Oxide electrodes in molten carbonates Part 2. Electrochemical behaviour of cobalt in molten Li + K and Na + K carbonate eutectics

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Journal of Electroanalytical Chemistry 391 (1995) 133-139

Oxide electrodes in molten carbonates Part 2. Electrochemical behaviour of cobalt in molten Li + K and Na + K carbonate eutectics P. Tomczyk 1, H. Sato, K. Yamada, T. Nishina, I. Uchida * Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Aramaki-Aoba, Sendai 980, Japan Received 18 October 1994; in revised form 30 January 1995

Abstract Spontaneous electrochemical processes occurring on Co and CoO in molten Li + K and Na + K carbonate eutectics saturated with a 0.9 02 + 0.1 CO 2 atmosphere were investigated at 1000 K. It was shown that oxidation of Co to CoO proceeds via formation of an unstable compound in Li2CO 3 + K2CO 3. There is no indication that such a compound is formed in Na2CO 3 + K2CO 3. CoO undergoes further oxidation in molten carbonates producing LiCoO 2 in Li2CO 3 + K2CO 3 and NaCoO 2 (presumably) in Na2CO 3 + K2CO 3. The oxidation processes increase the active area of the electrode as indicated by the measurements of exchange current densities, double-layer capacitances and impedances of electrodes. Keywords: Cobalt oxidation; Molten carbonates; Oxygen reduction

1. Introduction Dissolution of a lithiated NiO cathode is one of the major life-limiting factors of a molten carbonate fuel cell (MCFC) delaying commercial application of the device (see, for example, Refs. [1-4]) Therefore, extensive research aimed at replacing NiO with more suitable cathodic materials has been carried out [3-13]. From among the numerous candidates as perovskite-type compounds and mixed metal oxides, only LiCoO 2 exhibits a favourable performance and a dissolution rate considerably lower compared with that of NiO. These advantages of porous LiCoO 2 cathodes (formed by means of tape-casting and sintering of synthesised material) were verified during tests of a small-size laboratory MCFC [11-13]. In the present work, the spontaneous oxidation of Co in a molten Li + K carbonate eutectic was investigated. This process resulted in the formation of LiCoO 2 [14]. Then the oxygen reduction at these electrodes was studied. The

* Corresponding author. 1 On leave from: Institute of Physical Chemistry of the Polish Academy of Sciences, Molten Salts Laboratory, ul. Zagrody 13, 30-318 Cracow, Poland. 0022-0728/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved

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electrochemical behaviour of Co was also investigated in molten carbonate electrolyte which does not contain Li, namely in a Na2CO 3 + K2CO 3 eutectic.

2. Experimental Three types of smooth (ensuring semi-infinite diffusion) indicator electrode were employed in the investigations. (i) Made by electrodeposition of a Co thin layer (0.05-1.0 /xm) on a Au flag and oxidized in molten carbonates. These electrodes are further denoted as A u / C o (a notation A u / N i used in this article refers to the Ni thin layer electrodes investigated in Part 1 [15]). Co was electroplated from 0.38 to 0.44 M cobalt chloride aqueous solution, then the electrodes were dried and used immediately in the experiments. (ii) Made from thin plates of monocrystalline CoO (pure and doped ex situ with 2 at.% Li). These electrodes are denoted hereafter as [COO]me (the notation [NiO]mc refers in this article to the NiO monocrystalline electrodes investigated previously [15]). (iii) Gold flag-type electrodes, for some complementary studies.

P. Tomczyk et al. /Journal of Electroanalytical Chemistry 391 (1995) 133-139

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Construction, dimensions and experimental procedures applied to the electrodes were identical with those described in Part 1 [15]. The electrodes were investigated in two molten carbonate electrolytes: 62 mol.% Li2CO 3 + 38 mol.% K2CO 3 and 56 mol.% Na2CO 3 -t- 44 mol.% K2CO 3 saturated with a 0.9 0 2 + 0.1 CO 2 atmosphere at 1000 K. All the potentials in this work were measured with respect to the standard Au 10.33 0 2 + 0.67 CO 2 gas reference electrode. The melt purification, gas handling system, electrochemical cell, procedures of the measurements, experimental techniques and electronic equipment were the same as used previously [15].

