence of broad or well resolved peaks for the (0 0 6), (0 1 2) and (0 1 3), (1 2 0) lines. This prompts us to use the Raman spectroscopy which allows to get specific fingerprints for each structure.
Intensity /arb. units
3.1. XRD data
50
60
70
2θ/°
Fig. 1. XRD pattern obtained on a Ni foil recovered by a cobalt oxide deposit: (a) after heat-treatment at 500 8C in air during 4 h; (b) after 15 min of exposure to Li2CO3–Na2CO3 (52–48 mol%) at 650 8C in air/CO2 (70/30).
As mentioned earlier, LiCoO2 is reported to have two closely related structural modifications, layeredtype HT and spinel LT-LiCoO2 [22]. While the HTLiCoO2 phase with an hexagonal unit cell and a layered structure can be described by alternate pure Li and Co planes separated by oxygen layers, the LTLiCoO2 with cubic structure is characterised by a partial disorder between the Liþ and Co3þ ions. The distinction between these two phases by XRD is not easy because their powder pattern are almost identical, as demonstrated by Rossen et al. [23]. Conversely, Raman spectroscopy can be a useful tool to differentiate these two structures [24]. The factor group analysis on the space group R-3m and Fd3m for the HT and LT phases, respectively, predict two and four Raman bands, respectively. Comparison of the Raman spectra of our in situ LiCoO2 film with the conventional HT-LiCoO2 compound is reported in Fig. 4. Two strong Raman bands at 487 and 597 cm1 are observed for the HT-LiCoO2 (Fig. 4b) and four Raman bands are observed at 449, 484, 590 and 605 cm1 for our LT-LiCoO2 (Fig. 4a), in agreement with the assignment of the spinel LTLiCoO2 structure [24]. It is interesting to note that all the LiCoO2 films synthesised in molten carbonate media have been reported as HT compounds with the layered structure [19].
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
500
359
560
Intensity (a.u.)
400
730
a
b
300
400
500
600
700
800
-1
Wavenumber (cm ) Fig. 2. Raman spectra of the (a) Ni/NiO substrate; (b) a NiO pure sample.
For longer immersion times, the evolution of the Raman spectrum is illustrated in Fig. 5. The typical response of LT-LiCoO2 is maintained up to 6 h of immersion (Fig. 5a–c). However, the improved resolution of the overall Raman spectrum suggests an increase of the crystallisation with time. Beyond 590 605 484
Intensity (a.u.)
449
c 688
480 520 618
b a
300
400
500
600
700
800
-1
Wavenumber (cm )
Fig. 3. Raman spectra of (a) Ni/NiO substrate; (b) the heat-treated cobalt oxide deposit; (c) the oxidised deposit in molten carbonate electrolyte after 15 min immersion time.
12 h of immersion, the loss of the Raman LiCoO2 fingerprint (Fig. 5d) probably indicates the LT-LiCoO2 film is not mechanically stable. An EDS analysis confirms a significant loss of the cobalt content on the substrate, which decreases by 80% after 48 h of time immersion. The disappearance of the (0 0 3) XRD line is in accordance with the loss of LiCoO2 film. In fact, the Co3O4 electrochemical deposit encounters a chemical transformation leading to LiCoO2 formation according to this reaction [25]. Co3 O4 þ 3Liþ þ 2CO3 2 ! 3LiCoO2 þ 2CO2 þ e In the molten carbonate melt at 650 8C, a growth and crystallisation process of the grains takes place. Such a reaction induces an important volume expansion, as suggested by the very different density values: 6.11 and 2.5 (from [21]) for Co3O4 and LiCoO2, respectively, which can account for the poor mechanical stability of the deposit after chemical transformation. This phenomenon has already been observed, by Belhomme [21] and Yacisi and Selman [26] after in situ oxidation of metallic Co in LiCoO2. This effect is probably responsible for the loss of the mechanical
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L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
605 590
Intensity (a.u.)
449
484
a
597 487
b
300
400
500
600
700
800
-1
Wavenumber (cm ) Fig. 4. Raman spectra of (a) our LiCoO2 film; (b) HT-LiCoO2.
adherence, which was evidenced by Raman experiments showing the disappearance of LiCoO2. However, a dissolution process of LiCoO2 in molten carbonate melt cannot be discarded [4,27]: LiCoO2 þ 32 CO2 ! Liþ þ Co2þ þ 32 CO3 2 þ 14 O2
The spinel phase of LiCoO2 (LT-LiCoO2) is obtained in the present experiments due to the low synthesis temperature and to the crystallographic phase of the precursor, where structural filiations take place between the electrodeposited Co3O4 and the LT-LiCoO2 formed in the molten carbonate melt at 650 8C. In fact, the 590 605
484
Intensity (a.u.)
449 a
b
c d 300
400
500
600
700
800
Wavenumber (a.u.) Fig. 5. Raman evolution as a function of immersion time in molten carbonate electrolyte: (a) 15 min; (b) 1 h; (c) 6 h; (d) 12 h and more.
L. Mendoza et al. / Applied Surface Science 225 (2004) 356–361
Co3O4 and LT-LiCoO2 have very similar cubic lattice parameters (a), 0.8085 and 0.7995 nm, respectively [28,29]. Both compounds belong to the same space group: Fd3m. Recently, Shao-Horn et al. [29] have obtained the LT-LiCoO2 from the solid-state reaction of Co3O4 with Li2CO3 at 400 8C. Adhikary et al. [35] have used the same precursor at 300 8C in order to prepare the LT-LiCoO2 without traces of the high temperature phase. Conversely, when the synthesis temperature is high enough (more than 800 8C) and the precursors are: CoCO3 and Li2CO3 or Li2O [30,31], LiOH and CoCO3 [32,33] or Li2CO3 and CoCO3 [34], the HT-LiCoO2 is obtained.
4. Conclusion In this study, the structural characterisation of Co3O4 thin layers on nickel substrate was performed after direct exposure to the molten carbonate bulk under the usual MCFC cathode conditions. XRD experiments allowed to check the oxidation process of Co3O4 into LiCoO2 without indication on the cubic (LT) or hexagonal (HT) structure. Using Raman microspectrometry, it was possible to give an experimental evidence of the formation of a LT-LiCoO2 film, as indicated by the four well defined Raman bands observed at 449, 484, 590 and 605 cm1. The formation of LT-LiCoO2 thin films in molten carbonate at 650 8C has never been reported before in the literature; however, under the specific conditions used in this work, the mechanical stability of the cobalt oxide film did not exceed 12 h at 650 8C. These results are encouraging with respect to the protective role of LT-LiCoO2, if we consider that in a MCFC device, the molten carbonate electrolyte being immobilised in a solid inert structure of LiAlO2, the corrosive conditions are significantly less severe in comparison with the experiments performed directly in the bulk.
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