Physical Properties Of Molten Carbonate Electrolyte

T. Kojima, M. Yanagida, K. Tanimoto, Y. Tamiya, T. Asai, and Y. Miyazaki Osaka National Research Institute Midorigaoka 1-8-31. Ikeda City, Osaka 563, Japan

Introduction , '

Recently many kinds of compositions of molten carbonate electrolyte have been applied to molten carbonate fuel cell in order to avoid the several problems such as corrosion of separator plate and NiO cathode dissolution. Many researchers recognize that the addition of alkaline earth (Ca. Sr. and Ba) carbonate to Li2C03-Na2C03 and Li2C03-K2C03 eutectic electrolytes is effective to avoid these problems1'. On the other hand, one of the corrosion products, Cr042' ion is found to dissolve into electrolyte and accumulated during the long-term MCFC operation2'. This would affect the performance of MCFC.

There, however, are little known data of physical properties of molten carbonate containing alkaline earth carbonates and Cr042'. We report the measured and accumulated data for these molten carbonate of electrical conductivity and surface tension to select favorable composition of molten carbonate electrolytes.

2.EIcctrical conductivity

Electrical conductivity would directly affect the output of MCFC in the form of IR loss. The electrical conductivity of molten carbonate was measured using AC two probe technique3'.

The addition of alkaline earth carbonate resulted in decrease of the conductivity linearly with mole fraction of additives. The linear relationship between mole fraction of alkaline earth carbonate and conductivity makes it easy to calculate the conductivity of ternary mixture of Li-Na-alkaline earth and Li-K-alkaline earth from the data of alkali binary carbonate systems as shown in Fig. 1. as an example for SrC03 addition. These decrease of conductivity by the addition of alkaline earth carbonate may be due to the exchanging conductive alkali cations with less conductive alkaline earth cations, which strongly attract carbonate ions by their double charge.

Molten Li2C03-Na2C03(52:48mol%) added with alkaline earth carbonate by 10 mol% show about 1.3 times higher electrical conductivity than that of conventional molten LinCOj-KjC03 (62:38mol%) electrolyte at 923K. These electrolytes are promising from the viewpoints of not only low solubility of NiO", but also higher conductivity.

The addition of Cr042' to Li2C03-Na2C03(52:48mol%) and Li2C03-K2C03(62:38mol%) by 3 and 5 mol% was found to decrease the electrical conductivity of mixture as shown in Fig.2, and show less influence upon the conductivity of these mixtures than that of alkaline earth carbonate. Because the addition of Cr042' is only exchange less conductive carbonate ion with CrO/\

3.Surfncc tension

Surface tension would have influence upon gas-electrode reaction site and distribution of electrolyte among the cell components. The surface tension of molten carbonate was measured using maximum bubble pressure technique. The addition of alkaline earth carbonates into any of alkali (Li. Na and K) carbonate and their mixture resulted in the increase of surface tension of mixtures as shown in Fig.3. It is also because of double charged small alkaline earth cations attract carbonate ion stronger than alkali cations.

In the case of CrC>42' addition, it makes the surface tension of mixture lower as shown in Fig 4. The reason for the decrease of surface tension of mixtures is that larger CrQf'ion should have weaker coulombic force than carbonate ion.

4.ConcIusion

•The addition of all alkaline earth carbonate to alkali carbonate resulted in increase of the surface tension and decrease of the electrical conductivity of mixtures.

0 The addition of CrO*2' to alkali carbonate resulted in decrease of both surface tension and electrical conductivity of mixtures.

•Molten Li2C03-Na2C03(52:48mol%) added with alkaline earth carbonate is promising from the viewpoint of higher electrical conductivity than conventional molten Li2CC>3-K;C03 (62:38mol%) electrolyte.

5.Refcrcncc

1) K. Tanimoto, Y. Miyazaki, M. Yanagida, S. Tanase. T. Kojima, N. Olitori, H. Okuyama and T. Kodaina, Denki Kagaku, 63,316 (1995).

2)H. Kasai, Y. Hayakawa, H Ukai and S. Yajima. Denki Kagaku, 64. 526 (1996).

3) Y. Miyazaki, M. Yanagida. K. Tanimoto, T. Kodama and S. Tanase. J. Electrochem. Soc.. 133. 1402, (1986).

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