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Fig. 1. Dependence of the open circuit potential E of ( O ) A u / C o and (solid line) Au electrodes in a molten Li + K carbonate eutectic on the immersion time t. The simultaneously measured series resistance R s of ( O ) A u / C o and ( - - - ) Au electrodes are also presented on the graph. Thickness of the Co layer at the A u / C o electrode, 1 /~m.

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Fig. 2. Dependences of the open circuit potential E and series resistance R S of the A u / C o and Au electrodes in the molten N a + K carbonate eutectic on the time t which elapsed since the electrode immersion. Symbols as in Fig. 1. Thickness of Co layer at the A u / C o electrode, 1 /xm.

ently higher in a more acidic Na + K carbonate eutectic. There is also distinct behaviour differences of the [NiO]mc and [COO]me electrodes. Unlike [NiO]m~, the potential of [CoO]me electrode does not stabilise at that value attributed to the oxygen electrode ( - 0 . 0 6 V) but decreases abruptly to - 0 . 2 5 and - 0 . 1 5 V in molten Li2CO 3 4- K2CO 3 and Na2CO 3 + K2CO3, respectively (Fig. 3). The OCP plateaus at these potentials occur also for the A u / C o electrodes as can be seen in Figs. 1 and 2. No such wave has been observed for the A u / N i electrodes (Figs. 1 - 4 in Part 1 [15]). This finding allows one to conclude that CoO undergoes further electrochemical reaction in molten carbonates which is not the case for NiO. The product of this reaction was identified by X-ray diffraction (XRD) to be LiCoO 2 for the electrodes immersed in the molten Li + K carbonate eutectic [14].

3. Results and discussion 0

3.1. Oxidation of Co in molten carbonates

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Typical decays of the open circuit potential (OCP) recorded for the A u / C o and Au electrodes immersed in the molten Li + K and Na + K carbonate eutectics are shown in Figs. 1 and 2, respectively. Similar OCP data for the [CoO]me and Au electrodes are presented in Fig. 3. It seems reasonable to initiate the discussion of the plots seeking qualitative analogies and distinctions between the electrochemical behaviour of Ni [15] and Co in both melts investigated. The most striking similarity is the occurrence of a distinct OCP plateau at ca. - 0 . 4 V for the A u / N i and A u / C o electrodes in Li2CO 3 + K2CO 3 and the absence of such a potential arrest in N a 2 C O 3 -I- K 2 C O 3. The other is the rate of Ni and Co oxidation which is appar-

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t /s Fig. 3. Dependence of the open circuit potential E measured at the [CoO]me electrode on the time t which elapsed since the electrode immersion. (C), O ) Pure CoO and ( n , i ) CoO ex situ doped with 2 at.% Li. The open and closed symbols correspond to the data obtained with a LizCO 3 +K2CO 3 and Na2CO 3 +K2CO 3 melt, respectively.

135

P. Tomczyk et al. /Journal of Electroanalytical Chemistry 391 (1995) 133-139

To reveal the overall mechanism of the processes occurring at the A u / C o electrodes, it is helpful to compare the potentials of OCP plateaus with the standard electrode potentials calculated from the thermodynamic data and the potential of the oxygen electrode (Eqs. (1) and (2) in Part 1 [15], respectively). Such a comparison is presented in Table 1. According to the small difference between the experimental and calculated values, the cathodic process at approx. - 0 . 8 V can be attributed to the direct oxidation of the cobalt layer at the A u / C o electrode CO "4- C O 2 - ----}C o O q- C O 2 -~- 2 e -

(1)

Process (1) is complete in Na2CO 3 q - K E C O 3 whereas a competitive formation of an intermediate prevails in Li2CO 3 -t- K 2 C O 3. To identify the intermediate one should take into consideration those Co compounds which may be produced under the experimental conditions. The formation of CoCO 3 seems to be probable as the comparison of the calculated and experimental values in Table 1 indicates. There is also some probability concerning the formation of C 0 2 0 3. Although no thermodynamic data about this compound is available in the literature, it can be anticipated that the standard Gibbs energy change of C 0 2 0 3 formation may be similar to that of N i 2 0 3 [16]. Such an assumption yields a standard electrode potential equal to approx. - 0 . 4 V, the value of the OCP plateau in the Li2CO 3 + K2CO 3 melt. Therefore, the following processes of Co oxidation in the molten Li + K carbonate eutectic can be proposed Co + CO 2- ~ CoCO 3 + 2 e -

