Use Of Ceramic Porous Membranes In Molten Carbonate Fuel Cells

E. Passalacqua, S. Freni, A. Patti and G. Maggio. Institute CNR-TAE, via Saiita S. Lucia sopra Contesse 39, S. Lucia, Messina (ITALY).

Abstract

The diffusion of alkali vapours in the anode compartment of a DIR-MCFC produces the deactivation of the internal reforming catalyst. Sets of ceramic porous membranes purposed to limit the diffusion have been manufactured by different techniques and the influence of the preparation technique and of the preparative variables on the morphological characteristics of the membranes structures has been studied.

.Introduction

The direct internal reforming (DIR-MCFC) is one of the most promising configuration of molten carbonate fuel cells. The tendency of the internal reforming catalyst (Ni/Al203 or Ni/MgO) to react with traces of electrolyte vapours, like KOH, and thus to be poisoned [1-3], limit the complete development of this configuration. Besides of the development of catalysts for internal reforming containing nickel supported on an alkali-resistant support [4,5] or the introduction of a separator plate of Ni/Cu foil, positioned between the catalyst and the anode that permits the hydrogen diffusion but not the passage of carbonate [6], the carbonate transport towards the catalyst can be reduced by interposing, between catalyst and anode a porous ceramic membrane, composed by a wide variety of ceramic materials, like A1203, Zr02, MgO, B4N, SiC, Si02 [7], permeable to hydrogen, that will be a physical limit to the KOH diffusion.

This paper reports some interesting aspects emerging from a wide research carried out on the preparation techniques of the ceramic porous membranes based on material chemically inert to the alkali and thermally stable at the operational temperature of the molten carbonate fuel cells.

Experimental

The techniques of hot-rolling (HR) and tape-casting (TC) were selected to produce the samples of membranes and three different preparation procedures were investigated as a function of the binder and of the technique adopted.

The slurries were produced by dispersing, at room temperature and under stirring, the ceramic powder, plasticizer (polyethylene glycol) and the binder in the solvent. Then the slurries were heated at the solvent evaporation temperature under stirring, until a satisfactory homogeneous dispersion degree was reached. The obtained slips were hot-rolled (T=343 K) or tape-cast to produce green tapes with the desired size and thickness. Following this preparative procedure we have prepared the green tapes with methyl cellulose as binder and aqueous solvent by HR technique (MTC-HR); with MTC and aqueous solvent by TC technique (MTC-TC); and with polyvinylbutyral and non aqueous solvent (trichloroethylene/ethanol) by TC technique (PVB-TC).

Finally the green tapes underwent a thermal treatment cycle at a temperature ranging from 573 K to 1773 K, in an oven under nitrogen flow. The thermal profile of treatment conditions was determined by the thermogravimetric analyses carried out on samples prepared with MTC or PVB as binder by a STA 409 Netzsch simultaneous thermal analyzer in the temperature range between 293 K and 973 K.

The structure of the membrane samples was characterized by measurements of the thickness by means of a thickness gauge and of the hydrogen permeability by applying a pressure drop (Ap) to the sample.

Furthermore, porosimetric analyses were carried out by the mercury intrusion technique (Carlo Erba Porosimeter Mod.2000), in the range between 0.001 jim and 100 pm. From the porosimetric distribution curves, the total pore volume (Vp), the average pore radius (Rav) and the total porosity (EPS) of the samples were measured.

The capability of some ceramic membranes to resist alkali vapour diffusion in the cell operative conditions (923 K) was verified in simulated conditions. Tests of 150 hours were carried out wherein the porous membranes were placed between alkali doped (with a mixture of Li2/K2C03) alumina pellets and undoped one, under a nitrogen flow. At the end of each test, the K content was determined by atomic absorption spectroscopy analysis, carried out on an atomic absorption (AA) Perkin-Elmer model 4000 spectrophotometer.

Results and Discussion

Fig. 1 shows the TG profiles for a sample of 2E-3pm green tape. In the range between 293 K and 413 K, the sample presents a weight loss of 0.3%, attributable to the water evaporation. The burn-out of the material starts at 453 K and it is complete at 743 K, with a weight loss of 10.4%.

The main characteristics of the manufactured ceramic porous membranes are summarized in Table I.

Fig.l- TGA profile for a sample of 2E-^m green tape.

The TC technique allows a more accurate thickness control and the production of thinner ceramic tapes than the HR one. The porosimetric results show constant Vp (about 235 mm3/g for Al203-3nm and 340 mm3/g for SiC-5pm) as reported in table I. This parameter is independent on the nature of the binder, it appeared to depend mainly on the particle size of the starting material. In fact, samples prepared with larger particle size (3F-18nm) produce ceramic tapes with large pores and with a drastic lowering of the total pore volume and of the porosity.

The preparation technique could influence the value of the average pore radius. In Fig. 2 are reported the porosimetric curves of two samples of SiC membranes (3H-5pm and 3E-5pm), produced by the same starting powder and firing conditions but with different green preparation method. The HR sample has smaller pores and a more dense structure than the TC one. Moreover, the porosimetric analyses measured, also, a smaller total pore volume for the 3H-5pm sample that is in agreement with the earlier evidence. These differences of the structures could be the effect of the typical pressure exerted by the cylinders on the paste, during the hot rolling procedure.

TABLE I - Main characteristics of some porous membranes manufacture.

Samplc--particle size

Mater.

Procedure

Thermal Treatment

Thick, mm

Perm. *

Vp mm /g

Rav lim

0 0

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