(Invited) Key Contributions in the Fundamental Understanding of Carbonate Fuel Cells
Authors: Venkataraman Ramki , Yuh Chao-Yi 
Published: February 2020

With growing concern about greenhouse-gas induced global warming, fuel cells are expected to play an important role on reducing CO2 emissions due to their high efficiency while providing stable base-load power. Molten carbonate fuel cells (MCFC) are unique compared to other fuel cell technologies due to the role of carbonate ions as the charge transport species in the electrolyte. CO2 is an additional reactant in the cathode and an additional product in the anode, i.e., CO2 is transported across the electrolyte membrane. This makes the MCFC stacks very attractive for concentrating and capturing CO2 from the exhaust of fossil fuel power plants.
FuelCell Energy (FCE), Inc. in Danbury, CT is the world leader in developing and marketing carbonate fuel cells, with its Direct Fuel Cell (DFC®) power plants installed and operating at more than 50 locations worldwide. DFC®stacks are unique in the way they use internal reforming to efficiently produce hydrogen and remove waste heat during operation. FCE has a growing fleet with an installed capacity of > 180MW comprising several fuel cell parks including the largest in the world at 59MW. Customers of DFC power plants include wastewater treatment plants, telecommunications/data centers, manufacturing facilities, office buildings, hospitals, and universities.
Several research teams world over have worked on the MCFC technology over the past 5 decades. Among those efforts, contributions by Dr. Russ Kunz clearly stand out both in terms of the breadth of areas addressed and the simplicity of the mathematical treatment. Dr. Kunz [1,2] presented an elegant mathematical model of the cathode treating its structure as electrolyte-filled agglomerates. His fundamental half-cell electrochemical studies on cathode described various rate-controlling processes (kinetic, ionic and mass-transfer) and optimization approaches to enhance performance [3].
Electrolyte migration along the manifold seals and segregation within a cell are challenges for liquid-electrolyte fuel cells. His fundamental understandings on electrolyte ionic transport accurately captured migration and segregation, and his approaches are still in use today [4,5].
Ni internal shorting had been a major technical endurance barrier during the early days of the MCFC development. The dissolution of lithiated NiO cathodes results in a transport of metallic nickel into the electrolyte matrix and can result in an electronic short circuit between the electrodes. His theoretical analysis, accounting for the dissolution, diffusion, transference, and convection of nickel, accurately correlated with his experimental results and provided valuable mitigation approaches [6].
This paper will discuss the basic challenges in MCFC, importance of the contributions by Dr. Kunz and the current state of the technology.

References

Kunz, H. R., Bregoli, L. J, and Szymanski, S. T., J. Electrochem. Soc.,
131, 2815 (1984).

Kunz, H. R. and Murphy, L. A., J. Electrochem. Soc.,
135, 1124 (1988).

Bregoli, L. J. and Kunz, H. R., J. Electrochem. Soc.,
129, 2711 (1982).

Kunz, H. R., J. Electrochem. Soc., 134, 105 (1987).

Kunz, H. R., Tutorial in Electrochemical Engineering – Mathematical Modeling, Proceedings of the International Symposium, The Electrochemical Society, PV99-14, 191 (1999)

Kunz, H. R. and Pandolfo, J. W., J. Electrochem. Soc., 139, 1549 (1992).