(2)

CoCO 3 ~ CoO + CO 2

(3)

or

(4) (5)

2Co + 3CO 2- ---}C o 2 0 3 + 3CO 2 + 6 e C 0 2 0 3 ~ C o --t- C o O + 0 2

The objections concerning reliability of the calculated standard electrode potential of CoCO 3 formation and kinetic restrictions for reaction path (4) + (5) remain essentially the same as in the cases of NiCO 3 and Ni203, respectively [15]. When the potential of the A u / C o electrode was kept at V in the Na + K carbonate eutectic and at - 0 . 4 V in the Li + K carbonate eutectic for a prolonged time and XRD was then used to identify the compounds formed, no other products but CoO were found at the surface of electrodes [14]. The postulated reaction paths (1), (2) + (3) and ( 4 ) + (5) are consistent with this indication. Taking into account that LiCoO 2 was detected by XRD at the A u / C o electrode oxidized in Li2CO 3 + K 2 C O 3 at the oxygen electrode potential ( - 0.06 V), it can be concluded that the OCP plateau at approx. - 0 . 2 5 V can be assigned to the electrochemical reaction - 0 . 8

CoO + L i + + C O 2 - ~ LiCoO 2 + CO 2 "[- e -

(6)

and the OCP plateau observed at - 0 . 1 5 V in Na2CO 3 + K2CO 3 presumably to CoO + N a + + CO2- --* NaCoO 2 + CO 2 + e -

(7)

When the A u / C o electrode was withdrawn from the molten Na + K carbonate eutectic after the experiment, its surface was bright gold and no trace of any Co compound was detected there by XRD. That shows that the product of reaction (7) is very soluble in the melt. Unfortunately, because of the dissolution phenomenon the product of reaction (7) could not be identified unlike the stable LiCoO2 formed in reaction (6). Therefore, the formation of NaCoO 2 was assumed by analogy with reaction path (6). This dissolution phenomenon limited the testing of the [COO]me electrode in a Na2CO 3 + K 2 C O 3 melt to less than 2 h. Afterwards, the CoO monocrystal and gold-wire joint was damaged and the monocrystalline plate fell down to the bottom of the crucible.

Table 1 Potential plateaus observed at the experimental OCP curves for the Au/Co, [CoO]me and Au electrodes in molten carbonates and the theoretically predicted electrode potentialscalculatedvs. the 0.33 02 + 0.67 CO2 electrode from the thermodynamicdata and Nemst equation Experimental Calculated Electrode

E/ V (Li + K)2CO 3

E/ V (Na q" K)2CO 3

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- 0.85

- 0.75

-0.40 - 0.25 -

[CoO]me

- 0.05 - 0.25

Au

- 0.05

-

-0.15 - 0.05 -

-0.15 - 0.05

R e a c t i o n

Co+CO 2- ---}CoO + CO2 + 2 e3 Co + 4 CO2- ---}CoaO4 + 4 CO 2 + 8 eCo + CO2- ---}CoCOa + 2 eCoO + Li++ CO2- ---}LiCoO2 + CO2 + eCoO + Na ÷ + CO2- ---}NaCoO2 + CO2 + e1/2 02 + CO2 + 2 e - ~ CO2CoO + Li++ CO2- ~ LiCoO2 + CO2 + eCoO + Na + + CO2- ---,NaCoO2 + CO2 + e1/2 02 + CO2 + 2 e - ~ CO2-

a AG O has been estimated accordingto the data in Ref. [24].

b Calculatedfor the experimentalconditions: p(O 2) = 0.9 atm, p(CO 2) = 0.1, T = 1000 K.

