The electrolyte matrix plays a pivotal role on electrolyte management, gas sealing efficiency and durability of molten carbonate fuel cells (MCFCs). The matrix is manufactured by tape casting a slurry of ultrafine particles of LiAlO₂ and large size (>10µm) alumina crack arrestors to form a well-packed fine-pore microstructure. The electrolyte distribution and electrodes polarization in MCFC are controlled by the balance in capillary forces among the porous cell components. The electrolyte matrix needs to remain completely filled; therefore, its pores need to be stable and significantly finer than the electrodes to exert sufficient capillary force to prevent electrolyte loss and gas cross-over during endurance operation. A well balanced cell-component pore structure offers stable low cell resistance and enhanced triple phase boundary (TPB) in the electrodes.

Accelerated tests and long-term operations of FCE’s internal-reforming MCFC (Direct Fuel Cell, DFC^®) stacks demonstrated that the baseline electrolyte matrix meets the requirement for life greater than 5 years. However, development of more stable matrix and advanced electrolyte management designs are required to further extend cell life and enable accelerated MCFC deployment.

This paper will review baseline matrix stability in stack endurance tests and the progress on developing a unique matrix design that offers significant enhanced pore and material stabilities. Parameters affecting particles coarsening and phase transformation under reducing environment will be presented.

Experimental

Experiments were performed in 30W single cells (250 cm²) and 30 kW technology stacks. Each cell consisted of a porous Ni-alloyed anode, a porous in-situ lithiated NiO cathode and a porous matrix (LiAlO₂) filled with the alkali carbonate electrolyte. Tests were performed at different temperatures and gas atmospheres to investigate parameters and processes affecting matrix stability. Different techniques such as XRD, XPS, SEM, Mercury porosimetry, AC-impedance and steady-state polarization were carried out to evaluate and characterize matrix stability under DFC operating conditions.

Results and Discussion

Literature reported that matrix material LiAlO₂ coarsening increases with higher temperature, lower CO₂ partial pressure and in strong basic melt (1,2). Post-test analysis of accelerated FCE endurance single cell tests showed that the baseline matrix coarsening is more pronounced at the anode/matrix interface, whereas much slower at the center and the cathode side. A series of single cell tests were performed to investigate the benefits of newly developed “Smart” matrix in terms of pores and particles stability. Results showed that the advanced matrix offers >60% reduction of particles coarsening in anode side compared to the baseline (Figure 1). Porosimetry analysis showed essentially no pore-structure change (up to 6,000 hours). This improvement enables enhanced gas sealing efficiency, reduced electrolyte loss and extended cell life. Solubility and phase stability studies under different conditions will be also reported.

Acknowledgement

The advanced matrix research is funded by the Office of Energy Efficiency and Renewable Energy (EERE), U.S.

Department of Energy, under Award Number DE-EE0006606

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2. K. Takizawa, A. Hagiwara , J. Power Sources, 109, 127 (2002).