The molten carbonate fuel cell (MCFC) has emerged as one of the leading power-generation devices to efficiently convert chemical energy into electricity. To date ~ 4 Billion kWh of electricity has been produced commercially using this new technology. The matrix, a porous microstructure consisting mainly of ultrafine sub-micro LiAlO2particles sandwiched between two electrodes, immobilizes liquid electrolyte, isolates fuel from oxidant and facilitates ionic transport. The matrix holds a very important key to higher power density operation, longer service life and lower cost. Therefore, fine and stable pores are required to maintain sufficiently higher capillary force to retain and reduce electrolyte loss during endurance service.
Besides the improved pore structure stability, the electrolyte matrix has to remain substantially crack-resistance in order to provide effective gas sealing and eliminate cell resistance increase. FCE has made major advancements in matrix design and demonstrated > 5-year useful life through accelerated tests and field stack operation. However further improvements of matrix material stability (phase and particle size) are desired to extend cell life and accelerate MCFC deployment.
This paper will review the progress on developing an innovative matrix material and design that would enable stable performance and extended cell life. Mechanistic understanding of particles coarsening and schemes to enhance matrix material stability will be discussed.
Experimental
Experiments were performed with button and single cells (3 cm2 and 250 cm2, respectively). 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 under standard fuel (72.8%H2-18.2%CO2-9%H2O) and oxidant (18%CO2-12%O2-67%N2-3%H2O) conditions. Other specially designed out-of-cell tests (OCT) were also performed at different temperatures and gas atmospheres to investigate parameters and processes affecting matrix material 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 fuel cell operating conditions.
Results and Discussion
It has been reported in the literature that LiAlO2 particles generally coarsen faster at higher temperatures, under lower CO2 gas atmosphere, and in more basic electrolyte compositions (1-2). It is well understood that LiAlO2dissolves slightly in molten carbonate, via basic dissolution:
LiAlO2 + O2- → Li+ + AlO33-
Such dissolution-precipitation process may result in particle coarsening (Ostwald ripening) and pore growth.
FCE has developed an innovative stabilization approach to mitigate particles coarsening and enhance matrix pores stability for extended cell life. Figure 1 shows a comparison of the population of large pores (>0.2mm) of the advanced matrix to the conventional baseline design under accelerated conditions. The improved matrix offers consistent >60% reduction of pores larger than 0.2mm at the beginning of test compared to the baseline design and maintains pore size stability throughout the tests. These results were confirmed by high resolution SEM and specific surface area measurements (BET). This improvement is expected to enable enhanced gas sealing efficiency, reduced electrolyte loss and extended cell life. Additional test results from OCT and cells will be reported for mechanistic understanding.
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
1. S. Terrada, K. Higaki, I. Nagashima, Y. Ito, J. Power Sources, 83, 227 (1999).
2. E. Antolini, Ceramics International, 39, 3463 (2013).