E/ V

0.85 a 0.68 a - 0.35 a -

- 0.06 b - 0.06 b

P. Tomczyk et al. /Journal of Electroanalytical Chemistry 391 (1995) 133-139

136

Additional information about the compounds formed during the Co oxidation in molten carbonates can be deduced by examining the impedance of A u / C o electrodes, plotted together with the OCP decay in Figs. 1 and 2. As can be seen, the impedances of A u / C o electrodes are generally much lower during the whole oxidation process than the impedance of the Au electrode. Taking into account that the geometries of A u / C o and Au electrodes are identical, one can state that: (1) the electrochemically active area of the A u / C o electrode increases during oxidation of Co in molten carbonates; and (2) the chemical compounds thus produced are electrically good conductors. Conclusion (2) is justified by the fact that incorporation of an alkali metal into the CoO lattice structure enhances the conductivity of such formed compounds, i.e. CoO + Li20 solid solutions and LiCoO 2 [17,18]. The increase of the electrochemically active area of the A u / C o electrode during the oxidation in molten carbonates is likely to be caused by a rough structure of the compounds produced in reactions (1)-(7), which is also the case of the A u / N i electrodes. The previously mentioned dissolution of the product of reaction (7) also influences the impedance of the A u / C o electrode. In Fig. 2, this is confirmed by the increase in the electrode impedance during immersion times longer than 250 s. The distinction between the character of Ni and Co oxidation in molten carbonates can also be established by the comparison of the oxidation rates. For this purpose, the concept of transition time for the OCP decay has been introduced as previously [15,19]. The dependence of the transition time ~- so defined on the thickness of the Co layer l for the A u / C o electrodes immersed in the Li2CO 3 + K2CO 3 and Na2CO 3 + K2CO 3 melts are presented

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that obey the parabolic rate law, a linear dependence of ~on l is noted in all processes occurring at the A u / C o electrodes. The only deviation from such a behaviour can be seen in the case of the last process in the molten Na + K carbonate eutectic (Fig. 5). It may be assigned to the already described dissolution phenomenon. 3.2. Oxygen reduction at cobalt oxide electrodes The electrodes under study were first studied with linear scan voltammetry (LSV) to provide evidence for the electrode processes over the whole potential range available. Then, the coulostatic relaxation method (CR) was employed to investigate the kinetics of processes at the resting potential of the electrode. These studies were not aimed at determining the oxygen reduction mechanism but their purpose was to investigate phenomena specific for cobalt oxide electrodes. Typical LSV curves obtained at already oxidized A u / Co electrodes in the molten Li + K and Na + K carbonate eutectics are presented in Figs. 6(A) and 6(B), respectively. The voltammetric curves recorded for the A u / C o and A u / N i electrodes in Li2CO 3 + K 2 C O 3 show an apparent parallelism, i.e. similar, additional couples of peaks appear at the "background" usually observed for an Au electrode (these couples are indicated by 4, 4' in Fig. 6(A), and 3, 3' in Fig. 6(A) in Part 1 [15]). In Part 1 [15], couple 3, 3' has been attributed to the reversible surface reduction of either trivalent Ni entities produced during formation of a LixNi(II) l_2xNi(III)xO solid solution or NiCO 3 formed in accordance with equilibrium (6) in Part 1 [15]. Following analogous arguments, the occurrence of peak 4 may also be explained by the phenomena associated with: (a) lithia-

137

P. Tomczyk et al. /Journal of Electroanalytical Chemistry 391 (1995) 133-139

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tion of CoO; or (b) presence of CoCO a at the electrode surface. Considering alternative case (b), it is worth noting that the difference between the potential of peak 3 (approx. - 0 . 3 5 V) and the potential of peak 4 (approx. - 0 . 4 8 V) is approximately the same as the difference between the electrode potentials calculated from the thermodynamic data of NiCO 3 and CoCO 3 formation ( - 0 . 2 5 V and - 0 . 3 5 V, respectively). This seems to indicate that Ni and Co carbonates are involved in the overall reduction at the A u / N i and A u / C o electrodes, respectively. Whereas the presence of NiCO 3 at the A u / N i electrode at small negative overpotentials could be explained by the straight chemical equilibrium (6) in Part 1 [15], more complex oxidation-reduction processes should be considered for the justification of the presence of CoCO 3 at the A u / C o electrode. The elucidation of the voltammetric curves recorded for the A u / C o electrode in Na2CO 3 + K2CO a is even more ambiguous. Predominant peak 5 appears only during the first potential scan and cannot be renewed even after a prolonged immersion of the electrode in the melt. This behaviour and the characteristic spike-like, symmetric shape of the peak indicate the surface character of the electrode process [20]. Either the reduction is irreversible or the product is highly soluble in molten carbonates. The

