Development Of A New Electrolyte Matrix For Mcfc

I. Nagashima.K. Higaki, S. Terada, and T. Suemitsu i

Akashi Technical Institute, Kawasaki Heavy Industries, Ltd. 1-1, Kawasaki-cho, Akashi, 673 Japan

Introduction

To prolong the life of cell is one of the most important issues for MCFC to be brought into actual application. In this respect, investigators have been proposing the addition of tungstate salt such as K2W04 into MCFC electrolyte, which is supposed effectively to reduce the sintering of anode probably by precipitates formed through the reduction of tungstate with dissolved hydrogen near the anode surface (1).

In this research, such effect upon sintering of anode was quantitatively examined by out-of-cell tests and the validity of above assumption for the mechanism was confirmed. Also other effects of tungstate salt addition into electrolyte, such upon corrosion of separator, solubility of cathode, stability of matrix substrates (LiA102) were investigated.

Effect on sintering of anode

Fig.l is an example of the results of 100cm2 class cell operating test using pure Ni anode with 1.4 weight % K2W04 addition into electrolyte, which shows more than 10,000 hours operation was possible (1), although 2 to 3 thousands hours operation, at most, was possible in case of no addition of tungstate salt due to sintering of pure Ni anode. Fig.2 is an example of pore volume change of pure Ni anode using electrolyte with/without tungstate salt addition, which was obtained by simulated cell operation with 10cm2 effective electrode area. It shows that tungstate salt addition into electrolyte has some effect of restricting the microstructural change of the anode. Fig.3, obtained by an EPMA analysis, shows the distribution of W concentration across the anode and the electrolyte matrix after 10,000 hours cell operation. Clearly the concentration of W at the anode portion is higher than that at the matrix portion. This means that tungstate ion ,W042", converts into some other insoluble compounds at the anode portion as a sink and the diffusion of W042" ion from the matrix portion to the anode portion occurs. Such compound of W is supposed to be formed through the reduction of W042" ion by dissolved hydrogen in the electrolyte and to precipitate around the anode particles, accordingly to restrict the sintering of Ni anode.

Effect on the corrosiveness of electrolyte

In general, oxi-anions including carbonate ion tend to dissociate oxide ion, O2", according to the ambient condition and to affect the basisty of the melts. Tungstate ion, W042\ is supposed to act similarly and possibly to have some effects on corrosion of separator material by melts or cathode (NiO) dissolution in melts. Fig.4 shows the solubility of NiO vs partial pressure of C02 in Li/K carbonate system and Li/Na carbonate system both with and without K2W04 addition and shows that the effect of K2W04 addition on the solubility of NiO is small. On the other hand, as shown in Fig.5, results of corrosion test using SUS316L test pieces at cathode ambient shows that K2W04 addition has some effect on the corrosiveness of the melt and minimum corrosion was attained at 1.4 wt% K2W04 addition. These could not be explained merely by the basisty of the melt and further investigation is required.

Stability of matrix substrate (LiA102)

Phase transition and particle growth of LiA102 particle, observed during longer cell operation, is a serious problem to overcome in order to attain more than 40,000 hours of life of MCFC for it causes pore coarsening of porous matrix substrate and less capillary force to retain the electrolyte in its pores (2). Out-of-cell tests, in which LiA102 powders were impregnated in melts, were conducted and the effect of various parameters on phase transition and particle growth of LiA102, including tungstate salt addition into melt, were investigated. Fig.6 is an example of phase transition of LiA102 at C02 ambient and shows that tungstate salt addition has some influence on the phase stability of LiA102. In case of no addition of tungstate salt, increase of a phase and decrease of ,3 phase and y phase were remarkable, whereas such phase transition were small and especially slight increase of y phase was observed in case of tungstate salt addition. These mechanism of phase transition and particle growth, including the effects of additives other than tungstate salt, are now under investigation.

Conclusion

Effects of tungstate salt addition into MCFC electrolyte were investigated by out-of-cell tests. Anode sintering was reduced by tungstate salt addition, which was supposed to originate in the precipitation of W containing compounds around the anode particles formed by the reduction of tungstate by dissolving hydrogen. Solubility of cathode (NiO) was not affected by tungstate salt addition but the corrosion of stainless steel was minimum at 1.4 wt% addition. Further, phase transition of LiAI02 was reduced by tungstate salt addition.

Acknowledgement

This work was conducted under a contract from NEDO (New Energy and Industrial Technology Development Organization) and MCFC Research Association as a part of the New Sunshine Program of MTTI (Ministry of International Trade and Industry). We appreciate their advice and support.

Reference

1.Terada et a!., "Development of a New Electrolyte Matrix for MCFC", Fuel Cell Seminar Program and abstracts, ppl28-131, Nov.28-Dec.l, (1994)

2.Tanimoto et al., "Long term Operation of 100 cm2 class Single Cell of MCFC", Proc. 2nd IFCC, pp207-210, Fev.5-8, (1996)

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