10000

Operating pressure: Atmosphere, Current density: 150mA/ cm 2 Anode Gas Composition: H2/C02=80/20(50 C Humidity)

cells were the same as the previous cells used for the accelerated tests with the exception of the matrix thickness. The y -LiAIOî powder used for the matrices in these cells lias a purity of over 98% ( a content < 2%) and a sub-micron particle diameter. After disassembling the cells that were operated, particle diameter and phase composition of the LiAIOa were examined with a scanning electron microscope (SEM) and an x-ray microdiffractometer. Both examinations were performed at three positions in the cross section of the matrix ( near the cathode, at the middle layer, and near the anode) in each cell. The collimator with a 50 u m 0 pin hole was used for the x-ray microdiffractmeter measurement.

RESULTS AND DISCUSSIONS The optimum thickness and IR loss of the matrix

Optimum thickness is the minimum thickness necessaiy for operating cells for 40000 hours without shorting in this paper. Researchers reported that the shorting time is approximately proportional to the second power of the matrix thickness and the reciprocal of the cathode CCh partial pressure (2). as shown in Eq. [1],

A of Eq. [1] is the coefficient which depends on the cell specifications. The calculated number of A for various cells is shown in Table 1. The optimum thickness of the matrix can be estimated with Eq. [1] for various operating conditions. Fig. 1 shows the calculated optimum thickness. The Pco2_avg was assumed to be 0.5 atm in a 5 atm pressurized conditioa These results show that the improvements of electrolyte and matrix stmcture are veiy effective for the reduction of F'S-1 Calculated results of optimum thickness and IR loss of matrices matrix thickness for 40000 hours of operation without Ni-shorting (Pco2_avg=0.5atm)

The internal resistance (IR) loss of a matrix can be calculated using Archie's equation shown in Eq. [2] (7).

Electrolyte

Li/K

Li/Na

Li/Na

Li/Na

Matrix Tvpe

Conventional

Conventional

Imoroved

Advanced

(Electrolyte volume fraction') • (Matrix thickness / cm) (Electrolyte conductivity / ( Q -cm) )

The calculated IR loss of the endurable matrices over 40000 hours is also shown in Fig. 1. The empirical number was used as the coefficient B (8), and the current density was 150 mA/cm 2 . As seen in Fig. 1, the IR loss of a matrix with Li/Na electrolyte is below half of that of one with Li/K electrolyte. This is caused by the thinner matrix and high conductivity of Li/Na. In addition, the IR loss of the improved matrix is lower than that of a conventional matrix. The advanced matrix which is shown in Fig. 1 is now being developed to achieve a further reduction of the resistance. This is because the advanced matrix has a higher porosity than the others. From these results, it is expected that the pressurized operation of the MCFC becomes possible with the improvements of the electrolyte composition and the matrix structure, even if the nickel oxide cathode is used.

In-cell stability of theLiAlOi

Fig. 2 shows the results of post test analyses of the maximum particle diameters of LiAICh at different positions in the cross section of the matrix. LiAICk particles grew near the cathode with passage of time. On the other hand, fine particles were kept at the middle layer even in the cell operated for 20000 hours. There was little influence brought about by differences in the electrolyte and cathode gases on particle growth, although it was reported that particle growth was influenced by the basicity of electrolyte (5).

Fig. 3 shows the a -LiA102 content as determined by x-ray diffraction. The transformation of y to a was remaikable near the cathode where the particle growth was accelerated. These results suggest that there is a relation between phase transformation and particle growth. It was observed that the particle growth was accelerated in out-of-cell tests, when the packing of the matrix was looser (9). This particle growth might be caused by the decrease in packing density due to the phase transformation.

Fig. 4 shows die changes in internal resistance and gas cross-over in the cell operated for 20000 hours. The amount of cross-over didn't increase. This is because of the fine pore layer in the middle of matrix layer which retained electrolyte. However, the resistance increased gradually after 16000 hours. Particle growth causes the cell resistance to increase due to the lowering of electrolyte retention These results suggest that it is necessary to take some action to inhibit particle growth for longer operation.

0 0

Post a comment