potential of peak 5 is the same as the potential of the second wave of oxygen reduction at the Au electrode in Na2CO 3 + K2CO 3 [15,21,22]. Therefore, there is some uncertainty whether this response can be attributed to the reduction of either Co compound formed during oxidation in molten carbonates or oxygen entities adsorbed at a modified electrode surface. After completion of the LSV experiments, the CR method was applied to determine the kinetic parameters of oxygen reduction at the resting potential of the electrodes under study. The resting potentials of the thin layer electrodes corresponded to those of the oxygen electrode ( - 0 . 0 6 V) and were approached at the end of the OCP decay. In the case of [COO]me, the potential of the oxygen electrode was not reached even after several hours of electrode immersion. This behaviour could be accounted for by the practically infinite thickness of [COO]me as compared with a thin layer of CoO formed at the A u / C o electrode. Therefore, the CR measurements at the [COO]me electrode in Li2CO 3 + K2CO 3 were performed at ca. - 0 . 2 5 V. No data has been obtained in the Na2CO 3 + K2CO 3 melt because of vigorous dissolution of the CoO monocrystal in the melt that resulted in the damage of the electrode before initiation of the CR experiment. For the reasons discussed previously [15], the exchange currents and double layer capacities were calculated from the CR data with the assumption that two electrons are involved in process 1. Then, the values determined were normalised vs. the geometric areas of the electrodes. They are denoted further in this article by J0 and C d, respectively; J0 and C d determined for the A u / C o , [COO]me and Au electrodes are presented in Figs. 7 and 8, respectively.

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Fig. 7. Dependence of the current exchange density Jo on the thickness l of the Co layer at the Au/Co electrodes ( zx, •). Jo determined for the [CoO]me (U) and Au (0, ©) electrodes are also drawn on the plot. The open and closed symbols correspond to the data obtained with a Li2CO3 + K2CO3 and NazCO3 + K2CO3 melt, respectively.

138

P. Tomczyk et al. /Journal of Electroanalytical Chemistry 391 (1995) 133-139 1000

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(NaCoO 2 presumably) was found to be highly soluble in the melt unlike the LiCoO 2 formed in Li2CO 3 4- K2CO 3. The measurements of exchange current densities, double-layer capacitances and impedances of the electrodes indicate an increase in the electrode active area during the oxidation of Co in molten carbonates.

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One of the authors (PT) wishes to express his gratitude to the Japan Society for the Promotion of Sciences for sponsoring his fellowship at Tohoku University in Sendai where this work was carried out. The authors are very grateful to J. Ob/~kowski, M.Sc., Ing. (Technical University of Mining and Metallurgy, Institute of Material Science in Cracow) for kindly supplying them with CoO monocrystals. This work was also partially sponsored by KBN Research Project No. 2 P303 044 04.

References Examination of Figs.7 and 8 shows that J0 and C d are independent of l for the oxygen reduction in the molten Na + K carbonate eutectic. This effect can be explained by the fact that the CR measurements, performed at the end of the experimental series, were not made with the electrode covered by NaCoO 2 but in fact on bare Au. The rapid dissolution of Co oxides in acidic carbonate melt has already been reported in the literature [23]. In the case of the A u / C o electrodes in the molten Li + K carbonate eutectic, the linear dependences of J0 and C d on l are obeyed. Similar linear dependences have been reported for the A u / N i electrodes [15]. This behaviour has been attributed to the considerable increase of the electrode active area during oxidation, enhanced by the formation of an intermediate Ni compound. However, the oxidation of Co electrodes in Li2CO 3 + K2CO 3 (unlike Ni electrodes) does not end with the formation of CoO and advances to LiCoO 2. This final process (reaction (6)) causes an additional increase of the electrode active area as is indicated by the high values of J0 and C a determined for the [COO]me electrode.

4. Conclusions Spontaneous oxidation of Co in molten carbonates proceeds via either three or two electrochemical steps in the molten L i - t - K and Na + K carbonate eutectics, respectively. Formation of an intermediate during oxidation of Co to CoO is postulated in less acidic carbonate melts analogous to that for the A u / N i electrodes in Part 1. The final product of Co oxidation in Na2CO 3 -t- K2CO 3

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Ota, 34th Battery Syrup., Hiroshima, 1993, Abstract No. 2B05, p. 137. [24] I. Barin, Thermochemical Data of Pure Substances, VCH Verlagsgessellschaft mbH, Weinheim, Germany, 1989.