Fluidized Bed Production of Surface Functionalized Powders for Solid Oxide Fuel Cell Cathodes

Tuesday, 25 July 2017: 11:40
Grand Ballroom West (The Diplomat Beach Resort)
N. Sbrockey (Structured Materials Industries, Inc.), M. Aindow, B. Deljoo (University of Connecticut), H. Ghezel-Ayagh, A. Torabi (FuelCell Energy, Inc.), and G. Tompa (Structured Materials Industries, Inc.)
Solid oxide fuel cells (SOFC's) offer significant energy benefits, due to their high efficiency and capability to utilize either hydrogen or hydrocarbon fuels. Presently, there is considerable incentive to develop medium temperature (600 C to 800 C) SOFC's, in order to reduce material compatibility issues and to reduce the engineering challenges associated with high operating temperatures. Developing cathode materials with both high electrical conductivity and high catalytic activity at intermediate temperatures is one of the key challenges for SOFC development. La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is the present state of the art material for intermediate temperature SOFC cathodes. While monolithic LSCF cathodes have sufficient electrical conductivity, overall fuel cell efficiency can decrease at intermediate temperatures due to reduced electrochemical kinetics.

One approach to enhance intermediate temperature reaction kinetics is to use "surface functionalized" cathode materials, in which a thin layer of a high catalytic activity material, such as La0.8Sr0.2MnO3 (LSM), is deposited on the surface of a high conductivity material such as LSCF. Prior works have demonstrated the benefits of surface functionalized cathode materials [1,2]. These studies used liquid infiltration from a nitrate solution to deposit LSM onto the surface of post-sintered LSCF cathodes. An optimum surface loading of LSM on LSCF was identified, beyond which cathode performance was reduced. This effect was attributed to decreased conductivity of the surface layer, as the LSM thickness increased. Despite the demonstrated benefits of surface functionalized cathode material, the liquid infiltration process has not been widely implemented in SOFC production, mainly due to concerns about cost and manufacturing complexity.

This presentation will describe a technique for producing surface functionalized cathode powders by fluidized bed chemical vapor deposition (FBCVD). The FBCVD process combines two well established technologies, fluidized bed processing and chemical vapor deposition of oxide films. The starting material in this study was a commercial LSCF cathode powder (0.7 μm average diameter). Various LSM surface loadings were deposited by FBCVD, using the metal organic precursors La(thd)3, Sr(thd)2 and Mn(thd)3 along with an O2/Ar gas mixture. The resulting LSM coated LSCF powders were screen printed onto scandia stabilized zirconia electrolyte disks, and sintered to prepare symmetric half cells. A similar cell was prepared using uncoated LSCF powder for comparison. Electrochemical impedance spectroscopy was done to characterize performance of the half cells under open circuit potential, at 600 C, 700 C and 800 C in air. In each case, the LSM coated powders outperformed the uncoated LSCF, as indicated by a lower polarization resistance. An optimum surface loading was observed at 0.19 moles of LSM per kg of LSCF powder, which resulted in minimum polarization resistance, for each temperature tested. High resolution transmission electron microscopy shows the LSM surface functionalized layer to consist of uniformly dispersed fine particles on the LSCF surface.

The FBCVD process is economical, and can be easily scaled to high volume production. The resulting surface functionalized cathode powders represent a drop-in replacement for conventional cathode powders in SOFC manufacturing. The FBCVD process is shown to be an effective and economical technique to produce surface coatings on powders for solid oxide fuel cells, and many other applications.

1. Ze Liu, Mingfei Liu, Lei Yang, Meilin Liu, Journal of Energy Chemistry, Vol. 22, 555 (2013).

2. M.E. Lynch, L. Yang, W. Qin, J.-J. Choi, M. Liu, K. Blinn, M. Liu, Energy & Environmental Science, Vol. 4(6), 2249 (2011).