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Title

WETTING OF FUEL CELL MATERIALS BY MOLTEN CARBONATE: OBSERVATION OF SPREADING AND PENETRATION

Creator

Gao, Liangjuan

Date

2018, 2018-05

Description

The molten carbonate fuel cell (MCFC) continues to attract significant attention due to its high performance over a lifetime of three to five...
Show moreThe molten carbonate fuel cell (MCFC) continues to attract significant attention due to its high performance over a lifetime of three to five years. The wetting of fuel cell materials by the molten carbonate is key to the long-term performance. Therefore, the wetting behavior under MCFC operating conditions was studied by means of the sessile drop method using a digitized optical analysis system. Specifically the spreading of molten carbonate on dense and porous materials was determined, as well as the penetration into porous materials. Observations were made of the melting and spreading of a solid carbonate pellet upon controlled temperature increase, placed on top of the dense or porous substrate, under either a reducing atmosphere (80%H2+20%CO2 humidified at 45oC), pure CO2 atmosphere, or oxidizing atmosphere (1%O2+99%N2). To provide a relatively simple base case, an extensive study of wetting of dense Ni foil was made. It was demonstrated that the water-gas shift reaction occurred at the interface of the Ni surface and molten carbonate under reducing atmosphere but not under pure CO2 and oxidizing atmospheres. The contact angle was affected by the mass of the carbonate pellet under reducing atmosphere but not under pure CO2 atmosphere. The molten carbonate spread rapidly under oxidizing atmosphere due to the surface oxidation of Ni. The wetting of porous Ni substrate was influenced by the porosity, the amount of carbonate in relation to the empty pore volume available (expressed as degree-of-filling), and the thickness of the substrate. The spreading of molten carbonate on the surface of the porous substrate, as well as penetration into the pores of the substrate was observed and the rates of these two processes were measured as accurately as possible. A linear velocity averaged over a pore was expressed in terms of the absorption rate. A simple model containing the formation of film on the pore walls and the bulk pore filling was established. The wetting of dense and porous Ni-Al alloy substrate was investigated. It revealed that the wettability of Ni-Al substrate was improved by increasing the content of Al under both pure CO2 and reducing atmospheres. The absorption rate of porous Ni-Al substrate was significantly larger than that of a porous Ni substrate of compatible porosity. The absorption rate was significantly slowed down only when the volume of molten carbonate exceeded 1.3 times the volume of empty pores inside the substrate. It was demonstrated that the mechanical strength of α-LiAlO2 matrices is improved by heat-treating at 800oC under ambient gas atmosphere. The non-heat-treated and heat-treated samples were completely wetted by molten carbonate and exhibited the same wetting behavior. A non-heat-treated α-LiAlO2 sample cracked during the wetting investigation, however, the heat-treated α-LiAlO2 matrices did not crack, presumably due to their enhanced mechanical strength.
Ph.D. in Materials Science and Engineering, May 2018
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